braincase evolution in suchian archosaurs (reptilia: diapsida

Zoological Journal of the Linnean Society

, 2002,

136

, 49–76. With 10 figures

© 2002 The Linnean Society of London,

Zoological Journal of the Linnean Society,

2002,

136

, 49–76

49

Blackwell Science, Ltd

Oxford, UK

ZOJZoological Journal of the Linnean Society

0024-4082The Linnean Society of London, 2002September 2002

136

1

4976Original Article

BRAINCASE OF

BATRACHOTOMUSD. J. GOWER

E-mail: [email protected]

Archosaurian anatomy and palaeontology. Essays in memory of Alick D. Walker.

Edited by D. B. Norman and D. J. Gower

Braincase evolution in suchian archosaurs (Reptilia: Diapsida): evidence from the rauisuchian

Batrachotomus kupferzellensis

DAVID J. GOWER

Department of Zoology, The Natural History Museum, London SW7 5BD, UK

The osteology of an almost complete braincase of the rauisuchian archosaur


Gowerfrom the Middle Triassic of Germany is described. There is a possibly discrete epiotic ossification, the metotic fissureis undivided by bone (i.e. there is a metotic foramen), the medial wall of the otic capsule is mostly ossified, the cere-bral branch of the internal carotid artery entered the lateral surface of the parabasisphenoid, the ventral ramus ofthe opisthotic is more prominent laterally than a strong subvertical ridge on the exoccipital and basioccipital that liesposterior to the external foramen for the hypoglossal nerve, and the perilymphatic foramen faces away from theotic capsule in a posterior direction. Braincase morphology in the rauisuchians

Saurosuchus galilei

,

Postosuchuskirkpatricki,

and

Tikisuchus romeri

is reviewed. A matrix of 27 braincase characters for 12 archosaurian taxa isanalysed. The most parsimonious hypothesis is consistent with the currently orthodox view of archosaurian phy-logeny, except in that aetosaurians are more closely related to crocodylomorphs than is any rauisuchian. This phy-logeny is used in a brief interpretation of the evolution of derived braincase features present in extant crocodilians.© 2002 The Linnean Society of London,

Zoological Journal of the Linnean Society

, 2002,

136

, 49–76.

ADDITIONAL KEYWORDS: – Crurotarsi – Crocodylia – morphology – phylogeny – character evolution.

INTRODUCTION

Birds and crocodilians are the only extant members ofthe large diapsid clade Archosauria, a group that alsoincludes dinosaurs, pterosaurs and many less familiarfossil groups of the Mesozoic. The origin of birds andtheir morphology has received much recent interest(e.g. see detailed reviews by Witmer (1991) and Padian& Chiappe (1998)) but the ancestry of crocodilian mor-phology has been relatively neglected in comparison.It is currently recognized that extant crocodiliansare members of a clade, Crocodyliformes, that alongwith the Mesozoic Sphenosuchia comprise the cladeCrocodylomorpha. The immediate relationships ofCrocodylomorpha to other major clades of Mesozoicsuchians such as Aetosauria, Ornithosuchia, andRauisuchia (if this is indeed a clade – see Discussion inGower, 2000) remain rather less resolved (Gower &Wilkinson, 1996).

Extant birds and crocodilians are clearly very dif-ferent in gross form, but they share a number ofderived similarities of the neurocranium, particularlyof the otic region (e.g. Walker, 1972, 1985, 1990), a sub-stantial proportion of which appear to have evolvedindependently in the two groups (Gower & Weber,1998). Exactly where, when, and in what sequence thederived features of bird and crocodilian braincaseswere acquired requires a more detailed knowledgeof archosaurian phylogeny, and also of braincasemorphology in fossil taxa. Braincase osteology ofnon-crocodylomorphan suchians is particularly poorlyknown. Detailed descriptions and/or figures are onlyavailable for a very few parasuchians (phytosaurs; e.g.Camp, 1930, 1942; Chatterjee, 1978), rauisuchians(Chatterjee, 1985; Alcober, 2000), and aetosaurians(Case, 1922; Walker, 1961, 1985; Parrish, 1994; Gower& Walker, 2002), and no ornithosuchians.

The aim of this paper is to provide a detailed descrip-tion of a well preserved braincase of the rauisuchianarchosaur


Gower. The

50

D. J. GOWER



2002,

136

, 49–76

osteology of rauisuchians is poorly known and theirrelationships remain unclear and controversial(Gower, 2000). In addition to providing data on theearly evolution of the suchian braincase, this descrip-tion also aims to improve our knowledge of the oste-ology of rauisuchians and to increase the data availablefor future phylogenetic analyses of Archosauria.

T

ERMINOLOGY

AND

A

BBREVIATIONS

Braincase structure terminology follows Gower &Weber (1998). Archosauria is used in its traditionalsense (e.g. Juul, 1994; Benton, 1999), with archosaursin the sense of Gauthier (1986) referred to as crown-group archosaurs. The crown group has recently beennamed Avesuchia by Benton (1999). Systematic confu-sion means that Rauisuchia is also used here in itstraditional sense (Gower, 2000), even though generaand species usually referred to this taxon might notconstitute a monophylum.

Institutional abbreviations used are: BMNH, TheNatural History Museum, London; ISI, Indian Statis-tical Institute, Calcutta; SMNS, Staatliches Museum

für Naturkunde, Stuttgart; TTUP, Texas TechUniversity Palaeontology Collection, Lubbock; UCMP,University of California Museum of Palaeontology,Berkeley; UMMP, University of Michigan Museum ofPalaeontology, Ann Arbor.

Abbreviations used in more than one figure are asfollows: aar – anterior ampullary recess (external andanterior vertical semicircular canals open into thisconcavity), aur – auricular recess, bk – break in spec-imen, bo – basioccipital, bolr – lateral ridge on basio-ccipital, bor – basioccipital recess, bpt – basipterygoidprocess, bptf – basipterygoid facet, bs – parabasisphe-noid, btbo – basal tuber of basioccipital, btbs – basaltuber of parabasisphenoid, cc – osseus common crus,cp – cultriform process of parabasisphenoid, cpr –crista prootica, dhv – position of opening for dorsalhead vein plus associated sinus, dv – dorsal part ofvestibule, eo – exoccipital, eolr – lateral ridge on exoc-cipital, eop – exoccipital pillar, eov – possible route ofexternal occipital vein, ep – epiotic, fo – fenestra ova-lis, g.pVII – groove probably for palatine ramus offacial nerve + palatine artery, l. – left, mcv – path ofmiddle cerebral vein, mf – metotic foramen, np – noto-chordal pit, ob – otic bulla (ossified medial wall of ves-tibule of otic capsule), oc – occipital condyle, op –opisthotic, par – posterior ampullary recess (externaland posterior vertical semicircular canals also openinto this concavity), pp – paroccipital process, ppt –ridge possibly for attachment of protractor pterygoi-dei, pr – prootic, r. – right, s.ls – surface for articula-tion with laterosphenoid, so – supraoccipital, ug –unossified gap, V – position of foramen for trigeminalnerve, VII – position of passage of facial nerve, vrop –

ventral ramus of the opisthotic. In all figures, diagonalshading denotes a fracture surface, cross-shadingdenotes areas of matrix or artificial material.

MATERIAL AND METHODS

The braincase of


de-scribed here is housed at the Staatliches Museumfür Naturkunde, Stuttgart and catalogued as part ofspecimen SMNS 80260. The braincase is also knownin SMNS 80261, but this is a heavily crushed, andmuch less informative example.

B. kupferzellensis

de-rives from Middle Triassic (Lettenkeuper, Anisian) de-posits in Baden-Württemberg, southern Germany. Foran overview of the known material and a descriptionof the cranial osteology of this taxon see Gower (1999)and references therein.

The material was prepared mechanically, mostlysoon after it was excavated in 1977. At this time, thetwo main pieces (dorsal and ventral) of the braincaseof SMNS 80260 were bonded together artificially.These were separated in 1996 and a small amount ofremaining matrix was removed using a blunt needle.

During the course of this study, comparisons weremade with both original material of extant and fossildiapsids and with the literature. Chief sources of dataare given in parentheses for the following taxa:

Sphenodon

(Gower & Weber, 1998; Säve-Söderbergh,1947); other extant non-archosaurian diapsids (Säve-Söderbergh, 1947; Oelrich, 1956; pers. obs.);

Euparke-ria capensis

(Evans, 1986; Welman, 1995; Gower &Weber, 1998); proterosuchids and erythrosuchids(Gower & Sennikov, 1996a,b; Gower, 1997; pers. obs.);phytosaurs (Camp, 1930, 1942; Chatterjee, 1978; pers.obs. of BMNH, ISI, and TTUP material); aetosaurians(Case, 1922; Walker, 1961, 1985; Parrish, 1994; Gower& Walker, 2002);

Postosuchus kirkpatricki

(Chatterjee,1985; Parrish, 1993; pers. obs. of TTUP material);

Sau-rosuchus galilei

(Alcober, 2000); sphenosuchian cro-codylomorphs (Walker, 1990; Wu & Chatterjee, 1993);crocodyliform crocodylomorphs (Iordansky, 1973;Walker, 1990; pers. obs. of dried skulls in BMNH); birds(Walker, 1972, 1985, 1990; pers. obs. of dried skulls ofa limited range of taxa).

DESCRIPTION OF SMNS 80260

The organization of this description approximately fol-lows that given for the braincase of

Sphenosuchus acu-tus

by Walker (1990: 31–49). Firstly, the externalsurfaces including the inner ear are described, fol-lowed by the internal surface of the endocranial cavity.A horizontal fracture passes clean through the brain-case of SMNS 80260 at a level just above the floor ofthe endocranial cavity. The braincase of this specimenis therefore currently in two pieces, termed here the‘dorsal’ and ‘ventral’ pieces.

BRAINCASE OF

BATRACHOTOMUS

51



2002,

136

, 49–76

The braincase of SMNS 80260 is partly crushed butmost of the three-dimensional details are preservedintact, if occasionally a little distorted. The left side ofthe braincase has been pushed in toward the midlinemore than the right, so that the specimen is nowslightly asymmetrical. Additional preservational dis-tortion means that the dorsal and ventral pieces of thespecimen do not fit together precisely along the breakthrough the right exoccipital pillar. The parabasis-phenoid has been squashed so that its lower part,including the basipterygoid processes, has beenpushed back almost to the level of the basal tubera ofthe element. The surface of the bone is generally wellpreserved apart from the numerous shear-cracks thatrun through the specimen in association with the dis-tortion undergone during preservation. The hind partof the braincase of SMNS 80260 is largely completeexcept for some loss of bone from some of the edgesbordering the main horizontal fracture through thespecimen. Anteriorly, however, neither the cultriformprocess of the parabasisphenoid nor the laterosphe-noids are preserved.

E

XTERNAL

SURFACE

Basioccipital

The basioccipital forms almost all of the hemisphericaloccipital condyle, upon which a shallow notochordalpit is visible (Fig. 1). There is no great development of

a condylar stalk, in that the base of the condyle liesonly a short distance behind the posterior face of thebasal tubera and the rest of the main body of thebasioccipital (Figs 2, 3). The basal tubera of the basi-occipital are a pair of ventrally projecting, bilobedstructures separated by a median cleft. The moremedial of each pair of lobes is larger and projects fur-ther ventrally. It also bears a ridge on its posterior sur-face that runs along the longitudinal axis of the lobe.Medially, between these larger lobes, the posteroven-tral edge of the basioccipital carries a small crescentic‘basioccipital recess’, close to the suture between thiselement and the parabasisphenoid.

The smaller lobe of each basal tuber contacts theventral ramus of the opisthotic as well as the basaltuber of the parabasisphenoid, and its lateral edgeforms a subvertical ridge that is confluent with a lat-eral crista on the exoccipital (lateral exoccipital ridge;see below). A cleft marks the division between thelarger and smaller tuberal lobe of each side of thebasioccipital. Along with the basal tuber of the para-basisphenoid, this cleft forms the entrance to a gapthat passes up between these elements (and perhapsthe ventral surface of the ventral ramus of theopisthotic) an unknown distance towards the floor ofthe endocranial cavity. The inner surface of this chan-nel is not covered with a layer of firmly ossified bone.Similar openings and channels in other diapsids havebeen termed ‘pseudolagenar recess’ in basal archo-saurs (Gower & Sennikov, 1996a,b, 1997) and ‘unossi-

Figure 1.


Gower. Ventral part of braincase of SMNS 80260 in posterior view. Scalebar = 30 mm mpr = median pharyngeal recess.

52

D. J. GOWER



2002,

136

, 49–76

fied gap’ in a wider range of diapsids (Gower & Weber,1998). It is unclear whether this channel in SMNS80260 extends anteriorly between elements farenough to enter the ventral end of the lagenar/cochlear recess. The anterior surface of the smallerlobe of each basal tuber of the basioccipital forms partof a large concavity that lies at the ventral end of themetotic foramen. At least part of this area probablyhoused the perilymphatic duct.

Exoccipital

No sutural demarcation between exoccipital andopisthotic is detectable in SMNS 80260. Each elementis described separately here with the assumption that,as in other diapsids (where known), the exoccipital islargely restricted to the pillar positioned between theforamen magnum and the metotic fissure/foramen.The ventral parts of the exoccipitals are in contact formost of their length so that the basioccipital is largelyexcluded from the ventral border of the foramen mag-num. The preservational fracture that runs approxi-mately horizontally through the braincase of SMNS

80260 passes through the bases of the exoccipital pil-lars, and in doing so it reveals the path of a singlechannel on each side for the hypoglossal nerve (Fig. 4).This channel runs in a slightly anterior as well as lat-eral direction, in passing from the endocranial cavityto the external surface of the braincase.

