cranial osteology of the african gerrhosaurid angolosaurus skoogi

Original article

Cranial osteology of the African gerrhosaurid Angolosaurusskoogi (Squamata; Gerrhosauridae)

HOLLY A. NANCE

Department of Geological Sciences, Jackson School of Geosciences, The University of Texas at Austin,Campus Mail Code: C1100, Austin, Texas, 78712, USA

Present address: Department of Biological Sciences, 132 Long Hall, PO Box 340326, ClemsonUniversity, Clemson, South Carolina, 29634-0326, USA.

[email protected]

Abstract.—Phylogenetic relationships both within and between the scincomorph familiesGerrhosauridae and Cordylidae are in need of re-examination. Currently the basal gerrhosaurid isunknown, although Angolosaurus skoogi previously was proposed as the sister taxon to mainlandAfrican gerrhosaurids. Many details of the cranial osteology of A. skoogi are also unknown because ofthe rarity in museum collections and elusive lifestyle of this dune-dwelling lizard, endemic to the NamibDesert. In this study, High-Resolution X-ray Computed Tomography (HRXCT) was used to study indetail the cranial osteology of A. skoogi. Results of this study enabled completion of the first anatomi-cal description of the skull and mandible of A. skoogi. Throughout the description, reference is made toa fully labeled cross-sectional HRXCT data set, available online as supplementary material. Detailedinformation on cranial osteology obtained from these data can help resolve the contentious placement ofA. skoogi within Gerrhosauridae. Unlike other Cordyliformes (Gerrhosauridae + Cordylidae), the post-frontal in A. skoogi is not extensive and does not extend posteriorly beyond the orbital margin. Thesquamosal is not bifurcated anteriorly in A. skoogi, unlike other gerrhosaurids. In the braincase of A.skoogi, there is no separate foramen for passage of the glossopharyngeal nerve (cranial nerve IX). Thismorphological study emphasizes the utility of both HRXCT scans and disarticulated skeletal material asvaluable sources of phylogenetically-informative data.

Key words.—Angolosaurus skoogi; Cordyliformes; Namib Desert; anatomy; skull, mandible

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African Journal of Herpetology, 2007 56(1): 39-75. ©Herpetological Association of Africa

The acquisition and subsequent study ofrare, extant species, such as the coelacanth

or the tuatara, have provided many insightspertaining to both the phylogenetic relation-ships of related taxa and their morphologicalcharacter states. However, there are inherentchallenges associated with studying the mor-phology of fauna that are poorly represented inmuseum collections. Among these are smalltaxon sample sizes (of particular concern to

systematists), and obtaining data on detailedanatomy while maintaining the integrity of thelimited number of specimens available forresearch. Both of these challenges are encoun-tered by squamate morphologists and systema-tists who study the enigmatic Angolosaurusskoogi, a gerrhosaurid lizard endemic to theNamib Desert in southern Angola and northernNamibia (Fig. 1) and known to spend up to 24hours a day buried beneath the sand dunes

(Steyn 1963; Mouton & van Wyk 1997). Sucha limited geographic range and elusive lifestyle make A. skoogi a difficult animal to study(FitzSimons 1955). While the biology andecology of this dune-dwelling gerrhosauridhave been examined (Hamilton & Coetzee1969; Pietruszka et al. 1986; Mitchell et al.1987; Pietruszka 1988; Seely et al. 1988; Nagyet al. 1991), information regarding its detailedcranial osteology is minimal (Lang 1991;Lochetto 2002). This is due in part to theextremely limited number of skeletal speci-mens available for study. Of these few speci-mens, most are whole-body preparations pre-served in alcohol, making detailed study of thecranial osteology, and particularly the brain-case, impossible. Data pertaining to its anato-my are important because the phylogeneticposition of A. skoogi within Gerrhosauridae, afamily of lizards endemic to sub-SaharanAfrica and Madagascar, has historically beenproblematic.

Angolosaurus skoogi previously was hypothe-sised to be the sister taxon to mainland Africangerrhosaurids (Lang 1991) (Fig. 2), althoughmore recent analyses suggested A. skoogi isnested within Gerrhosaurus (Lochetto 2002;Lamb et al. 2003). Its potential basal positionwithin African Gerrhosauridae renders

A. skoogi is a key taxon in addressing the larg-er question regarding relationships among thescincoid families Gerrhosauridae, Cordylidae,and Scincidae. However, attaining morpholog-ical data necessary to clarify the phylogeneticposition of this gerrhosaurid is difficult for tworeasons. First, as previously mentioned,detailed anatomy of A. skoogi is known fromrelatively few specimens due to the rarity ofthis extant gerrhosaurid in museum collections.Second, a lack of detailed cranial osteologicaldata on A. skoogi results from the layer ofosteoderms present in all gerrhosaurids andcordylids. In the cranial region, these ossifica-tions often fuse to the bones of the skull, ren-dering osteological features like bone-to-bonecontacts and sutures difficult, if not impossibleto see in articulated specimens (Fig. 3).Although these ossifications are less numerousin A. skoogi than in other cordyliforms (e.g.,Gerrhosaurus and Cordylus), their presenceobstructs a clear view of the cranial anatomy.To circumvent these two problems, I utilizedHRXCT (see Methods) to digitally remove theosteoderms from scanned images of the skull ofthis rare lizard and study its anatomy in detail.

Cranial osteology has been recognized as animportant source of taxonomically significantdata for over 150 years (e.g., Cope 1864, 1871,

AFRICAN JOURNAL OF HERPETOLOGY 56(1) 2007

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Figure 1. Geographic range of Angolosaurus skoogi,shaded in grey on the map.

Figure 2. Preferred generic relationships amongCordyliformes (Cordylidae + Gerrhosauridae) fromLang (1991).

1892). Among gerrhosaurids, many studiesdevoted to cranial osteology focused on thegenus Gerrhosaurus (e.g., Siebenrock 1892;Brock 1935; Broom 1935; Malan 1941). Thefollowing anatomical description is the firstdetailed cranial osteological study of A. skoogi,which can be used to help resolve its phyloge-netic position within Gerrhosauridae.

MATERIALS AND METHODS

Biological material.—Two formalin-fixed andalcohol-preserved specimens of Angolosaurusskoogi (Fig. 4), an adult (California Academyof Sciences [CAS] 206978) and a juvenile(CAS 206977) were scanned at The Universityof Texas High Resolution X-ray ComputedTomography Facility (UTCT). Both specimenswere captured near the Odondujengo River inNamibia on July 27, 1998 by A. M. Bauer, A.C. Lamb, W. R. Branch, and J. L. Wright.

Scanning parameters.—Both specimens of A.skoogi were scanned using the ultra-high reso-lution subsystem at the UTCT. High resolutionX-ray computed tomography uses the differen-tial attenuation of X-rays as they pass throughmaterials of different density and composition(such as scales, flesh, cartilage, and bone) togenerate a series of digital images or ‘slices,’ inwhich grayscale values correspond to densityand elemental contrast. Series of these two-

dimensional images are stacked to render athree-dimensional image of the object and itsinternal structure (Rowe et al. 1999; Ketcham& Carlson 2001). This technique enables digi-tal serial-sectioning of an object at a slice reso-lution of 20 microns or less (Rowe et al. 1999),and offers a non-destructive means to study theinterior of an object (e.g., Rowe et al. 1997;Rowe et al. 1999; Maisey 2001; Polcyn et al.2002; Tykoski et al. 2002; and Brochu 2003).The specimens were mounted in a plastic cylin-der, and positioned to acquire a series of slicesalong the transverse (= coronal) plane. Theadult specimen is nearly twice as large as thejuvenile, so different scanning parameters wereused. The adult specimen was scanned using anX-ray energy set to 100 kV and 0.24 mÅ, witha slice thickness and inter-slice spacing of 0.53mm. The image field of reconstruction was 18mm. The image was reconstructed as a 512 x512 16-bit TIFF file, resulting in an in-planepixel resolution (i.e., the distance between eachpixel) of 0.035 mm. A total of 402 transverseslices through the head and neck wereobtained. The juvenile specimen was scannedwith an X-ray energy set to 120 kV and 0.27mÅ. The slice thickness and inter-slice spacingwas 0.05 mm, and the field of reconstructionwas 10 mm. The image of the juvenile wasreconstructed using the same parameters as theadult, except with an in-plane pixel resolutionof 0.02 mm.

NANCE .— Cranial Osteology of A. skoogi

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Figure 4. Specimens of A. skoogi: top, adult (CAS206978); bottom, juvenile (CAS 206977).

Figure 3. Three-dimensional HRXCT volumetricreconstruction of the skull depicting lateral view ofA. skoogi (CAS 206978) with osteoderms present.

Image processing.—To create serial cross-sec-tional slices of the cranium and mandible of A.skoogi, the transverse stack was digitallyresliced in both the frontal (= horizontal) andsagittal planes. Three digit file names for eachstack were drawn into the upper left-hand cor-ner of each image using Slicorama®. Thetransverse stack was sequentially numberedfrom the tip of the snout to the posterior base ofthe skull, the frontal stack was numbered fromthe dorsal surface of the skull to the ventral sur-face, and the sagittal stack was numbered fromthe right side of the skull to the left side. Tonalrange, or brightness, contrast, and color, of theHRXCT images were adjusted in AdobePhotoshop® to optimize visibility of anatomi-cal detail.

QuickTime Pro® was used to create cut-awaymovies of each anatomical plane. For thesemovies, the HRXCT images were imported andexported at a speed of 15 frames per second.The parameters ‘graphics compression,’‘grayscale depth,’ and ‘best quality’ were usedto reduce file size while maintaining the quali-ty of the images in the movies.

Adobe Illustrator® was used to label the dataset of the adult specimen, adopting the UTCTconvention of labeling bones red, and process-es and foramina yellow. Teeth are labelled blueand additional structures are green. Every fifthslice in the transverse plane is labelled, as wellas additional slices in the transverse, frontal,and sagittal planes when necessary to depictmorphological details. Abbreviations used inlabeling the HRXCT data are listed in Table 1.Labeled figures in the text are black and white,with bones written in all uppercase letters andprocesses and foramina in lowercase.

Osteological description.—Several previousstudies on squamate anatomy were referenced,including those by Cope (1892), Siebenrock(1892), Malan (1941), van Pletzen (1946),Säve-Söderbergh (1946, 1947), Oelrich (1956),

Baird (1960), Kamal (1965, 1966a,b, 1969),Edmund (1969), Wever (1978), Bellairs &Kamal (1981), de Queiroz (1987), and Evans(in press). Unless otherwise noted, the termi-nology used here was adopted by Evans (inpress) for both the labelled HRXCT dataset andthe anatomical description. Structures namedby Evans (in press) with a cardinal direction (e.g., ‘posterior process’) were referred to by theirpreviously published names or a new name ifnone existed. Although the HRXCT imagesonly show detailed osteology (and not softanatomy), reference to some soft anatomicalfeatures was made in the description of A. skoo-gi. While tentative, these references (e.g., pas-sage of nerves or blood vessels through foram-ina) were made based on descriptions of theclosely related Gerrhosaurus (Malan 1941) andCordylus (van Pletzen 1946), and the more dis-tantly related iguanids Ctenosaura (Oelrich1956) and Iguana (de Queiroz 1987). However,these soft anatomical features were not visiblein the HRXCT images of A. skoogi.

When applicable, character states discussed inthe phylogenetic analyses of Estes et al. (1988)and Lang (1991) were referenced. The charac-ter numbers used in this description are thesame as those used by the original authors, pre-ceded by a ‘C’ (for ‘character’) if Estes et al.(1988), and proceeded by a ‘CC’ if


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Table 1. Abbreviations used in HRXCT data set.

Abbreviation Full terminology

ant. anteriordent. dentaryf. fenestra, foramen, fossahor. horizontalinf. inferiormax. maxilla, maxillaryp. processpost. posteriorpremax. premaxilla, premixillary

Lang (1991). The additional ‘C’ in the lattercase reflects the designation of ‘cranial’ char-acters by Lang (1991). The polarity for thecharacter state given corresponds to that usedby those authors.

The cranial description is based on the adultspecimen of A. skoogi (CAS 206978).Individual elements of the skull are describedstarting with bones located at the tip of thesnout and working towards the back of theskull. Individual elements comprising theosseus braincase and the mandible are thendescribed, followed by the dentition. In theosteological description, reference is made tothe appropriate labeled HRXCT slices in whichstructures can be seen. These slices can beviewed as QuickTime movies at theHerpetological Association of Africa’s home-page (http://www.wits.ac.za/haa/). These refer-ences are made parenthetically, indicating inwhich anatomical plane and on which num-bered slices the structures appear. The follow-ing abbreviations were used: transverse plane =Tra, frontal plane = Fro, and sagittal plane =Sag.

Despite having only two specimens available tostudy, preliminary observations of ontogeneticvariation in A. skoogi were made by comparingthe HRXCT images of the adult and the juve-nile. Designation of the larger of the two spec-imens as an adult was based on development ofthe frontals and parietals, fusion between theprootic and basisphenoid, and fusion betweenthe prootic and supraoccipital being to theextent of that found in most mature squamates(Maisano 2001, 2002). To compare the twospecimens of A. skoogi, previous work relatedto squamate ontogenetic variation was refer-enced, including studies by de Queiroz (1987),Barahona and Barbadillo (1998), Barahona etal. (1998, 2000), and Evans (in press).Characters known to vary between adult andjuvenile iguanids, as well as those that differbetween the two specimens of A. skoogi, are

discussed. Additional variation between thetwo specimens regarding the stapes and denti-tion was also observed and discussed here.

