revision of the aÏstopod genus phlegethontia (tetrapoda: lepospondyli)

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REVISION OF THE AÏSTOPOD GENUS PHLEGETHONTIA (TETRAPODA:LEPOSPONDYLI)Author(s): JASON S. ANDERSONSource: Journal of Paleontology, 76(6):1029-1046. 2002.Published By: The Paleontological SocietyDOI: http://dx.doi.org/10.1666/0022-3360(2002)076<1029:ROTAGP>2.0.CO;2URL: http://www.bioone.org/doi/full/10.1666/0022-3360%282002%29076%3C1029%3AROTAGP%3E2.0.CO%3B2

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1029

J. Paleont., 76(6), 2002, pp. 1029–1046Copyright q 2002, The Paleontological Society0022-3360/02/0076-1029$03.00

REVISION OF THE AISTOPOD GENUS PHLEGETHONTIA(TETRAPODA: LEPOSPONDYLI)

JASON S. ANDERSONBiology Group, Erindale College, University of Toronto, Mississauga, Ontario L5L 1C6, Canada

ABSTRACT—The aıstopod family Phlegethontiidae is restudied based on new specimens from Pit 11 of Mazon Creek, Illinois, and thecoal shales of Nyrany, Czech Republic, as well as most available specimens from North America. Phlegethontiids have highly fenestrateskulls, with orbits placed just anterior their skull’s mid point. Dermal skull bones are greatly reduced in number and limited in extent,whereas the endochondral braincase is hyperossified. The frontals are fused medially and enclose the parietal foramen and anteriorsagittal crest. As in most other aıstopods, the quadrate, pterygoid, and epipterygoid are fused into a composite bone, the palatoquadratecomplex. Details of cranial anatomy contradict a previous model of cranial kinesis by severely limiting the skull’s potential mobility.Remnants of the pectoral girdle are present, perhaps due to the presence of an operculum–opercularis-like connection to the stapes. Noremnants of the pelvis are present.

Three species are recognised within the family. Phlegethontia linearis has short anterior vertebrae, high neural spines on at least theanterior four vertebrae, and vertebrae number between 230–250 in total. Phlegethontia longissima has low neural spines throughoutthe column, anterior vertebrae that are twice as long as P. linearis, and only 200–210 total vertebrae. Sillerpeton permianum, knownfrom a single braincase and an unassociated string of vertebrae, is distinguished from Phlegethontia by the retention of a separateforamen for the passage of the occulomotor nerve. Phlegethontia ‘‘phanerhalpa’’ is a tiny braincase fragment that differs from theother species of Phlegethontia only in the placement of the jugular foramen relative to the centre of the foramen magnum. This isprobably a size-related feature, and P. ‘‘phanerhalpa’’ is considered a nomen dubium.

OF THE three recognised families of aıstopods (Phlegethonti-idae, Ophiderpetontidae, and Lethiscidae, whose names are

used throughout this paper for ease of discussion without stipu-lating their monophyly), the phlegethontiids are the most derivedcompared to both the typical ‘‘labyrinthodont’’ and lepospondylconditions. The first aıstopods were described from the coal shales(Westphalian A) of Jarrow, Ireland by Huxley and Wright (1867).Two genera were named: Ophiderpeton (see Milner, 1994, Car-roll, 1998a and Anderson, in press b for recent discussions), and‘‘Dolichosoma.’’ Unfortunately, these specimens were never ad-equately prepared, and have been reported to have degraded dra-matically due to pyrite disease (McGinnis, 1967:37). Cope (1868,1871, 1874, 1875) named many species of aıstopods, includingPhlegethontia linearis (Fig. 1) and P. ‘‘serpens,’’ from the West-phalian D of Linton, Ohio, and established the family Phlege-thontiidae. Fritsch (1875, 1879) named several species of ‘‘Dol-ichosoma’’ from the ‘‘Gaskohle’’ deposits (Westphalian D) ofNyrany and Kournova, Czech Republic (further discussed bySchwartz, 1908). These specimens added many anatomical detailsunavailable in the specimens from Jarrow. Nyrany phlegethontiidswere reduced to one species by Steen (1938), although she ac-knowledged that there were few differences between specimensfrom any of the localities. Turnbull and Turnbull (1955) assigned‘‘Dolichosoma’’ to the Phlegethontiidae, citing the similarly pro-portioned skulls and similar dermal ossification. Additionally,Cope’s (1875) Phlegethontidae has priority over Lydekker’s(1889) Dolichosomatidae.

Gregory (1948) named the next new species of phlegethontiid,Phlegethontia ‘‘mazonensis,’’ from the slightly older (lower West-phalian D) deposits of Mazon Creek, Illinois. Unlike the coalshales of Linton and Nyrany, which preserve fossils flattened dor-soventrally and sometimes laterally, Mazon Creek’s nodules pre-serve fossils in three dimensions, frequently with impressions ofsoft tissues. Although he opted to name a separate species, Greg-ory acknowledged that the apparent difference between the Lintonphlegethontiids and the specimen from Mazon Creek might bepreservational in nature. He wrote, ‘‘. . . it seems highly probablethat Phlegethontia mazonensis was closely related to if not iden-tical with P. linearis [from Linton]. The new specific name isgiven in recognition of the slight difference in age and locality,and to designate this far better preserved specimen.’’ In 1955,

Turnbull and Turnbull described a second specimen referred toPhlegethontia ‘‘mazonensis’’ from Mazon Creek that displayedareas not preserved in the type specimen. Further preparation ofthis specimen reveals it to be a new genus (Anderson, in pressa).

The first major review of aıstopods was provided by Baird(1964), in which he clarified many taxonomic issues. First, hetreated ‘‘Dolichosoma’’ as a nomen nudum, citing the inadequatedescription of the type by Huxley (through a layer of matrix), andtransferred Fritsch’s ‘‘Dolichosoma’’ longissima to Phlegethontia.He also recognised Phlegethontia ‘‘serpens’’ as a junior synonymof P. linearis.

Shortly after Baird’s review of the group, McGinnis (1967)published her study of a new phlegethontiid from the LowerPermian fissure fill deposits at Richards Spur (Ft. Sill of Mc-Ginnis) in Oklahoma, which included a revision of the family.She completed the pendulum swing from the extreme splitting ofCope and Fritsch to the more conservative (and, in my opinion,justified) lumping of all North American species within the singleprovisional species Phlegethontia cf. P. longissima. Lund (1978)subsequently transferred these to P. cf. P. linearis, the speciesname with priority. McGinnis observed little difference betweenspecimens from the various localities, and could not justify theseparation of species simply because they are found in differentlocations.

Lund’s (1978) paper named two new genera. One, Sillerpeton,is based on the braincase from Richards Spur described byMcGinnis. The other, ‘‘Aornerpeton,’’ represents recognition ofGregory’s (1948) Phlegethontia ‘‘mazonensis’’ as a distinct genus.Lund also proposed an elaborate system of cranial kinesis, in-spired perhaps by the rather snake-like skull of phlegethontiids.Finally, Thayer (1985) described a very small, fragmentary brain-case from the Swisshelm Mountains in Arizona that he namedPhlegethontia ‘‘phanerhalpha.’’ This specimen is notable in thatit may preserve sutures in the occiput; all other phlegethontiidshave no trace of sutures anywhere in the braincase.

The availability of latex casts of previously unstudied, completespecimens from Nyrany in the collections of the NaturhistorischesMuseum, Vienna warrants a reconsideration of the morphologyand relationships of Phlegethontia. In addition, new casts weremade of Linton and Mazon Creek specimens, and galvanotypes

1030 JOURNAL OF PALEONTOLOGY, V. 76, NO. 6, 2002

FIGURE 1—Phlegethontia linearis, AMNH 6966, from Linton, Ohio. Scale bar 5 mm.

of Fritsch’s material were restudied. The available materials allowa limited investigation into patterns of allometry and developmentin these aıstopods. The new specimens, together with the previ-ously described materials, provide a clearer picture of phlege-thontiid morphology, diversity and evolution.

MATERIALS AND METHODS

Abbreviations.—Institutional.—AMNH: American Museum of Natural History.CM: Carnegie Museum of Natural History.CGH: National Museum, Pilzen.

FMNH: Field Museum of Natural History.MCZ: Museum of Comparative Zoology, Harvard University.NMNH: National Museum of Natural History, Smithsonian Insti-

tution.NMW: Naturhistorisches Museum Wien.UCMP: University of California Museum of Paleontology, Berke-

ley.UMMP: University of Michigan Museum of Paleontology, Ann

Arbor.Anatomical.ak, anterior keel of braincase; ang, angular; bpp,

basipterygoid process; ca, foramen for the carotid artery; clei,

1031ANDERSON—REVISION OF PHLEGETHONTIA

cleithrum or fused cleithrum and clavicle; ctp, cultriform process;cp, costal process of rib; d, dentary; dps, dorsal process of squa-mosal; ecpt, ectopyerygoid; en, external naris; ep, epipterygoidportion of palatoquadrate complex; f, frontal; gas, gastralia; II,foramen for the optic nerve; ins sq, insertion for the dorsal processof the squamosal; j, jugal; jf, foramen for the jugular vein; lr,longitudinal ridge; m, maxilla; mfn, median frontal notch; mhg,medial haemal groove; mpn, medial parietal notch; n, nasal; nc,nuchal crest; ns, neural spine; oc, occipital complex; op, odontoidprocess of the atlas; orb, orbit; ot, foramen for the olfactory tract;p, parietal; pa, proatlas; pal, palatine; parf, parietal foramen; pc,palpebral cup; pe, posteromedial element; pf, postfrontal; pm, pre-maxilla; pop, posterior process of rib; poz, postzygopophysis; pq,palatoquadrate complex; prf, prefrontal; prz, prezygapophysis; pv,pituitary vein; pz, postzygapophysis; qc, quadrate condyle; sc,sagittal crest; snf, foramen for the spinal nerve; sq, squamosal; st,stapes; tp, transverse process; tr, transverse ridge of the stapes;tub, tuberculum; V, foramen for the trigeminal nerve; vf, ventralflange of the frontal; vsn, ventral notch for the squamosal.