The lateral surface of the exoccipital is dominatedby a conspicuous subvertical crista that forms the pos-terior margin of the metotic foramen (Figs 1

−

3). Thecrest (‘lateral exoccipital ridge’) extends further verti-cally than the rest of the exoccipital and contacts andcontinues onto the basal tuber of the basioccipital. Thecrest effectively divides the lateral surface of the exoc-cipital into two concave surfaces – anterolaterally andposterolaterally. The external hypoglossal foramenlies on the anterolaterally directed surface, close tothe metotic foramen. This superficially resembles thecondition in the early suchians

S. acutus

(Walker,1990),

Dibothrosuchus elaphros

(Wu & Chatterjee,1993), and

Stagonolepis robertsoni

(Walker, 1985),except that in these taxa the external foramina for thehypoglossal nerve lie posterior to the crest while in

B. kupferzellensis

the opposite is true.

Figure 2.


Gower. Ventral part of braincase of SMNS 80260 in left lateral view. See Figure 3for scale. cn = condylar neck/stalk.

BRAINCASE OF

BATRACHOTOMUS

53



2002,

136

, 49–76

The metotic fissure of SMNS 80260 shows no sign ofhaving been subdivided by bone, and the entire poste-rior margin of the resulting metotic foramen is formedby the anterolateral face of the exoccipital pillar.

Supraoccipital

The supraoccipital of SMNS 80260 is complete. It is anapproximately triangular element in posterodorsalview, being much wider than long (Fig. 5). Posteriorly,it forms the dorsal border of the foramen magnum.The dorsal surface of the element bears a midlinedorsal ridge and a pair of anterolateral depressedfacets for articulation with the ventromedial surfaceof the parietals. The lateral surfaces of these facetsare irregular. Behind the facets for the parietal, thesupraoccipital extends a lateral process over theproximodorsal edge of each of the proximal endsof the paroccipital processes of the opisthotics. Thesupraoccipital is also exposed on the anterior, unfin-ished surface of the braincase of SMNS 80260(Fig. 6), and it might have made brief contact with thelaterosphenoids.

Epiotic

The epiotic has only a very small exposure on theexternal surface of the braincase. It forms a thin,irregular strip between the lateral surface of thesupraoccipital and prootics, immediately adjacent tothe facet on the former element that articulates withthe parietal. Indeed, it seems probable that the epioticalso contacted the parietal at this point, but the suturewith the supraoccipital is difficult to trace, and it maybe that these two elements are partially fused. Theanterior view of the braincase of SMNS 80260 (Fig. 6)indicates that the epiotic also articulated with thelaterosphenoids.

Opisthotic

The paroccipital processes of SMNS 80260 are largelyformed by the opisthotic. They are relatively short andhave strongly expanded, flattened distal ends. Theposterior surface of each paroccipital process bears alow subhorizontal ridge near its dorsal edge, extend-ing laterally away from the suture with the supra-occipital. When the parietal is articulated with the

Figure 3.


Gower. Ventral part of braincase of SMNS 80260 in right posterolateral view.Scale bar in mm.

54

D. J. GOWER



2002,

136

, 49–76

braincase, the post-temporal fenestra is a narrow slitbetween the ventral surface of the parietal, the lateraltip of the supraoccipital and the dorsal edge of theparoccipital process of the opisthotic immediatelyabove the subhorizontal ridge. Anteriorly (Fig. 6), onecan envisage the post-temporal fenestra communi-cating with an anteromedially extending groove thatlies along the supraoccipital-opisthotic suture. At itsmedial extent, at the junction of supraoccipital,opisthotic, prootic, and perhaps epiotic, there is a deep

subtriangular pit. This is seen on the left of SMNS80260, but is obscured by crushing on the right side.The groove and pit are here interpreted as the possibleroute of the external occipital vein.

The anterior surfaces of the paroccipital processes ofthe opisthotics bear subvertical, narrow oval de-pres-sions just lateral to midway along their lengths(Fig. 6). A small amount of surface bone seems to havebeen stripped off at least the depression on the leftside of SMNS 80260. It is unclear if these features are

Figure 4.


Gower. Stereopair photographs and diagram of ventral part of braincase ofSMNS 80260 in dorsal view i.e. normal to fracture surface between ventral and dorsal pieces. Anterior is to the top of thefigure. Scale bar = 15 mm cr = cohlear/lagenar recess; lr = lateral ridge; pn = notch in break through ventral ramus of theopisthotic for passage of perilymphatic duct; VI = foramen for abducens nerve; vv = ventral part of vestibule; XII = foramenfor hypoglossal nerve.

BRAINCASE OF

BATRACHOTOMUS

55



2002,

136

, 49–76

natural or if they have been developed or exaggeratedduring preservation. They perhaps represent a contactzone between the paroccipital process and the poste-rior surface of the central ridge of the dorsal end of thequadrate. The anterior surface of the paroccipital pro-cess lateral to this feature exhibits subhorizontalstriae, most strongly developed towards the loweredge.

The posterior surface of the narrow ventral edge ofthe paroccipital process harbours the stapedial grooveand, at its proximal end, the openings of the metoticforamen and fenestra ovalis. The ventral ramus of theopisthotic, separating the fenestra ovalis from themetotic foramen, is well defined in SMNS 80260. Itcan be seen externally in posterior (Fig. 1) and lateral(Figs 2, 3) views, although this is not as prominentas in some non-crown-group archosauromorphs (e.g.Evans, 1986; Gower & Sennikov, 1996a, 1997; Gower,1997; Gower & Weber, 1998). The distal end of thisramus is moderately expanded and the suture itshares with the basioccipital is oblique, with the lat-eral side of the ventral ramus of the opisthotic extend-ing further ventrally than the medial side.

Inner ear

The bony structure of the inner ear, the ossified part ofthe otic capsule, consists essentially of two partiallydifferentiated cavities, the vestibule and the cochlearrecess. The braincase of SMNS 80260 is slightly dam-aged on both sides just lateral to the cochlear recesses,but it seems fairly certain that the lateral surface ofthe braincase did not bear any clear indication of theposition or orientation of the cochlear recess in theform of a ‘cochlear prominence’, such as is presentin crocodylomorphs (e.g. Walker, 1990). Instead, theupper part of the recess is clearly positioned medial toa substantial, undifferentiated lateral part of theprootic that ends at the crista prootica, and, at thelower end, the parabasisphenoid also contributes tomasking its location in external view.

The recess itself is much deeper, more tubular, andmore clearly defined than that documented for non-crown-group archosaurs (Walker, 1990; Gower & Sen-nikov, 1996a; Gower, 1997; Gower & Weber, 1998). It issubcircular in cross-section (Fig. 4) and descendsapproximately 11 mm vertically below the vestibule.Preparation or exposure of the ventral end is perhaps

Figure 5.


Gower. Dorsal part of braincase of SMNS 80260 in posterior view. Scalebar = 30 mm fm = foramen magnum; sg = stapedial groove; s.p = surface for articulation with parietal.

56

D. J. GOWER



2002,

136

, 49–76

not complete, but there is nothing to suggest that itdid not remain fairly straight for the whole of itslength. In SMNS 80260, the dorsal end of the subcir-cular cross section of the recess is divided into approx-imately two halves – the curved anterior margin isformed largely by the prootic, and the less stronglycurved posterior margin is formed by the anterior sur-face of the ventral ramus of the opisthotic beneath theposition of the perilymphatic foramen. There are alsoclear indications, at the ventral end of the recess, thatcontributions are made by the parabasisphenoid,anterolaterally, and by the basioccipital, posterolater-ally. The parabasisphenoid contribution occurs as aresult of the dorsal wing of the element passingbetween the distal end of the ventral ramus of theopisthotic and the crista prootica, while the basioccip-ital contribution is associated with the oblique suturethis element forms with the ventral ramus of theopisthotic. Despite this, the full extent of these contri-butions is not yet clear. There is no indication ofwhether the floor of the lagenar/cochlear recess is com-

pletely ossified or whether it is confluent with theunossified gap lying between the basioccipital, basi-occipital, and parabasisphenoid. Such a continuousunossified channel is present in

Sphenodon

(Gower &Weber, 1998) and the erythrosuchid

Garjainia prima

(=

Vjushkovia triplicostata

; see Gower & Sennikov,2000) and was termed the ‘pseudolagenar recess’ byGower & Sennikov (1996a,b) because the potentiallycochlear end was not well defined and should probablynot therefore be interpreted as decisively indicatingthe presence of an elongated cochlea.

In SMNS 80260, the recess appears to maintainapproximately the same cross-section for all of itslength. While the exact length of the lagena/cochlea isdifficult to estimate, the elongate and well defined,tubular nature of the recess is interpreted as indicat-ing that a more elongated, cochlear-like organ haddeveloped, in comparison to the sac-like and rela-tively short lagena of extant lepidosauromorphs (e.g.Oelrich, 1956) and presumably non-crown-grouparchosaurs (e.g. Gower & Sennikov, 1996a; Gower,

Figure 6.


Gower. Dorsal part of braincase of SMNS 80260 in anterior view. Scalebar = 30 mm.

BRAINCASE OF

BATRACHOTOMUS

57



2002,

136

, 49–76

1997; Gower & Weber, 1998) and perhaps even phyto-saurs (Case, 1928; Camp, 1930; Walker, 1990).

The dorsal limit of the cochlear recess is marked bya transversely concave surface on the posterolateraledge of the prootic. At this dorsal limit, the recessjoins the anterior part of the lower end of the vesti-bule. The boundary between these cochlear (or lage-nar) and vestibular parts of the otic capsule is clearlyindicated by a low ridge, but it is not demarcated bythe presence of a definite crest of bone (Fig. 4). In thismanner,

B. kupferzellensis

resembles non-crown-grouparchosaurs and contrasts with, for example, extantlizards (e.g. Oelrich, 1956), the extinct crocodylo-morph

S. acutus

(Walker, 1990), and extant birds andcrocodilians (A.D. Walker, pers. comm.; pers. obs.).Hasse, 1873) termed the dividing crest in crocodiliansthe crista vestibuli, and Oelrich (1956) termed thesimilar structure in

Ctenosaura pectinata

the lagenarcrest. Another difference between this region in

B. kupferzellensis

and extant crocodilians, is that inthe former the lagenar/cochlear recess extends essen-tially ventrally from the bottom of the vestibule, whilein the latter the recess is geniculate and lies ventro-lateral to the vestibule.

Except for a small part of the anteroventral cornerof its floor, the vestibule is preserved in the dorsalpiece of SMNS 80260 (Fig. 7). Here it is essentially asubspherical, three-cornered chamber, with the sur-faces linking these corners being decidedly convex.The corners are situated in approximately anterior,posterior and dorsal positions, and each one is markedby concavities that presumably harbour fossae andforamina that indicate the position of ampullae andthe path of the semicircular canals. On the basis ofcomparisons with extant and extinct diapsids, wherethe underlying pattern is very conservative (pers.obs.), interpreting the relationship between these ves-tibular features and the soft tissues that they relate tois relatively simple. The dorsalmost corner lies withinthe epiotic and it probably houses what was the ven-tral opening of the sinus superior utriculi, i.e. it holdsthe osseous crus communis. The anteroventral cornerlies in the prootic and it contains a large concavitythat holds the anterior opening of the external semi-circular canal and the ventral opening of the anteriorvertical semicircular canal (both visible on both sides),as well as a space that would have been occupied bythe anterior ampulla. The posteroventral fossa isformed by opisthotic, and it is interpreted as probablyhousing the ventral opening of the posterior verticalsemicircular canal and the posterior opening of theexternal semicircular canal, as well as a recess for theposterior ampulla.

Allowing for a little loss of bone during preservationand preparation, the medial wall of the vestibule canbe considered to be very nearly completely ossified by

the epiotic, prootic, and opisthotic, with these ele-ments meeting at a Y-shaped suture (Figs 7, 8). Thislevel of ossification resembles the condition in manycrown-group archosaurs and squamates, and con-trasts with non-crown-group archosauromorphs and

Sphenodon

in which this medial wall is ossified to amuch lesser degree (Gower & Weber, 1998)

As a result of the horizontal break running throughthe braincase of SMNS 80260, it is difficult to inter-pret the detailed form of the ventral ramus of theopisthotic with great confidence. However, the breakhas revealed a notch in the broken dorsal surface ofeach ramus on the ventral piece of the braincase(Fig. 4). This lies just medial to the mid point of thewidth of each ramus and is interpreted as the ventralborder of the perilymphatic foramen. This notchpasses back through the opisthotic in a posterior orperhaps slightly posteromedial direction. On bothsides, it is not entirely clear whether the piece of theramus medial to the notch is finished or broken – andthus whether the bony part of the opisthotic loop wascomplete (with a loop closure suture; Walker, 1990) orincomplete and perhaps finished with cartilage in life(as in

Sphenodon

). There is a suggestion, particularlyon the right side, that the subhorizontal surface of theventral ramus medial to the notch was finished, indi-cating that a loop closure suture was probably notpresent, but confirmation of this interpretation willprobably require better preserved material. Whatis clear, is that the perilymphatic notch/loop is in amore medial position than the expanded and morelaterally deflected perilymphatic foramen of aetosau-rians (Walker, 1985, 1990; Gower & Walker, 2002) andcrocodylomorphs (Walker, 1990). The perilymphaticforamen in

B. kupferzellensis

is also smaller relative tothe size of the ventral ramus of the opisthotic than inaetosaurians and crocodylomorphs.