RESULTS

GENERAL SKULL ARCHITECTURE

The overall shape of the skull of Angolosaurusskoogi reflects its dune-dwelling lifestyle. Theanterior end of the snout is depressed, causingthe entire skull to be shovel-shaped in lateralview (Fig. 3). In dorsal view, the snout appearsdepressed laterally, widening more posteriorly(Fig. 5). At the level of the coronoid process ofthe coronoid, the skull reaches its widestbreadth, which it maintains toward the posteri-or end of the skull in dorsal view.

The skull can be divided into six regions basedon their relative positions; the snout, palate,orbital region, temporal region, osseus brain-case, and the mandible. The snout is bound bythe median premaxilla anteriorly, and thepaired maxilla anterolaterally, nasal dorsally,and vomer ventrally. The paired septomaxillalies within the cartilaginous portion of the nasalchamber. The palate includes the paired vomer,palatine, pterygoid, and ectopterygoid. Theorbital region consists of the paired prefrontal,lacrimal, jugal, frontal, postfrontal, and postor-bital. The temporal region of the skull houses


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Figure 5. Dorsal view of the skull of A. skoogi (CAS206978) with osteoderms removed.

large temporal muscles (Oelrich 1956), andhence requires large fenestrae into which thesemuscles can expand as the jaw is adducted.These openings are discussed below with theother openings in the skull. The bones compris-ing the temporal region and forming the mar-gins of these fenestrae are the unpaired, medi-an parietal, and the paired squamosal,supratemporal, and quadrate. The osseus brain-case is located in the posterior region of theskull and includes the paired prootic and otooc-cipital, and the median sphenoid, basioccipital,and supraoccipital. The mandible, or lower jaw,lies ventral to the lateral margins of the palateand consists of six paired elements: the dentary,splenial, coronoid, angular, surangular, andarticular.

OPENINGS IN THE SKULL

At the anterior end of the snout is the pairedexternal naris (fenestra exonarina of Oelrich(1956)). This fenestra is oval-shaped in lateralview in A. skoogi (Fig. 6), and is bordered ante-riorly by the premaxilla, laterally by the maxil-la, posteriorly by the nasal, and ventrally by thevomer. More posterior is the large, paired orbit(Fig. 6). Each large, nearly circular opening iscovered dorsally by osteoderms, and has palpe-bral ossifications confined to the anterior mar-gin (Tra 108-133; Estes et al. 1988: C36 (1)).The orbit is bordered by the prefrontal andlacrimal anteriorly, the frontal medially, themaxilla and jugal ventrally, and the postorbitaland postfrontal posteriorly. In A. skoogi, theanteroventral border is formed by the maxillain lateral view, because the anterior portion ofthe jugal is confined to the medial surface ofthe maxilla (Estes et al. 1988: C31 (0); Lang1991: CC4 (1)). Medial to each orbit is a bonyring of individual scleral ossicles surroundingthe eye (Tra 134-238). The number of individ-ual ossicles comprising each scleral ring andtheir imbrication pattern varies withinSquamata (Gugg 1939; Underwood 1970,1984; de Queiroz 1987; Estes et al. 1988). The

resolution of the scanned images is inadequateto determine the number and pattern of scleralossicles present in A. skoogi.

Posterior to the orbit, in the temporal region, isthe paired upper temporal fenestra (supratem-poral fossa of Oelrich (1956)). This fenestra isclosed in other gerrhosaurids (Malan 1941), butis present as a slit bordered by the postorbital,parietal, supratemporal, and squamosal in A.skoogi (Lang 1991: CC8 (0)). Each fenestra iscovered dorsally by osteoderms, and boundedby the postorbital anteriorly, the parietal medi-ally, and the supratemporal and squamosal pos-teriorly (Fig. 7). The paired lower temporalfenestra (infratemporal fossa of Oelrich (1956))is bound anterodorsally by the jugal, pos-terodorsally by the squamosal, and posteriorlyby the quadrate and supratemporal (Fig. 6).Each fenestra is open ventrally in A. skoogi, asin all squamates (Evans, in press). The pairedpost-temporal fenestra (posterior temporalfossa of Oelrich (1956)) is situated more poste-riorly. This narrow opening is between the pari-etal and ossified braincase (Fig. 8). The medi-an, posterior opening of the osseus braincase,where the spinal cord exits the skull, is the fora-men magnum (Fig. 8). Its margins are formedby the supraoccipital dorsally, the paired exoc-cipital components of the otooccipital laterally,and the basioccipital ventrally.

The paired choana (fenestra exochoanalis ofOelrich (1956)) is located ventrally within thepalate. The choana is an oval-shaped openingbordered anteromedially by the vomer, antero-laterally by the maxilla, and posterolaterally bythe maxillary process of the palatine (Fig. 9).The paired suborbital fenestra (inferior orbitalfenestra of Oelrich (1956)) is located more pos-teriorly. Each fenestra is bordered anterolater-ally by the maxilla, posterolaterally by theectopterygoid, and medially by the palatine andpterygoid (Fig. 9). This fenestra transmits thesuperior alveolar nerve and artery (Oelrich1956). Posterior to the suborbital fenestra is the


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Figure 7. Labeled dorsal view of the skull of A. skoogi (CAS 206978) with osteoderms removed.

Figure 6. Labeled lateral view of the skull of A. skoogi (CAS 206978) with osteoderms removed.

paired subtemporal fenestra. This fenestra isbound anteriorly by the ectopterygoid, medial-ly by the pterygoid, and posteriorly by thequadrate (Fig. 9). The lateral margin of the sub-temporal fenestra is open, and this fenestra pro-vides room for the mandibular adductor mus-cles (Evans, in press).

The paired coronoid recess is more of a hollowspace than an opening or fenestra (Fig. 9). Thisspace is bound laterally by the jugal, mediallyby the ectopterygoid and pterygoid, andreceives the coronoid process of the coronoidbone when the mandible is adducted (Evans, inpress).

The interpterygoid vacuity (palatal sinus andpyriform recess of Oelrich (1956); pyriformrecess of Estes et al. (1988)) is located at theventral midline of the skull. This elongatedopening (Fig. 9) is narrow at its anterior-mostpoint, where it separates the posterior ends ofthe paired vomers medially (referred to as thepalatal sinus by Oelrich (1956)). It continuesposteriorly, widening between the palatines andthe pterygoids (referred to as the pyriformrecess by Oelrich (1956)). The posterior marginof the vacuity is formed laterally by the pairedbasipterygoid process of the sphenoid, andmedially by the sphenoid bone.

INDIVIDUAL ELEMENTS

Premaxilla.—The premaxilla is a median,unpaired, bilaterally symmetrical bone (Tra 15-60, Figs. 6-8), and forms the anteriormost ele-ment of the snout. The premaxilla laterally con-tacts both maxillae, posterodorsally meets theanteromedial edges of the nasals, and pos-teroventrally meets the anterior margins in thevomers. The premaxilla has a pair of lateralprocesses, called the palatal processes, whichborder the external nares and meet the antero-medial margin of the maxillae laterally. Thenasal process of the premaxilla lies mediallybetween the external nares, is wide laterally,and extends posteriorly, tapering to a point just

posterior to where it meets the nasals. Thisprocess is smooth dorsally, and appears flat-tened dorsoventrally in transversal view. Theincisive process on the ventral side of the pre-maxilla is a small, anteriorly-rounded projec-tion extending anteriorly toward the posteriormargins of the premaxillary teeth. The premax-illa is broad and curved anteriorly, taperingposteriorly to form an anchor-shaped bone(Oelrich 1956).

The anteroventral surface of the premaxillabears a row of seven unicuspid teeth; one toothlies along the midline and both the right andleft sides bear three teeth each. The anterodor-sal surface (anterior, rostral body of Oelrich(1956)) is pierced by a pair of foramina (Tra20-27). These foramina transmit the medialethmoid nerves (cranial nerve V [CN V])(Oelrich 1956). At the base of the nasalprocess, located on the midline of the premax-illa, is a single oblong foramen piercing thedorsal surface (Tra 30-41). This foramen doesnot penetrate the ventral surface, nor does itcontinue either anteriorly or posteriorly as acanal in the bone.

Septomaxilla.—The paired septomaxilla in A.skoogi is a short and narrow bone, lying nearthe midline of the skull between the premaxillaand the vomer (Tra 41-92, Fig. 7). It is locatedjust posterior to the posteroventral border of thepremaxilla, and meets the medial edge of thevomerine process of the maxilla anteriorly, andthe pointed anterior tip of the vomer posterior-ly. There is no contact between the septomaxil-la and the premaxilla in A. skoogi. Medially, theseptomaxilla nearly meets its twin on the mid-line, forming a raised crest (Estes et al. 1988:C40 (1)). The septomaxilla extends within thenasal vestibule, and is mainly supported by sur-rounding cartilage in other lizards (Malan1941; van Pletzen 1946; Oelrich 1956).

The septomaxilla is broad anteriorly and has aventral, posteriorly-projecting septal process


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Figure 8. Labeled posterior view of the skull of A. skoogi (CAS 206978) with osteoderms and cervical ver-tebrae removed.

Figure 9. Labeled palatal view of the skull of A. skoogi (CAS 206978) with jaw disarticulated.

that nearly meets its opposite medially as ittapers to a point between the vomers posterior-ly. Anterior to this septal process is thevomeronasal organ. The dorsal surface of theseptomaxilla is expanded and convex (Estes etal. 1988: C41 (1)), but its ventral surface isconcave dorsally, allowing the bone to bothroof the vomeronasal organ and floor the ante-rior nasal vestibule (Malan 1941; van Pletzen1946; Oelrich 1956). The function of the sep-tomaxilla is thought to be for protection of thecartilages between the anterior chamber of thenasal capsule and the vomeronasal organ(Oelrich 1956).

Maxilla.—The large, mediolaterally-flattenedmaxilla (Tra 30-175, Fig. 9, 10, 11) forms thelateral side of the snout. Each maxilla has threeprocesses: the premaxillary, the orbital (Oelrich1956), and the facial. The maxilla meets thepremaxilla anteromedially, and the jugal andlacrimal posteriorly. Along the posteromedialside of the maxilla, just dorsal to the tooth row,this bone meets the anteriormost tip of theectopterygoid and the lateral margin of themaxillary process of the palatine. The facialprocess of the maxilla, located on its dorsal sur-face, contacts the lateral edge of the nasal ante-riorly. Posteriorly, it meets the ventral marginof the prefrontal. The ventral surface of themaxilla bears a single row of pleurodont (Esteset al. 1988: C84 (0)), monocuspid and polycus-pid teeth (described in detail below). Theorbital process of the maxilla extends posteri-orly to contribute to the anterior half of the ven-tral margin of the orbit in lateral view (Estes etal. 1988: C27 (0), C31 (0); Lang 1991: CC4(1)). This same condition was scored in othercordyliforms by Estes et al. (1988; their‘Cordylidae’ included both gerrhosaurids andcordylids as used in my study). The conditionscored by Lang (1991) differed in that the jugalwas said to form the anteroventral margin ofthe orbit, although that description does notspecify from which view the character wasscored.

The dorsal side of the premaxillary process(Tra 24-55) is flattened and slightly concave.The premaxillary process lies dorsal to thepalatal process of the premaxilla and meets theanterior margin of the vomer medially.

The orbital process of the maxilla (Tra 180-200) contacts the jugal and lacrimal posterior-ly, and the ectopterygoid medially. This processextends posteriorly beneath the orbit in A.skoogi, forming the anteroventral margin of theorbit. The medial side of this process forms thelateral margin of the wide suborbital fenestra.The lateral part of this process contributes tothe rim of the suborbital fenestra and serves asattachment for the inferior orbital membrane inother lizards (Oelrich 1956).

The facial process, located between the pre-maxillary and orbital processes of the maxilla(Tra 62-107), is a wide dorsal extension of themaxilla forming the lateral wall of the nasalchamber. This process tapers to a point dorsal-ly, separating the nasal and the prefrontal. Justventral to the facial process, the maxilla ispierced by five of seven foramina located alongits inferior lateral border (Tra 59-132). Theseforamina transmit cutaneous branches of thesuperior alveolar nerve (maxillary branch ofCN V) and the maxillary artery (Oelrich 1956).Anterodorsal to this row of foramina are threeto four small foramina piercing the medial wallof the maxilla (the precise number is difficult todiscern from the scanned images). Theseforamina transmit cutaneous branches of thelateral ethmoidal nerve (CN V) (Oelrich 1956).A large, single foramen, called the anteriorinferior alveolar foramen (Oelrich 1956), ispresent at the anterior base of the facial process(Tra 88-95). This anteriorly-directed foramentransmits the terminal branches of the maxil-lary artery and superior alveolar nerve to thenasal chamber (Oelrich 1956). The medial sideof the facial process meets the lateral side ofthe cartilaginous nasal chamber (Oelrich 1956).


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Figure 10. Dorsal view of the skull of A. skoogi (CAS 206978) detailing parietal morphology.

Figure 11. Labeled lateral view of the left mandible of A. skoogi (CAS 206978).