Specimens studied.—Phlegethontia linearis.—From Linton, Jefferson Co., Ohio. Can-neloid shale below Upper Freeport Coal, Allegheny Group, lateWestphalian D.—AMNH 6966 and counterpart 6886 (Turnbull, 1958). Complete

specimen (type).Phlegethontia (‘‘Dolichosoma’’) longissima.—From Nyrany(Nurschan) Plzen Basin, Bohemia, Czech Republic, late West-phalian D.—NMW 1896 II 34 Nearly complete specimen, part and counter-

part.UMMP 22278 Galvanotype of CGH 129, skull and anterior 16

vertebrae.UMMP 22289 Galvanotype of CGH 129 (figured by Fritsch as

two pieces), 130 vertebrae in a broken series.From Linton, Jefferson Co., Ohio. Canneloid shale below UpperFreeport Coal, Allegheny Group, late Westphalian D.—AMNH 2564 Skull and anterior 38 vertebrae.AMNH 6899 Type of Phlegethontia ‘‘serpens.’’AMNH 6913 Vertebrae and ribs.CM 44759 Small skull and attached jaws in lateral view, part and

counterpart.CM 68307 Forty-one vertebrae and ribs.CM 68336 Disarticulated skull, part and counterpart.CM 68338 Disarticulated skull and 14 vertebrae.MCZ 2038 Skull and anterior 4 vertebrae, part and counterpart.MCZ 2334 Ventral view of braincase and skull roof.USNM 4484 Disarticulated skull and anterior vertebrae.From Mazon Creek (Pit 11 spoil heaps), Grundy Co., Illinois.Francis Creek Shale above Morris (no. 2) Coal, Carbondale For-mation, Westphalian C-D boundary.—FMNH PR 1358 (Part and counterpart) Skull and anterior verte-

brae in lateral view, in epoxy.FMNH PR 624 (Part and counterpart) Skull and anterior half of

specimen.FMNH PR 831 (Part and counterpart) Nearly complete specimen,

diminutive individual.FMNH PR 1145 (Part and counterpart) Skull and anterior half of

specimen, faint body impression, diminutive individual.MCZ 2204 Skull and anterior 9 vertebrae.USNM 17097 (Part and counterpart) Nearly complete specimen

(type).

Sillerpeton permianum.—From fissure fill deposit in Arbuckle Limestone, Dolese BrothersLimestone Quarry, Richards Spur (Fort Sill), Oklahoma.—UCMP 62580 Braincase (type).

SYSTEMATIC PALEONTOLOGY

Class TETRAPODA Goodrich, 1930Subclass LEPOSPONDYLI Zittel, 1888

Order AISTOPODA Miall, 1875Family PHLEGETHONTIIDAE Cope, 1875

Revised diagnosis.Aıstopods with pointed snouts. Frontalsfused and encompass the parietal foramen except caudally. Pos-terior portion of the skull ossified as a single unit; parietals, post-parietals, tabulars, exoccipitals, basioccipital, and parasphenoidnot present as separate ossifications. Lower temporal bar formedby squamosal and jugal. Approximately sixty-four precaudal ver-tebrae, ribs present to at least the 85th vertebra. Tail extremelylong (100–190 vertebrae). Dorsal osteoderms absent, gastraliathin, elongate and widely spaced.

Genus PHLEGETHONTIA Cope, 1871Type Species.Phlegethontia linearis.Revised diagnosis.Phlegethontiids that lack a separate fora-

men for cranial nerve III in the braincase wall.

PHLEGETHONTIA LINEARIS Cope, 1871Revised diagnosis.Phlegethontiid with neural spines of equal

or greater height as the neural arch pedicels on at least the anteriorfour vertebrae. Centrum length of anterior four vertebrae 6–7%the length of the skull. Approximately 230–250 vertebrae inlength.

Holotype.AMNH 6966 (counterpart AMNH 6886), onlyknown specimen.

PHLEGETHONTIA LONGISSIMA Fritsch, 1875Phlegethontia serpens COPE, 1871.Dolichosoma longissimum FRITSCH, 1875.Dolichosoma angustatum FRITSCH, 1875.Phlegethontia mazonensis GREGORY, 1948.Phlegethontia cf. P. longissima (FRITSCH, 1875) MCGINNIS, 1967.Phlegethontia cf. P. linearis (COPE, 1871) LUND, 1978.Aornerpeton mazonense (GREGORY, 1948) LUND, 1978.

Holotype.CGH 129.Revised diagnosis.Phlegethontiid with neural spines that are

less than a quarter of the height of the neural arch pedicelsthroughout the column. Anterior vertebrae approximately 12% thelength of the jaw. 200–210 vertebrae present in the column.

Genus SILLERPETON Lund, 1978Revised diagnosis.Phlegethontiid with a fossa for articulation

of the dorsal end of the epipterygoid on the lateral surface of thebraincase; cultriform process not projecting beyond the edges ofthe braincase midorbitally; single foramen for cranial nerves Vand VII, separate foramina for II and III.

SILLERPETON PERMIANUM Lund, 1978Aıstopod GREGORY, PEABODY AND PRICE, 1956.Phlegethontia cf. P. longissima MCGINNIS, 1967.

Holotype.UCMP 62480.Revised diagnosis.As for Genus.

PHLEGETHONTIIDAE nomen dubiumPHLEGETHONTIA PHANERHALPHA Thayer, 1985

DESCRIPTION AND COMPARISONS

Skull.This section describes the genus Phlegethontia; refer-ences to Sillerpeton, only known from the braincase (McGinnis,


1967, fig. 5), will be made where relevant. Since P. linearis isrepresented by only a single specimen and since there are nodiagnostic differences in cranial anatomy between the two speciesthe majority of the description will be based on P. longissima.The description is based primarily on AMNH 6966 (Fig. 1),AMNH 2564 (Fig. 2.1), MCZ 2204 (Fig. 4), NMW 1896 II 34(Fig. 3), and USNM 17097 (Figs. 5 and 9), with others notedwhere applicable.

The endochondral portions of the skull are hyperossified or areextensively coossified with the otherwise absent dermal bones ofthe posterior skull roof and occiput, whereas the remaining dermalbones are reduced to narrow, strut-like structures, giving the skulla superficially snake-like appearance. The light construction ofthe phlegethontiid skull makes breakage and disarticulation com-mon. Additionally, the dermal bones only achieve tight articula-tion in larger specimens, making interpretation difficult. Skullsare usually found with the rostrum either displaced posteriorly orabsent due to a zone of dissolution common at the margins ofMazon Creek nodules (Figs. 4, 5 and 6). However, the lower jawsusually remain in place, which provides an estimate of skull size.Dermal ornamentation is limited to thin lines (Gregory, 1948; Fig.5) and nutrient foramina. Lateral line canal grooves are absent.

The nasal ramus of the premaxilla is present (contra McGinnis,1967) in specimens from Nyrany (UMMP 22278, NMW 1896 II34; Fig. 3.1), Linton (AMNH 2564; Fig. 2.1) and Mazon Creek(FMNH PR 1358) and is equal in length to the maxillary ramus.The rostrum is narrowly rounded and rapidly flares laterally, giv-ing the premaxillae a V-shaped appearance in dorsal view. At thejunction of the nasal and maxillary rami the posteromedial surfaceis concave, forming the anterior, dorsal and ventral margins ofthe external naris. No septomaxilla is present. Nutrient foraminapierce the lateral walls of the maxillary ramus in larger specimens.There is space for seven teeth. The maxillary ramus of the pre-maxilla abuts the maxilla with an oblique suture.

The maxilla is a long, rectangular bone that bore a maximumof 14 teeth. Like the premaxilla, foramina are present on the lat-eral surface. The maxilla forms most of the ventral margin of theorbit and extends from just posterior to the external naris to justbeyond the orbit. It maintains a constant depth throughout itslength, unlike the maxilla in the ‘‘ophiderpetontid’’ Oestocephal-us, which gradually tapers posteriorly (Carroll, 1998a).

The single element present in the antorbital region probablyrepresents the prefrontal based upon its shape, orientation, and theabsence of a lacrimal duct. It is a roughly triangular bone (Figs.2.1 and 3.1), with a wide contact with the nasal and frontal dor-sally and a narrow contact with the maxilla ventrally. The ‘‘hy-potenuse’’ of the triangle is concave and forms the anterior marginof the orbit.

The nasals are the least well-preserved bones of the skull (Figs.2.1 and 3.1). Judging from the shapes of the anterior and posteriorbounding ends of the premaxilla and frontal, it was a small, di-amond-shaped bone that widens as it extends along the taperedposterior end of the premaxilla then contacts its mate over a shortdistance before wedging sharply into the anterior end of the fron-tal.