Prootic

The prootic forms much of the lateral wall of theendocranium. The anterior margin of the element isbroadly notched by its contribution to the border ofthe trigeminal foramen. The lateral fossa surroundingthis foramen is bordered dorsally and ventrally by apair of subhorizontal ridges that mark a transition tomuch thicker areas of bone. The more ventral of theseridges has been recorded also in noncrown-grouparchosauromorphs where it has been interpreted asthe site of origin of the protractor pterygoidei (e.g.Gow, 1975; Benton, 1983; Gower & Sennikov, 1996a).A small cleft harboured by the overhang of the ridgedorsal to the trigeminal foramen, seen most clearly onthe right side of SMNS 80260 (Figs 6, 8), partly sub-divides the trigeminal foramen. Based on its spatialrelation to features on the internal surface of the

58

D. J. GOWER



2002,

136

, 49–76

Figure 7.


Gower. Stereopair photographs and diagram of dorsal part of braincase of SMNS80260 in ventral view i.e. normal to fracture surface between ventral and dorsal pieces. Anterior is to the left of the figure.Scale bar = 30 mm.

BRAINCASE OF

BATRACHOTOMUS

59



2002,

136

, 49–76

Figure 8.


Gower. Stereopair photographs and diagram of dorsal part of braincase of SMNS80260 in left ventrolateral view. Anterior is to the left of the figure. pcv = probable path of posterior cerebral/cephalic vein(this is associated with a depression for a sinus on the wall of the endocranial cavity). The diagram is of a slightly more lat-eral view than the stereo pair photographs, such that the latter show part of the interior of the left vestibule. Scalebar = 20 mm.

60

D. J. GOWER



2002,

136

, 49–76

endocranium (see below) and comparisons with livingtaxa (e.g. crocodilians, pers. obs.;

Ctenosaura pectinata

,Oelrich, 1956), the more dorsal and slightly posteriorpart can be interpreted as marking the passage of themiddle cerebral vein, and the lower and slightly moreanterior part as for the trigeminal nerve.

Posteriorly, the prootic extends back over the prox-imolateral surface of the paroccipital process as a thinsheet. Dorsal to the trigeminal foramen, the prooticcontacts the epiotic and supraoccipital along an irreg-ular suture in a region that articulates with ventro-medial ‘sockets’ on the parietals (Gower, 1999).

The crista prootica has been partly destroyed by thehorizontal fracture through the specimen. Ventrally, itseems to have been barely raised above the surround-ing lateral braincase surface but this might at leastpartly be the result of compression during preserva-tion. Its anteroventral overlapping (laterally) of theparabasisphenoid can be clearly detected (Figs 2, 3).The position of the foramen of the facial nerve can belocated on the right side of the ventral part of SMNS80260 (Fig. 4), behind the crista prootica at a pointlevel with the posterior end of the horizontal lateralridge below the trigeminal foramen. A small pit withinthe broken lateral surface of the prootic probablymarks its position on the left side of the ventral part ofthe specimen (Fig. 2). Grooves along the ventral frac-ture surface on the dorsal part of SMNS 80260 indicatethe passage of the facial nerve parallel to the ventraledge of the paroccipital process (Figs 7, 8). There is nointrinsic evidence that there was more than a single,combined opening for the hyomandibular and palatineand branches of the facial nerve, but the horizontalfracture through the specimen and loss of some bone atthis point makes this a detail that ideally would bechecked against better preserved material.

Behind the path of the bony channel for the facialnerve, and beneath the floor of the fenestra ovalis, theposteromedial part of the prootic forms the cochlearrecess with the ventral ramus of the opisthotic, asdescribed above (see inner ear).

Anteriorly, the thickened part of the prootic beneaththe trigeminal foramen meets its antimere along themidline and forms the roof to the hypophyseal fossa.This part of the prootic is not upturned (see character4 of Gower & Sennikov, 1996a) and the externalabducens foramen, while not visible, probably lies onthe underside of a horizontal surface because it cannot be located on the better preserved anterior surfaceof the prootic (that additionally probably articulatedwith the laterosphenoid). Whether the external fora-men lies wholly within prootic (or partly in paraba-sisphenoid) is also unknown because of fractures inSMNS 80260 that pass from the dorsal end of thehypophyseal fossa back through the ventral ends ofthe prootics.

Parabasisphenoid

The parabasisphenoid of

B. kupferzellensis

is distinctlyverticalized, although this has clearly been exagger-ated by preservational distortion in SMNS 80260, inwhich the basipterygoid processes lie almost directlybelow the basal tubera. In posterior view (Fig. 1), thebasal tubera resemble a pair of roughly finished scoops– the upper concave surfaces of which contribute to theunossified gaps between this element and the basio-ccipital (basal tubera) and opisthotics (ventral rami).The dorsolateral edge of each basal tuber of the para-basisphenoid is sandwiched between the anteroventralsurface of the ventral ramus of the opisthotic and theposteromedial surface of the prootic. It thus forms atleast a small part of the floor of the fenestra ovalis, andprobably the distal end of the cochlear recess.

Posteromedially, between the basal tubera and justbelow its suture with the basioccipital, the paraba-sisphenoid holds a deep depression of small diameter(Fig. 1). This is generally rounded, but there is a smallindication both dorsally and ventrally that its floorbecomes partially subdivided by low midline ridges.This apparently blind pit is interpreted as the medianpharyngeal recess (Witmer, 1997). Similar depressionsin other suchians have often been termed the medianeustachian foramen, following the nomenclature fre-quently employed for crocodyliforms, where the appar-ently equivalent depression (which may be at leastpartly within basioccipital) is not blind, but insteadcommunicates with the true (lateral) eustachiantubes. The basioccipital recess is partially subdividedbut blind in the crocodylomorph

Sphenosuchus acutus

(Walker, 1990). Wu & Chatterjee (1993) reported thatthe subdivided basioccipital recess in the crocodylo-morph

Dibothrosuchus elaphros

communicates withthe eustachian tubes, but it is unclear whether this isan inference based on the condition in crocodyliformsor if this was actually observed in the slightly crushedmaterial they studied. The equivalent concavity in atleast some aetosaurians is a large, deep, hemispheri-cal to conical, closed depression that occupies much ofthe area between the closely spaced basal tubera andbasipterygoid processes (e.g. Case, 1922; Parrish,1994).

In posterior view (Fig. 1), the parabasisphenoid isstrongly waisted below the basal tubera and above thebasipterygoid processes. The latter processes areapproximately the same size as the basal tubera, andthus do not resemble the relatively much larger com-parative structures in crocodilians and the sphenosu-chian crocodylomorphs

S. acutus

(Walker, 1990) and

D. elaphros

(Wu & Chatterjee, 1993). There is no indi-cation of any foramina for the entrance of the cerebralbranches of the internal carotid arteries on the poste-rior surface of the parabasisphenoid, such as is seen innon-crown-group archosaurs and archosauromorphs

BRAINCASE OF BATRACHOTOMUS 61

© 2002 The Linnean Society of London, Zoological Journal of the Linnean Society, 2002, 136, 49–76

(e.g. Benton, 1983; Evans, 1986; Gower & Sennikov,1996a; Gower & Weber, 1998). Instead, the foramen forthis blood vessel is interpreted as lying on the lateralsurface of the parabasisphenoid, in a groove that arcsfrom behind the posterior edge of the crista prootica onto the posterodorsal edge of the basipterygoid process,on the lower edge of the broad notch that separates thebasal tuber of the parabasisphenoid from the basiptey-goid process. The groove passes ventromedially aroundthe base of the basipterygoid process before extendinganterodorsally along the lateral surface of the base ofthe cultriform process of the parabasisphenoid. Thereare no foramina along that part of the groove thatpasses around the basipterygoid process.

Based on comparisons with Sphenodon (Säve-Söder-bergh, 1947; pers. obs.) and reconstructions of soft tis-sues proposed for a range of fossil archosaurs (e.g.Walker, 1961, 1990; Gower & Sennikov, 1996a, 1997;Gower, 1997; Gower & Weber, 1998), this groove andthe probable position of an entrance foramen allow areconstruction of the paths of the internal carotidartery and palatine branch of the facial nerve. Thepalatine branch of the facial nerve is interpreted asbeing transmitted to the base of the cultriform processof the parabasisphenoid outside of bone. Thus therewere no bony Vidian canals in B. kupferzellensis. Thepalatine artery would have probably branched fromthe cerebral branch of the internal carotid near thepoint that the latter vessel entered the braincase. Thepalatine artery would have passed to the base ofthe cultriform process in an open (not enclosed inbone) channel in association with the palatine branchof the facial nerve.

The basipterygoid process bears a well defined facetfor articulation with the pterygoid. This facet com-prises an oval region on the ventrolateral surface ofthe process that wraps around the ventral edge andonto a small area of the posterior end of the ventro-medial surface of the process. The articulation withthe pterygoid was evidently non sutural. The cultri-form process is incomplete except for its tall, laterallycompressed base that bears a dorsal gutter formingthe floor of the hypophyseal fossa (Fig. 9). Here, theventral end of the hypophyseal fossa bears a deep,incompletely prepared pit that probably houses theforamina for the entrance of the cerebral branches ofthe internal carotid arteries into the fossa. The vastmajority of the hypophyseal fossa is formed by theparabasisphenoid as it passes up between the ven-trolateral wings of the paired prootics. There is noevidence that the parabasisphenoid would havecontacted the laterosphenoid in the region preformedby the pila antotica.

LaterosphenoidThe laterosphenoid is not preserved in SMNS 80260,but there is good evidence of its existence. This ele-ment seems to be ossified in all archosaurs and anarea for its articulation with the ventral surface ofthe frontal of SMNS 52790 and SMNS 80260 canclearly be detected. The long and narrow form of thisarea on the frontal has been interpreted (Gower, 1999:19) as an indication that the laterosphenoid ofB. kupferzellensis is relatively long and slender in com-parison with other known suchian archosaurs.

Figure 9. Batrachotomus kupferzellensis Gower. Ventral part of braincase of SMNS 80260 in anterior view. See Figure 1for scale. g. ic = groove for cerebral branch of internal carotid artery; hf = hypophyseal fossa.

62 D. J. GOWER


INTERNAL SURFACE

(FIGS 4, 7, 8)

BasioccipitalThe basioccipital is not exposed at the posterior part ofthe floor of the endocranial cavity because of midlinecontact between the exoccipitals. It does appear to beexposed further anterolaterally, between the antero-lateral margin of each exoccipital and the lateral sur-face of the base of the ventral ramus of the opisthotic.Thus the basioccipital contributes to the floor of themetotic foramen. Although the basioccipital is exposedhere, at the lateral edge of the endocranial cavity,exposure along the midline is prevented by contactbetween the posterior end of the prootics, and betweenthe prootics and exoccipitals.

ExoccipitalThe exoccipitals make midline contact on the posteriorpart of the floor of the endocranial cavity. Here thefloor is flat to slightly concave transversely. The ante-rior end of the exoccipitals are tapered. Their anteriormargins mark the point of an abrupt drop of approx-imately 5 mm down onto the middle part of the floor ofthe endocranial cavity and, laterally, a greater dropdown onto the floor of the metotic foramen. The inter-nal foramen for the hypoglossal nerve lies slightly pos-terior to the external foramen, so that the nervepassed through the exoccipital pillar in a slightly pos-teromedially anterolaterally aligned channel.

The metotic foramen is a tall, narrow, subverticalslit (Figs 2, 3). Above its dorsal end, the internal aspectof the exoccipital exhibits a very distinctive feature,most clearly seen on the right side (Fig. 8). A smallforamen passes through the braincase immediatelyabove the dorsal end of the metotic foramen. Thispasses downwards and slightly backwards out throughthe braincase wall, originating within a well markeddepression on the inner wall of the endocranial cavityon the concave posterodorsal surface of the otic bulla.This depression has a gently tapering anterodorsalend and an incised, semicircular posteroventral mar-gin that is formed by a narrow splint of bone that formsthe ventral border to the foramen by bridging acrossthe dorsal end of the metotic foramen. The depressionis interpreted as part of a vascular sinus, and the fora-men as possibly transmitting the posterior cerebral/cephalic vein or an equivalent vessel.