In contrast to the smooth, lateral side of themaxilla, the medial side has a shallow, medial-ly-extending palatal shelf (Lang 1991: CC19(0)) running along its length. This shelf floorsthe nasal chamber (Oelrich 1956), and dorsallybears 14 tooth positions, each with a functionaltooth or replacement tooth coming in (Tra 34-180). Along the labial margin of these teeth arelarge resorption pits (Estes et al. 1988: C85(0)), which mark the development of replace-ment teeth. These teeth have no basal infolding,and do not appear striated in A. skoogi (Estes etal. 1988: C86 (0)). There is no offset in thetooth margin of the maxilla (Estes et al. 1988:C87 (0)). The portion of the palatal shelf poste-rior to the palatine/maxilla suture forms the lat-eral margin of the choana, while the portion ofthe palatal shelf anterior to this suture forms thelateral margin of the fenestra vomeronasalisexterna (Oelrich 1956). Dorsal to this shelf isthe hollow superior alveolar canal, whichtransmits the maxillary artery and the superioralveolar nerve (Oelrich 1956). Both the nerveand the artery enter through the posterior alve-olar foramen, which pierces the anteroventralmargin of the facial process. Many small dentalforamina that transmit both dental nerves andarteries (Oelrich 1956) pierce the ventral sur-face of this hollow tube in the palatal shelf.

Nasal.—The nasal is a flat bone overlying theanterior part of the cartilaginous nasal chamber(Malan 1941; Oelrich 1956). The paired nasalbone (Tra 45-100, Fig. 7) lies midway betweenthe premaxilla and frontal. The right and leftnasals are medially sutured to each other forabout half of their entire length posteriorly.Each nasal meets the nasal process of the pre-maxilla anteromedially, the facial process ofthe maxilla anterolaterally, the prefrontal pos-terolaterally (Estes et al. 1988: C4 (0)), and thefrontal posteriorly. The nasal process of thepremaxilla separates the nasals anteriorly,while the anteromedially projecting process ofthe frontal separates the posterior extremities ofthe nasals, forming a V-shaped notch along the

midline of the pair, with the pointed tip of theV directed anteriorly. The posterior edge of thenasal overlies a thin, short shelf located at theanteriormost edge of the frontal. Each nasal issomewhat triangular-shaped, with the anteriorborder of the nasal perpendicular to the sagittalplane, while both lateral and medial borderstaper posteriorly to a point near the frontal.Along the anterior margin of the nasal, there isan anteriorly-concave excavation made by theexternal naris. This concavity results in antero-medial and anterolateral extensions of thenasal. The more medial of the two is referred tohere as the premaxillary process and the morelateral of the two as the maxillary process. In A.skoogi, the maxillary process is the longer ofthe two. The anterior border of the nasal con-tributes to the posterior margin of the externalnaris.

About two-thirds of the way toward the poste-rior end of the bone, lying along the midline ofeach nasal in dorsal view, is a foramen (Tra 77-80). In other lizards, this foramen transmitscutaneous branches of the lateral ethmoidalnerve (CN V1) and venous tributaries of theorbital sinus (Oelrich 1956). The remainingdorsal surface of the nasal is smooth.

Vomer.—Each vomer (Tra 75-127, Fig. 9) has aconvex ventral surface and concave dorsal sur-face. The posterior end of the vomer meets thedorsal surface of the vomerine process of thepalatine, while the anterior end meets the medi-al surface of the premaxilla. The anterior two-thirds of the vomer contact its twin medially,while the posterior third is separated by theanterior extension of the interpterygoid vacuity(median palatal sinus of Oelrich (1956)), aspace running anteroposteriorly between thetwo bones. There is no contact between thevomer and the maxilla in A. skoogi, thus theposterior border of Jacobson’s organ is notclosed by bone, representing the ‘paleo-choanate’ condition (Estes et al. 1988: C42 (0);Evans, in press). Each vomer is relatively large,


50


51

exceeding half the length of the maxillary toothrow in A. skoogi (Estes et al. 1988: C39 (1)). Indorsal view, the anterior-most portion of thevomer is narrow and triangular, and contactsthe other vomer medially (Fro 185-190).Posterior to this triangular portion, the vomernarrows laterally, while maintaining contactwith the other vomer medially. These pairedpalatal bones are not inseparably fused in theadult A. skoogi (Estes et al. 1988: C38 (0)). Forthe remaining two-thirds of its length, thevomer broadens laterally while the spacebetween the two vomers, the interpterygoidvacuity, broadens as well. The lateral border ofthe vomer is not constricted anteriorly, anddoes not form a laterally-concave margin in A.skoogi.

A large portion of the posterolateral border ofthe vomer contributes to the medial margin ofthe choana. In the iguanid lizard, Ctenosaurapectinata, the groove for the lacrimal duct runsalong the anterolateral border of each vomer(Oelrich 1956); however, a groove is not visi-ble in the scanned images of A. skoogi. Thisduct opens into and supplies fluid to thevomeronasal organ in lizards (van Pletzen1946). The median border of the fenestravomeronasalis externus (Oelrich 1956) is posi-tioned where the anterior end of the lacrimalgroove should be in A. skoogi. Runningbetween the anterolateral borders of eachvomer is a groove for the paired ramus medi-alis (palatine branch) of CN VII (Oelrich1956). There is no heart-shaped depressionalong the midline of the two vomers posterior-ly (Lang 1991: CC14 (0)).

Palatine.—The palatine (Tra 110-197, Fig. 9)contacts the palatine process of the pterygoidposteriorly, the palatine process of the maxillalaterally, and the posterior end of the vomeranteriorly. There is no contact between thepalatine and the ectopterygoid (Estes et al.1988: C45 (0)), or the jugal (Lang 1991: CC3(0)). The palatine has three processes: the ante-

rior vomerine process, the posterior pterygoidprocess, and the lateral maxillary process(Oelrich 1956). There is no medial extensionfrom the ventrolateral edge of the palatine con-tributing to a bony secondary palate (Estes etal. 1988: C43 (0)), and there is no medialextension from the anteromedial margin of thepalatine. The palatine forms a large portion ofthe palate, the floor of the orbit, and the floor ofthe posterior nasal chamber (Oelrich 1956).There are no teeth on the surface of the palatinein A. skoogi (Estes et al. 1988: C82 (1)).

The thin vomerine process (Tra 108-127)tapers slightly anteriorly, is concave dorsally,and forms the floor of the posterior nasal cham-ber. The anterior-most end of the processslightly overlaps the dorsal surface of the pos-terior end of the vomer (Tra 108-126). The lat-eral border of the vomerine process forms theposteromedial border of the choana. This open-ing is relatively prominent in relation to thesize of the palatine bone in A. skoogi (Estes etal. 1988: C44 (1)). In other lizards, the medialpalatine nerve runs along the midline of thedorsal surface of this process (Oelrich 1956).

The large, quadrangular pterygoid process ofthe palatine meets the palatine process of thepterygoid posteriorly, forming an interdigitat-ing suture (Tra 170-197). This process extendsposteroventrally from the main body of thepalatine. The concave dorsal surface forms thefloor of the orbit and anteriorly contains agroove running anteroposteriorly (Tra 145-165). This groove contains the main branch ofthe palatine plexus of cranial nerve VII(Oelrich 1956). The medial portion of thepterygoid process forms the lateral border ofthe interpterygoid vacuity, a V-shaped spacebetween the two palatines and the two ptery-goids with the wide, open end of the V direct-ed posteriorly. The anterior, pointed end of thisV-shaped space continues forward between themedial margins of the paired vomers. Thisspace is broad in A. skoogi (Estes et al. 1988:

C48 (1)). The lateral side of the process formsmost of the medial rim of the wide suborbitalfenestra.

The short maxillary process extends anterolat-erally from the junction of the pterygoid andvomerine processes (Tra 115-143). Its lateralborder contacts the palatine process of the max-illa ventrally, and the anteroventral palatineprocess of the prefrontal dorsally. The palatinedoes not meet the ectopterygoid posteriorly.Behind the contact of the maxillary processwith the prefrontal, is the palatine foramen (Tra128-136), which is more of a canal in A. skoo-gi. This canal emerges within the nasal cavityand carries the intermediate branch of nerveVII (Oelrich 1956).

The lateral border of the maxillary process con-tributes to the suborbital fenestra, which trans-mits the superior alveolar nerve and artery(Oelrich 1956). A ridge situated at the junctionof the maxillary and vomerine processes (Tra128-131) provides attachment for theorbitonasal membrane (Oelrich 1956). Anteriorto this ridge is an indentation between the max-illary and vomerine processes. This indentationforms the posterior rim of the choana. The ven-tral surface of the palatine is relatively smooth.

Ectopterygoid.—The ectopterygoid (Tra 172-218, Fig. 9) is a short, triradiate bone with dor-somedial and ventromedial processes that claspthe dorsal and ventral surfaces of the pterygoidflange. I am calling these processes the dorsalpterygoid process and the ventral pterygoidprocess (medial process of Oelrich (1956)),respectively. These processes are approximate-ly equal in length. A laterally-directed process,referred to here as the jugal process, extendshorizontally and contacts the ventromedial sur-face of the jugal. The jugal process also meetsthe posteromedial margin of the maxilla, butdoes not contact the palatine. The anteriormostend of the jugal process forms a distinct point.The ectopterygoid is a slender bone in A. skoo-

gi, as in all Cordyliformes, and does not restrictthe size of the suborbital fenestra (Estes et al.1988: C46 (0)). Its posterior end is broader thanits anterior end, but both ends of the bone arebroader than the narrow middle isthmus of thebone. The bone is oriented at an anterolateralangle, connecting the palate with the externalroofing elements (Oelrich 1956).

The ectopterygoid is an attachment site formany oral membranes (Oelrich 1956). The dor-sal side forms part of the orbit floor andreceives the orbital temporal membrane(Oelrich 1956). The anterior edge of the bonereceives the inferior orbital membrane, whichfills the suborbital fenestra. The ventromedialsurface of the posterior end of the ectoptery-goid is concave and bounds the coronoid recesswith the anterior tip of the transverse process ofthe pterygoid. This recess receives the coronoidprocess of the mandible (Oelrich 1956).

Pterygoid.—The pterygoids, comprising theposterior portion of the palate, (Tra 177-303,Figs. 8, 9) are a pair of dorsally-concave bonesthat are broad anteriorly and narrow posterior-ly. Anteriorly, the pterygoid meets theectopterygoid and the palatine. More posterior-ly, the pterygoid meets the epipterygoid on itsdorsal surface, and the basispterygoid processof the sphenoid bone on its medial surface. Atits posteriormost end, the pterygoid contactsthe medial margin of the quadrate. The broadanterior portion of the pterygoid is somewhattriangular shaped, while the posterior portion isa laterally-inflected and laterally-flattened, nar-row process.

The broad anterior end of the pterygoid has twoprocesses; the palatine process and the ptery-goid flange. The palatine process (Oelrich1956) dorsally meets the posterior end of thepalatine (Tra 170-195). This contact is quiteragged, resembling two V-shaped extensionson the pterygoid interlocking with the W-shaped end of the palatine. The ventrolateral


52

surface of the palatine process is convex and asin other gerrhosaurids, houses ventrally-extending pterygoid teeth in A. skoogi (Estes etal. 1988: C83 (0); Lang 1991: CC15 (1)) (Tra206-220). Medial to this convex surface arethree foramina (Tra 197-214), which transmitpalatine branches of CN VII (Oelrich 1956).

An anterolateral process of the pterygoid,called the pterygoid flange (Evans, in press;‘transverse process’ of Oelrich (1956)), con-tacts the ectopterygoid laterally (Tra 193-218),and forms the posterior bony limit of the oralcavity. This process is shorter and narrowerthan the palatine process and extends anterolat-erally from the posterolateral portion of thepalatine process of the pterygoid to meet theectopterygoid.

The sole posterior projection of the pterygoid isthe quadrate process. This process articulatesposteriorly with the ventromedial surface of thequadrate (Tra 290-302); anterodorsally it con-tacts the anteroventral end of the epipterygoid(Tra 240-245). That contact occurs within thefossa columellae of the quadrate process of thepterygoid. This fossa is a short and narrow pitrunning lengthwise toward the anterior end ofthe quadrate process. The medial wall of thispit extends more dorsally than the lateral wall.

The quadrate process of the pterygoid articu-lates anteromedially with the anterior edge ofthe basipterygoid process of the sphenoid bone(Tra 250-260) at the pterygoid notch (Oelrich1956). Located medial to the fossa columellaeof the quadrate process is the V-shaped ptery-goid notch, a synovial joint between thebasipterygoid process of the sphenoid bone andthe pterygoid (Oelrich 1956). Posterior to thisnotch, along the medial side of the quadrateprocess, is a medially concave groove runningposteriorly toward the posterior tip of theprocess (Tra 287-300). This groove is the sitefor insertion of the protractor pterygoideusmuscle (Oelrich 1956). The lateral side of the

quadrate process is smooth and convex lateral-ly. A small facet at the ventrolateral end of theprocess contacts the medial side of the quadrateas the pterygoid extends posteriorly and curvesslightly around the medial edge of the quadrate.

Epipterygoid.—The epipterygoid is present(Estes et al. 1988: C47 (0)) as a thin, columnarbone in A. skoogi (Tra 233-271). Its ventral sur-face sits in the fossa columellae, a concavity inthe dorsal surface of the pterygoid (Tra 233-248). Its dorsal end meets the membranousbraincase (Evans, in press), and the lateral mar-gin of the parietal via ligaments of thepseudotemporalis superficialis muscle (Oelrich1956). The slender epipterygoid bone is posi-tioned in the skull at an angle, its ventral endmore anterior than its dorsal end. The ventralportion of the epipterygoid is broader than therest of the bone, and flattened mediolaterally.The dorsal end of the bone tapers slightly as itextends posterodorsally and approaches, butdoes not contact the alar process of the prooticand the ventral downgrowths of the parietal.The space lateral to the dorsal end of theepipterygoid is called the cavum epiptericum.This space contains the trigeminal nerve gan-glion (Evans, in press). In coronal view, theepipterygoid is slightly curved, bending lateral-ly toward its dorsal end.