The frontals are fused into a single large ossification, as insauropleurine and some diplocaulid nectrideans (Bossy and Mil-ner, 1998). In dorsal view the anterior margin is deeply W-shaped,forming three anteriorly oriented points, between which lie theposterior halves of the nasals (Fig. 2.1). The frontal forms theentire dorsal margin of the orbit. The frontal is narrow betweenthe orbits, but doubles in width just posterior to the orbits, thennarrows to a point on the midline except for a deep cleft thataccommodates the sagittal crest (Figs. 2.1 and 4.1). This cleft isrelatively deeper in larger individuals. At its rostral end the notch

expands laterally, mirroring an expansion in the dorsoanterior por-tion of the braincase, forming the parietal foramen. This mor-phology is similar to that in the extant amphisbaenid Amphisbae-na alba (Montaro and Gans, 1999; Fig. 6). The sagittal crest ex-tends into the area of the foramen, and contacts the deepest an-terior portion of the frontal notch. A ledge on the highly ossifiedbraincase further supports the frontal. On the ventral surface ofthe frontal two parallel, medial ridges arise anterior to the mainbody of the braincase (MCZ 2334, MCZ 2204; Fig. 4.2) andextend anteriorly to at least the orbit. They accommodate the dor-sal edge of the anterior keel of the braincase.

McGinnis (1967) identified the frontal as a fusion of the fron-tals and parietals, yet, in all other aıstopods the parietal foramenis located well within the parietals. Furthermore, in no other le-pospondyl is the parietal foramen found on the parietal-postpar-ietal suture, as would be required following McGinnis’ hypothe-sis. Conversely, the parietal foramen is found at or near the fron-tal-parietal suture in other lepospondyls such as microsaurs (e.g.,Odonterpeton; Carroll and Gaskill, 1978) and nectrideans (e.g.,Ptyonius; Bossy and Milner, 1998), so the interpretation that thissingle ossification represents a fusion of just the frontals is pre-ferred. Additionally, most aıstopods have a posterior projectionof the frontal into the parietal that lies to the side of the midlineand parietal foramen. The frontal processes increase in lengthphylogenetically until they surround the parietal foramen in phle-gethontiids (Anderson, in press a, b).

No other bones of the skull roof and occiput can be identified.Lund (1978) claimed that the presence of parietals, tabulars, anda supraoccipital distinguished Phlegethontia from ‘‘Aornerpeton’’(the Mazon Creek phlegethontiids), which lacks these elements.However, I could find no separate bones in the areas indicated inhis description in any specimen. A two-dimensional representa-tion of AMNH 2564 (Fig. 2.1) might give the appearance of sep-arate parietals, but examination of the specimen shows the rele-vant areas to be the flattened shelf-like facet for the frontal artic-ulation, and a ventrolateral distortion due to crushing.

The postfrontal and jugal form the posterior border of the orbit(Fig. 2.1). The former is a roughly triangular bone with its widestsurface oriented dorsally and sutured to the frontal. There is nocontact with the prefrontal anteriorly because of the frontal’s par-ticipation in the dorsal margin of the orbit. In AMNH 2564 (Fig.2.1) the descending arm of the postfrontal appears to be surround-ed laterally, posteriorly, and medially by the jugal, forming atongue-and-groove joint, although this does not seem to be thecondition in the Mazon Creek specimens (e.g., MCZ 2204, Fig.4) and may be either a taphonomic artifact or polymorphism. Thejugal is an irregularly triradiate bone. It overlaps the dorsal sur-face of the maxilla for a short distance anteriorly, then becomesdorsally inflected and rises into the main body of the bone, thusgiving the anterior process the appearance of a small distal‘‘foot.’’ A short, robust process extends posteriorly. Its ventralsurface is concave for reception of the anterior process of thesquamosal.

The squamosal is also triradiate. Its longest process is directedanteriorly, where it underlies the posterior process of the jugal.The dorsal process of the squamosal narrows substantially beforeinserting into a rounded notch on the braincase near the junctionof the sagittal and nuchal crests (Figs. 2.1 and 4.1). The ventralprocess covers the quadrate portion of the palatoquadrate laterally.Study of the Turnbull aıstopod (FMNH PR 281; Anderson, inpress a) suggests that the phlegethontiid squamosal incorporatesthe quadratojugal, but there is no evidence that the latter existedas a separate ossification even early in the ontogeny of Phlege-thontia. Neither is there evidence of a suture on the anterior pro-cess, as suggested by Lund (1978). In fact, with the exception of


FIGURE 2—Phlegethontia longissima. 1, AMNH 2564, from Linton, Ohio; 2, USNM 4484, from Linton, Ohio. Scale bars 10 mm.


FIGURE 3—Phlegethontia longissima. 1, NMW 1896 II 34, from Nyrany, Czech Republic. Left lateral close-up of the skull, showing the palpebralossifications, nasal ramus of the premaxilla, and a mediolateral curvature of the quadrate ramus of the palatoquadrate. Scale bar 5 mm; 2, NMW1896 II 34, from Nyrany, Czech Republic. Scale bar 10 mm.

UMMP 22278, there is no crack or ridge in that position in anyspecimen that preserves that area.

No vomers or palatines are preserved in any phlegethontiid.The pterygoid is fused to the quadrate, forming a palatoquadratecomplex (Figs. 3.1, 4.1, 4.2). Lund (1978) identified this elementas the ‘‘pterygoquadrate’’ because he thought he had a separatepalatine. Gallup (1983) adopted this term in his description of theophiderpetontid Coloraderpeton because he recognized a distinctpalatine (identified by Anderson, in press b, as the palatal ramus

of the palatoquadrate complex). The palatoquadrate complex hasthree regions: the palatal ramus, the ‘‘quadrate,’’ and the epipter-ygoid. The palatal ramus, which lacks teeth in Phlegethontia, runsclosely parallel to the cultriform process, leaving slit-like interp-terygoid vacuities. The lateral edge begins anteriorly at a point,flares laterally to achieve its greatest width at about its midlength,then narrows again as it approaches the region of the quadrateramus. There is no descending process or flange. The loose artic-ulation with the basipterygoid process is at the junction of the


FIGURE 4—Phlegethontia longissima, MCZ 2204, from Mazon Creek, Illinois. 1, Dorsal view; 2, ventral view; 3, right lateral view of braincase,slightly off-set from horizontal. Scale bar 5 mm.

quadrate and palatal rami. The quadrate ramus extends laterallyfrom the basipterygoid process a short distance to the quadratecondyle. The anterior face of the quadrate ramus is deeply con-cave, as seen in NMW 1896 II 34 (Fig. 3.1), possibly for theorigin of jaw adductor muscles. The quadrate condyle is semi-cylindrical and is directed at nearly a right angle to the sagittalplane.

The epipterygoid is a thin, roughly rectangular sheet of bonethat rises from the dorsal surface of the palatal ramus of the pal-atoquadrate and is tightly appressed to the lateral wall of thebraincase. It continues forward to a level just anterior to the fo-ramen for cranial nerve II (optic foramen). Posteriorly it sur-rounds the prootic foramen on all but its posterior margin. It con-tinues subhorizontally to the optic foramen above the prooticforamen. The epitperygoid descends below the optic foramen,leaving its upper margin free. Just anterior to the optic foramen

the epipterygoid extends as an anterodorsal process (called theanterior process by McGinnis, 1967). In smaller specimens (e.g.,USNM 17097, Fig. 5) the epipterygoid is incompletely ossifiedand does not maintain a subhorizontal upper border. Rather, itdescends anteroventrally but fails to reach the foramen for theoptic nerve.

The endochondral bones of the phlegethontiid braincase areindistinguishably co-ossified at an early growth stage, forming amammal- or snake-like calvarium. This is best seen in MCZ 2204,a large specimen from Mazon Creek (Fig. 4). Further preparationof this specimen reveals many significant new features. For con-venience, the areas where a separate element would have existedare referred to by that bone’s name, but it should be noted thatnone of these bones is a distinct element at any stage of ontogeny.The only exception is the specimen described by Thayer (1985),which may show a suture between the basi- and exoccipitals.


FIGURE 5—Phlegethontia longissima, USNM 17097. Scale bar 5 mm.

The braincase dominates the posterior half of the skull. It iswidest just below the skull table, with the walls converging ven-trally to the parasphenoid and posterodorsally to the sagittal crest.In the largest individuals it is ossified from occiput to orbit. It isa tall structure that is keeled anteriorly in the region where thesphenethmoid is usually located (Fig. 4.3). This ‘‘sphenethmoidkeel’’ rises from the cultriform process dorsoanteriorly to meetthe frontal just in front of the orbits, where it fits into the frontal’sposterior recess above and paired longitudinal flanges below (alsoseen in MCZ 2334). Gregory (1948) and Carroll (1998b) sug-gested that the ventral descending flanges on the frontal protectedthe olfactory tracts; however, there are no foramina on the anteriorsphenethmoid keel that would permit passage of the tracts. Theolfactory bulbs probably exited from the braincase through thetwo anteriorly facing foramina located on the ventral keel justabove the base of the cultriform process.

Two large foramina pierce the lower half of the braincase (Figs.4.2, 4.3, and 5). The larger, posterior foramen just anterior to theotic capsule is the prootic foramen, for the trigeminal nerve (V),and, according to McGinnis (1967), the hyomandibular branch ofthe facial (VII) nerve and probably the abducens (VI) nerve. Thesmaller, anterior foramen may have accommodated the opticnerve (II). This foramen marks the anterior extremity of Siller-peton’s braincase. Anterior and ventral to the optic foramen andlateral to the base of the cultriform process in MCZ 2204 (Fig.4.3) is a very small foramen. McGinnis tentatively suggested thatit was for the passage of the occulomotor nerve (III), while cau-tioning that III usually exits posterior to the optic foramen (as inSillerpeton). Considering the small size it is more probable thatit served as the exit for the pituitary vein. This interpretationwould require that III exits through either the prootic foramen orthe foramina dorsal to the cultriform process. Regardless, the ab-sence of a distinct foramen for III distinguishes the braincase ofPhlegethontia from that of Sillerpeton. On the midline just abovethe cultriform process is a large anteriorly directed foramen sub-divided by a median septum (or possibly, two closely associatedforamina), that McGinnis suggested accommodated the optic orbasilar arteries, or the pituitary vein. However, since the anteriorkeel of the braincase tightly articulates with the frontal, it is morelikely that this foramen was for the passage of the olfactory tract.A prominant ridge extends along the lateral surface of the brain-case about midheight and dorsal to the foramina for II and V (Fig.4.2 and 4.3).