The route of the posterior cerebral/cephalic vein indiapsids is somewhat confused in the literature. How-ever, Dendy (1909: 413) reported that in Sphenodonthe posterior cephalic vein passes “outwards, down-wards, and backwards, above and behind the ductusendolymphaticus, to the jugular foramen, throughwhich it leaves the cranial cavity just behind the audi-

tory capsule, in company with the ninth, tenth, andeleventh cranial nerves”. Dendy also reports that thisvessel adheres close to the inner surface of the cranialwall and that its position is easily recognized afterremoval of the brain. The same vein has been reportedas leaving the braincase via the foramen magnum in,for example, lizards (e.g. Dendy, 1909) and crocodil-ians (e.g. Grosser & Brezina, 1895; Bruner, 1907), andeven as shifting from the metotic fissure to the fora-men magnum in later ontogenetic stages of crocodil-ians (Hochstetter, 1906; van Gelderen, 1924−25). Thusthe reality, consistency, and significance of this possi-ble difference does not appear to be emphatic from abrief investigation of the literature. Walker (1990: 48,89, fig. 30) described a ‘reniform cavity’ in S. acutus,positioned above the internal dorsal end of the metoticforamen. This holds several small foramina, and agutter passes from its lower end in to the dorsal end ofthe metotic foramen. Walker (1990: 89) found onlyslight indications of the presence of the depression inskulls of extant crocodilians. I can confirm this, andreport that in at least some specimens, one of the sev-eral foramina in this region is larger than the othersand forms a decidedly ventrally and laterally directedpassage that bears at least a superficial resemblanceto the lower end of the depression and the foramendescribed in B. kupferzellensis. A hair passed throughthis foramen in dried skulls of crocodilians emergesinto the middle ear cavity below and behind theperilymphatic foramen. The wall of the endocranialcavity above and behind the otic bulla of extant croc-odilians is associated with several venous sinuses, andthus it is possible that the foramina observed incrocodilian skulls, in S. acutus (including the gutter),and in SMNS 80260 were associated with venousdrainage, although this clearly represents an areafor further investigation. Whatever its soft–tissueassociations, the presence of a small foramen here inB. kupferzellensis is perhaps a derived state amongarchosaurs. A similar feature can also be detected inPostosuchus kirkpatricki (see below).

No suture can be detected between the exoccipitaland opisthotic on the internal surface of the braincaseof B. kupferzellensis. The only slight indication of apossible suture lies on the right side of SMNS 80260,where the anterior end of the splint of bone that formsthe ventral margin of the venous sinus (describedabove) meets the posterior wall of the otic bulla.However, this might instead be a suture within acompound opisthotic + exoccipital as it encloses thepassage of the vein. A suture exists within the com-pound opisthotic + exoccipital here in at least someextant crocodilians (pers. obs. of Crocodylus spp.),between the posterior surface of the otic bulla and theanterodorsal end of the ‘exoccipital pillar’, enclosingthe larger foramen that occurs in this region.



SupraoccipitalThe demarcation of supraoccipital from epiotic is notvery clear on the internal surface of the braincase. Thesupraoccipital certainly forms the ceiling to the pos-teriormost part of the endocranial cavity because itcontributes to the dorsal border of the foramen. Itprobably also forms the ceiling to the anteriormostpart of the endocranial cavity that is preserved inSMNS 80260 (Figs 6, 8). The supraoccipital may evenform the whole of the midline of the ceiling, but this isnot entirely clear. The epiotic certainly forms themajority of the upper part of the walls of the posteriorpart of the endocranial cavity. Anterior to the supraoc-cipital, the parietals probably formed the endocranialceiling.

EpioticThe epiotic forms a considerable portion of the dorso-lateral walls of the interior surface of the endocranialcavity. It forms a little more than the upper third ofthe ossified medial wall of the otic bulla, that is visiblethrough the foramen magnum (Fig. 5). This medialwall is better preserved on the right, where the con-fluence of the epiotic, opisthotic, and prootic probablymarked the location of the endolymphatic foramen.The basis for this suggestion is largely that an alter-native position for this foramen cannot be detected, aswell as comparison with sphenosuchian and crocodyli-form crocodylomorphs (Walker, 1990; pers. obs.). Ante-rior to the vestibule, the medial surface of the epioticbears the dorsal limit of the shallow auricular (floccu-lar) recess as it extends up from the prootic.

Each of the dorsolateral corners of this portion ofthe endocranium of SMNS 80260 holds a deep subtri-angular pit within a fossa. On the right side, thisappears to lie within the anterodorsal corner of theepiotic (Fig. 8), but on the left it is possible that thesupraoccipital forms its anterodorsal edge. Similarpits are seen in a wide range of extant and extinctdiapsids, and in B. kupferzellensis this probably repre-sents the passage of the dorsal head vein, with thefossa surrounding the pit probably indicating the posi-tion of a venous sinus.

OpisthoticThe opisthotic forms the posteroventral part of the oticbulla. The ventral ramus of the opisthotic forms theperilymphatic foramen, which is described above(inner ear section). The medial edge of the ventralramus is thinner than the lateral edge. The suturebetween the ventral ramus and the basioccipital isoblique, such that below the position of the perilym-phatic foramen, the ventral ramus becomes less andless medially extensive.

ProoticThe prootic forms part of the floor and lateral walls ofthe mid part of the endocranial cavity, and contributesto the anteroventral portion of the otic bulla. It alsoholds a little more than half of the auricular recess.Both the left and right recesses of SMNS 80260 hold asingle well marked, presumably vascular foramenthat enters the bone in a posteroventral direction. Agroove passes around the anterior edge of the auricu-lar recess and down to the trigeminal foramen. This isinterpreted as indicating the passage of the middlecerebral vein. There was probably communicationbetween the upper part of this groove and the presum-ably sinus-holding fossa that surrounds the foramenfor the dorsal head vein on the anterodorsal part of theepiotic. Indication of this venous connection is weak inSMNS 80260, but it might have partly been preservedon the posterior edge of the medial surface of the(missing) laterosphenoid.

The ventral part of SMNS 80260 shows the oppositeprootics meeting along the midline on the floor of theendocranial cavity (Fig. 4). Here the floor is concavetransversely, more so than the posterior part of thecavity where the floor is formed by the exoccipitals.The prootics extend back to meet the anterior end ofthe exoccipitals along the midline. Immediately lat-eral to this and medial to the cochlear/lagenar recess,the posterior edge of the prootic contacts the anteriorend of the basioccipital. This is visible on the right sideof SMNS 80260, but obscured by crushing on the left ofthe specimen. The part of the floor formed by theprootics is marked by a pair of foramina for theabducens nerves, which seem to lie entirely withinthese elements. Posterolateral to the abducens fora-men, the prootic forms an overhang in the ventrolat-eral edge of the endocranial cavity. The posterior partof this overhang is formed by the anteroventral limitof the otic bulla, and it harbours the internal foramenfor the passage of the facial nerve through the brain-case wall. There are no clear indications of foraminathat might have transmitted branches of the acousticnerve, probably as a result of loss of detail along thefracture surface between the dorsal and ventral piecesof SMNS 80260.

ParabasisphenoidThe parabasisphenoid does not seem to be exposed inthe walls or floor of the endocranial cavity of SMNS80260.

BRAINCASE STRUCTURE IN OTHER RAUISUCHIANS

Rauisuchian osteology in general is poorly known(Gower, 2000), and braincase structure has been fig-

64 D. J. GOWER


ured and/or described for only a handful of taxa.Braincase structure is here reviewed for those rauisu-chians in which it is best known.

SAUROSUCHUS GALILEI

Saurosuchus galilei Reig is known from the UpperTriassic Ischigualasto Formation of Argentina. Brain-case elements were not among the original materialdescribed by Reig (1959) or in two subsequent descrip-tive accounts (Reig, 1961; Sill, 1974), but a good andlargely complete braincase was preserved in the spec-imen figured and described in detail by Alcober (2000).In overall form, the braincase of S. galilei resemblesthat of B. kupferzellensis rather more than that of therauisuchians Postosuchus kirkpatricki and Tikisuchusromeri, in that the parabasisphenoid is fairly shortfrom its dorsal to ventral edges (plesiomorphic condi-tion). S. galilei braincase characters that are plesio-morphic for crown-group archosaurs include theabsence of a semilunar depression and parabasisphe-noid intertuberal plate, a lateral position on the para-basisphenoid for the entrance of the cerebral branch ofthe internal carotid artery, and a metotic foramen (i.e.an undivided metotic fissure). The lateral edge of theexoccipital bears a subvertical ridge, but this is not asprominent as in B. kupferzellensis. This ridge lies pos-terior to the single external foramen for the hypoglo-ssal nerve, and there is some indication (left side ofAlcober’s fig. 7A) that it extends down onto the dorsalpart of the basioccipital. The posterior surface of thebraincase shows a large unossified gap between thebasal tuber of the basioccipital, ventral ridge of theopisthotic, and parabasisphenoid. The opposite exoc-cipitals and prootics meet along the midline. The pilaantotica was ossified by prootic and parabasisphenoid,so that there is no contact between laterosphenoid andparabasisphenoid. Alcober’s fig. 8 shows that the baseof the cultriform process of the parabasisphenoidbears a groove on its lateral surface, in an equivalentposition to that described for B. kupferzellensis andinterpreted as indicating the route of the palatinebranch of the facial nerve and perhaps the palatineartery.

Alcober identified the presence of a ‘presphenoid’ inS. galilei, articulating with the laterosphenoid andprootic anterior to the single undivided opening for themiddle cerebral vein and trigeminal nerve. Indeed,the limits of this element were identified as includinga border to the trigeminal opening. Examination ofAlcober’s figures shows the ‘presphenoid’ to be in aposition occupied by the anteroventral portion of thelaterosphenoid in a wide range of other Triassic archo-saurs. Thus the suture between laterosphenoid and‘presphenoid’ might alternatively represent a break inthe (incomplete) posterior portion of laterosphenoid,

extending from a potential zone of weakness at thetrigeminal foramen. Fractures in this position havebeen observed in some Triassic archosaurs, but thepattern described by Alcober is consistent on bothsides of the specimen, and I have not examined thematerial first hand.

POSTOSUCHUS KIRKPATRICKI

Postosuchus kirkpatricki Chatterjee is known from theUpper Triassic of the south-western United States ofAmerica (Chatterjee, 1985; Long & Murry, 1995). Ithas a confused taxonomic history (Long & Murry,1995; Gower, 2000), but it seems that the documentedbraincase material from the type locality can be rela-tively confidently associated with the species nameP. kirkpatricki. Chatterjee (1985: fig. 7) figured threeviews of a composite restoration of the braincase ofP. kirkpatricki and described and interpreted aspectsof the osteology. The braincase of the holotype (TTUP9000) is currently largely obscured by paint and plas-ter applied during the restoration of the skull. Myobservations of this specimen were aided by accessto photographs (provided by S. Chatterjee) taken ofthe specimen prior to restoration. I also examined theother TTUP P. kirkpatricki braincase, no. 9002.

My observations agreed with the view presented byChatterjee (1985) in terms of the general proportionsand in many of the sutural relations and detailedfeatures. I concur that there is a single opening forcranial nerves IX-XI (metotic foramen), a laterallypositioned entrance of the cerebral branch of the inter-nal carotid artery, and an unossified gap between thebasal tubera of the basioccipital and parabasisphenoidthat was probably filled with cartilage in life. Someother details merit further discussion

There is a strong subvertical ridge on the lateralsurface of the exoccipital and upper part of the basi-occipital, but this lies posterior, not anterior, to thesingle external foramen for the hypoglossal nerve(seen on the left side of TTUP 9002), so that thisforamen is not (contra Chatterjee, 1985: fig. 7) visiblein posterior view. This ridge runs down onto thedorsolateral edge of the basal tubera of the basioccip-ital. It is stronger than the corresponding ridge inB. kupferzellensis and, as in Stagonolepis robertsoni(Gower & Walker, 2002) and crocodylomorphs (Walker,1990), it extends further laterally than the ventralramus of the opisthotic. Unlike the situation inB. kupferzellensis, the basal tubera of the basioccipitalare barely bilobed, and there is no clear recess on theposterior surface of the basioccipital between thetubera. The sutural relations between opisthotic andprootic on the paroccipital process are not clear. Theforamina for the cerebral branches of the internalcarotid arteries are within depressions that lie close to



the midline, where the parabasisphenoid is very nar-row. Chatterjee identified a separate presphenoid ossi-fication articulating with the laterosphenoid andparabasisphenoid. Chatterjee’s (1985: fig. 7) recon-struction of this region presents a highly unusualarrangement for a Triassic archosaur. The known partof the laterosphenoid is shown as sandwiched betweenthe prootic and the base of the cultriform process ofthe parabasisphenoid, and as forming the anteroven-tral half of the border to the trigeminal foramen andeven part of the crista that overhangs that part of theparabasisphenoid pierced by the foramen for the cere-bral branch of the internal carotid foramen. Thisregion is incompletely preserved in TTUP 9002, and itdoes not provide support for this reconstruction. Thesurface of TTUP 9000 (photographs and direct obser-vation) is poorly preserved and difficult to interpret.At least that part of the laterosphenoid-prootic suturedepicted as lying immediately posteroventral to thetrigeminal foramen might be instead interpreted as anartefactual crack. I suggest that determination of theexact pattern of elements here, including the possiblepresence of a presphenoid, requires additional mate-rial. There are several features for which it is not pos-sible, in the known specimens, to ascertain thecondition with any confidence including the presence/absence of a discrete epiotic, the exact path of theabducens and facial nerves through the braincasewall, and the entire shape of the cultriform process ofthe parabasisphenoid.