Lacrimal.—The lacrimal (Tra 110-121) is asmall, anteroposteriorly-flattened bone con-fined to the anterolateral margin of the orbit(Estes et al. 1988: C28 (0)). This element meetsthe lateral margin of the prefrontal, but is notfused to it (Estes et al. 1988: C29 (0)). It meetsthe maxilla anterolaterally and the anteriormosttip of the jugal posteriorly. The medial marginof the lacrimal comes very close to, but doesnot contact, the lateral margin of the maxillaryprocess of the palatine, thus forming a smallspace between the two bones (Tra 128-130). Asingle lacrimal foramen (Tra 128) pierces theposteromedial surface of the lacrimal (Estes etal. 1988: C30 (0)). In Ctenosaura, this foramen


53

transmits a vein that drains the skin closelycovering the lateral surface of the bone(Oelrich 1956). In Gerrhosaurus (Malan 1941)and Cordylus (van Pletzen 1946), the naso-lacrimal duct lies in a groove between thelacrimal and the prefrontal. The lacrimal isobscured in lateral view by the posterior por-tion of the facial process of the maxilla (Lang1991: CC2 (1)).

Jugal.—The jugal (Tra 120-248, Figs. 6, 7, 9)is an L-shaped bone in lateral view. It meets themaxilla anteriorly in a tongue-and-groove artic-ulation, the ectopterygoid medially, the postor-bital posteromedially, and the squamosal pos-terolaterally. This latter contact with thesquamosal was not found in the cordyliformsscored by Estes et al. (1988: C18 (1)), but isclearly present in A. skoogi. The anterior por-tion of the jugal contributes to the posteroven-tral margin of the orbit in lateral view. In bothdorsal and medial views, this contribution tothe orbit extends farther anteriorly. The jugal isdivided into two processes: the anterior maxil-lary process and the posterior temporal process(Oelrich 1956).

The maxillary process extends anteriorly alongthe medial wall of the maxilla. The dorsal sur-face of the maxillary process is slightlyexpanded mediolaterally, to help form the ven-tral wall of the orbit. In life the orbital fasciaattaches to this process of the jugal (Oelrich1956). The anterior portion of the lateral sur-face of the maxillary process is pierced by sev-eral small foramina (Tra 179, 193). These sub-orbital foramina transmit cutaneous branchesof the maxillary nerves (CN V) (Oelrich 1956).

The temporal process of the jugal extends pos-terodorsally and contributes little to the tempo-ral region in A. skoogi. However, the jugal doesmeet the postorbital to form a complete jugal-postorbital bar in A. skoogi (Estes et al. 1988:C32 (0)). Just anterior to where the temporalprocess of the jugal and the postorbital meet is

a tiny foramen piercing the medial side of thejugal (Tra 209-212). In Ctenosaura, a similar-ly-placed foramen transmits branches 1 and 2of the maxillary nerve (CN V2) (Oelrich 1956).A portion of the temporal process also providesattachment for the infratemporal fascia(Oelrich 1956).

Prefrontal.—The prefrontal is a roughly pyra-mid-shaped bone forming most of the anteriororbital margin (Tra 99-145, Fig. 6). The antero-lateral border of this bone meets the posteriormargin of the nasal process of the maxilla. Indorsal view, the anteromedial border of the pre-frontal contacts the lateral edge of the nasal,while the posteromedial border of the pre-frontal meets the anterior edge of the anterolat-erally projecting process of the frontal. Theposterolateral border of the prefrontal con-tributes to the anterior margin of the orbit. Theposterolateral surface of the prefrontal meetsthe tiny lacrimal bone. The ventromedial exten-sion of the prefrontal, called the palatineprocess (Oelrich 1956), meets the dorsal edgeof the maxillary process of the palatine. Thereis no contact between the prefrontal and anyposterior orbital elements (e.g., the postfrontalor the postorbital) in A. skoogi (Estes et al.1988: C5 (0)), as is the case in allCordyliformes (Cordylidae + Gerrhosauridaefrom Lang (1991)). The prefrontal precludescontact of the facial processes of the maxillaand the anterolateral margins of the frontal inA. skoogi.

The palatine process of the prefrontal and theventrally-descending process of the frontalform the orbitonasal fenestra (Tra 127-130), anopening between the orbital and nasal cavities(Oelrich 1956). The borders of this fenestraprovide attachment for the orbitonasal mem-brane (Oelrich 1956). The posterolateral edgeof the prefrontal is much like that of othercordyliforms and like that in Gerrhosaurus(Malan 1941). It is concave posteriorly,smooth, and curved to form the anterolateral


54

rim of the orbit. In A. skoogi, the posterolateraledge of this surface articulates with the flatlacrimal bone, but does not contribute to for-mation of the lacrimal foramen, unlike inGerrhosaurus where the prefrontal contributesto both the foramen and the lacrimal canal(Malan 1941). The posterior rim of the pre-frontal bone also provides attachment for theorbital fascia (Oelrich 1956). The lateral, medi-al, and dorsal surfaces of the prefrontal are rel-atively smooth.

Frontal.—The paired frontal bones (Tra 90-229, Fig. 7) meet along the midline. They arenot inseparably fused in A. skoogi, much likeother gerrhosaurids (Estes et al. 1988: C6 (0);Lang 1991: CC6 (0)). The frontal bone meetsits opposite medially, with the contact betweenthe pair forming a straight line running alongthe sagittal plane. The frontal bone also con-tacts the prefrontal anterolaterally, the nasalanteriorly, the parietal posteriorly, the post-frontal posterolaterally, and contributes to thedorsal margin of the orbit laterally. The frontalbone does not contribute to the bony externalnaris in A. skoogi (Estes et al. 1988: C2 (0)),and thus is similar to the case in other cordyli-forms. The paired frontal is dorsoventrally flat-tened. The posterior margin is broad, while thecentral and anterior portions of the sutured pairare narrow. When paired, the lateral borders ofthe frontals are roughly parallel to one another(Estes et al. 1988: C7 (0)).

The anterior tip of the frontal lies ventral to theposterior end of the nasal (Tra 90-103), but nobroad frontal shelf is present (Estes et al. 1988:C8 (0)). Posteriorly, the frontal meets the pari-etal, and overlies anterolateral parietal tabs,which were scored as both present and absentin cordyliforms (Estes et al. 1988: C22 (1)).These tabs are thin, triangular-shaped, anterior-ly-projecting extensions of the parietal oneither side of its anterior border. There are notabs on the posterior rim of the frontal (Estes etal. 1988: C11 (0)).

The ventral surface of the frontal is smooth,and has a ventrally-extending ridge, called thesupraorbital ridge (Oelrich 1956), runningalong its lateral edge for the length of the bone(Tra 135-215). This ridge is short for most of itslength, except at the anterior-most edge of thebone, where the ridge extends ventrally towardthe palatine (Estes et al. 1988: C9 (1)). Thisventral extension is called the crista cranii (Tra127-234). In other lizards, a thin part of the car-tilaginous solium supraseptal attaches to thecristae cranii (Oelrich 1956; Evans, in press).In A. skoogi, the cristae cranii do not contactone another below the olfactory tracts (Estes etal. 1988: C10 (0)), nor are they pierced ven-trally by a small foramen present in other ger-rhosaurids. Medial to the supraorbital ridge is ashallow groove. This groove is the olfactorytract (olfactory canal of Oelrich (1956)) andholds the olfactory stalks and olfactory bulbs ofthe brain (Oelrich 1956). In other lizards, a pairof narrow excavations in the posterior half ofthe frontal, located just medial to the supraor-bital ridges (Tra 195-215), provide attachmentfor the thick part of the cartilaginous soliumsupraseptal (Oelrich 1956). Just posterior tothese excavations is the attachment site for theepioptic membrane (Oelrich 1956). The poste-rior portion of the frontal also covers part of themembranous braincase. There is no contribu-tion to the parietal foramen by the frontal in A.skoogi (Estes et al. 1988: C25 (0)).

Palpebrals.—The palpebral ossifications in A.skoogi are located at the anterior-most cornerof each orbit (Tra 108-133). They meet the pos-terolateral margin of each prefrontal anteriorly,and meet the ventral margin of the supraorbitalosteoderms that cover the orbits dorsally. Theseossifications within the eyelid are known inother cordyliforms and scincids (Estes et al.1988: C36 (1)).

Postfrontal.—The postfrontal is a small boneconfined to the posterior rim of the orbit (Tra219-230, Fig. 6) (Estes et al. 1988: C15 (1)).


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This bone was said to be “extensive” and “notconfined to the orbital rim” in Cordyliformes(Cordylidae in Estes et al. 1988: p. 129).However, the postfrontal in A. skoogi is smalland does not extend posteriorly beyond themargin of the orbit. The medial margin of thepostfrontal meets the posterolateral tip of thefrontal and the anterolateral tip of the parietaljust anterior to where the frontal and parietalmeet. There is no contact between the post-frontal and the posterior end of the jugal. Thepostfrontal is not forked where it contacts thefrontal, and does not clasp the frontoparietalsuture in A. skoogi (Estes et al. 1988: C13 (0);Lang 1991: CC7 (1)). This is a reversal of ascleroglossan synapomorphy (Estes et al.1988). This state reversal was scored similarlyfor A. skoogi by Lang (1991); however, in theanalysis by Estes et al. (1988), the state scoredfor all Cordyliformes was that in which thepostfrontal clasps the frontoparietal suture.

Laterally, the postfrontal overlies the medialedge of the postorbital. Osteoderms over theorbit and upper temporal fenestra obscure thistiny bone in dorsal view, but cross-sectionalimages (Tra 220-230) show that it is not fusedto the postorbital and, thus, is a separate ele-ment (Estes et al. 1988, C12 (0), C14 (0)). Thissmall bone functions as a splint between thefrontal, parietal, and postorbital, to reinforcethe postorbital arch (Oelrich 1956). In A. skoo-gi, the postfrontal is not the primary elementrestricting the upper temporal fenestra (Estes etal. 1988: C20 (0)).

Postorbital.—The postorbital (Tra 218-243,Figs. 6, 7) is present in A. skoogi (Estes et al.1988: C16 (0)) as a flat, boomerang-shapedbone contributing to the posterior rim of theorbit. Its lateral side meets the posteromedialborder of the temporal process of the jugal,while its medial side meets the lateral marginof the postfrontal. The postorbital has a strong,anteroventrally-directed process that runsalong the medial margin of the jugal, forming

approximately one-half of the posterior marginof the orbit (Lang 1991: CC5 (1)). This charac-ter state for the postorbital bone was scored dif-ferently for cordyliforms by Estes et al. (1988:C17 (1)), as being primarily a temporal boneforming less than one half of the posterior rimof the orbit when viewed either medially or lat-erally. Although the upper temporal fenestra isoverlain by osteoderms in A. skoogi, it is clearfrom the transverse slices that the postorbital isthe primary restricting element of the anterolat-eral rim of this fenestra (Estes et al. 1988: C19(1)).

The anterior rim of the postorbital, whichforms the posterior margin of the orbit, is con-cave anteriorly. This rim also provides attach-ment for the infratemporal fascia and the ante-rior portion of the origin of the levator angu-laris oris muscle (Oelrich 1956). The dorsal andlateral surfaces of the bone are smooth.

Parietal.—The parietals (Tra 213-366, Figs. 6,8) fuse ontogenetically to form a single roofingelement lying along the midline of the skull andcovering the osseus and membranous compo-nents of the braincase (Evans, in press).Anteriorly, it meets the frontal, and its antero-lateral tip just meets the posterolateral marginof the postfrontal. Laterally, the parietal formsthe medial margin of the upper temporal fenes-tra. Its posterolateral extensions, the pairedpostparietal processes (supratemporal process-es of Oelrich (1956)), contribute to the posteri-or rim of the upper temporal fenestra dorsallyand the dorsal rim of the post-temporal fenestraventrally. Their lateral margins articulate withthe medial margins of the supratemporal bonesdistally. The posterior margin of the parietalforms a long, forked process, called the parietalfork (Oelrich 1956), which is directed posteri-orly and braces the supraoccipital crest (Lang1991: CC11 (1)) (Tra 300-366).

The parietal in A. skoogi is slightly concaveventrally and somewhat rectangular in shape.


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The postparietal processes and the parietal forkborder the prominent emargination of the pari-etal in A. skoogi (Estes et al. 1988; Lang 1991),causing the posterodorsal surface of the osseusbraincase to be exposed in dorsal view (Estes etal. 1988: C24 (1); Lang 1991: CC18 (0)).Prominent emargination of the parietals wasnot present in all members of Cordyliformesexamined by Estes et al. (1988). I found noemargination in Cordylus, Pseudocordylus, andPlatysaurus. The anterior rim of the parietalhas two triangular-shaped parietal tabs (Estes etal. 1988: C22 (1); Lang 1991: CC10 (1)).These are thin, anteriorly-directed extensionsoverlain by the posterior border of the frontal(Tra 213-225). Although Estes et al. (1988)noted that presence of parietal tabs is variablein Cordyliformes, Lang (1991) scored them asabsent in A. skoogi. Laterally, a V-shapedextension of the parietal extends into the medi-al margin of the upper temporal fenestra, withthe pointed tip of the V directed laterally (Fig.10). For the most part, the dorsal parietal sur-face is smooth. In Gerrhosaurus andCtenosaura, the dorsolateral surface providesattachment for the origin of the jaw muscles,the pseudotemporalis superficialis, and theadductor externus medius (Malan 1941;Oelrich 1956).