A sagittal crest is developed on the braincase of larger individ-uals of Phlegethontia (e.g., MCZ 2204, Fig. 4.1). Although Sil-lerpeton possesses an incipient crest (McGinnis, 1967), smallerspecimens of Phlegethontia (USNM 17097, Fig. 5; FMNH PR831, Fig. 6) do not, despite being much larger than Sillerpeton.The crest begins anteriorly within the posterior notch of the fron-tal and ends posteriorly behind the otic capsules. Here it splitsinto two ridges that continue laterally along the posterior marginof the capsules, forming a nuchal crest presumably for insertionof epaxial musculature. There is a swelling on the sagittal crestjust posterior to the parietal foramen (AMNH 2564, Fig. 2.1;MCZ 2204, Fig. 4.1) that may form a posterior brace for thefrontal.

In occipital view the phlegethontiid braincase is high and oval,with a large, round foramen magnum. Its border is thinnest dor-sally, suggesting that a supraoccipital ossification is not incorpo-rated into the co-ossified structure. The central occiput is recessedfrom the most posterior levels of the otic capsules. The skull iswidest across the otic capsules. In the smallest specimens of Phle-gethontia (USNM 17097, Fig. 5; FMNH PR 831, Fig. 6) and inSillerpeton the semicircular canals are visible as ridges on theexterior surface of the capsules.

There is no indication on the occiput of a site for articulation


FIGURE 6—Phlegethontia longissima, FMNH PR 831, from Mazon Creek, Illinois. Left lateral view. Scale bar 1 mm.

FIGURE 7—Dorsal view of the skull of Amphisbaena alba. The parietalreceives the sagittal crest in a manner similar to the frontal of Phle-gethontia (from Montaro and Gans, 1999).

FIGURE 8—Reconstruction of cranial anatomy of Phlegethontia longis-sima. Composite based primarily on AMNH 2564 and MCZ 2204. 1,dorsal view; 2, ventral view; 3, lateral view. Phlegethontia linearis doesnot differ from P. longissima in cranial anatomy.

with the proatlas. The jugular foramen pierces the exoccipital (theextent of which is determined from the location of the possiblesuture in P. ‘‘phanerhalpha’’) in Sillerpeton and the opisthotic inPhlegethontia (revealed after further preparation of MCZ 2204).In P. ‘‘phanerhalpha’’ (Thayer, 1985) the jugular foramen piercesthe opistotic lateral to the base of the foramen magnum, ratherthan lateral to the center of the foramen magnum as in Sillerpetonand P. longissima, which might be a size related difference. Theoccipital condyle (actually a cotyle) is characteristic of all aısto-pods. It is circular and concave with a notochordal pit in its center.Unlike osteolepiforms, which have a superficially similar occipitalarticulation, the notochord does not penetrate into the braincasein Phlegethontia.

In ventral view (Fig. 4.2) the dominant features are the large,oval fenestrae ovales. They are located on the posteroventral sur-face of the otic capsules, have their long axes oriented antero-medially, and are narrower medially. Internally they have thickwalls. In USNM 17097 and MCZ 2204 the plate-like stapes fitstightly into the fenestrae ovalis (Fig. 4.2). The only other tetrapodgroups that have similarly large plate-like stapes are caeciliansand amphisbaenids. The stapes has neither a columella nor a sta-pedial foramen. A crescentric ridge that is anteriorly convex and

posteriorly bevelled runs mediolaterally across the surface of thestapes and is highly suggestive of a muscle scar.

The parasphenoid is indistinguishably fused with the endo-chondral elements of the braincase (Fig. 4.2). Most of the basalplate has been eliminated to accommodate the increased size ofthe fenestra ovalis. The parasphenoid is so modified as to promptMcGinnis (1967) to describe it as absent in Sillerpeton. Becausethe specimen of Sillerpeton studied by her represents such an


early growth stage, she argued, the parasphenoid had not yet su-tured to the braincase and had become disarticulated and lost. Theview here is that it had already fused to the occipital complex inSillerpeton, as in Phlegethontia. The general outline of the par-asphenoid is present as a swelling on the surface of the endo-chondral braincase, and a cultriform process is present betweenthe foramina for the carotid arteries. In the largest Phlegethontiaspecimen preserving the braincase (MCZ 2204, Fig. 4.2), the gen-eral configuration is the same as that of Sillerpeton, but moredeveloped in detail. The cultriform process is distinct and extendsanteriorly from the braincase, but the basal plate region is no moredistinct than in Sillerpeton.

MCZ 2204 (Fig. 4.2) has a ventrally pronounced cultriformprocess. The carotid artery foramina have developed into longgrooves that run parallel to the midline a little less than half thelength of the process from the basipterygoid processes until theirentrance into the braincase. They may exit again further anteriorlywhere the cultriform process meets the ventral base of the sphe-nethmoid keel. In Sillerpeton the prootic foramen is positionedventrally, and the basipterygoid process is located on its posteriormargin. In Phlegethontia the prootic foramen is more dorsal andlateral, and the base of the basipterygoid process is positionedfurther posterior to it. From a broad base, the basipterygoid pro-cess projects ventrally and somewhat posterolaterally. It ends ina roughened head that was presumably capped by soft tissue inlife. The anteromedial surface of the process is slightly concave.

Phlegethontia has reduced the number of ossifications in thelower jaw to two. The anterior ossification is the dentary (Mc-Ginnis, 1967). All teeth are borne on the dentary, and like themaxilla and premaxilla it is pierced laterally by foramina belowthe base of the teeth (MCZ 2204). The posterior ossification,named the ‘‘posterior element’’ by McGinnis (but more preciselycalled the posteromedial element), includes the areas occupied bythe articular, angular, surangular, and splenial in more primitivetetrapods (Fig. 3.1). The lateral surface of the posteromedial el-ement extends anteriorly from the articular about one third of thelength of the jaw. Medially it extends to near the symphysis,which is formed entirely by the dentary.

The ophiderpetontid Oestocephalus (Carroll, 1998a) has a lat-eral depression below the ‘‘coronoid’’ process on the lateral sur-face of the jaw, analogous to the masseteric fossa of mammals,for the insertion of jaw adductor muscles. Phlegethontia, in con-trast, has neither a depression nor a coronoid process. It is slightlyinflected dorsally at the articular cotyle, which is a simple saddle-shaped structure. It is not as elevated above the tooth row as inOestocephalus. The articular cotyle is the posterior termination ofthe jaw as there is no retroarticular process. Because the jaw isso highly coossified it tends not to disarticulate, providing a betterindication of overall skull length than the skull itself, because thepremaxillae tend to become lost or posteriorly displaced (e.g., Fig.3.1).

There are spaces for a total of 20–23 teeth with simple, pointedcones. In larger specimens they are slightly recurved toward thetip. The first few teeth at the symphysis and the most posteriorteeth are smallest, but no specimen shows ‘‘caniniform’’ teethexcept perhaps FMNH PR 1358, an individual of intermediatesize. All teeth are ankylosed at their bases. As observed byMcGinnis (1967), when laterally crushed the teeth split along theirlong axes. She also described a pattern of tooth replacement inwhich alternating alveoli are occupied, and where the bases ofreplacement teeth are poorly anklyosed, a typical pattern for tet-rapods (Emonds, 1960). This pattern was also noted in Oestoce-phalus by Carroll (1998a). The new specimens from Nyrany (e.g.,NMW 1896 II 34) have teeth occupying all the alveoli. In otherspecimens, where there are isolated tooth crowns, there is no newcusp growing into the empty space. It is uncertain whether the

isolated crowns are in fact from the bone in question and so arecusps of unankylosed replacement teeth, or if they belong to an-other bone.

Within the posterior portion of the orbits in phlegethontiidsfrom Nyrany, Linton, and Mazon Creek are more than six elon-gate rectangular ossifications arranged roughly horizontally (Fig.3.1), each overlapping the bone immediately above. Most previ-ous authors (Gregory, 1948; McGinnis, 1967) have referred tothese as sclerotics. However, it seems impossible to articulate suchlong bones into a functional ring within the orbit of Phlegethontia.Also, they are not arranged in a circular pattern, as would beexpected of a disarticulated sclerotic ring, but rather they arealigned as a continuous sheet-like surface (like Venetian blinds).Lund (1978) and Carroll (1998a; fig. 98a) referred to these as‘‘cranial osteoderms’’ or ‘‘orbital osteodermi.’’ Among other le-pospondyls, many microsaurs have palpebral ossifications (Carrolland Gaskill, 1978). In some the palpebral cups consist of a singlelongitudinal ossification (Llistrofus), whereas others have multipleossifications either arranged into an elongate cup (Hapsidopar-eion, Rhynchonkos) or a mass of small granules (Asaphestera,Crinodon). Given that in Phlegethontia these structures are al-ways arranged horizontally in the posterior of the orbit rather thanin a disassociated ring, and considering the orbital structures de-scribed in other lepospondyls, these elements were presumably atype of palpebral cup of a design unique to phlegethontiids.

Vertebrae.Phlegethontiid vertebrae (Figs. 1–6, 9) are similarto those of other aıstopods. They are holospondylous and deeplyamphicoelous, with the single neural arches indistinguishablyfused to the centra at an extraordinarily small size. They are cy-lindrical, although compression sometimes gives the vertebrae asquare shape in dorsal or lateral view. The centra are narrowlywaisted, thereby constricting the notochord. In the smallest indi-viduals the neural arches dominate the vertebrae, which have rel-atively tiny centra that superficially resemble those of caeciliansand small salamanders (Wake, 1966; Duellman and Trueb, 1986).The posterior articular surface of each centrum is slightly largerthan the anterior end of the succeeding centrum, and when closelyarticulated the latter fits inside the former, producing what Gallup(1983) has called ‘‘functional opistocoely.’’