The endocranial cavity is exposed in TTUP 9002.The exoccipitals meet along the midline and, ante-riorly, they appear to be separated here from theprootics by exposure of the parabasisphenoid andbasioccipital. As in B. kupferzellensis, there is a abruptdrop in the level of the floor of the endocranial cavityfrom the hind part formed by the exoccipitals to themid part in front of this. On each side, close to the lat-eral edge of the floor of the endocranial cavity, thereare two clearly defined cup-like depressions. The moreposterior of these (visible in Parrish, 1993: fig. 7A) liesjust in front of the base of the exoccipital pillar and ishere interpreted as the floor of the metotic foramen.The more anterior depression lies a short distance infront of this and is here interpreted as the lagenar/cochlear recess. This is not as deep as in crocodylomor-phs, but it is better defined than in non-crown-grouparchosaurs and phytosaurs (Walker, 1990; Gower &Weber, 1998). Its upper, anterior limit is not markedby any sign of a crista demarcating the border betweenthe lower end of the vestibule and the upper end of thelagenar recess. On the left side of TTUP 9002, thelagenar/cochlear recess lies along the basioccipital-parabasisphenoid suture. Anteriorly, in the same spec-imen, the opposite prootics seems to make midlinecontact. TTUP 9002 also presents evidence that

P. kirkpatricki closely resembled B. kupferzellensis inpossessing a separate foramen possibly for the pas-sage of the posterior cerebral/cephalic vein. Seen beston the left side, the wall of the endocranial cavityimmediately above the metotic foramen bears a cres-cent-edged depression that is interpreted as theposition of a venous sinus. A foramen within thisdepression passes through the thin anterior edge ofthe exoccipital pillar to emerge immediately above theexternal opening of the metotic foramen. This is alsovisible on the right side of TTUP 9002, but the medialsurface is partly broken.

In addition to TTUP 9000 and TTUP 9002,Chatterjee (1985: 407) referred a third braincase toP. kirkpatricki, UMMP 7473. This had previously beenfigured by Case (1922: pl. 13), who referred thisisolated braincase to the dinosaur Coelophysis.Chatterjee considered it to differ from the braincase ofKnown Coelophysis and instead found that UMMP7473 “corresponds so well” with the TTUP braincasesreferred to P. kirkpatricki that he proposed that it rep-resented the same taxon. I have examined UMMP7473 and believe that Chatterjee’s taxonomic identifi-cation is not correct. The specimen is incomplete (seefigures in Case, 1922), consisting of the occipitalcondyle, the endocranial cavity, and the proximalparts of the parabasisphenoid and paroccipital pro-cesses. The region of the fenestra ovalis, ventralramus of the opisthotic and perilymphatic foramenhas been obliterated during preservation/preparationon both sides and is marked now by only a single largehollow. The suture between exoccipitals and thecondylar portion of the basioccipital cannot be de-tected. The structures labelled ‘bpt’ by Case are notthe basipterygoid process as such, but rather theincomplete parts of the parabasisphenoid that wouldhave supported these processes. The part of the para-basisphenoid that has been preserved does resemblethat of P. kirkpatricki in that it consists of a pair of thinsubvertical plates of bone that border a transverselynarrow but deep median pharyngeal recess. Addition-ally, a small depression is positioned just above thisrecess and between the ventral edge of the basaltubera of the basioccipital, resembling a similar fea-ture in TTUP 9000 and 9002. The endocranial cavity ofUMMP 7473 is also exposed. Features observable hereinclude well ossified otic bullae, well marked fossaeindicating the position of venous sinuses and the pas-sage of the dorsal head veins, shallow auricular fossae,and an abrupt step down on the floor of the endocra-nial cavity anterior to the front end of the exoccipitals.These features are consistent with this specimenbeing P. kirkpatricki, but they are also found in a widerrange of Triassic archosaurs.

Foramina that are interpreted as being for theentrance of the cerebral branches of the internal

66 D. J. GOWER


carotid arteries are positioned on the lateral surfacesof the parabasisphenoid – one of the few indications(Gower & Weber, 1998; Parrish, 1993) that the brain-case of UMMP 7473 probably belongs to a crown-group archosaur. However, the one important differ-ence between this specimen and the known braincasesof P. kirkpatricki lies in the position of the externalforamen for the hypoglossal nerve. This is posterior toa very strong subvertical ridge on the lateral surface ofthe exoccipital. Among crurotarsans, this arrange-ment is known to certainly occur only in aetosauriansand crocodylomorphs (see below; Gower & Walker,2002). It is not possible for me to completely rule outthat UMMP 7473 might be from an ornithodiran, butthis may partly reflect my unfamiliarity with brain-case osteology in taxa from that group.

Apart from Chatterjee’s (1985) original figures ofthe braincase of P. kirkpatricki, two other interpreta-tions have been published (Chatterjee, 1991: fig. 28b),Parrish, 1993: fig. 7). Chatterjee (1991: fig. 28b) pre-sented a modified version of his previous (1985) recon-struction of the braincase of P. kirkpatricki in lateralview. Sutural relations remained largely the same(except for a smaller prootic contribution to the paroc-cipital process), but a few changes in detail weremade. A small foramen was now indicated positionedabove the metotic foramen, this was identified as theposterior tympanic recess (Chatterjee, 1991: 335,fig. 28b), a pneumatic diverticulum of the tympaniccavity. This is here interpreted as possibly represent-ing the passage of the posterior cerebral/cephalic veinthrough the braincase (see above). In addition, twofurther possible pneumatic depressions were accentu-ated relative to the earlier reconstruction, posterodor-sal (not visible in TTUP 9002, pers. obs.) to thetrigeminal opening, and in the region of the foramenfor the cerebral branch of the internal carotid artery.

Parrish (1993: 300) claimed that P. kirkpatrickipossesses a ‘wedgelike, dorsoventrally expandedparasphenoid rostrum’ and that this is a derivedfeature shared with rauisuchian and crocodylomorphsuchians including Lotosaurus and Dibothrosuchus.The exact meaning of ‘wedgelike’ in this context isunclear. This description has also been used todescribe the condition seen in crocodilians where therostrum does not taper proximally, but rather expandsdorsoventrally before ending at an abrupt blunt edge.This contrasts with the condition in which the rostrumis longer than it is high and it tapers to a point prox-imally. This latter condition is probably plesiomorphicfor suchians (and archosaurs as a whole) because it ispresent in erythrosuchids and proterosuchids (Gower& Sennikov, 1996a; Gower, 1997), Euparkeria capensis(Welman, 1995; Gower & Weber, 1998) and phytosaurs(Camp, 1930; Chatterjee, 1978; pers. obs.). The para-basisphenoid rostrum is incomplete in the known

specimens of P. kirkpatricki (including that figured byParrish, 1993: fig. 7) as well as of Dibothrosuchuselaphros (Wu & Chatterjee, 1993: 68). The rostrum ofthe crocodylomorph Sphenosuchus acutus tapers ante-riorly in lateral view and is not wedge-shaped (Walker,1990).

Parrish (1993: 302) also identified a synapomorphyfor his clade Paracrocodylomorpha (comprising Cro-codylomorpha + Poposauridae, the latter representedby P. kirkpatricki): “Foramen at basioccipital/basisphe-noid juncture junction . . . In crocodylomorphs andPostosuchus, this is the exit for the eustachian tubes”.A similar opening/pit (present in many, even non-crown-group archosaurians) is more probably anunossified gap that was plugged with cartilage in life(see review by Gower & Weber, 1998).

Long & Murry (1995) proposed a revision of mate-rial referred to P. kirkpatricki. This included referral ofadditional braincase material to this species. How-ever, these specimens (e.g. UCMP A269/27479; Long &Murry, 1995: fig. 125) are fragmentary and I suggestthat their identification must remain in question forthe time being.

TIKISUCHUS ROMERI

Tikisuchus romeri Chatterjee & Majumdar is a rauisu-chian from the Upper Triassic Tiki Formation of India,known from a single incomplete specimen, ISI R 305.The osteology of this taxon has been described onlybriefly by Chatterjee & Majumdar (1987). Theseauthors figured a restoration of the braincase inposterior view and presented the following descrip-tion: ‘braincase very deep with elongated and wellpronounced basipterygoid processes; supraoccipitaltapers dorsally and makes a movable contact withparietal’ (Chatterjee & Majumdar, 1987: 788). Fur-thermore, they (p. 787) included ‘ossified laterosphe-noid’ in a list of archosaurian features possessed bythe specimen.

I have re-examined this material as part of thisstudy. The surface preservation of ISI R 305 is verycrumbly, and sutures and other details are hard tomake out. In addition, several regions of the braincaseare not preserved or could not be located at the time ofthis study. This makes it impossible to comment on thelaterosphenoids (which could not be located), exoccipi-tals, metotic region, fenestra ovalis. The paroccipitalprocesses are fairly short with strong distal expan-sions. The basioccipital bears dorsal facets that indi-cate that the ventral ends of the exoccipitals met alongthe midline to form the ventral border of the foramenmagnum. The basal tubera of the basioccipital arebarely bilobed. There is no sign that the entranceforamina for the cerebral branches of the internalcarotid arteries were located on the posterior or pos-



teroventral wall of the parabasisphenoid. The antero-lateral surfaces of the same element are even lessperfectly preserved, but I tentatively interpret thatthis is where the foramina for the cerebral branch ofthe internal carotid artery are positioned.

The braincase of T. romeri resembles that ofP. kirkpatricki, particularly in the elongated region ofthe parabasisphenoid between the basipterygoid pro-cesses and the basal tubera, and in the way this regionharbours a dorsoventrally elongate median pharyn-geal recess. This is a derived feature within Suchia,and it represents a potential synapomorphy shared byP. kirkpatricki and T. romeri to the exclusion of otherknown suchians, including B. kupferzellensis (althoughthe braincase UMMP 7473 appears to possess a simi-lar feature, see above). One clear difference betweenthe parabasisphenoids of P. kirkpatricki and T. romerilies in the presence of paired short hook-like (in thetransverse plane) processes on the parabasisphe-noid of the latter taxon. These are not apparent inChatterjee & Majumdar’s (1987) figure 2, but they canbe clearly detected on the posterior surface of theelement between the bases of the basipterygoidprocesses.

In summary, rauisuchian braincases are slowlybecoming better known. The braincases of B. kupfer-zellensis, S. galilei, P. kirkpatricki and T. romeri sharemany features. Most of these are apparently plesio-morphic for Archosauria (e.g. undivided metotic fora-men, external foramina for hypoglossal nerve notposterior to subvertical ridge), some are probablyderived for Archosauria (e.g. well ossified otic bullae,lateral foramen for the cerebral branch of the inter-nal carotid artery), but few if any seem to be bothderived for Suchia and restricted to rauisuchians. Theelongate median pharyngeal recess of P. kirkpatrickiand T. romeri is perhaps the only such character.B. kupferzellensis and P. kirkpatricki share a separateforamen possibly for the posterior cerebral/cephalicvein above the dorsal end of the metotic foramen, buta probable equivalent is also present in at least somecrocodilians, and the condition in S. galilei andT. romeri is unknown.

CRUROTARSAN PHYLOGENY AND THE EVOLUTION OF SUCHIAN BRAINCASES

There are two intimately related issues here –whether the patchy current knowledge of crurotarsanbraincase osteology provides any potentially usefulevidence for resolving the phylogenetic interrelation-ships of Crurotarsi, and also how much we can learnabout early suchian braincase evolution by optimizingcharacter states on a framework phylogeny for cruro-tarsans. The approach taken to explore these issues isone of constructing and analysing a character-taxon

matrix for braincase characters among some non-crown-group archosaurs and some crurotarsans. Infocusing on these taxa, an initial assumption has beenmade that the archosaur evolutionary tree consists ofa crown group comprising a bird- and a crocodilian-lineage, plus a series of non-crown-group taxa (Gower& Wilkinson, 1996). It has also been accepted that cer-tain higher taxa are members of the crocodilian ratherthan bird-lineage (i.e. they are crurotarsans ratherthan ornithodirans), for example, phytosaurs, rauisu-chians, aetosaurians, and sphenosuchian crocodylo-morphs. Among crurotarsan and noncrown-grouphigher taxa, Ornithosuchia and Proterochampsidaewill not be considered any further here because satis-factory braincase specimens or published descriptionsare currently unavailable to me. Similarly, some sin-gleton taxa with poorly preserved and/or documentedbraincases have not examined for this study andare thus excluded from further consideration. Theseinclude Gracilisuchus stipanicicorum and Erpetosuchusgranti. Braincase evolution in early ornithodirans isbeyond the scope of this study and will not be consid-ered further here.

The approach adopted here means that braincasecharacters identified through this and previousstudies are used both to construct a phylogenetichypothesis, and to investigate crurotarsan braincaseevolution by optimizing them on the very same phylo-genetic hypothesis. Some workers might be concernedhere with issues of circularity in argumentation, but Ido not share these concerns in this instance because(1) the phylogenetic hypothesis derived from brain-case data is broadly concordant with currently ortho-dox views of archosaur phylogeny (see below); (2) thisstudy is intended to be an exploration of braincasedata, rather than to be the final word on either archo-saurian phylogeny or archosaurian braincase evolu-tion; (3) rather than being viciously circular, futureworkers are free to start from an entirely new positionand set of assumptions.

TAXA

The terminal taxa employed here are the protero-suchid Proterosuchus fergusi, the erythrosuchidGarjainia prima, the euparkeriid Euparkeria capen-sis, phytosaurs, the rauisuchians B. kupferzellensis,S. galilei, P. kirkpatricki, and T. romeri, aetosaurians,and sphenosuchian (D. elaphos; S. acutus) and cro-codyliform (Crocodylus sp.) crocodylomorphs. Sourcesof data are largely as given in Material and Methods.Characters are scored for phytosaurs on the basis ofinformation in Chatterjee (1978) and personal obser-vations of BMNH R38037 and R42745 (both probablyNicrosaurus meyeri, A. Hungerbühler, pers. comm.).Ingroup variation among phytosaurs has not been

68 D. J. GOWER


studied in detail. Similarly, the aetosaurian terminaltaxon has been scored largely on the basis of data forStagonolepis robertsoni (Walker, 1961, 1985; Gower &Walker, 2002), but also for Desmatosuchus spurensis(Case, 1922) and Longosuchus meadei (Parrish, 1994;Gower & Walker, 2002). As with the phytosaurs, itmay be that some of the scorings here do not repre-sent the only or even the plesiomorphic condition forthe clade as a whole. The included erythrosuchid,G. prima, is the senior subjective synonym of Vjushk-ovia triplicostata (Gower & Sennikov, 2000). Data forthis taxon were taken from Gower & Sennikov(1996a). Crocodyliformes and crocodilians are repre-sented through personal observation of dried skulls ofCrocodylus sp.