The dorsal surface of the parietal is penetratedby a distinct parietal foramen (Estes et al. 1988:CC26 (0)), in the very center of the bone (Esteset al. 1988: C25 (0)) (Tra 250-253). The pari-etal foramen is obscured from view when theosteoderms are present, but digital removal ofosteoderms reveals the presence of this fora-men (Fig. 10).

The ventral surface of the parietal is concave,causing the whole bone to be slightly round andconvex dorsally, in lateral view (Lang 1991:C12 (0)). Along the midline of the ventral sur-face, just anterior to the base of the parietalfork, is an excavation called the parietal fossa(Oelrich 1956). This fossa (Tra 277-287),

receives the anterior end of the processusascendens of the tectum synoticum, a cartilagi-nous process that projects from the supraoccip-ital (Malan 1941; Oelrich 1956; Evans, inpress). The parietal fossa is partly floored ven-trally by a plate of bone originating from theanterior margin of the fossa, and has a finger-like process projecting anteriorly along its ven-tral surface. Anteriorly, on either side of theparietal fossa, are shallow grooves extending tothe anteroposterior midpoint of the bone. Inother lizards, these grooves house the taeniamarginalis cartilage (Malan 1941; Oelrich1956). Extending anterolaterally from the pari-etal fossa, and continuing anteriorly, are ven-trally-directed ridges on either side of the mid-line of the bone (Tra 225-285). These ridgesrun along the length of the parietal and meet thefrontal/postfrontal contact. These ventral ridgesprovide attachment for the origin of the levatorpterygoideus muscle (Malan 1941; Oelrich1956). However, the anterior portions of theseridges do not extend far ventrally toward thedorsal end of each epipterygoid (Estes et al.1988: C23 (1); Lang 1991: CC9 (1)). Lateral tothese ridges, on either side of the bone, areslight excavations within its ventral surface.These are for insertion of the cartilaginousextension of the paired epipterygoid bone(Malan 1941; Oelrich 1956).

The posterior border of the parietal bone cre-ates the dorsal rim of the post-temporal fenes-tra. This rim also provides attachment for sev-eral occipital muscles, including the depressormandibularis, the spinalis capitis, and the epis-ternocleidomastoideus (Oelrich 1956).

Squamosal.—The squamosal (Tra 243-321,Figs. 6, 8) is present in A. skoogi (Estes et al.1988: C33 (0)) as a flat, paired bone best seenin lateral view. This bone contributes to the lat-eral margin of the upper temporal fenestra andthe posteriodorsal margin of the lower temporalfenestra. Its anterior-most extension comes to apointed tip and meets the posterior end of the


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jugal. The posteroventral surface of thesquamosal meets the dorsal surface of thequadrate, while the posteromedial edge con-tacts the medial margin of the postorbital andthe anterior edge of the supratemporal bone.

This flat bone is tadpole-shaped in lateral view,with the round, sagittally-expanded end of thebone directed posteriorly and a pointed exten-sion projecting anteriorly. This basic shape ispresent in all gerrhosaurids examined.However, all other gerrhosaurids examinedhere have a bifurcated anterior end, while A.skoogi has a single anterior extension. Theanterior process of the squamosal meets thejugal anteriorly and contributes a small portionto the posterior rim of the lower temporal fen-estra. The posterior portion of the squamosal isexpanded dorsoventrally, but there is no superi-or process of the squamosal in A. skoogi (dor-sal process of Estes et al. 1988: C34 (0)).

In other lizards, the squamosal is held in placeby ligaments and, thus, is potentially kinetic(Oelrich 1956). The lateral surface is smooth,providing attachment for the levator angularisoris muscle and the adductor externus superfi-cialis muscle, whereas its medial side providesattachment for the adductor externus medialismuscle (Oelrich 1956).

Supratemporal.—The supratemporal (Tra 302-327, Figs. 6-8) is present in A. skoogi (Estes etal. 1988: C35 (0)) as a paired element at theposterior end of the skull. It contributes a smallportion to the lateral rim of the post-temporalfenestra and the posterior rim of the upper tem-poral fenestra. This bone contacts the lateralside of the postparietal process of the parietal.The posterior process of the supratemporalcurves ventrally to meet the posteromedial tipof the cephalic condyle of the quadrate. Thisbone anteriorly meets the posterior edge of thesquamosal, and ventromedially meets the later-al border of the paraoccipital process of theotooccipital.

The supratemporal is shaped somewhat like aslender tadpole in lateral view. Two regionswere previously recognized (Oelrich 1956); thepointed anterior region (Tra 302-314), whichmeets the postparietal process of the parietal,and the posteriorly directed process (Tra 315-327), which sits on the dorsal surface of thequadrate. The latter process appears to bewedged between the otooccipital, quadrate, andsquamosal in posterior view (Fig. 8). Thisprocess, directed ventrally in A. skoogi, is con-cave on both its medial and lateral marginswhere it meets the paraoccipital process of theotooccipital and the posterior end of thesquamosal, respectively. In other lizards, theanterior portion of the supratemporal bone pro-vides attachment for the origin of the adductorexternus medius muscle laterally, and providesattachment for the origin of the adductor exter-nus profundus muscle medially (Oelrich 1956).In Ctenosaura, the supratemporal bone sup-ports the posterolateral portion of the parietal,and serves as the center of motion between theoccipital and maxillary segments of the skull(Oelrich 1956).

Braincase.—The osseus braincase, or oticooc-cipital region, is comprised of five bones, thesphenoid and basioccipital, midline boneswhich form the floor of the braincase; theprootic and otooccipital, paired elements form-ing the walls of the cranial cavity; and thesupraoccipital, the midline roofing bone of thebraincase. This bony region of the braincasehouses the posterior part of the brain and theinner ear. As in other lizards, there is a mem-branous component to the braincase in A. skoo-gi, called the orbitotemporal region. The onlyossified element to this region is theorbitosphenoid. Due to the density contrastbetween fully ossified material and the remain-ing cartilaginous, membranous component,adequate detail of the orbitotemporal region isnot visible in the scanned images and only the


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orbitosphenoid is discussed.

Orbitosphenoid.—The orbitosphenoid is apaired, ossified element suspended in the mem-branous orbitotemporal region of the braincase(Tra 224-239). The bone is crescent-shaped inlateral view and concave posteriorly. In large,adult Iguanine lizards, the orbitosphenoid bearsfour processes; an anterodorsally-projectingprocess that connects to the planum suprasep-tale, a posteriorly-projecting process that joinsthe pila accessoria and the pila antotica, aninferior process that is confluent with the carti-laginous portion of the pila metoptica, and aventrally-projecting process that contributes tothe anterior portion of the pituitary region(Oelrich 1956; de Queiroz 1987). In the adultspecimen of A. skoogi, the orbitosphenoid lacksall of these well-developed processes exceptthe posteroventrally-directed inferior process.It is continuous with the hypochiasmatic carti-lage of the pila metoptica. This overall shapemore closely resembles that of juvenile, large-bodied iguanines and adult, small-bodied igua-nines (de Queiroz 1987). The anterodorsal endsof the two orbitosphenoids may fuse to eachother medially in large-bodied iguanines (deQueiroz 1987), but are not fused in A. skoogi(Fig. 12; the image was difficult to render dueto low density of the element).

Sphenoid.—The sphenoid (Tra 181-309, Fig. 9)forms the anterior floor of the braincase. Thiselement is comprised of an inseparably fusedbasisphenoid and dermal parasphenoid in adultlizards (Bellairs & Kamal 1981; Evans, inpress). These two components are distinct indeveloping skulls. The sphenoid bone shares astraight articulation with the basioccipital pos-teriorly and a dorsolateral suture to the prootic.Both of these sutures lack overlap between thestructures. It has three anteriorly-directedprocesses. The thin, dermal parasphenoid ros-trum (parasphenoid process of Oelrich (1956))is fused to the median ventral border of thesphenoid underneath the ossified portion of the

paired trabecular cartilages, called the cristaetrabeculares (Tra 181-260). At the ventrolateralangles of the sphenoid, the paired basiptery-goid processes (Tra 247-275) extend anteriorly;their broad, distal ends articulate with themedial edge of the quadrate processes of thepterygoids. In A. skoogi, the basipterygoidprocesses are short and broad, as is the case inall gerrhosaurids (Lang 1991: CC16 (0)). Inbetween the paired basipterygoid processes, thenarrow parasphenoid rostrum supports the ossi-fied bases of the trabeculae cranii (Evans, inpress). The sphenoid is broad anteriorly andtapers somewhat posteriorly.

On the dorsal surface of the sphenoid is thedorsum sella (Säve-Söderberg 1947), an anteri-orly-concave, vertical plate of bone towards the


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Figure 12. Three-dimensional HRXCT reconstruc-tion of orbitosphenoid in A. skoogi (CAS 206978).Top: Paired orbitosphenoid in anterior view withdorsal margin toward the top of the figure. Bottom:Lateral view of left orbitosphenoid with anteriormargin toward the left of the figure.

anterior end of the sphenoid (Tra 268-273). Thetop of the dorsum sella forms a deep, dorsally-concave crest called the crista sellaris (Tra 265-270). More laterally, the dorsum sella extendsdorsally into the bases of the anterior inferiorprocesses of the prootic. The abducens nerve(CN VI), the vidian branch of cranial nerve VII,the palatine artery, and the carotid artery passthrough this wall via a series of canals. Theanterior end of the vidian canal (Tra 265-293),through which the vidian nerve and the palatineartery pass (Oelrich 1956), is formed mediallyby the sphenoid bone and ventrally by theparasphenoid (Evans, in press). The anteriorexit of this canal, located at the base of thebasipterygoid process, is lateral to the anteriorexit of the carotid canal (Tra 270-276), which isthe passage for the internal carotid artery(Oelrich 1956). Posteriorly, these two canalsmerge inside the sphenoid bone, and exit poste-riorly, ventrolateral to the anterior auditoryforamen (Tra 290-296). The entire length of thevidian canal is bound medially and ventrally bythe sphenoid (Estes et al. 1988: C53 (0)).

Dorsal to the vidian and carotid canals, theabducens canal pierces the dorsum sella (Tra269-272), and carries the abducens nerve(Oelrich 1956). This canal is located just underthe crista sellaris in the retractor pit, a recesswhich gives rise to the bursalis muscle and theretractor bulbi muscle (Säve-Söderbergh 1946;Oelrich 1956). Posteriorly, this canal opens intothe cranial cavity. The ventral surface of thesphenoid bone is smooth.

Prootic.—The paired prootic (Tra 267-325)forms the anterolateral wall of the braincase.The prootic contacts the sphenoid ventrally, thesupraoccipital dorsally, and the otooccipitalposteriorly. The prootic also forms the anteriormargin of the fenestra ovalis, and contributes tothe anterior portion of the vestibule, which con-tains the cochlear cavity, the anterior semicir-cular canal, and the horizontal semicircularcanal (Evans, in press).

The prootic has three major processes, calledthe alar, the anterior inferior (Evans, in press),and a posteriorly-directed process which I callthe otooccipital process. The most anterior ofthese paired processes are the short, broad alarprocesses (Tra 266-288), each extendinganterodorsally in A. skoogi (Estes et al. 1988:C49 (1)). The anterior edge of each process iscalled the crista alaris (Evans, in press). Justposterior to the alar process, the prootic meetsthe supraoccipital and borders the acousticrecess anteriorly. The interior of the alarprocess is hollow, and opens posteriorly intothe anterior and horizontal osseus semicircularcanals.

The anterior inferior process if the prootic islocated ventral to the alar process (Tra 267-280). The anteriorly-concave notch betweenthese two processes is called the incisura proot-ica (trigeminal notch of Oelrich (1956)) (Tra279-281). The median head vein and thetrigeminal nerve (CN V) pass through thisnotch; the ganglion for CN V lies lateral to it inthe cavum epiptericum (Evans, in press). Insome squamates (e.g., iguanians), there is ananterodorsally-projecting process extendingfrom the incisura prootica. However, thisprocess, called the supratrigeminal process, isnot present in A. skoogi (Estes et al. 1988: C50(0)).

The otooccipital process extends posterolater-ally to overlap and meet the anterolateral sur-face of the paraoccipital process of the otooc-cipital (Tra 295-305). This posterolaterally-projecting process provides a facet anteroven-trally for the paraoccipital process and housesthe horizontal osseus semicircular canal as itcourses posteriorly through the prootic (Evans,in press).

The prootic contributes to the vestibule, whichis formed anteriorly by the prootic and posteri-orly by the otooccipital, and houses the organsof the inner ear (Wever 1978). Also housed


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inside the vestibule is the large, dense, crystal-lized statolithic mass (Tra 294-333). Thesestructures are present in all vertebrates and aidin detecting movement and maintaining properorientation (Wever 1978). Anteroventral to thevestibule is a laterally-concave recess withinthe medial wall of the prootic called theacoustic recess. This recess (Tra 290-304)houses the anterior auditory ganglion (Oelrich1956). Ventrolateral and posterior to theacoustic recess and ventral to the vestibule isthe lagenar recess (Tra 293-324). This recesshouses the lagena, a major component of thecochlear duct in lizards (Wever 1978). Like thevestibule, the lagenar recess is formed anterior-ly by the prootic and posteriorly by the otooc-cipital.