Most phlegethontiid specimens have very low neural spines.However, Phlegethontia linearis is distinguished from P. longis-sima by having neural spines as tall as the neural arch pedicle onat least the anteriormost four vertebrae (Fig. 1), while P. longis-sima has low nerual spines that are less than a third as high asthe neural arch pedicel. Zygapophyses are directed laterally, andslightly ventrally. There is a gap between the prezygapophyses,producing a v-shaped notch in the anterior margin of the neuralarch. There is sometimes a similar notch between the postzyga-pophyses, but it is not usually as well developed. McGinnis(1967) described a longitudinal ridge running from the pre- to thepostzygapophysis in isolated vertebrae from Richards Spur, but itis not present in the specimens from other localities. The precau-dal centra are slightly keeled (Fig. 9). Intercentra or haemal archesare not known in any aıstopod.

The transverse process extends laterally from the base of theneural arch pedicel to a distance twice that of the zygapophysis,although this varies allometrically. In small individuals the trans-verse process extends only a little beyond the span of the zyg-apophysis. Diapophyses are borne on the transverse process, andPhlegethontia does not have distinct parapophyses. Just posteriorto the transverse process a foramen for passage of the spinalnerves opens on the lateral wall of the neural arch pedicel on allvertebrae except the atlas, where it is anterior to the transverseprocess. Distribution of spinal nerve foramina is the same in Oes-tocephalus (Anderson, in press b). Lethiscus seems to restrict spi-nal nerve foramina to more posterior trunk vertebrae (Wellstead,


FIGURE 9—Phlegethontia longissima, UMMP 17097, from Mazon Creek,Illinois. Transition from precaudal to caudal vertebrae, in 1 ventral and2 dorsal views. Note the development of the median haemal groove onthe first caudal vertebrae, and the greater angle the rib head makes tothe rib shaft with that in more anterior and posterior vertebrae.

FIGURE 10—Reconstruction of the anterior postcranial skeleton of 1 Phle-gethontia longissima and 2 P. linearis, showing the diagnostic differ-ence between the length of the anterior vertebrae and height of theneural spines.

1982), but the specimen studied by Wellstead is poorly preservedso this observation should be considered tentative.

The first 20 vertebrae in Phlegethontia longissima graduallyincrease in overall dimensions, where they achieve their largestsize. Maximum size is maintained for the next 30–40 vertebrae.After this point size begins to gradually decrease to the end ofthe tail. Phlegethontia linearis has a greater degree of regionaldifferentiation in vertebral size: the ratio of length of atlas cen-trum to longest dorsal centrum is 0.38, compared with 0.60 forP. longissima. However, the proportion of the largest vertebra’slength to the lower jaw length (lower jaw length is equal betweenthe species) is about the same in both species: 0.19 in P. linearisand 0.20 in P. longissima, indicating that the difference betweenthe two species is in the smaller ‘‘cervical’’ vertebrae in P. li-nearis. It is unknown where the transition occurs between theshorter centra of P. linearis and those centra of equal proportionsin both species. Phlegethontia longissima has 64 precaudal ver-tebrae (USNM 17097; the first preserved caudal in FMNH PR831 is the 66th). The location of the first caudal vertebra in P.linearis cannot be determined, but it is assumed to be similarlypositioned. Phlegethontia longissima has 177 vertebrae present inthe most complete specimen (NMW 1896 II 34), but it is missingthe distal portion of the tail, giving an estimated total vertebralcount of 200–210 using the pattern of terminal central reductionin P. linearis as a guide. Phlegethontia linearis (AMNH 6966)has 220 preserved vertebrae, but it is missing a couple of bodyloops of undetermined length, and its estimated total count is

230–250. If the Mazon Creek phlegethontiids are as long as theNyrany specimens and assuming that P. linearis has a similarnumber of precaudal vertebrae, then the tails of Phlegethontia areamong the longest known of all limbless tetrapods, numberingapproximately 135 in P. longissima and 190 in P. linearis.

Phlegethontiids lack the accessory vertebral articulations pre-sent in nectrideans and some ophiderpetontids, although FMNHPR 831 seems to have a secondary set of articulations below theprezygapophyses on the third and fifth vertebrae. If these are ac-cessory articulations, they articulate upon a flat surface on theposterior centrum. This pattern is opposite of that seen in nectri-deans, which have accessory articulations above the zygapophy-ses, as do snakes (Bossy and Milner, 1998). Phlegethontiids alsodo not possess basipophyses.

A proatlas can be seen in MCZ 2204 (Fig. 4) and FMNH PR831 (Fig. 6). Previously, the only aıstopod known to have a pro-atlas was the Mazon Creek ophiderpetontid Oestocephalus sp.(MCP 323; Carroll, 1998a). In common with the proatlas in Oes-tocephalus, that of Phlegethontia is a single median element, witha convex dorsal margin that bridges the gap between the occiputand atlas. There are no remnants of midline sutures, although itwas presumably derived from paired elements, as is common inPaleozoic ‘‘labyrinthodonts’’ and present in the oldest microsaurfrom Goreville (Lombard and Bolt, 1999). Unlike the proatlas ofOestocephalus, which is a short, wide strap-shaped bone, that ofPhlegethontia is longer than wide, and over twice as long as theodontoid process of the atlas. Assuming the proatlas is in naturalarticulation, there would be a short gap between the occiput andatlas. Gregory (1948) noted this gap in the larger USNM 17097(Fig. 5). In Oestocephalus, the atlas is disarticulated, but the rel-atively shorter proatlas would permit a closer articulation with theskull.

The ventral portion of the anterior margin of the centrum ofthe atlas of Phlegethontia tapers to a rounded point, producing ashort odontoid process (USNM 17097, Fig. 5; FMNH PR 831,Fig. 6). An atlantal odontoid process (of varying degrees of ex-pression) is also present in microsaurs, adelospondylids, lysoro-phids, and nectrideans (where it is progressively lost in derivedkeraterpetontids). The odontoid process in Phlegethontia is a ta-pered anterior continuation of the centrum, giving the centrum aprocoelous appearance. It is a smaller structure than that in mi-crosaurs, but relatively larger than that in the nectridean Diplo-ceraspis (Beerbower, 1963). Unlike microsaurs and nectrideans,the anterior centrum is not laterally expanded to articulate withexoccipital condyles.

The atlantal centrum is slightly shorter than the second verte-bra. The transverse processes are located posterior to the midpoint


of the centrum and are directed caudally about 108 from lateral.Neither the atlas nor the second vertebra bears ribs. Transverseprocesses on vertebrae 2–4 are also caudally directed, whereasthe fifth has laterally directed transverse processes. A web of boneconnects the distal end of the transverse process to the posteriorrim of the centrum. The atlas is ventrally keeled from the anteriormargin to near the midlength of the centrum. There are no pre-zygapophyses for articulation with the proatlas. The second ver-tebra is not morphologically distinct from those subsequent to it.The transverse processes are just caudal to the midpoint of thecentrum, but more cranial than those of the atlas.

On the fourth centrum the transverse processes are located atthe midlength of the centrum, and are angled slightly caudally.Beginning on the 8th (USNM 17097, Fig. 5) to 12th (MCZ 2204)vertebra, the distal end of the transverse process rotates forwardso that it angles cranially. In the very diminutive FMNH PR 831,however, the transverse processes are laterally directed until the38th vertebra. When anteriorly directed, as in most of the dorsalvertebrae and the anterior most caudals, the transverse processesare inclined 30 degrees from lateral. Like the posteriorly directedatlantal transverse processes, the inclined transverse processes arejoined to the anterior centrum by a web of bone. Posterior to thelast rib-bearing vertebra the transverse processes lose the anteriorangulation and are again laterally directed, as in the posteriorcaudal vertebrae of USNM 17097. Also posterior to the last rib-bearing vertebrae, the transverse processes gradually diminish un-til only a swelling remains.

Serial changes can also be noted in the location of the proximaltransverse process. In USNM 17097 (Fig. 5) the bases of thetransverse processes progressively migrate to a more cranial po-sition, beginning with the 4th vertebra. In other specimens(FMNH PR 831) this does not occur until the anterior caudalvertebrae.

The first caudal vertebra is identified as the most anterior ver-tebra that has paired haemal flanges extending along the ventralsurface of the centrum (Gregory, 1948; Zidek and Baird, 1978;Fig. 9). In other tetrapods, haemal arches do not begin for severalvertebrae posterior to the sacrum. If the haemal flanges werestrictly homologous with the haemal arches, the first appearanceof the flanges would be posterior to the actual beginning of thecaudal series. Although there is no true sacral vertebra becausethere is no pelvis, the first caudal vertebra’s ribs have an unusualkink that orients their head at a 708 angle to the shaft (Fig. 9).This angle is much more acute than is seen anywhere else in thecolumn, and it may indicate that this vertebra is modified froman ancestral sacral vertebra. If so, then the first occurrence of thehaemal flanges gives an accurate count of the total number ofcaudal vertebrae. Terminal vertebrae are only known in AMNH6966 (Fig. 1).

Ribs.Ribs begin on the third vertebra. Aıstopods resemblemodern apodous squamates in two respects. Firstly, all apodoussquamates have ribs starting on or before the fourth vertebra.Secondly, most modern limbless squamates have accessory pro-cesses anterior and posterior to the rib-vertebra articulation(Hoffstetter and Gasc, 1969). Additionally, all ribs of modernlimbless squamates except scincids are functionally single headed.The aıstopod rib has a short, robust anterior or costal process (the‘‘capitulum’’), a tuberculum, and a longer posterior process (theposteromedial process of Baird, 1964, McGinnis, 1967, and Lund,1978) that overlaps successive ribs (Fig. 2.1). In modern limblesssqumates the area called here the tuberculum in aıstopods is theconjoined tuberculum and capitulum.