CHARACTERS

Some of the characters used here have been employedpreviously elsewhere, while others represent a firstattempt to explore potential phylogenetic informationrepresented by observed variation among taxa. Thecharacter-taxon matrix is shown in Table 1.

(1) Foramina for entrance of cerebral branches of inter-nal carotid arteries into braincase positioned on theposterior (0), posterolateral (1), or anterolateral (2) sur-face of the parabasisphenoid.

In phytosaurs, the foramina are on the lateral surfaceof the parabasisphenoid, but only just anterior to thenotches between the basal tubera and basipterygoidprocesses (Chatterjee, 1978; pers. obs.). This is mor-phologically intermediate between the condition innon-crown-group archosaurs (foramina on posteriorsurface, Parrish, 1993: character 7; Gower & Weber,

1998) and more derived suchians (foramina in a moreanterodorsal position, on the lateral surface of theparabasisphenoid). The partially lateral position inphytosaurs is here assumed to be partly homologouswith the more lateral position in those suchians moreclosely related to crocodylomorphs. This character isordered 0-1-2, following the recommendation ofWilkinson (1995).

(2) Lateral surface of exoccipital without subverticalcrest (0), with clear crest lying anterior to externalforamina for hypoglossal nerve (1), or with clear crestpresent anterior to external foramina for hypoglossalnerve (2).

In the plesiomorphic archosaurian condition (e.g.Gower & Sennikov, 1996a), there is no lateral exoccip-ital crest or sharp ridge; the lateral surface is insteadsmoothly rounded. This condition appears to beretained in phytosaurs (although faint indications of acrest are perhaps sometimes present) and S. galilei(Alcober, 2000). A crest is present in some otherrauisuchians, and perhaps in all aetosaurians and cro-codylomorphs, with only the latter two taxa havingexternal hypoglossal foramina positioned behind thisridge. This character is ordered 0-1-2 in this analysis,on the basis that the hypoglossal foramina lie on ananterolateral surface on the anterior of the exoccipitalpillar in those taxa lacking a ridge.

(3) Ventral ramus of the opisthotic extends further lat-erally than lateralmost edge of exoccipital (0), or theconverse arrangement (1).

Although the plesiomorphic condition of this characterfor archosauromorphs and archosaurians is clearlya laterally extensive ventral ramus of the opisthotic(Gower & Sennikov, 1996a), it seems likely that there

Table 1. Taxon-character matrix showing distribution of braincase character states among archosaurian taxa. See text fordetails of characters, taxa, and sources of data

123451

67890 123452

67890 12345 67

Proterosuchus fergusi 00000 000?? 00000 00000 ?000? 00Garjainia prima 00000 00000 00001 00000 ?0000 00Euparkeria capensis 00000 00000 00000 10010 00010 01Phytosaurs 10000 00100 00000 10010 ?0010 01Saurosuchus galilei 21000 011?? 0000? 10000 ?001? 01Batrachotomus kupferzellensis 21000 01110 0000? 00010 00101 11Tikisuchus romeri 2??00 011?? 0?00? ?10?? ???? ? ?1Postosuchus kirkpatricki 21100 01110 0000? ?101? ???1? 11Aetosaurians 22111 011?? 0000? ?00?? 1111? 01Sphenosuchus acutus 22111 11111 11112 10111 11111 01Dibothrosuchus elaphos 22110 111?? 11112 10111 11??? 01Crocodylus sp. 22111 11111 11112 10101 1111? 11



is some homoplasy in this character, at least withinArchosauria. For example, within ErythrosuchidaeG. prima (= V. triplicostata) possesses the plesio-morphic state, but Erythrosuchus africanus andShansisuchus shansisuchus possess the derived state(Gower & Sennikov, 1996a: character 5; Gower, 1997).Within Crurotarsi, the derived state is found in cro-codylomorphs and aetosaurians, but also marginallyin P. kirkpatricki (pers. obs.).

(4) Region of braincase between fenestra ovalis andposterior edge of occipital condyle relatively shortalong longitudinal axis (0), or elongated (1).

There are clearly difficulties associated with delineat-ing and recognizing ‘short’ from ‘elongated’, but thischaracter is employed as an initial attempt to capturethe observed variation. The derived state (1) is presentin aetosaurians (e.g. Gower & Walker, 2002) and cro-codylomorphs (e.g. Walker, 1990; pers. obs.), and thuscovaries with the presence of exoccipital pillars inwhich the external hypoglossal foramina lie posteriorto subvertical lateral ridges. The presence of thederived state does not covary with an elongated condy-lar stalk. In phytosaurs, for example, the stalk sup-porting the condyle is long but the actual exoccipitalpillar and the region between the end of the condyleand the fenestra ovalis are not here considered to beelongated. A morphometric analysis has not been con-ducted, and future work might scrutinize this charac-ter in more detail.

(5) Ventromedial surfaces of opposite exoccipitals do(0), or do not (1), meet along the midline on the floor ofthe endocranial cavity.

Among the taxa considered here, the plesiomorphicstate (0) covaries with the absence of a basioccipitalcontribution to the border of the foramen magnum(not always the case – see character 17 of Gower &Sennikov, 1996a), and midline contact between theopposite prootics. A single character is used here.

(6) Pneumatization of bony elements of the middle earcavity absent or restricted (0), or well developed (1).

Well developed middle ear pneumaticity is here con-sidered to be confined to sphenosuchian and crocodyli-form crocodylomorphs. Large median pharyngealrecesses (e.g. P. kirkpatricki, aetosaurians) and depres-sions on the lateral surfaces of the parabasisphenoid(e.g. P. kirkpatricki) are not considered to be unequiv-ocal indications of extensive middle ear pneumatiza-tion, and are not here incorporated as part of thederived state (1) of this character. The sphenosuchianS. acutus, in contrast, is scored as derived because ithas clear pneumatic sinuses that pass right throughthe parabasisphenoid and invade the basipterygoidprocesses (Walker, 1990).

(7) Medial wall of vestibule incompletely ossified (0) oralmost completely ossified (1).

As Gower & Weber (1998) point out, this character isproblematic in terms of delineating clear characterstates because of largely unknown ontogenetic andindividual variation in most taxa. However, while non-crown-group archosauromorphs have largely medi-ally open vestibules (that were probably finished inunossified tissue), crocodylomorphs, aetosaurians, andrauisuchians have well ossified medial vestibule wallsthat form bony otic bullae. The situation in phytosaursis not entirely clear, but individuals with an essen-tially completely ossified medial vestibule wall havenot been observed or apparently reported, and so thistaxon is scored as exhibiting the plesiomorphic state(0).

(8) Semilunar depression on lateral surface of basaltuber of parabasisphenoid present (0), or absent (1).

This structure is known only in non-crown-grouparchosauromorphs (see Gower & Sennikov (1996a:character 11), Gower, 1997, Gower & Weber, 1998).

(9) Well defined recess for lagena/cochlea absent orshort and strongly tapered (0), or present, and elon-gated and tubular (1).

As discussed above, recognition of the derived state (1)is complicated by the probable continuation of the ven-tral end of the lagenar recess into the unossified gapbetween the ventral ramus of the opisthotic and thebasal tubera of basioccipital and parabasisphenoid.Thus, while G. prima (= V. triplicostata) might be inter-preted as having a potentially elongated lagenarrecess, this is not a well defined structure that isclearly tubular in nature (Gower & Sennikov, 1996a)and this taxon is therefore scored here as possessingthe plesiomorphic state.

(10) Crista vestibuli absent (0), or present (1).

The crista vestibuli is a term established by Hasse(1873: 721–722, pl. XXXIII, fig. 11) for the thin crest ofbone that demarcates and partially separates thelagenar/cochlear portion of the ossified part of the oticcapsule from the vestibular portion. Plesiomorphicallyin archosaurs (e.g. Walker, 1990; Gower & Weber,1998), the lagenar region is at most only poorly differ-entiated. Even in crown-group archosaurs such asB. kupferzellensis, where the lagenar region is clearlyobservable (and a cochlea may even have beenpresent), the demarcation between vestibule and lage-nar/cochlear recess is nothing more than a change inangle of an essentially continuous surface. Crocodil-ians and the sphenosuchian S. acutus (the pronouncedrim described by Walker (1990: 37, fig. 22)) share thederived presence of a crista vestibuli. The condition is

70 D. J. GOWER


unknown in many fossil taxa, including aetosaurians.The derived condition is present also in at least extantbirds (where it is variably developed; A. D. Walker,pers. comm.) and lizards (e.g. lagenar crest of Oelrich,1956; inferior cisternal crest of Baird, 1970: 202). Cur-rently orthodox views of amniote phylogeny suggestthat these multiple occurrences (crocodilians, birds,lizards) are convergent, but this is worthy of furtherstudy.

(11) Lagenar/cochlear prominence absent (0), orpresent (1).

The cochlear prominence (Walker, 1990) is an exter-nal feature present on the prootic (and opisthotic) ofextinct and extant crocodylomorphs (Walker, 1990;pers. obs.) whereby the lateral surface of the part ofthe braincase that forms the lateral wall of the lage-nar/cochlear recess is locally thickened so as to markexternally the position of the recess. Walker (1990:95) reports that it is also present in birds. No suchfeature is present in non-crown-group archosaurs orapparently in non-crocodylomorph crurotarsans (pers.obs.).

(12) Distal end of the ventral ramus of the opisthoticdoes not, or barely, makes contact with prooticanteroventral to fenestra ovalis (0), or has extendedcontact with prootic (1).

In the plesiomorphic condition (state 0; e.g. G. prima= V. triplicostata, Gower & Sennikov, 1996a; Erythro-suchus africanus, Gower, 1997; Euparkeria capensis,Welman, 1995), the ventral end of the fenestra ovalisis usually closed by brief (if any) opisthotic-prooticcontact. This may be accompanied by much greatercontact between opisthotic and parabasisphenoid. Inthe derived condition (state 1), there is much greatervertical contact between the prootic and the anteriorsurface of the distal end of the ventral ramus of theopisthotic immediately below the fenestra ovalis (e.g.Sphenusuchus acutus, Walker, 1990: fig. 28a).

(13) Eustachian tubes not enclosed by bone (0), or par-tially or fully enclosed (1).

In diapsids plesiomorphically, there is no indication inbony elements of the route taken by the eustachiantubes. This plesiomorphic condition is present in non-crown-group archosauromorphs and archosaurs (e.g.Gower & Weber, 1998) and is retained in many cruro-tarsan lineages. Within Suchia, only crocodylomorphsseem to show bony indications of the positions of thetrue eustachian tubes. In the sphenosuchian Spheno-suchus acutus there are open grooves (Walker, 1990),while the sphenosuchian Dibothrosuchus elaphroswould appear to have at least partially enclosed lat-eral tubes (Wu & Chatterjee, 1993). The tubes are fullyenclosed in bone in extant crocodilians.

(14) Contact between quadrate and prootic absent (0),present (1).

Prootic-quadrate contact is seen only in crocodylomor-phs among non-crown-group archosaurs and crurotar-sans. Sphenosuchians have nonsutural contact (e.g.Walker, 1990; Wu & Chatterjee, 1993), while a suturalarticulation is present in extant crocodilians.

(15) External foramen for abducen nerves betweenparabasisphenoid and prootic (0), within prootic only(1), within parabasisphenoid only (2).

This character is unordered here because the relation-ship between the three states is unclear, at leastwithin Archosauromorpha, where state 0 would seemto be the plesiomorphic condition (see also Gower &Sennikov, 1996a: character 3).

(16) External foramina for passage of abducens nerveson underside of a horizontal surface (0), or on the ante-rior of a more vertical, upturned process (1).

See character 4 of Gower & Sennikov (1996a).

(17) Parabasisphenoid relatively short dorsoventrally(0), or substantially elongated in the region between thebasal tubera and the basipterygoid processes, such thatthe median pharyngeal recess is dorsoventrallyextended and trough-like (1).

Parrish (1993: 300) hypothesized that the derivedstate “posteroventrally open trough formed by elon-gated and conjoined basipterygoid processes” is a syn-apomorphy of his Rauisuchia (= his Rauisuchidae +Crocodylomorpha + Gracilisuchus stipanicicorum).Contrary to Parrish, this condition is not present inB. kupferzellensis (see above) or crocodylomorphs, andinstead I consider it restricted to P. kirkpatricki andT. romeri among documented forms.

(18) Basipterygoid processes of moderate size (0), ormarkedly enlarged (1).

In Archosauromorpha plesiomorphically, the basip-terygoid processes and basal tubera of the paraba-sisphenoid are of comparable size. The basipterygoidprocesses of crocodylomorphs are much enlarged, to apoint where they are clearly larger than the basaltubera of the parabasisphenoid.