The prootic is pierced by several foramina. Theanterior-most opening along the medial wall ofthe prootic is the facial foramen (Tra 292-295),through which passes the facial nerve (CN VII)(Oelrich 1956). This foramen is locatedanteroventral to the vestibule and anterior tothe acoustic recess. Within the acoustic recessare two other foramina. The anteriormost of thetwo is the anterior auditory foramen (Tra 290-296). It transmits the anterior branch of theanterior ramus of the acoustic nerve (CN VIII)(Oelrich 1956), and opens into the otic capsule(Tra 281-336). Posterior to this foramen in theacoustic recess is the posterior auditory fora-men (Tra 303-305), which opens into the lage-nar recess. Dorsal and posterolateral to thelagenar recess is the fenestra ovalis (Tra 307-322), formed anteriorly by the prootic and pos-teriorly by the opisthotic portion of the otooc-cipital. The footplate of the stapes sits withinthis fenestra.

The lateral surface of the prootic is character-ized by a prominent crest running posteriorly,called the crista prootica (Tra 276-295). Inother lizards, this crest provides attachment forthe protractor pterygoidei muscle, and helpsform the roof and lateral wall of the tympanic

cavity (Evans, in press). In A. skoogi, the cristaprootica is well-developed ventrally, creating adeep canal between its medial surface and thelateral surface of the prootic in ventral view.Just ventral to this crest is the lateral exit of thefacial foramen, and a groove through whichruns the principle head vein (Evans, in press).

Otooccipital.—The otooccipital (Tra 317-357,Fig. 8) is formed by fusion early in ontogeny ofthe exoccipital and the opisthotic bones(Bellairs & Kamal 1981; Evans, in press).These bones are inseparably fused in mostadult squamates (Evans, in press), as is the casein A. skoogi. The anterior edge of the otooccip-ital meets the posterior edge of the prootic dor-sally where they together enclose the fenestraovalis, and ventrally where they form the lage-nar recess. The posteromedial portion (theexoccipital contribution) forms the lateral rimof the foramen magnum (Tra 325-336), andarticulates ventrally with the basioccipital. Thelaterally expanded posterior face of the otooc-cipital is called the crista tuberalis (Säve-Söderbergh 1947). In A. skoogi, the cristatuberalis does not extend far enough laterally toobstruct view of the anterior margin of the lat-eral aperture of the recessus scala tympani(LARST), when viewed posteriorly. The otooc-cipital forms the posterolateral and posteriorwall of the braincase, and the posterior half ofthe vestibule. The exoccipital components formthe ventrolateral portions of the occipitalcondyle in A. skoogi.

The otooccipital has one laterally-projectingprocess: the paraoccipital process (Tra 305-335). This process abuts the supratemporalbone dorsolaterally, and the dorsomedial aspectof the quadrate ventrolaterally. In coronal view,the paraoccipital process is slightly broader atits distal end than at its proximal end. Ventrally,this process bears a crest, called the interfenes-tral crest (crista interfenestralis of Säve-Söderbergh (1947)), which separates the fenes-tra ovalis from the secondary tympanic mem-


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brane housed within the lateral opening of therecessus scala tympani (Evans, in press).Running through the proximal end of thisprocess is the horizontal semicircular canal(Oelrich 1956) (Tra 299-336). This canalextends anterolaterally and courses around theauditory chamber.

There are a number of foramina piercing theotooccipital. The vagus foramen marks theboundary between the opisthotic and exoccipi-tal components of the otooccipital, while thehypoglossal foramina pass entirely through theexoccipital component of the otooccipital(Kamal 1966b). Three hypoglossal foraminaprovide passage for CN XII (Sag 158-172, 228-237) and are visible on the posteromedial sideof the otooccipital just dorsal to its suture withthe basioccipital, and can be seen from the pos-terolateral side of the bone as well. The oval-shaped vagus foramen, which transmits cranialnerve X, is located dorsal to the hypoglossalforamina and posterior to the auditory capsulewithin the otooccipital (Sag 150-160, 236-243).

Posterior to the lagenar recess, the otooccipitalcontributes to the recessus scala tympani. Thishollow cavity is floored by the basioccipitaland houses the terminal end of the perilym-phatic sac, which exits the vestibule via the per-ilymphatic duct through the perilymphatic fora-men (Wever 1978). The round, medial openingof this recess, called the medial aperture of therecessus scala tympani (MARST) (Tra 315-322), is slightly anterior the LARST (Tra 317-326). While the MARST is formed entirely bythe otooccipital, the LARST is formed dorsallyby the otooccipital and ventrally by the basioc-cipital. In A. skoogi, the MARST is about halfthe size of the LARST. The recessus scala tym-panum is connected with the vestibule by theperilymphatic foramen (Tra 315-324), whichlies medial to the fenestra ovalis and on theventral floor of the vestibule. This foramen isenclosed entirely within the otooccipital, and

carries the perilymphatic duct away from thebraincase (Evans, in press).

The HRXCT images of A. skoogi show no indi-cation of a separate foramen for the passage ofthe glossopharyngeal nerve (CN IX). Withinsquamates, there are three known courses CNIX takes as it exits the braincase (Kamal 1969).In a purely intracapsular course, the nervepasses through the internal glossopharyngealforamen (Kamal 1969) in the medial wall of thevestibule, traverses it, exits the vestibulethrough the perilymphatic foramen, traversesthe recessus scala tympani, and finally exits thebraincase via the lateral opening of the recessusscala tympani. In a purely extracapsular course,CN IX exits the cranial cavity through themedial opening of the recessus scala tympani,traverses the recessus, and exits the braincasethrough the lateral opening of the recessusscala tympani (Kamal 1969). The third courseis neither completely intra- nor extracapsular.Along this course, the glossopharyngeal nerveexits the cranial cavity through the glossopha-ryngeal foramen (Kamal 1969), located on theventromedial edge of the vestibular wall. Thenerve traverses the prootic, without enteringthe vestibule, to enter the recessus scala tympa-ni, and then exits the recessus through the lat-eral opening of the recessus scala tympani. Thecourse in A. skoogi appears to be purely extra-capsular, with the nerve traversing the recessusscala tympani via its medial and lateral open-ings. Neither a separate foramen piercing themedial wall of the vestibule, nor a canal tra-versing the medial wall, are visible.

In Gerrhosaurus major, a separate foramen forCN IX was found in an articulated skull exam-ined in this study (CAS 204767). This samespecimen was scanned using HRXCT, and theresulting images also revealed a separate glos-sopharyngeal foramen. However, locating thisforamen was difficult and required carefulsearching through all three anatomical sliceplanes to corroborate its position. Because the


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foramen is so difficult to find in the HRXCTscan, and considering that the skull of thatspecimen of G. major is nearly twice the size ofthe scanned specimen of A. skoogi, it is con-ceivable that a separate glossopharyngeal fora-men is present in A. skoogi, but the scan reso-lution is insufficient to reveal it.

Basioccipital.—The basioccipital is anunpaired element that forms the posterior floorof the cranial cavity (Tra 300-359, Figs. 8, 9).It is sutured anteriorly to the sphenoid, antero-laterally to the prootic, and posterolaterally tothe otooccipital. Posteriorly, it forms the centralcomponent of the occipital condyle and theventral rim of the foramen magnum.

In lateral view, the basioccipital is concave dor-sally, with its posterior edge raised to form themedial portion of the occipital condyle. Theventral surface is, for the most part, convex;with a ventrally-directed ridge running alongthe midline of the bone (Tra 310-322). The ven-tral surface also bears two ventrally-projectingcrests (Tra 301-315), called the basal tubera(Evans, in press) (sphenoccipital tubera ofOelrich (1956)). These crests provide attach-ment for hypaxial muscles (Evans, in press). InA. skoogi, posterior extensions of the sphenoidcontribute to the anterior portions of the basaltubera (Lang 1991: CC17 (0)).

Supraoccipital.—The supraoccipital is anunpaired element that forms the posterior roofof the braincase (Tra 288-343, Fig. 8). It artic-ulates anterolaterally with the prootic and pos-terolaterally with the otooccipital. It also con-tributes to the dorsal rim of the foramen mag-num. The supraoccipital is comprised laterallyand ventrally of the upper portions of thevestibules. The supraoccipital is convex dorsal-ly and doubly concave ventrally where it roofsboth vestibules. Medially, the supraoccipitalcovers the medulla and cerebellum.

The most prominent feature of the supraoccip-

ital is the large, posterodorsally-extending crestlocated on the dorsal surface of the bone (Tra302-343). The lateral faces of this crest aresmooth, and provide attachment for the liga-mentum nuchae (Oelrich 1956). The dorsal-most tip of this crest is braced by the long pari-etal fork in A. skoogi (Tra 316-343).

The supraoccipital is intimately associated withthe membranous labyrinth of the inner ear. Thesupraoccipital contributes a large portion of theanterior semicircular canal (Tra 272-310) con-tinuing into the prootic, and the posterior semi-circular canal (Tra 310-340) continuing into theotooccipital. Located toward the posterior por-tion of the vestibule is the osseus common crus,a vertically-directed canal forming the junctionof the anterior and posterior semicircular canals(Tra 310-320). The osseus common crus opensinside the vestibule, just posterior to theendolymphatic foramen. This foramen piercesthe medial wall of the supraoccipital, and is themedial opening for the endolymphatic canal(Tra 332-333). This canal transmits theendolymphatic duct from the cranial cavity tothe vestibule (Wever 1978).

Stapes.—The stapes (Tra 307-319) is a slenderrod derived from the hyoid arch (Bellairs &Kamal 1981; Evans, in press), expanded at itsproximal and distal ends. The slender stapedialshaft lies within the middle ear, with theexpanded medial footplate resting in the fenes-tra ovalis. The expanded lateral end contactsthe tympanic membrane just medial to thequadrate.

Quadrate.—The quadrate (Tra 268-323, Figs.6, 8) is a paired bone positioned at the rear ofthe skull. It facilitates motion between the cra-nium and the lower jaw (Oelrich 1956; Evans,in press). The quadrate meets the ventral mar-gin of the squamosal and the ventral end of theposterior process of the supratemporal dorsally.Its medial surface contacts the ventrolateral tipof the paraoccipital process of the otooccipital


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and, more ventrally, contacts the anterolateral-ly-inflected tip of the quadrate process of thepterygoid. Ventrally, the quadrate meets thecondylar facet (Oelrich 1956) of the articularbone of the lower jaw. Unlike the conditionfound in most reptiles, this articulation in A.skoogi is more like a ball-and-socket joint, withthe knob-like facet of the articular fitting intothe cotyle of the quadrate. This condition pre-viously was observed in other gerrhosaurids(Malan 1941). The quadrate is united withthese other elements via a synovial joint(Malan 1941; Oelrich 1956), allowing for agreat degree of mobility. This type of joint ischaracteristic of borrowing forms, like A. skoo-gi (Malan 1941). The entire anterior face of thequadrate contributes to the posterior margin ofthe lower temporal fenestra.

The quadrate resembles a T-shaped columnforming a 90° angle with the horizontal axis ofthe skull, or parietal table, in lateral view. Onits ventral surface is a concave, mandibularcotyle that meets the condylar facet of the artic-ular bone (Tra 283-304). The mandibularcotyle of the quadrate is divided into a medialand lateral half by a median ridge. These twohalves of the cotyle are of approximately equalsize. When viewed posteriorly, the medial andlateral margins of the quadrate are nearly paral-lel to one another.

The dorsal surface of the quadrate also forms asomewhat convex cephalic condyle. Its lateralside lies slightly anterior to the medial side ofthe condyle (Tra 271-323). The lateral side alsohas a dorsally-projecting tubercle at its anteriorend. The posterior end of the condyle slopesdownward, creating a rounded appearance. Thecephalic condyle of the quadrate contacts theparaoccipital process of the otooccipital medi-ally, the supratemporal posteriorly, and thesquamosal anterolaterally. Attachments to thesebones are made by thick fibrocartilage, or‘intercalary cartilage’ (Versluys 1912). In otherlizards, the dorsal surface of the quadrate also

serves as attachment for the origin of theadductor mandibularis superficialis and themedius muscles (Oelrich 1956).

The medial side of the anterior face of thequadrate extends farther anteriorly than the lat-eral side. The anterior face is broader dorsally,tapering slightly toward the mandibularcondyle. The dorsolateral portion of the anteri-or face has an anteriorly-projecting tubercle inA. skoogi. This condition is present in othergerrhosaurids examined (e.g., Gerrhosaurus,Zonosaurus). In the iguanid, Ctenosaura pecti-nata, there is a small foramen, the quadrateforamen, piercing the dorsolateral region of theanterior face of the quadrate (Oelrich 1956).However, no such foramen is visible in thescanned images of A. skoogi. Additionally, nosuch foramen is seen in the other examinedspecimens of gerrhosaurids and cordylids, andthis foramen is not mentioned by Malan (1941)or van Pletzen (1946) in their studies ofGerrhosaurus and Cordylus.

A dorsoventral ridge called the curved centralpillar (posterior crest of Oelrich (1956)),divides the posterior face of the quadrate intomedial and lateral halves. The ridge is presentfor most of the length of the bone, flatteningout just dorsal to the mandibular cotyle. Thelateral half of the posterior face of the quadrateis concave posteriorly. Like the anterior face, itis broader dorsally and tapers ventrally justabove the mandibular cotyle. Towards the ven-tral end of the medial half of the posterior faceof the quadrate lies the contact between thequadrate and the quadrate process of the ptery-goid. There is no pterygoid lappet of thequadrate (Estes et al. 1988: C37 (1)). The later-al side of the quadrate has a prominent cres-cent-shaped tympanic crest running dorsoven-trally. This crest (Tra 284-296) provides attach-ment for portions of the tympanum, and the ori-gin of the adductor mandibularis externussuperficialis muscle, and for skin (Oelrich1956).