The posterior process is present in the six anteriormost ribs inAMNH 2564 (Fig. 2.1), seven in MCZ 2204 (Fig. 4), and eightin USNM 17097 (Fig. 5, although it is much smaller on theeighth). On the 10th (MCZ 2204) or 13th (USNM 17097, Fig. 5)

vertebra the rib shafts change orientation from directed ventro-laterally to more posteriorly, which persists for the remainder ofthe column. In USNM 17097 thin, short posterior processes arepresent about a third of the length down the shaft on posteriorprecaudal vertebrae, beginning with the 36th vertebra and con-tinuing through the anterior caudals (Fig. 9).

Rib shafts are generally slender in Phlegethontia. The anteriorribs are the most robust, but by the 30th vertebra the shafts be-come very thin. In the posterior precaudal and anterior caudalvertebrae rib shafts become hairlike structures. The posterior ex-tent of rib bearing vertebrae is difficult to determine. AMNH 6966does not show ribs well (Fig. 1), so a full count from P. linearisis lacking. The only specimen of P. longissima that shows ribsclearly along the length of the column is USNM 17097, whichhas ribs to the 82nd vertebra. This would leave over 100 caudalvertebrae without ribs, making the phlegethontiid tail a long,whip-like structure, which is very different from that in ophider-petontids (Zidek and Baird, 1978; Anderson, in press b). AMNH6966 definitely has no ribs on its terminal 61 vertebrae, at whichpoint overlying vertebrae obscure the distal series.

USNM 4484 (Fig. 2.2) is anomalous because it has robust ribs,which led McGinnis (1967) to suspect it represented a new spe-cies. However, the patterns of rib shape and proportions are thesame in this specimen as in all other phlegethontiids. The ratioof jaw length to anterior vertebrae is the same as in Phlegethontialongissima. Further, other bones, namely the pectoral elementsand some vertebrae, are similarly swollen, and the skull is highlydisarticulated, almost as if the elements exploded outward fromtheir actual positions. These features are seen in fossils in whichthe bones have been in-filled with anhydrous minerals, such asgypsum, that are subsequently exposed to water. No charactersother than those attributable to taphonomic factors distinguish thisspecimen from any other phlegethontiid, therefore the robust ribsof USNM 4484 are not considered diagnostic of a different spe-cies.

Pectoral girdle.Located in the area of vertebrae three to sixis an elongate, sigmoidal bone of controversial identity (Figs. 1,2, 4). It has been previously described as ‘‘sickle shaped,’’ how-ever, the ‘‘handle’’ is also gently curved. In lateral view (Fig. 5)it is inclined anteroventrally. The dorsally placed and longitudi-nally directed ‘‘handle’’ is shorter than the ventral ‘‘blade.’’ Itscurve overlies the ribs near the level of the transverse processes.The ‘‘blade’’ is concave ventrally, and the point is directed towardthe skull. A similarly shaped and positioned bone is also presentin ophiderpetontids (Baird, 1964; Milner, 1994; Carroll, 1998a;Anderson, in press b).

This element has been previously identified as the hyoid. How-ever, this seems unlikely, in as much as in modern amphibiansand reptiles the hyoids occupy the space between the lower jawsand in salamanders beneath the occiput (Duellman and Trueb,1986). In adelospondylids (Andrews and Carroll, 1991) and ly-sorophids (Wellstead, 1991) the main hyoid apparatus is moreposterior, but, as in salamanders, is associated with the occiputand not far back on the vertebral column. In the temnospondylDvinosaurus (Bystrow, 1935) there are hyoid elements posteriorto the occiput, but they are closely articulated with other bran-chials that are anterior to the occiput. In all of these examples thehyoid elements are stout and rather short bones. In contrast, inphlegethontiids it is well posterior to the occiput and is longerand more gracile. Its positioned lateral to the ribs which wouldbe expected of the pectoral girdle. In most microsaurs and in thenectridean Scincosaurus the pectoral girdle is positioned at thethird vertebra, and in almost all other lepospondyls the pectoralgirdle is located at the fifth vertebra, the same region in whichthe aıstopod element is found (Figs. 1, 2, 4 and 5). The longnarrow element found in phlegethontiids corresponds most closely


FIGURE 11—Phlegethontia longissima, FMNH PR 624, from MazonCreek, Illinois. Specimen shows a body impression 1, within whichcan be seen a cast of the intestinal tract 2, possibly the stomach. Scalebars 10 mm.

in position and orientation with the cleithrum in microsaurs, al-though it tends to be shorter in microsaurs. The length and si-nusoidal shape is best matched if the phlegethontiid element isformed from a fusion of the cleithrum and clavicles of microsaurs.If correct, the ‘‘blade’’ of the sickle corresponds with the curveof the clavicle. This hypothesis is supported by the presence of aswelling at the point where the two elements would contact andfuse in AMNH 2564 and USNM 4484 (Fig. 2). A break occursat this point in USNM 4484 (Fig. 2.2), suggesting a possibleweakness or incomplete fusion in this specimen.

Gastralia.As in ophiderpetontids, the ventral gastralia are ar-ranged in a chevron pattern, with the apex of the chevron lyingon the ventral midline and directed cranially (Fig. 5). Ophider-petontids have short, stout ‘‘wheat-shaped’’ ventral gastralia anddiamond-shaped dorsal osteoderms that covered the dorsal bodyand the adductor chamber of the skull. Phlegethontiids, in con-trast, do not have dorsal osteoderms, and the ventral gastralia arefine and elongate.

Soft tissue preservation.Due to the unusual type of preser-vation common in Mazon Creek fossils, a few specimens of Phle-gethontia show the impression of the body around the skeletalmaterial (Figs. 5 and 11). Dorsally the body extends posteriorlyfrom just above the occiput and lies immediately above the ver-tebrae, within one half the height of the neural arch. Ventrally thesoft body outline begins below the lower jaw and continues wellbelow the ribs by almost another full rib length. The gastraliaalways closely correspond to the soft body impression, as wouldbe expected of ossicles within the dermis.

FMNH PR 624 shows an elongate, cylindrical mass within theventral portion of the body cavity (Fig. 11) approximately 9.6mm long, and lies at about the level of vertebrae 21–30. It issomewhat jumbled and broken, but does not appear spiral or oth-erwise wrapped.

Lund (1978) described the presence of dark pigments in theanterior body and head of FMNH PR 1358. Dark patches arepresent in the areas indicated by Lund but are not present in anyother specimens. In other amphibians from Mazon Creek, such asSaurerpeton (Godfrey, 1997, fig. 19.1) dark pigments of the retinaare preserved. This phenomenon is known from other localitiesas well, but is not present in phlegethontiids. No such body col-oration is present in any other aıstopod specimen.

DISCUSSION

Species resolution.The question of species resolution in phle-gethontiids remains difficult to answer, but details are becomingclearer based on new specimens. Phlegethontia linearis, knownfrom only a single specimen (AMNH 6966, Fig. 1, and its coun-terpart 6886; Turnbull, 1958), is distinguished from P. longissimaby having high neural spines on at least the first four vertebrae,at least the first four centra reduced in length, and a much longervertebral column.

Vertebral counts in modern amniotes (Arnold, 1987), amphib-ians (Jockusch, 1997), and fish (Lindsey and Arnson, 1981), canvary due to environment or change with growth. These factorsmay have also affected Phlegethontia, but the picture remainscomplicated because no completely articulated vertebral columnsare currently known. However, two complete ophiderpetontidspecimens are known from Nyrany (Anderson, in press b), whichshow that during growth from a skull length of 5 to 8 mm (anincrease of 38%) no vertebrae were added to the column. Thissuggests that differences in vertebral count could have systematicimportance in aıstopods.

Sillerpeton is recognized here as a distinct genus, whereas‘‘Aornerpeton’’ is reduced to subjective junior synonymy withPhlegethontia. Sillerpeton, despite its geologically younger agethan the species of Phlegethontia, may more basal due to the

retention of a separate foramen for cranial nerve III posterior tothe foramen for II, although knowledge of additional features ofits anatomy is required to be certain of this interpretation. Theforamen for III in Phlegethontia has either migrated anterior tothe foramen for II and has become much reduced in size, or ithas been lost (McGinnis, 1967). Sillerpeton is further distin-guished by the jugular foramen piercing the area usually occupiedby the exoccipital, whereas in Phlegethontia, including ‘‘P. pha-nerhalpa,’’ which may show a suture separating the exoccipitaland opisthotic, it pierces the opistotic.

Identifying aıstopod species is difficult because derived char-acters occur throughout the body, but different portions of thebody are preserved from different localities. The most complete(in terms of numbers of vertebrae) are from the coal shales ofNyrany and Linton (NMW 1896 II 32 and AMNH 6966). Un-fortunately, these are preserved as two-dimensional fossils anddetails of important features of structure are lost or obscured. Forexample, the haemapophyseal flanges of the caudal vertebrae,which might clarify whether the Mazon Creek specimens areshorter (the longest series of preserved vertebrae is 130) than thecoal shale fossils (177 and 220 vertebrae are preserved respec-tively), are not visible in the flattened specimens.