(19) Supraoccipital excluded from dorsal border offoramen magnum by dorsomedial midline contactbetween opposite exoccipitals (0), or supraoccipital con-tributes to border of foramen magnum (1).

(20) Pila antotica ossified mainly by prootic and lat-erosphenoid, such that laterosphenoid-basisphenoidcontact is absent (0), or pila antotica ossified largely bylaterosphenoid and basisphenoid, with contact occur-ring between these two elements anterior to the trigem-inal foramen in the adult braincase (1).



The derived state 1 is present in crocodilians, birdsand lepidosaurs. A laterosphenoid ossification isabsent in the latter group, but the basisphenoid con-tributes to the ossification of the pila antotica (e.g.Evans, 1986). This might be another multiple brain-case convergence in these three extant groups (seebrief discussion by Gower & Weber, 1998: 399).

(21) Perilymphatic foramen with an incompletely ossi-fied border (0), or border entirely ossified such that theventral ramus of the opisthotic forms a perilymphaticloop (Walker, 1985), incorporating a loop closure suture(Walker, 1990: 37) with itself (1).

State 0 is plesiomorphic for archosaurs based on thecondition in, for example, Sphenodon, rhynchosaurs(Benton, 1983; pers. obs.), Prolacerta (Evans, 1986)and non-crown-group archosaurs (e.g. Walker, 1990;Gower & Weber, 1998). Among crurotarsans, thederived state is present with certainty only in aeto-saurians (Gower & Walker, 2002) and crocodylomor-phs (Walker, 1990). A completely bony border to theperilymphatic foramen is also present (presumablyconvergently) in squamates and birds (Walker, 1990).

(22) Perilymphatic foramen in a medial position andorientated so as to transmit the perilymphatic duct outof the otic capsule in a posteromedial or posterior direc-tion (0), or foramen positioned more laterally so thatduct is transmitted posterolaterally/laterally and theforamen is at least partly visible in lateral view (1).

Walker (1985, 1990; see also Gower & Walker, 2002)recognized and highlighted this character. The plesio-morphic state (0) is present in Sphenodon amongextant diapsids (pers. obs.) and in non-crown-grouparchosaurs (e.g. Gower & Weber, 1998; pers. obs.). Itis also retained in several early crurotarsans includ-ing phytosaurs and rauisuchians. The presumablyderived state is observed, among suchians, in aetosau-rians (Gower & Walker, 2002) and crocodylomorphs(Walker, 1985, 1990). The condition in incompletelyprepared or preserved fossil material can be deter-mined from the orientation of the ventral ramus ofthe opisthotic.

(23) Foramen for trigeminal nerve and middle cerebralvein combined and undivided (0), or at least partiallysubdivided by prootic (1).

In Archosauria plesiomorphically (state 0), the prooticcontribution to the trigeminal foramen is smooth andundifferentiated, even where the route of the middlecerebral vein can be detected on the inner wall of theprootic (e.g. Erythrosuchus africanus, Gower, 1997). Inthe aetosaurian Stagonolepis robertsoni (Walker, 1961,1990), the anterior of the prootic has two embayments– for separate openings for the trigeminal nerve andmiddle cerebral vein. Crocodylomorphs (e.g. Walker,

1990; pers. obs. of dried crocodilian skulls) have asingle foramen that shows some indication of atleast a partial bony subdivision of the exits of thevein and nerve. A partial subdivision is also seen inB. kupferzellensis. The emphasis for this character ishere placed on the prootic because the laterosphenoidis less well known in many fossils. Bony subdivision ofnerve and vein represents some ossification within theregion that lies between these structures. Future workmight investigate how this varies (perhaps withontogeny) within taxa.

(24) Prominent subhorizontal ridge on lateral surfaceof prootic below trigeminal foramen present (0), orabsent (1).

This ridge has been interpreted as a possible site oforigin for the m. protractor pterygoidei. It might bedifficult to determine the state of this character insmall or poorly preserved material (e.g. Gower &Weber, 1998: 377), but there is a clear differencebetween larger and well preserved material with, forexample, G. prima possessing, and S. acutus lackingthe ridge. The absence of the ridge (the derived state)might have different causes in different taxa. Forexample, both Erythrosuchus africanus (Gower, 1997)and S. acutus (Walker, 1990) lack the ridge, but inS. acutus this might be associated with the greatlyreduced area between the trigeminal foramen and thecrista prootica. This is another character that wouldbenefit from further study, particularly drawing oninformation from extant taxa. This character wasemployed by Gower & Sennikov (1996a: character 6).

(25) Auricular recess largely restricted to prootic (0), orextends onto internal surface of epiotic/supraoccipital(1).

The plesiomorphic condition is clearly present in, forexample, E. africanus (Gower, 1997), and less so inphytosaurs (pers. obs. of BMNH 38037) where thefossa seems to reach the supraoccipital, but withoutreally extending onto it. In the derived condition (e.g.B. kupferzellensis, Fig. 8), the fossa extends a long wayonto the epiotic/supraoccipital. In extant crocodilians,the auricular fossa is very shallow, if actually pre-sent, so that this taxon is scored as uncertain for thischaracter.

(26) Additional foramen passing above and into thedorsal end of the metotic foramen absent (0), or present(1).

This foramen is interpreted as a possibly discretepassage for the posterior cerebral/cephalic vein (seeabove). It is present in at least B. kupferzellensis,P. kirkpatricki, and probably extant crocodilians (seeabove). This character is somewhat problematic onseveral levels. This area is often not well preserved,

72 D. J. GOWER


prepared or clearly visible in fossil material. Addition-ally, its configuration in those taxa where it has beenobserved suggests that its presence might only bedependent on a small amount of additional ossifica-tion. For example, there seems to be an open groovehere in S. acutus (Walker, 1990) and it is debatablehow different this situation is to that in, for example,B. kupferzellensis other than slightly more ossificationof the soft-tissue separating the vein from other struc-tures passing through the metotic foramen, in the lat-ter taxon. However, this feature seems clearly to beabsent in, for example, Sphenodon, rhynchosaurs (e.g.Benton, 1983; pers. obs.), and non-crown-group archo-saurs (e.g. Gower & Sennikov, 1996a; Gower & Weber,1998), so that the derived condition, foramen present,might have evolved (perhaps more than once) onlywithin Suchia.

(27) Parabasisphenoid intertuberal plate present (0),absent (1).

See character 2 of Gower & Sennikov (1996a).Several characters that are not immediately perti-

nent to the scope of this study have not been includedhere. These are characters for which at least one of thestates would be scored in only a single taxon includedin this analysis and thus are parsimony uninforma-tive. These include the large hemispherical medianpharyngeal recess present in some aetosaurians (seeParrish, 1994: character 14; Gower & Walker, 2002);the median parabasisphenoid tubercle of G. prima (=V. triplicostata, see character 23 of Gower & Sennikov,1996a); the small transverse hook-like processes onthe posteromedial edge of the basipterygoid processesof T. romeri (see above); the subdivided metotic fissureof crocodilians (see Gower & Weber, 1998), and manyother characters present only in crocodilians amongthose taxa considered here.

Some other potentially informative characters areexcluded here because of difficulties in formulatingand/or recognizing states, particularly in small sam-ples of imperfectly preserved fossils. These include thenumber of external foramina for the hypoglossalnerves, the presence of a discrete epiotic ossification,and the presence of a ‘presphenoid’ ossification.

ANALYSIS

These 27 characters were scored for 12 taxa and anal-ysed using parsimony as implemented in PAUP* Ver-sion 4.0b6 (Swofford, 1998). Trees were rooted withProterosuchus. A parsimony matrix randomizationpermutation tail probability test (Faith & Cranston,1991) on 100 permutations of the complete data setresulted in a minimum possible score of 0.01. Thisallows rejection of the null hypothesis that the dataare no more hierarchically structured than are com-

parable but random, phylogenetically uninformativedata – a recommended minimum requirement of anymatrix to be used for phylogenetic inference (Faith &Cranston, 1991). A Branch and Bound (BB) searchrecovered five most parsimonious trees (MPTs) of 37steps in length. The strict consensus of these MPTs isshown in Figure 10. Support for each node on thestrict consensus was measured using the decay index(Bremer, 1988; Donoghue et al., 1992) and bootstrapproportions (Felsenstein, 1985), the latter based on1000 BB replicates and a maximum of 100 trees heldfor each iteration. Significance of length differencesbetween optimal and suboptimal hypotheses wastested for in PAUP using Templeton tests (Templeton,1983).

PHYLOGENY

The phylogeny (Fig. 10) recovered from analysis of thebraincase data presented in Table 1 is largely consis-tent with the currently orthodox view of archosaurphylogeny (Gower & Wilkinson, 1996; see also subse-quent analysis by Benton, 1999). Thus, crocodiliansand sphenosuchians comprise a crocodylomorph clade;rauisuchians, aetosaurians and crocodylomorphs forma clade to the exclusion of phytosaurs and noncrown-group archosaurs; Euparkeria is more closely relatedto the included crown-group taxa than is the represen-tative erythrosuchid (G. prima). The outstanding areaof disagreement between the present analysis andthe current orthodoxy lies in the relative positionof rauisuchians, aetosaurians, and crocodylomorphs.There is currently consensus (Gower & Wilkinson,1996; Benton, 1999) that crocodylomorphs are moreclosely related to at least some rauisuchians (at leastPostosuchus) than to aetosaurians. Braincase dataanalysed here instead support a sister-group relation-ship between aetosaurians (Stagonolepis) and crocody-lomorphs, among the taxa included in this study. Themost parsimonious interpretation of the braincasedata analysed here does not support rauisuchianmonophyly. However, this is the weakest part of thetree and some character incongruence, combinedwith missing data especially for P. kirkpatricki andT. romeri, mean that alternative arrangements of therauisuchians can be enforced with small additions totree length.

Bootstrap proportions and decay indices are goodto reasonable (considering the size of the data set)for all nodes except those immediately adjacent tothe rauisuchians B. kupferzellensis, P. kirkpatricki, andT. romeri. The most robust groupings in the uncon-strained MPTs (those with a decay index > + 2) are asignificantly (P £ 0.1) better fit to the data than are atleast some of the shortest trees not including thesegroupings (see Fig. 10).



BRAINCASE EVOLUTION

Although not all parts of the phylogenetic hypothesisrecovered here (Fig. 10) are as robust as would bewished, this tree is used as the framework for a pre-liminary interpretation of crurotarsan, and especiallysuchian, braincase evolution. Missing data allows sev-eral alternative equally parsimonious optimizationsfor some of the characters on this tree. Focus here is onthe acquisition of those characters that are derived forextant crocodilians but that are absent in non-crown-group archosaurs and in phytosaurs (the first branchoff the crurotarsan lineage among those taxa consid-ered here). Assuming that the currently orthodox viewof the content of Crurotarsi and Ornithodira is correct,this therefore covers derived braincase charactersthat evolved after the divergence of the crocodilian-and bird-lineages from their last common ancestor.The conclusions presented here partly echo some

themes presented by Walker (1985, 1990; see alsoGower & Walker, 2002).

Some derived crocodilian braincase features appar-ently absent in the last common ancestor of birds andcrocodilians (see Gower & Weber, 1998) were probablyacquired by the ancestral suchian, after the phyto-saurs branched off the crocodilian lineage. Theseinclude a fully lateral position on the parabasisphe-noid for the entrance of the cerebral branches of theinternal carotid foramina, and a more completely ossi-fied medial wall of the vestibular part of the otic cap-sule. A second suite of characters were acquired atsome unknown point between the origin of the branchleading to phytosaurs and the origin of the last com-mon ancestor of crocodylomorphs and aetosaurians.These are a well-defined and elongated lagenar/cochlear recess, a partial bony subdivision of the fora-men for the passage of the trigeminal nerve and themiddle cerebral vein through the braincase wall, and a

Figure 10. Strict consensus tree of five most parsimonious trees (MPTs) retrieved by parsimony analysis of data inTable 1. These MPTs have a length of 37 steps, a consistency index of 0.811, and a retention index of 0.889. Numbers aboveand below nodes represent decay indices and bootstrap proportions, respectively. Node A represents Crurotarsi (and allcrown-group archosaurs here, in the absence of any ornithodiran taxa), node B represents Crocodylomorpha. The two nodesmarked with medium thickness internal branches indicate that some (but not all) of the shortest trees not including thosenodes are a significantly worse fit to the data than are the unconstrained MPTs (as measured by the Templeton test; P £0.1). The single node (B) marked with the thickest internal branch indicates that all of the shortest trees not including thatnode are a significantly worse fit to the data than are the unconstrained MPTs. Constrained shortest trees lacking othernodes (thin internal branches) are not a significantly worse fit to the data.

74 D. J. GOWER


subvertical crest or ridge on the lateral surface ofthe exoccipital (and basioccipital). Some importantderived features of the crocodilian otic region wereacquired after the origin of rauisuchian lineages andbefore the origin of the last common ancestor of aeto-saurians and crocodylomorphs. These include a com-plete perilymphatic loop, a posterolaterally directedperilymphatic foramen, greater prominence of theexoccipital lateral ridge (also acquired, perhaps inde-pendently, in P. kirkpatricki), passage of the hypoglos-sal nerve posterior to this exoccipital ridge, and anelongated region between the fenestra ovalis and theoccipital condyle. Several derived crocodilian brain-case features were acquired after the origin of theaetosaurian lineage but before the origin of the lastcommon ancestor of crocodylomorphs, including acochlear prominence, quadrate-prootic articulation,enlarged basipterygoid processes, extensive tympanicpneumaticity, and at least partial bony enclosure ofthe eustachian tubes. Subsequent to the origin ofsphenosuchian crocodylomorphs, ancestors to crocodil-ians evolved a subdivided metotic fissure (and associ-ated discrete secondary tympanic window), a suturalarticulation between the prootic and quadrate and thepterygoid and basipterygoid process, and many otherfeatures (e.g. geniculate cochlear recess and others;see Walker, 1990).