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Mandible.—The mandible (Fig. 11) consists ofsix paired elements: the dentary, splenial, coro-noid, angular, surangular (supra-angular ofBaur (1891); de Beer 1937; Oelrich 1956), andarticular. These six bones surround a tubularcanal that courses through them called theMeckelian fossa. This structure houses theremains of the embryonic Meckel’s cartilage.The mandible articulates with the dorsal por-tion of the skull via the articulation between thearticular and the quadrate.

Dentary.—The dentary is a paired bone com-prising the majority of the anterior portion ofthe mandible (Tra 27-217, Fig. 11). The twodentary bones meet one another medially attheir anterior ends. More posteriorly, along thelength of its medial side, each dentary articu-lates with the thin splenial bone. There is exten-sive bone-to-bone contact between these twoelements (Estes et al. 1988: C67 (0)). At itsposterior end, the dentary meets the coronoiddorsally, the surangular laterally, and the angu-lar ventrally. The dentary bone is hollow, withthe Meckelian fossa coursing through itslength. As in Ctenosaura pectinata (Oelrich1956), the posterior elements of the jaw in A.skoogi form the continuation of this hollowtube because their anterior ends join the den-tary (Estes et al. 1988: C64 (0)).

The dentary is a long bone, flattened mediolat-erally toward its posterior end, and morerounded anteriorly. It is the only tooth-bearingbone of the lower jaw. Each dentary bears 15pleurodont teeth (Estes et al. 1988: C84 (0)),filling each of the 15 tooth positions. The firstfive are unicuspid, while the remaining ten arepolycuspid, with two to five cusps per tooth. Amore detailed description is discussed below(see Dentition). These teeth are situated alongthe medial surface of the bone above the dentalshelf, and along their lingual side are largeresorption pits (Estes et al. 1988: C85 (0)). Atthe base of the teeth is a large, prominent ridge,called the subdental shelf (Estes et al. 1988:

C58 (1), C59 (0)).

The lateral surface of the dentary is relativelysmooth and convex laterally. Anteriorly, sixforamina in a row pierce the lateral surface, justventral to the base of the teeth (Tra 36-128).These foramina transmit terminal branches ofthe mandibular nerve (Evans, in press) (inferi-or alveolar nerve of Oelrich (1956)) to the skin.These foramina continue medially through thedentary bone and open into the canal for themandibular nerve, which passes through thedentary just lateral to the Meckelian fossa.

The medial surface of the dentary is convexwhere the splenial is attached along its length(Tra 108-245). The splenial forms the medialmargin of the Meckelian fossa (Tra 50-275),but does not extend to the anterior end of thedentary. Near the anterior end of the dentary,the Meckelian fossa is incompletely closedmedially (Tra 50-107). If the splenial wereremoved, the Meckelian fossa would beexposed medially for its entire length (Estes etal. 1988: C57 (0)). In A. skoogi, the dentaryforms an open groove, not a closed, fused, orsutured tube around Meckel’s cartilage (Esteset al. 1988: C55 (0)).

Inside the dentary bone, the Meckelian fossa isseparated from the canal for the mandibularnerve by a thin plate of bone, which dorsoven-trally traverses the sagittal plane, and is calledthe intramandibular septum (Tra 60-237). Thisseptum is anterior to the most posterior toothposition in A. skoogi (Estes et al. 1988: C56(0)). The Meckelian fossa runs along the medi-al portion of the dentary, while the canal for themandibular nerve runs along the lateral portion.Only briefly do these two canals merge medial-ly as the septum dividing them is reduced (Tra140-157).

The dentary is bifurcated posteriorly in lateralview, forming dorsal and ventral posteriorly-pointed extensions. The large, dorsal extension


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extends dorsally onto the dorsolateral surfaceof the coronoid in A. skoogi (Estes et al. 1988:C60 (1)). On the posterolateral surface of thedentary, there is no surangular or coronoidnotch present in A. skoogi (Estes et al. 1988:C63 (0)).

Splenial.—The splenial is a thin, flat, pairedelement contributing to the medial margin ofthe Meckelian fossa (Tra 108-245). Most of thesplenial has extensive bone-to-bone contactwith the medial surface of the dentary bone(Estes et al. 1988: C67 (0)). It extends wellanterior to the midpoint of the tooth row in A.skoogi (Estes et al. 1988: C65 (0)), and extendsposterior to the apex of the coronoid (Estes etal. 1988: C66 (0); Lang 1991: CC21 (1)) tomeet the angular and the surangular posteriorlyalong their medial margins. This posteriorextension beyond the coronoid apex previouslywas observed in some cordyliforms (Estes etal. 1988), but not in A. skoogi (Lang 1991).Both the anterior and posterior ends of the sple-nial taper distally; however the anterior end ismore severely tapered and becomes narrowerthan the posterior end. The middle portion ofthe splenial is expanded dorsoventrally.

The splenial is pierced by two foramina. Themore anterior of the two is situated dorsally(Tra 134-152). This large foramen transmits thelingual branch of the mandibular nerve (Evans,in press). The more posterior of the two foram-ina is situated more ventrally along the splenial(Tra 162, 166). This foramen, called the anteri-or mylohyoid foramen (Oelrich 1956), trans-mits the anterior mylohyoid nerve (Malan1941; Oelrich 1956).

Coronoid.—The coronoid is a paired bonelying towards the middle of the jaw on its dor-sal surface (Tra 180-243, Fig. 11). The coro-noid meets the dentary anteriorly, the surangu-lar posteriorly, and the splenial ventrally. Thecoronoid bone is a dorsally-directed element onthe jaw, with the base of the ascending portion

located just posterior to the last tooth positionin the dentary and processes reaching anteriorto the last tooth position. The posterolateralsurface of the coronoid provides attachment forthe adductor mandibularis externus and mediusmuscles, and the bodenaponeurosis (Oelrich1956). The posterodorsal tip of the lateral sideof the dentary overlies the lateral side of thecoronoid (Tra 182-200) (Estes et al. 1988: C68(0), C71 (1); Lang 1991: CC22 (1)). The coro-noid has five processes, named by Evans (inpress) as the dorsal, labial, anteromedial, pos-teromedial, and posterior process. However,here the dorsal process is referred to as thecoronoid process, the anteromedial process asthe dentary process, the posteromedial as thesplenial process, and the posterior as the suran-gular process.

The coronoid process of the coronoid bone isbroad at its base, concave laterally, and extendsand tapers dorsally to a rounded end (Tra 200-230). The height of the coronoid process isequal to the height of the lateral face of the den-tary bone at its anteroposterior midpoint in A.skoogi (Lang 1991: CC23 (0)). Both the labial(Tra 184-197) and the dentary (Tra 185-204)processes of the coronoid extend anteroventral-ly. These two processes clasp the posterior por-tion of the dentary (Estes et al. 1988: C70 (0)).This state of having two processes clasp thedentary differs from that discussed by Estes etal. (1988), because there is more coronoidoverlap on the medial surface of the dentarythan on the lateral surface of the dentary in A.skoogi (i.e., the two processes clasping eitherside of the dentary are not of equal length). Thedentary process is slightly concave laterallyand conforms to the medial margin of the den-tary bone (Estes et al. 1988: C69 (0)).

The splenial process lies along the medial sideof the jaw (Tra 218-235). This process contactsthe splenial anteroventrally and the surangularposteriorly, and contributes to the rim of theadductor fossa (mandibular foramen of Oelrich


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(1956)). This fossa (Tra 244-268) is of moder-ate size in A. skoogi (Estes et al. 1988: C81 (0);Lang 1991: CC26, (0)), meaning the fossa isnot expanded to accommodate the extension ofthe adductor mandibulae muscle and does notcover the entire length between the coronoidand the anterior margin of the retroarticularprocess. The short surangular process extendsposteriorly to meet the dorsal margin of thesurangular (Tra 236-244).

Angular.—The angular is present in A. skoogias a paired element located on the ventral sur-face of the mandible (Tra 210-250, Fig. 12)(Estes et al. 1988: C72 (0)). This bone insertsinto the posteroventral end of the dentary, andmeets the splenial anterodorsally, the surangu-lar dorsally, and the articular posteriorly. Theanterior end of the angular extends anteriorlypast the apex of the coronoid process of thecoronoid bone, but does not reach the last toothin the dentary (Lang 1991: CC25 (1)). Theangular is a dorsoventrally-flattened bone thatcontributes to the ventral curve of the jaw. It isvisible on both the lateral and medial sides ofthe mandible in A. skoogi (Lang 1991: CC24(0)).

Along the ventromedial side of the bone, atabout the mid-point of the bone, is a small fora-men (Tra 224-227). This foramen, called theposterior mylohyoid foramen, transmits theposterior mylohyoid nerve (Oelrich 1956). Theventral surface of the angular also providesattachment for fibers of the mandibulohyoid Imuscle (Oelrich 1956).

Surangular.—The surangular is a paired ele-ment that lies near the posterior end of thelower jaw (Tra 200-306, Fig. 11). Its taperedanterior end (Estes et al. 1988: C61 (0)) insertsinto the posterior end of the dentary, forming atongue-and-groove articulation. It also contactsthe coronoid anterodorsally, the angular ven-trally, and the articular posteriorly. The suran-gular encloses most of the posterior portion of

Meckel’s canal, although the canal is open dor-sally as it traverses this bone (Tra 244-270).Both the dorsal and lateral surfaces of thesurangular are smooth, and each providesattachment for various muscles in the jaw(Oelrich 1956).

The surangular is pierced by two foramina. Theanteriormost of the two is located just ventral tothe coronoid process of the coronoid bone, isdirected anteriorly (Tra 206-233), and isreferred to here as the anterior surangular fora-men (anterior supra-angular foramen of Oelrich(1956)). The other foramen is located moreposteriorly near the suture between the suran-gular and the articular (Tra 274-278), and isreferred to here as the posterior surangularforamen (posterior supra-angular foramen ofOelrich (1956)). Both of these foramina trans-mit cutaneous branches of the inferior alveolarnerve (Malan 1941; van Pletzen 1946; Oelrich1956).

Articular.—The articular is a paired boneforming the posterior end of the mandible (Tra223-353, Fig. 11). The articular meets the sple-nial anteriorly, the angular ventromedially, andthe surangular dorsally and laterally. The artic-ular has a thin, anteriorly-projecting process, aposterior condyle, and a posteriorly-directedretroarticular process (Oelrich 1956). The ante-riorly-projecting process, called the prearticu-lar process here (anterior process of Evans, inpress) develops from dermal prearticular, whilethe remainder of the bone develops from theendochondral articular (Evans, in press).

The prearticular process of the articular is nar-row at its most anterior point, which extendsanterior to the posterior end of the coronoidbone near the anterior-most point of the suran-gular (Estes et al. 1988: C62 (0)). There is noprearticular crest on the medial edge of thisprocess in A. skoogi (Estes et al. 1988: C73(0)). This process broadens posteriorly towardits base, which is just anterior to the condyle.


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This process forms the lateral margin ofMeckel’s canal. Just anterior to the adductorfossa, along the medial wall of the prearticularprocess of the articular, is a small foramen (Tra238-241). In other lizards, this foramen trans-mits the anterior part of the chorda tympaninerve, a branch of cranial nerve VII (vanPletzen 1946; Oelrich 1956).

The condyle of the articular inserts into theventral cotyle of the quadrate. The contactbetween these two elements connects the lowerjaw to the rest of the skull. The condyle is tri-angular-shaped in transversal view with theapex of the triangle directed dorsally (Tra 283-305). This triangular projection, called thecondylar facet (Oelrich 1956), fits into the dor-sal surface of the ventral cotyle of the quadrate.In most squamates, the nature of the contactbetween the articular and the quadrate is theopposite of what is described here, with theventral end of the quadrate having a condylethat inserts into a cotyle in the articular (Malan1941). However, the condition in A. skoogidescribed here is also present in Gerrhosaurus(Malan 1941). Just posterior to the condylarfacet, the articular becomes concave dorsally.The lateral and medial margins broaden slight-ly mediolaterally as they continue posteriorlytoward the end of the bone (Estes et al. 1988:C78 (1)). Ridges surrounding the condylarfacet provide attachment for the ligaments ofthe articular capsule (Oelrich 1956).

Posterior to the condylar facet is the posterior-most extension of the articular, called theretroarticular process (Tra 305-353) (Oelrich1956). This process is inflected medially in A.skoogi (Estes et al. 1988: C75 (1)). The depres-sor mandibulae and cervicomandibularis mus-cles attach to the medial edge of this process(Evans, in press). The dorsal surface of thisprocess has a deep pit or sulcus (Estes et al.1988: C74 (0); Lang 1991: CC27 (1)). Bothpresence and absence of this pit previouslywere observed in Cordyliformes (Estes et al.

1988). On the dorsomedial side of the retroar-ticular process is a tiny foramen (Tra 311-316).This foramen transmits the posterior part of thechorda tympani nerve and the posterior condy-lar artery (Oelrich 1956). There is also a smallflange on the posteromedial margin of theretroarticular process (Estes et al. 1988: C76(1)). There is no angular process extendingfrom the condyle in A. skoogi (Estes et al.1988: C80 (0)), and no offset forming a waist tothe process (Estes et al. 1988: C77 (0)). Thereis posterior twisting of the process in A. skoogi(Estes et al. 1988: C79 (0); Lang 1991: CC28(1)).