Growth.A sufficient size range of phlegethontiid specimensis now available from Mazon Creek to construct a plausiblegrowth series. Table 1 lists Mazon Creek specimens in order ofsize, which reveals a few notable anatomical trends. First, by anextremely small size (,1 mm in length), vertebrae are fully os-sified. This fact led Baird (1965) to conclude that aıstopods weredirect developers, not passing through a larval stage. Supportingthis hypothesis is the observation that no specimen has been found


TABLE 1—Phlegethontia longissima specimens representing growth seriesfrom very young to mature. An * indicates individuals for which an ac-curate measurement of skull length is impossible.

Specimen numberJaw length

(mm)Total skull

length (mm)Number ofvertebrae

FMNH PR 831FMNH PR 624USNM 17097FMNH PR 1358MCZ 2204NMW 1896 II 34AMNH 2564USNM 4484

1.6*4.157.65*8.75

12.1*14.818.1526.2*

3.36*5.324.0*

10.85?

15.6616.2419.8

1161001056213

1703014

FIGURE 12—Preliminary hypothesis of aıstopod relationships, to be dis-cussed in detail elsewhere (Anderson et al., 2001).

that shows the presence of gills, gill supports, or lateral line canalgrooves. Such structures are preserved in other taxa at MazonCreek (e.g., Godfrey, 1997), so their absence in phlegethontiidsmay be significant.

The smallest specimen (FMNH PR 831, Fig. 6) lacks ribs, gas-tralia, cleithra and teeth. The braincase is completely ossified pos-teriorly, indicating that ossification of the anterior portion lagsbehind during development and is only fully ossified in largerindividuals. FMNH PR 831 shows a similar degree of ossificationas Sillerpeton. The appearance of the anterior keel in the largestspecimens appears to coincide with the cessation of additionalossification.

Ossification of the dermal skull bones lags behind that of en-dochondral elements, with only the frontal, prefrontal, maxilla andboth jaws, and possibly the palatal ramus of the palatoquadrateevident in FMNH PR 831 (Fig. 6). In the somewhat larger USNM17097 (Fig. 5) the jugal appears, the squamosal had begun toossify, and the postfrontal is present in limited form, but all arenot fully ossified. The frontal is better defined than FMNH PR831, but the posterior prongs are still relatively short. No bonesare present anterior to the prefrontal, but this might be due to azone of dissolution common at the edges of Mazon Creek nod-ules, where the snout unfortunately lies. The epipterygoid hasgrown around the lateral walls of the braincase, but has notreached its full height. In the largest Mazon Creek phlegethontid,MCZ 2204, the full suite of dermal bones is present, the epipter-ygoid is fully formed, and the sagittal crest is developed.

This trend of earlier ossification of endochondral bone and de-layed ossification of dermal bone may be a phyletic character ofaıstopods. According to preliminary phylogenetic analysis pre-sented here (Fig. 12), Ophiderpeton and Lethiscus are the mostbasal aıstopods, followed by a dichotomy formed, on the onehand, by Oestocephalus and Coloraderpeton, and on the other bythe Turnbull specimen (Turnbull and Turnbull, 1955; Anderson,in press a) and a monophyletic Phlegethontiidae. Ophiderpeton-tids have fully roofed skulls, but open cheek region. They haveextensive dermal osteoderms that thickly cover the body early ingrowth. Their braincase anatomy is inadequately known becauseof the cover of the parasphenoid basal plate, but the otic capsulesappear incompletely ossified (Carroll, 1998a, fig. 3c, d). TheTurnbull aıstopod retains a full compliment of skull roofing bones,but the skull has become narrower than that of ophiderpetontids(Anderson, in press a). Dermal osteoderms are absent, and thegastralia have achieved the reduced state of phlegethontiids. How-ever, while the braincase is becoming more fully ossified, the oticcapsules appear to be incompletely ossified. There seems to havebeen a decoupling of the commencement of ossification of en-dochondral versus dermal bones that is magnified phylogeneti-cally, leading toward the phlegethontiid state. Hanken (1983,

1984) has discussed how growth can be terminated at an extreme-ly small size by accelerating the completion of epiphyseal ossi-fication in long bones. This can lead to gross changes in the mor-phology of other systems because they were unable to achievecomplete ossification. Hanken also discussed how hyperossifica-tion in one system (in this case the endochondral skull and ver-tebral column) is usually accompanied by a reduction in ossifi-cation in another (here, dermal skull bones and dorsal ossicles).

Functional morphology.Lund (1978) proposed an elaboratemechanism of cranial kinesis, modeled after basal snakes (e.g.,Frazetta, 1970), for the skull of Phlegethontia. In his model, thebraincase formed a fixed bar in a four-bar kinematic chain. Thepalatoquadrate and ‘‘palatine’’ served as struts for force transferto the mobile premaxilla. The lacrimal, maxilla, postfrontal and‘‘orbital segment of the braincase’’ formed a dependent chain,linked to the principal chain (i.e., the premaxilla and frontals) bythe premaxillary-maxillary joint and ‘‘possibly the palatine, andprobably by connective tissue associated with the adductor man-dibularis complex to the pterygoquadrate’’ (Lund, 1978, p. 70).He also hypothesized a ligamentous connection between the pos-terior margin of the maxilla and the mandible. The result wasthat, when the palatoquadrate was protracted, the premaxillawould rise and rotate the maxilla about its articulation with the‘‘lacrimal’’ while the jugal would slide ventrally on its articulationwith the postfrontal (Lund also recognized the tongue-and-groovenature of this articulation). Continuing, he proposed a potentiallaterodorsal rotation of the cheek reinforced by the ‘‘postorbitalosteoderms’’ (palpebral ossifications in this paper) and the squa-mosal-‘‘postorbital’’-‘‘quadratojugal’’ complex.

Lund’s (1978) functional interpretation does not concur withthat proposed here. The contact between the dorsal process of thesquamosal and braincase is here interpreted as immobile, ratherthan as a potential pivot point. Lund’s ‘‘quadratojugal’’ is part ofthe posterior palatoquadrate. His ‘‘postorbital’’ is interpreted hereas the anterior process of the squamosal. I have not been able tofind even a crack in any specimen in the right position to suggestthe presence of such a suture, except perhaps in UMMP 22278.Furthermore, Lund identifies palatines and incorporates them asan important component of his kinetic chain, yet I have beenunable to confirm the presence of these elements in any specimen.


Correct homologies aside, Lund’s (1978) kinetic model failsbecause the palatoquadrate is immobile. As described by him, thepalatoquadrate is the main force-transmitting bar of the model.However, the epipterygoid is firmly fixed to the braincase. Duringontogeny the epipterygoid grows around the prootic foramen, al-most enveloping it. If the epipterygoid were to protract, it wouldclose off the prootic foramen and place great pressure on thetrigeminal nerve, if not severing it outright. Thus, if the palato-quadrate could not move, then the squamosal could not pivot, andthe premaxillae could not rise. There is evidence to suggest thatthere might be a more limited kinetic mechanism at work. Forinstance, the maxillae do not have definite sutural articulationswith the surrounding bones, and the premaxillae are often dis-placed dorsoposteriorly. However, given the relatively small sam-ple, it may be that these features are simply the result of defor-mation of the small, lightly constructed phlegethontiid skull.

The unusual rib morphology of aıstopods has provoked muchfunctional speculation, from a possible lateral arrangement (Baird,1964) to the hypothesized ‘‘rib-walking’’ in the ophiderpetontidColoraderpeton (Gallup, 1983). K-shaped ribs are restricted to theanterior six rib bearing vertebrae in Phlegethontia, so rectilinearlocomotion (Gans, 1986), referred to by Gallup as rib-walking, isunlikely to have been a primary means of propulsion (see An-derson, in press b for a further discussion). Additionally, the elon-gate, rather thin body of Phlegethontia contradicts the pattern inmodern limbless animals, in which thick bodies characterize rec-tilinear locomotion.

The posterior process of the rib is properly positioned and ori-ented to provide a large area for the insertion of M. iliocostalis.Ribs in which this process is prominent seem to be frequentlydisplaced posteriorly and dorsally, closer to the origin of the M.iliocostalis, the lateral most member of the epaxial musculature.This suggests that the process is relatively important in aıstopodlocomotion. In snakes (Moon, 1999) and amphisbaenians (Gasc,1981) M. iliocostalis has its origin from either a tendon (mostsquamates) or a tendonous sheet (Amphisbaena alba) arising fromthe anteroventral half of M. longissimus dorsi. It spans many seg-ments (.10) and inserts as a tendon on the posterolateral surfaceof the rib, about one third of the length of the shaft from the head.In Amphisbaena the insertion of M. iliocostalis is joined by theinsertion of M. supracostalis ventralis superior (Gasc, 1981).These muscles assist in the lateral flexure of the animal, whichforms the basis for most types of lateral undulation (Gans, 1985;Moon and Gans, 1998). Caecilians have a unique form of loco-motion termed ‘‘internal concertina’’ (Gans, 1973) in which thevertebral axis moves within a muscular sheath, forming anchorpoints with the substrate (usually burrows) by pressing out withthe posterior body wall and using that anchor to propel itselfforward. It then establishes a new anchor point with the anteriorportion of the body and draws the rest of the body forward, setsa new anchor with the posterior body, and repeats the cycle. Somecaecilians are now known to supplement this locomotive strategywith a hydrostatic mechanism (O’Reilly et al., 1997). As a result,caecilian muscular anatomy is very different from squamates.They exhibit less muscular differentiation, with the M. dorsalistrunci and M. subvertebralis being the dominant internal trunkmusculature (Naylor and Nussbaum, 1980). Resegmentation islimited to myosepta spanning three to four vertebrae. Muscle in-sertions on ribs are not an important factor in locomotion, sincecaecilian ribs seem to lie between different layers of the lateralmuscle wall, whereas the muscles attach to the myosepta or otherfascia.

The costal process of the phlegethontiid rib matches closely theanteroventral process on the rib of Amphisbaena alba (Gasc,1981). In Amphisbaena, the anteroventral process is joined to the

tuberculum of the preceding rib by M. tuberculocostalis profun-dis.