DISCUSSION

There are several caveats associated with the findingsof the current study. Firstly, the phylogeny of Mesozoicarchosaurs is clearly still in need of further attention.Most of the characters analysed here not been usedin previous studies of basal archosaur phylogenyand, while the phylogenetic hypothesis presented inFigure 10 might not be correct, these braincase dataare certainly worthy of further investigation. For now,it might be noted that the unorthodox proposal thataetosaurians are more closely related to crocodylo-morphs than are any rauisuchians also receives sup-port from at least two detailed characters of the skull,namely the form of the palatine and the presence of aventromedial process of the prefrontal (see Gower &Walker, 2002). Other caveats are that the promisingphylogenetic signal in these braincase data has yet tobe subjected to a wider range of archosaurian taxa.The phylogeny derived from braincase data mightalso be perturbed by the addition of other braincasecharacters. A further caveat is the preliminary natureof this study because braincase morphology is stillnot well understood in basal archosaurs. Good brain-case specimens have not been found, prepared ordescribed for many taxa and understanding of individ-ual and ontogenetic variation is extremely poor even

in most of those taxa where good specimens have beendocumented.

These caveats aside, the main findings of the cur-rent study are that braincase morphology representsa relatively untapped resource of phylogenetic data. Itsuggests that aetosaurians are closer relatives ofcrocodylomorphs than any well known rauisuchian.Mapping braincase character states onto this phyloge-netic framework suggests that some of the manyderived features of the braincase of crocodilians wereacquired successively in the evolution of the ancestorsof crocodilians and successively larger clades includ-ing them and sphenosuchians, aetosaurians, rauisu-chians, and phytosaurs, and that these evolutionarychanges can be detected in the fossil record. Addi-tional work is required to further flesh out and/or testthis interpretation, and to extend the study to basalornithodirans.

ACKNOWLEDGEMENTS

I thank Rupert Wild for providing hospitality andaccess to the Stuttgart collection, and for entrustingme with the study of the SMNS rauisuchian materialfrom Kupferzell. For access to comparative material,I thank Sandra Chapman and Angela Milner(London), Sankar Chatterjee (Lubbock), Jenny Clackand Ray Symonds (Cambridge), and Gregg Gunnell(Michigan). Some of the photographs presented hereare the work of Harry Taylor (London). This studybenefited greatly from comments on earlier drafts by,assistance from, and discussions with, James Clark,Axel Hungerbühler, Rainer Schoch, Jayc Sedlmayr,Hans-Dieter Sues, Erich Weber, Mark Wilkinsonand Larry Witmer. Some of this work was carriedout in Tübingen with the support of a Royal SocietyEuropean Research Fellowship (1995, 1996). Addi-tional assistance and support during this phase wasgenerously provided by Hans Hagdorn, WernerKugler, Wolf-Ernst Reif and Frank Westphal.

The late Alick Walker (and his papers) helped mewith many aspects of this project by providing invalu-able information, guidance, and encouragement.

REFERENCES

Alcober O. 2000. Redescription of the skull of Saurosuchusgalilei (Archosauria: Rauisuchidae). Journal of VertebratePaleontology 20: 302–316.

Baird IL. 1970. The anatomy of the reptilian ear. In: Gans C,Parsons TS, eds. Biology of the Reptilia, Vol. 2. London:Academic Press, 193–275.

Benton MJ. 1983. The Triassic reptile Hyperodapedon fromElgin: functional morphology and relationships. Philosophi-cal Transactions of the Royal Society of London B 302: 605–720.



Benton MJ. 1999. Scleromochlus taylori and the origin ofdinosaurs and pterosaurs. Philosophical Transactions of theRoyal Society of London B 354: 1423–1446.

Bremer K. 1988. The limits of amino acid sequence data inangiosperm phylogenetic reconstruction. Evolution 42: 795–803.

Bruner HL. 1907. On the cephalic veins and sinuses of rep-tiles, with description of a mechanism for raising the venousblood-pressure in the head. American Journal of Anatomy 7:1–117.

Camp CL. 1930. A study of the phytosaurs, with description ofnew material from North America. Memoirs of the Universityof California 10: 1–174.

Camp CL. 1942. California mosasaurs. Memoirs of theUniversity of California 13: 1–68.

Case EC. 1922. New reptiles and stegocephalians from theUpper Triassic of western Texas. Carnegie Institute ofWashington Publication 321: 7–84.

Case EC. 1928. An endocranial cast of a phytosaur fromthe Upper Triassic beds of Western Texas. Journal ofComparative Neurology 45: 161–168.

Chatterjee S. 1978. A primitive parasuchid (phytosaur) rep-tile from the Upper Triassic Maleri Formation of India.Palaeontology 21: 83–127.

Chatterjee S. 1985. Postosuchus, a new thecodontian reptilefrom the Triassic of Texas and the origin of tyrannosaurs.Philosophical Transactions of the Royal Society of London B309: 395–460.

Chatterjee S. 1991. Cranial anatomy and relationships of anew Triassic bird from Texas. Philosophical Transactions ofthe Royal Society of London B 332: 277–342.

Chatterjee S, Majumdar PK. 1987. Tikisuchus romeri a newrauisuchid reptile from the late Triassic of India. Journal ofPaleontology 61: 787–793.

Dendy A. 1909. The intracranial vascular system ofSphenodon. Philosophical Transactions of the Royal Societyof London B 200: 403–426.

Donoghue MJ, Olmstead RG, Smith JF, Palmer JD. 1992.Phylogenetic relationships of Dipsacales based on rbcLsequences. Annals of the Missouri Botanical Garden 79:333–345.

Evans SE. 1986. The braincase of Prolacerta broomi (Reptilia:Triassic). Neues Jahrbuch für Geologie und Paläontologie,Abhandlung 173: 181–200.

Faith DP, Cranston PS. 1991. Could a cladogram this shorthave arisen by chance alone?: On permutation tests for cla-distic structure. Cladistics 7: 1–28.

Felsenstein J. 1985. Confidence limits on phylogenies: anapproach using the bootstrap. Evolution 39: 783–791.

Gauthier JA. 1986. Saurischian monophyly and the originof birds. Memoirs of the California Academy of Sciences 8:1–55.

van Gelderen C. 1924–25. Die Morphologie der Sinus duraematris. Zeitschrift für Anatomie und Entwickelungs-geschichte 73: 541–605, 74: 432–508, 75: 525–596.

Gow CE. 1975. The morphology and relationships ofYoungina capensis Broom and Prolacerta broomi Parrington.Palaeontologia Africana 18: 89–131.

Gower DJ. 1997. The braincase of the early archosaurian rep-tile Erythrosuchus africanus. Journal of Zoology, London242: 557–576.

Gower DJ. 1999. Cranial osteology of a new rauisuchianarchosaur from the Middle Triassic of southern Germany.Stuttgarter Beiträge zur Naturkunde B 280: 1–49.

Gower DJ. 2000. Rauisuchian archosaurs (Reptilia,Diapsida): an overview. Neues Jahrbuch für Geologie undPaläontologie, Abhandlungen 218: 447–488.

Gower DJ, Sennikov AG. 1996a. Morphology and phyloge-netic informativeness of early archosaur braincases.Palaeontology 39: 883–906.

Gower DJ, Sennikov AG. 1996b. Endocranial casts of earlyarchosaurian reptiles. Paläontologisches Zeitschrift 70: 579–589.

Gower DJ, Sennikov AG. 1997. Sarmatosuchus and theearly history of the Archosauria. Journal of VertebratePaleontology 17: 60–73.

Gower DJ, Sennikov AG. 2000. Early archosaurs fromRussia. In: Benton MJ, Kurochkin EN, Shishkin MA,Unwin DM, eds. The Age of Dinosaurs in Russia andMongolia. Cambridge: Cambridge University Press, 140–159.

Gower DJ, Walker AD. 2002. New data on the braincase ofthe aetosaurian archosaur (Reptilia: Diapsida) Stagonolepisrobertsoni Agassiz. Zoological Journal of the Linnean Society136: 000–000.

Gower DJ, Weber E. 1998. The braincase of Euparkeria, andthe evolutionary relationships of birds and crocodilians.Biological Reviews 73: 367–411.

Gower DJ, Wilkinson M. 1996. Is there any consensus onbasal archosaur phylogeny? Proceedings of the Royal Societyof London B 263: 1399–1406.

Grosser O, Brezina E. 1895. Über die Entwickelung derVenen des Kopfes und Halses bei Reptilien. MorphologischeJahrbuch 23: 289–328.

Hasse C. 1873. Zur Morphologie des Labyrinthes der Vogel. In:Hasse C, ed. Anatomische Studien, Vols 1 and 2. Leipzig:Wilhelm Engelmann, 189–224.

Hochstetter F. 1906. Beiträge zur Anatomie undEntwickelungsgeschichte der Blutgefässsystemes derKrokodile. In: Voeltzkow A, ed. Reise in Ostafrika, Vol. 4.Stuttgart: E. Scheizerbart, 1–139.

Iordansky NN. 1973. The skull of the Crocodilia. In: Gans C,Parsons TS, eds. Biology of the Reptilia, vol. 4. London: Aca-demic Press, 201–262.

Juul L. 1994. The phylogeny of basal archosaurs.Palaeontologia Africana 31: 1–38.

Long RA, Murry PA. 1995. Late Triassic (Carnian andNorian) tetrapods from the Southwestern United States.Bulletin of the New Mexico Museum of Natural History andScience 4: 1–254.

Oelrich TM. 1956. The anatomy of the head ofCtenosaura pectinata (Iguanidae). MiscellaneousPublications of the Museum of Zoology, University ofMichigan 94: 1–122.

Padian K, Chiappe LM. 1998. The origin and early evolutionof birds. Biological Reviews 73: 1–42.

76 D. J. GOWER


Parrish JM. 1993. Phylogeny of the Crocodylotarsi, withreference to archosaurian and crurotarsan monophyly.Journal of Vertebrate Paleontology 13: 287–308.

Parrish JM. 1994. Cranial osteology of Longosuchus meadeiand the phylogeny and distribution of the Aetosauria.Journal of Vertebrate Paleontology 14: 196–209.

Reig OA. 1959. Primeros datos descriptivos sobre nuevos rep-tiles arcosaurios del Triásico de Ischigualasto (San Juan,Argentina). Revista de la Asociación Argentina de Geología13: 257–270.

Reig OA. 1961. Acerca de la posición sistemática de lafamilia Rauisuchidae y del género Saurosuchus (Reptilia-Thecodontia). Publicaciones del Museo Municipal deCiencias Naturales y Tradicionales de Mar del Plata 1: 73–114.

Säve-Söderbergh G. 1947. Notes on the brain-case in Sphe-nodon and certain Lacertilia. Zoologiska Bidragen Tot deAnatomie 25: 489–516.

Sill WD. 1974. The anatomy of Saurosuchus galilei and therelationship of rauisuchid thecodonts. Bulletin of theMuseum of Comparative Zoology 146: 317–362.

Swofford DL. 1998. Paup: Phylogenetic Analysis usingParsimony, version 4.0b6. Champaign, Illinois: IllinoisNatural History Survey.

Templeton AR. 1983. Phylogenetic inference from restrictionendonuclease cleavage site maps with particular reference tothe evolution of humans and the apes. Evolution 37: 221–244.

Walker AD. 1961. Triassic reptiles from the Elgin area:

Stagonolepis, Dasygnathus and their allies. PhilosophicalTransactions of the Royal Society of London B 244: 103–204.

Walker AD. 1972. New light on the origin of birds and croco-diles. Nature, London 237: 257–263.

Walker AD. 1985. The braincase of Archaeopteryx. In: HechtMK, Ostrom JH, Viohl G, Wellnhofer P, eds. The Beginningsof Birds. Eichstätt: Freunde des Jura-Museums Eichstätt,123–134.

Walker AD. 1990. A revision of Sphenosuchus acutusHaughton, a crocodylomorph reptile from the ElliottFormation (late Triassic or early Jurassic) of South Africa.Philosophical Transactions of the Royal Society of London B330: 1–120.

Welman J. 1995. Euparkeria and the origin of birds. SouthAfrican Journal of Science 91: 533–537.

Wilkinson M. 1995. A comparison of two methods of characterconstruction. Cladistics 11: 297–308.

Witmer LM. 1991. Perspectives on avian origins. In: SchultzeH-P, Trueb L, eds. Origins of the Higher Groups ofTetrapods. Ithaka and London: Comstock PublishingAssociates, 427–466.

Witmer LM. 1997. Craniofacial air sinus systems. In: CurriePJ, Padian K, eds. The Encyclopedia of Dinosaurs. SanDiego: Academic Press, 151–159.

Wu X-C, Chatterjee S. 1993. Dibothrosuchus elaphros, acrocodylomorph from the Lower Jurassic of China and thephylogeny of the Sphenosuchia. Journal of VertebratePaleontology 13: 58–89.

braincase evolution in suchian archosaurs (reptilia: diapsida

Documents