Dentition.—The functional teeth of A. skoogiare present in the premaxilla (Tra 15-31, Figs.6, 9), the maxilla (Tra 35-177, Figs. 6, 9), andthe dentary of the lower jaw (Tra 28-186, Fig.11). The teeth are pleurodont (Estes et al. 1988:C84 (0)), meaning that the teeth are superficial-ly attached to the medial surface of the tooth-bearing bone (Estes et al. 1988). This conditionis present in all other gerrhosaurids andcordylids (Edmund 1969; Estes et al. 1988).The marginal tooth replacement type in A.skoogi is similar to the ‘iguanid’ type (Estes etal. 1988: C85 (0)). In this condition, thereplacement tooth develops along the lingualside of the exposed tooth in a large resorptionpit. This condition is also the same found in allother gerrhosaurids and cordylids (Edmund1969; Estes et al. 1988). There is no basalinfolding of these marginal teeth in A. skoogi(Estes et al. 1988: C86 (0)).

There are seven tooth positions in the premax-illa, each with a functional tooth in the adultspecimen of A. skoogi. The same number ofpositions was observed in certain members ofGerrhosaurus, Zonosaurus, Cordylus,Platysaurus, and Pseudocordylus by Edmund(1969). In A. skoogi, one tooth lies in the mid-line of the bone, and both the right and leftsides have three teeth. All teeth in the premax-illa are unicuspid.


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In the maxilla of A. skoogi, there are 14 toothpositions, all of which house functional teeth inthe adult specimen. The number of tooth posi-tions present in cordyliforms examined byEdmund (1969) ranged from 16 in Zonosaurusornatus and Gerrhosaurus major grandis, to 25in G. vallidus. In tooth positions 5, 6, 8, 11, and13 (with position 1 being the most mesial in themaxilla and position 14 being the most distal),replacement teeth are visible just medial to thebase of the current functional tooth. The num-ber of cusps on the teeth in the maxilla varies.Maxillary tooth 1 has one cusp, teeth 2 through4 each have two cusps, tooth 5 has three cusps,teeth 6 and 7 each have four cusps, teeth 8through 12 each have five cusps, tooth 13 hasfour cusps, and tooth 14 has three cusps (Tra35-178).

In the dentary of A. skoogi, there are 15 toothpositions. This number ranges from 18 to 28 ingerrhosaurids and cordylids examined byEdmund (1969). In the adult A. skoogi speci-men, twelve of these positions house function-al teeth in the left dentary and 13 house func-tional teeth in the right dentary. Only replace-ment teeth are in tooth positions 5, 7, and 12 inthe left dentary, although replacement teeth arevisible just medial to the base of the functionalteeth in positions 10, 13, and 15. In the rightdentary, only replacement teeth are present inpositions 5 and 7, while positions 10, 12, 13,and 15 have replacement teeth coming in medi-al to the base of the functional teeth. Like in themaxilla, the number of cusps on teeth in thedentary varies. In each dentary, teeth 1 through5 each have one cusp, tooth 6 has two cusps,teeth 7 and 8 each have three cusps, teeth 9 and10 each have four cusps, and teeth 11 through15 each have five cusps.

ONTOGENETIC VARIATION INANGOLOSAURUS SKOOGI

Ontogenetic variation in the cranial osteologyof squamates previously has been discussed in

the literature for only a few taxa (e. g., deQueiroz 1987; Barahona & Barbadillo 1998;Barahona et al. 1998, 2000; Evans, in press).Knowledge of such variation is relevant to anymorphological or systematic study, particularlywhen dealing with fossils in which the age ofthe individual may be difficult to determine. Adiscussion of the ontogenetic variation withinA. skoogi observed in this study is limited dueto a sample size of only two specimens, includ-ing one adult and one juvenile. However, thereare morphological differences between the twoscanned specimens. These differences permitsome preliminary comments on the ontogenet-ic variation in A. skoogi. Specific charactersand features known to vary ontogeneticallyamong iguanine and lacertid lizards (deQueiroz 1987; Barahona & Barbadillo 1998;Barahona et al. 1998) that also occur in A.skoogi are discussed here.

The overall skull shape differs between theadult and the juvenile specimens of A. skoogi.In the juvenile, the length of the skull from thetip of the snout to the posterior end of the skullis shorter, and the width of the skull is broaderlaterally (Fig. 13). The characteristic shovel-head shape of the adult A. skoogi is not as pro-nounced in the juvenile, and the snout is not asdepressed dorsoventrally (Fig. 14).

All bones are present in both the adult and thejuvenile; however, the degree of ossification inthese elements and in the osteoderms is less inthe juvenile. During ontogeny, there is a defi-nite sequence of ossification within the skull.The bones of the snout region are the first toossify, followed by the skull roof and posteriorregion of the frontal. Then, the anterior regionof the parietal ossifies, followed by the simul-taneous ossification of the remainder of theparietal and neurocranium (Barahona &Barbadillo 1998; Maisano 2001). As noted inthe juvenile lacertids studied by Barahona &Barbadillo (1998), the anterior and centralregions of the parietal, the posteromedial area


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of the frontals, the facial process of the maxil-la, the anterior portion of the nasal, the maxil-lary process of the palatine, and the rostrum ofthe parasphenoid are incompletely ossified inthe juvenile A. skoogi. This incomplete ossifi-cation results in a large parietal fontanelle inthe juvenile specimen of A. skoogi, and in otherjuvenile squamates (Maisano 2001).

The joints and contacts between many of thebones in the juvenile are separated by gaps. Inlacertids, these gaps in juvenile skulls are filledwith growing membranous connective tissue(Barahona & Barbadillo 1998). In the adultspecimen of A. skoogi, the frontals meet medi-ally throughout their entire length, while in thejuvenile, the pair is separated by a gap posteri-orly (Fig. 13). There is a gap between the pre-maxillary process of the maxilla and the pre-maxilla in the juvenile, whereas these two ele-ments are in contact in the adult A. skoogi.There is more space between the cotyle of thearticular and the condyle of the quadrate in thejuvenile than in the adult specimen. In themandible, the anterior end of the splenial isreduced in the juvenile, so a larger portion ofthe Meckelian fossa is open medially in thejuvenile than in the adult A. skoogi.

In the braincase of the juvenile, the basicranialfenestra (an opening between the posteriormargin of the sphenoid and the anterior marginof the basioccipital) is open, the bones sur-rounding the semicircular canals are incom-pletely ossified, and the distal end of thebasipterygoid process remains cartilaginousand does not contact the medial surface of thequadrate process of the pterygoid as closely asin the adult (Fig. 15). In large-bodied iguaninelizards, the paired orbitosphenoid bones arecrescent-shaped in lateral view in early stagesof ontogeny. During development, theorbitosphenoid extends processes anterodorsal-ly, posterodorsally, and posteroventrally,becoming more triradiate in lateral view. In theadult stage, the anterodorsal process bifurcates,

resulting in a quadraradiate bone in lateral view(Oelrich 1956; de Queiroz 1987; Evans, inpress). In both the adult and the juvenile speci-mens of A. skoogi, the orbitosphenoid is cres-cent-shaped and concave posteriorly in lateralview (Fig. 12), and lacks all but the pos-teroventral process found in adult, large-bodiediguanines. The stapedial shaft in the juvenile isbroader throughout its length than in the adult,and the lateral end of the stapes is incomplete-ly ossified in the juvenile A. skoogi.

The dentition of the premaxilla, maxilla, den-tary, and pterygoid in the juvenile differs fromthat in the adult (Fig. 15). In the premaxilla,both adult and juvenile have seven tooth posi-tions, but the medial functional tooth is absentin the juvenile.

In the maxilla, the adult has 14 tooth positions,whereas the juvenile has only twelve. Fewerteeth in the tooth row of a juvenile reptile com-pared to that of an adult is not uncommon; inone year, Iguana juveniles can add severaltooth positions in the posterior end of the toothrow (Edmund 1969). In both the right and leftmaxillae of the juvenile, all positions are filledwith nine functional teeth and three replace-ment teeth. The number of cusps per tooth inthe juvenile differs from that in the adult, but islikely due to the reduced number of teeth in thejuvenile, thus the tooth positions in the adultand the juvenile do not coincide. The cusps inthe adult were discussed above. In the juvenile,tooth 1 has one cusp, teeth 2 and 3 each havetwo cusps, tooth 4 has three cusps, teeth 5through 7 each have four cusps, teeth 8 through11 each have five cusps, and tooth 12 has twocusps.

There are 14 tooth positions in the dentary ofthe juvenile, whereas the adult has 15 positions.In both the right and left dentaries of the juve-nile, there are nine functional teeth and fivereplacement teeth filling these positions. As inthe maxilla, the reduced number of tooth posi-


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Figure 13. Dorsal view of the skull of juvenile A. skoogi (CAS 206977).

Figure 14. Lateral view of the skull of juvenile A. skoogi (CAS 206977).

Figure 15. Ventral view of the skull of juvenile A. skoogi (CAS 206977) with jaw articulated.

tions in the juvenile is the likely cause for thedifference in number of cusps per tooth. In thejuvenile, teeth 1 through 5 each have one cusp,tooth 6 has two cusps, teeth 7 and 8 each havethree cusps, teeth 9 and 10 each have fourcusps, and teeth 11 through 14 each have fivecusps.

On the ventral surface of the pterygoids in boththe adult and juvenile, pterygoid teeth are pre-sent. In lacertids, there may be a single row ofteeth in juveniles and multiple rows arrangedlaterally in adults (Barahona et al. 1998).However, both the adult and juvenile speci-mens of A. skoogi have only a single row ofpterygoid teeth. In the adult, there is a singlerow of three teeth, running anterior to posteri-or, down each pterygoid. In the juvenile, thereis a single row of two teeth on the ventral sur-face of each pterygoid.

DISCUSSION

With these new morphological data of A. skoo-gi, previously published characters (Estes et al.1988, Lang 1991) were rescored and in somecases character states defined by these previousauthors were changed. These changes, alongwith new characters found in A. skoogi andother disarticulated specimens of cordyliforms,were used to test current hypotheses regardingrelationships both within Gerrhosauridae andamong the closely related families Cordylidaeand Scincidae (Nance 2003). This preliminaryanalysis supports A. skoogi as the most basalmember of Gerrhosauridae (Fig. 16), not just ofmainland African gerrhosaurids as in Lang(1991). These results also differ from a recentanalysis based on mitochondrial DNA (Lamb etal. 2003), which suggests the genusAngolosaurus should cease to exist and A.skoogi should be renamed Gerrhosaurus skoo-gi. This analysis was based on mitochondrialDNA, which acts as a single gene in phyloge-netic studies and therefore represents a small

portion of an organism’s evolutionary history(Avise 2004). While the hypothesized relation-ships (Fig. 16) based on the cranial osteologydescribed here have low bootstrap support,they nonetheless warrant the examination ofmore specimens, inclusion of more taxa, andmore character data to further resolve relation-ships within these clades (Nance 2003).

Poor museum representation of rare or endem-ic taxa is a current problem in squamate sys-tematic studies. For taxa such as A. skoogi, thefew specimens that are available for study arelargely preserved in alcohol. This is becausemuch of squamate systematics is based on scalecounts. In light of the current phylogenetichypothesis for all of Squamata, which is large-ly based on the results of Estes et al. (1988), thediscovery of new character data seems espe-cially important now. Similar conclusions pre-viously were made based on the examination ofdetailed cranial osteology of other taxa (e.g.,Rieppel & Crumly 1997; Rieppel 1999).Although disarticulated cranial material ispoorly represented in museum collections, thistype of material is an excellent source of mor-phological data. Disarticulating the craniumand braincase of a specimen permits unob-


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Figure 16. 50% majority rule consensus tree depict-ing all 48 equally-parsimonious trees (208 stepseach), depicting hypothesized gerrhosaurid relation-ships based on cranial osteology data. Bootstrap sup-port greater than 50% indicated at nodes.

structed study of articulations and contactsbetween individual elements, foramina andprocesses associated with them, and detailedmorphology of the individual elements that isotherwise hidden in articulated material. Thisdetailed morphology provides phylogeneticallyinformative data. This study using HRXCT tostudy the rare gerrhosaurid lizard, A. skoogi,demonstrates the wealth of detailed osteologythat is attainable with this technology, as wellas the utility of osteology in squamate system-atics. Hopefully the new data provided by thisstudy will help contribute to the ultimate goalof attaining a well-supported phylogeny ofCordyliformes, increase the use of detailed cra-nial osteology in phylogenetic systematics, andprovide further insight into squamate morphol-ogy.

ACKNOWLEDGEMENTS

I thank Chris Bell for his guidance and super-vision throughout this research, and Tim Roweand James Sprinkle for their thoughtful insightconcerning this paper. Rich Ketcham and MattColbert scanned the specimens used in thisstudy. Matt Colbert and Jessie Maisano helpedextensively with HRXCT image processing.Chris Bell provided funding for the scans andThe University of Texas Geology Foundationfunded aspects of my research. Susan Evans,Trip Lamb, and Steve Lochetto shared theresults of their research prior to its publication.Peter Marko critiqued earlier drafts of thismanuscript.

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Received: 9 May 2006; Final acceptance: 11 May 2007.

cranial osteology of the african gerrhosaurid angolosaurus skoogi

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