Although it is impossible to reconstruct accurately the muscu-lature of fossil animals with no close modern relatives, the pres-ence of the posterior process, which is evidence for a highly de-veloped iliocostalis-like muscle, suggests that Phlegethontia wasan accomplished terrestrial lateral undulator and did not locomotelike caecilians. The large orbits and lightly built, fenestrate skullsimilarly argue against a fossorial habit. Gregory (1948) inter-preted Phlegethonia as fossorial, based upon the highly ossifiedbraincase, forward position of the jaws, large round stapedialfootplate, and low neural spines, which it has in common withamphisbaenids. Alternatively, limbless animals may have a solidlyossified braincase because a solid attachment is needed for theexpanded role of epaxial musculature to move the head. Limbedanimals can lift their heads using their forelimbs, but the headsof limbless animals are limited in the amount of excursion pos-sible vertically, unless compensated for by another system. All ofthis head motion must be executed by epaxial musculature if thepectoral girdle is reduced or lost, which requires a strong anchorand lever for force transmission.

Some have questioned why the cleithrum would be preservedeven as a vestage in a limbless vertebrate (Hook, 2000). Theanswer is twofold. Firstly, it may be that since the pectoral rem-nants are internal there is no disadvantage to their being present.Aıstopods are unlike snakes, which have changed the geneticidentity of the pectoral region to ‘‘thorax’’ and thus the signal tocommence limb development is not delivered (Cohn and Tickle,1999; Anderson, in press b). The pelvic region of snakes, how-ever, retains its genetic identity and is reduced following a distalto proximal pattern (Cohn and Tickle, 1999), and remnants of thepelvis are still present internally in basal members of the group.The pectoral girdle in aıstopods may be analogous to the pelvicremnants of snakes. This may be supported by the observationthat in other elongate lepospondyls showing limb reduction thereis first a reduction in the number of digits and the size of the limb(i.e., lysorophids; Wellstead, 1991). This is followed by the lossof the limb and the endochondral portion of the pectoral girdlebut the retention of the dermal girdle in an unmodified condition(adelospondylids; Andrews and Carroll, 1991).

Secondly, the retention of a cleithrum may be related to theinferred presence of an ‘‘operculum-opercularis’’ type complex.As mentioned above, there appears to be a muscle scar on theventral surface of the stapes that has invoked the reconstructionof a hypothetical muscle that is directed posteriorly and somewhatlaterally, toward the remnants of the pectoral girdle. This idea hasits basis in modern frogs and salamanders, which possess an os-sicle accessory to the footplate of the stapes (sometimes fused toit) called the operculum. In salamandrids the operculum fills theentire fenestra ovalis, similar to the condition found in Phlege-thontia. The operculum serves as the origin of a muscle, the op-ercularis, which inserts on the pectoral girdle and serves to trans-mit low frequency vibrations to the middle ear (Monath, 1965).This hypothesis is perhaps supported by the observation that cae-cilians, which lack any trace of the pectoral girdle, are the onlygroup of modern amphibian that does not have an opercularisprimitively (Duellman and Trueb, 1986).

Biogeography.Phlegethontiids are now known from Nyrany,Czech Republic; Linton, Ohio; Mazon Creek, Illinois; RichardsSpur, Oklahoma; the Swisshelm Mountains, Arizona; Jarrow, Ire-land and Five Points, Ohio (Baird, 1964; McGinnis, 1967; Thayer,1985; Hook and Baird, 1993; Carroll, 1998b). Isolated vertebraehave also been reported from the Lower Permian Sangre de CristoFormation in New Mexico (Berman, 1993), and the Pennsylva-nian Dunkard Formation of Pennsylvania (Hook and Baird, 1993).

A series of vertebrae described recently from the Stephanian


Basin de Montceau-les-Mines of the French Massif Central (Du-tuit and Heyler, 1994) were identified as ‘‘Dolichosoma’’ longis-simum. The authors reported not less than 100 vertebrae pre-served, including the end of the tail, of which the anteriormost15 vertebrae bear ribs. The close-up photograph of the rib-bearingvertebrae (Dutuit and Heyler, 1994; fig. 8) reveals the laterallyprojecting, anteriorly placed transverse processes characteristic ofphlegethontiid caudal vertebrae, and the approximately 85 rib freeposterior caudal vertebrae are consistent with this identification.

Phlegethontiids are present throughout the Permo-Carbonifer-ous ‘‘Edaphosaur-Nectridean’’ fauna of Milner (1993). The gen-eral morphology in the braincase varys little in those specimensspanning the Westphalian B of the Swisshelm Mountains to theLower Permian of Fort Sill, a history of approximately 75 millionyears. However, it cannot be said that this evolutionary conser-vatism included the rest of the phlegethontiid body.

Aıstopods, while widespread, were very rare constituents of thefaunas in which they are found. Only a few specimens are knownfrom the extremely fossiliferous terrestrial fissure fills of RichardsSpur, for example. The absence of lateral line canal grooves andthe relative scarcity of aıstopods in the aquatic deposits of Lintonand Nyrany compared with obligate aquatic vertebrates suggeststhat phlegethontiids were primarily terrestrial or lived on the mar-gins of the water in which their bodies were preserved (Milner,1980). However, none are known from the terrestrial deposits ofJoggins, Nova Scotia, which suggests that they either were notpresent in that environment, or that they were not prone to beingtrapped in the upright stumps that accumulated the largest portionof the fauna. Their large orbits and lightly built skulls are evi-dence against a subsurface habitus. Phlegethontia probably livedin the leaf litter around bodies of water.

Relationships.Relationships within the Aıstopoda will be dis-cussed elsewhere, however preliminary results are given in Figure12. This analysis places phlegethontiids as the most derived ais-topods, with the Turnbull aıstopod, the clade formed by Color-aderpeton and Oestocephalus, Ophiderpeton and Lethiscus form-ing successive outgroups.

The most recent investigation of lepospondyl relationships wasby Anderson (2001). This study suggested that aıstopods are thesister taxon to nectrideans, supported by the common presence offused, unipartite vertebrae without separate intercentra and withrelatively long transverse processes. Aıstopods and nectrideansalso share several characters that are not fully consistant withineither taxa. For example, intravertebral foramina for the spinalnerves posterior to the transverse processes in precaudal vertebraeare present in all aıstopods but known only in the Lower Permiannectrideans Crossotelos (Carlson, 1999) and Sauropleura bairdi(Bossy and Milner, 1998). Accessory articulations of the vertebraeare known in Oestocephalus (Baird, 1964; Anderson, in press b)and all nectrideans (Bossy and Milner, 1998). Their caudal ver-tebrae differ however, in that those of phlegethontiids exhibit aposterior displacement of the foramina, whereas the foramina mi-grate anteriorly in Crossotelos. Sauropleurine and diplocaulinenectrideans have fused frontals, similar to Phlegethontia. Mostsalamanders also possess intravertebral foramina for spinal nerves(Duellman and Trueb, 1986).

Anderson (2001) placed lysorophids as sister taxa to aıstopods,but suggested that this is due to convergence to a similar elongate-limbless morphotype. There are no convincing synapomorphieslinking lysorophids with aıstopods, and this group breaks downwith only one extra step. The phylogenetic analysis of Anderson(2001) differs from those of recent authors (Carroll, 1995; Laurinand Reisz, 1997) in not finding a close relationship between aıs-topods and adelospondylids. This may have been because of aneffort to limit the effect of correlates with an elongate-limblessmorphotype (Carroll counted approximately 15 characters that

were related to loss of limbs alone) by removing nonindependentcharacters associated with this bodyplan. However, all three stud-ies are in broad agreement regarding the general arrangement ofPaleozoic tetrapods. Further discussion may be found in Anderson(2001).

CONCLUSIONS

Two species of Phlegethontia are recognized. Phlegethontia li-nearis is distinguished by the shorter centra and higher neuralspines of its anterior most vertebrae. It also has a higher numberof vertebrae than P. longissima. Phlegethontia linearis is onlyknown from Cope’s type, and it coexisted with both P. longissimaand Oestocephalus in the coal swamps of Linton.

Contrary to previous work, the skull of Phlegethontia, thoughsuperficially snake-like, was not highly kinetic. The squamosalinserts onto the braincase, and the jugal has a tongue-and-groovejoint with the postfrontal. The epipterygoid portion of the pala-toquadrate is tightly attached to the braincase and surrounds thetrigeminal foramen, making the mobility of this composite bonehighly unlikely. Any possible kinesis would have been limited tothe premaxilla and maxilla.

ACKNOWLEDGMENTS

I would like to thank R. Carroll for his support and encour-agement through the preparation of this study. I thank, in alpha-betical order of institution, G. Gaffney and C. Hotton (AmericanMuseum of Natural History); D. Baird, D. Berman, and M. Daw-son (Carnegie Museum of Natural History); J. Bolt, W. Simpsonand the late S. McCarroll (Field Museum of Natural History); andF. Jenkins, C. Schaff, and W. Amaral (Museum of ComparativeZoology) for access to specimens. Figures were drawn by P. Gas-kill and myself, and final figure preparation was aided by D. Scott.I thank Verlag Dr. Friedrich Pfeil for graciously allowing me toreproduce figures used in the Lepospondyl volume of the Hand-book of Paleoherpetology. Previous drafts of this manuscript wereimproved by reviews from D. Berman, R. Carroll, R. Holmes, S.Modesto, and an anonymous reviewer. This work was supportedby the Natural Sciences and Engineering Research Council ofCanada.

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ACCEPTED 10 DECEMBER 2001

revision of the aÏstopod genus phlegethontia (tetrapoda: lepospondyli)

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