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A Middle Jurassic Sphenodon-like sphenodontian(Diapsida: Lepidosauria) from Huizachal Canyon,Tamaulipas, MexicoVictor-Hugo Reynoso aa Instituto de Geología, Universidad Nacional Autónoma de México, CiudadUniversitaria , Mexico, D.F., 04510Published online: 24 Aug 2010.

To cite this article: Victor-Hugo Reynoso (1996) A Middle Jurassic Sphenodon-like sphenodontian (Diapsida:Lepidosauria) from Huizachal Canyon, Tamaulipas, Mexico, Journal of Vertebrate Paleontology, 16:2, 210-221, DOI:10.1080/02724634.1996.10011309

To link to this article: http://dx.doi.org/10.1080/02724634.1996.10011309

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Journal of Vertebrate Paleontology 16(2):210-221, June 1996© 1996 by the Society of Vertebrate Paleontology

A MIDDLE JURASSIC SPHENODON-LIKE SPHENODONTIAN (DIAPSIDA: LEPIDOSAURIA)FROM HUIZACHAL CANYON, TAMAULIPAS, MEXICO

VICTOR-HUGO REYNOSOInstituto de Geologia, Universidad Nacional Aut6noma de Mexico, Ciudad Universitaria, Mexico D.E 04510.

ABSTRACT-A new Middle Jurassic North American sphenodontian, Cynosphenodon huizachalensis, gen . et sp. nov.is described on the basis of several dentaries and maxillae. It is characterized by its small size and its striking similarityto Sphenodon. The presence of a deep posteromedial wear facet on the caniniform tooth, a small denticle in theadditional tooth series, and an anterior small groove dorsal and convergent to the Meckelian Canal, are unique char­acteristics of this species. In adult individuals the caniniform tooth placed posteriorly to an edentulous ridge, and thepresence of wear facets on teeth and jaws caused by propalinal jaw action, suggest that C. huiza chalensis is closelyrelated to Sphenodon; the wear facet on the posteromedial face of the caniniform tooth, however, may suggest a differentarrangement of the anterior palatal region.

RESUMEN-Con base en varios dentarios y rnaxilas, se describe Cynosphenodon huizachalensis, gen. et sp . nov. unnuevo esfenodonte del Jurasico Medio de Norte America. Se caracteriza par sus pequefias dimensiones y su gransimilitud a Sphenodon. La presencia de una faceta de desgaste en la parte posteromedia del diente caniniforme, de undentfculo en la serie dental adicional, y de un pequefio surco dorsal y convergente al Canal de Meckel, son caracteresunicos de esta especie. El diente caniniforme ubicado posteriormente a un margen edentado en adultos, y la presenciade facetas de desgaste en dientes y mandfbulas causados por movimientos mandibulares propalfneos, sugieren que C.huizachalensis esta cercanamente relacionado a Sphenodon ; la faceta de desgaste en la cara posteromedia del dientecaniniforme, sin embargo, sugiere un arreglo diferente de la region palatal anterior.

INTRODUCTION

Beginning in 1985, preliminary descriptions of a Middle Jur­assic North American terrestrial vertebrate assemblage werepublished (Clark and Hopson, 1985; Fastovsky et aI., 1987;Clark et aI., 1991 ; Clark and Hernandez, 1994) . Close to onehundred sphenodontian specimens have been collected fromthis assemblage, making this unquestionably the most abundantgroup in the fauna. Much more material remains to be prepared.

The presence of sphenodontians in the Huizachal Canyonfauna was reported in 1987 (Fastovsky et aI., 1987 ; see alsoFraser, 1988, Clark et aI., 1991), but, no detailed descriptionhas yet been published. Reynoso (1992) considered the sphen­odontian collection to include at least three different taxa ofsphenodontians. The best known and most abundant taxon isdescribed below. Descriptions of two other taxa will be pub­lished elsewhere.

Huizachal Canyon is located 15 km south west of CiudadVictoria, in the state of Tamaulipas, Mexico (Fig. I) . Thesphenodontian remains occur at different localities in the low­er part of the La Boca Formation red beds. The fossils arevery small in size, green, and soft. Though some are crystal­lized or fragmented , most of them are very well preserved.They are enclosed in a very hard, silicified, hematitic mud­stone, and preparation is a slow mechanical procedure usingneedles .

Sphenodontians were a highly diverse group in the Late Tri­assic to Late Jurassic of Europe and China. Almost 30 differentspecies have been described to date (Huene, 1952 ; Cocude­Michel, 1963 ; Fabre, 1973, 1974; Evans, 1980 ; Fraser, 1982,1986 , 1988; Carroll, 1985; Fraser and Benton, 1989 ; Whiteside,1986 ; Wu, 1994) . The reported diversity of this group in NorthAmerica, however, is much lower, and fewer species have beendescribed: an unnamed genus from the Late Triassic of Con-

* Current address : Redpath Museum, McGill University, 859 Sher­brooke 51. West Montreal, Quebec, Canada H3A 2K6.

necticut (Sues and Baird, 1993); the Early Jurassic Clevosaurusbairdi (Sue s et aI., 1994); the Late Jurassic Opisthias rarus(Gilmore, 1909), Theretairus antiquus (Simpson, 1926), and Ei­lenodon robustus (Rasmussen and Callison, 1981); and the Ear­ly Cretaceous Toxolophosaurus cloudi (Throckmorton et aI.,1981). Realizing GUnther's (1867) prediction, many fossil spe­cies have proved to be closely related to Sphenodon, so theworld wide abundance of the Rhynchocephalia (sensu Gauthieret aI., 1988) makes this taxon very well known. Today onlytwo species of the genus Sphenodon survive (Daugherty et aI.,1990) .

Abbreviations-IGM: Instituto de Geologia, Ciudad Univ­ersitaria, Mexico; KU , Kansas University; RM Redpath Mu­seum, McGill University.

SYSTEMATIC PALEONTOLOGY

LEPIDOSAURIA Dumeril et Bibron, 1839SPHENODONTIA Williston, 1925SPHENODONTIDAE Cope, 1869

SPHENODONTINAE Nopcsa, 1928

Diagnosis-Sphenodontians bearing caniniform dentitionposterior to an edentulous ridge at the anterior end of the den­tary. Sphenodon, Theretairus (Simpson, 1926), and an unnamedsphenodontian from Kirtlington, Oxfordshire, England (Evans,1992) are included. Here the name Sphenodontinae is restrictedto Sphenodon and its closest relatives. Clevosaurus and Opis­thias (see Fraser, 1986) are excluded.

Genus CYNOSPHENODON. gen. nov.

Type Species-C. huizachalensisEtymology-From the Greek cyno (=dog; cynodon = fang),

and Sphenodon. In reference to its anterior caniniform tooth andto its similar appearance (and close relationship) to Sphenodon.

Diagnosis-As for the type species.

210

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REYNOSO-SPHENODON-LIKE SPHENODONTIAN 211

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101

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FIGURE 1. A, Locatio n of Hu izachal Canyon, and B, stratigra ph ic rela tions hips of La Boca Forma tion outcrops, showing the level wheresphe nodo ntian fossi ls were co llected.

Species C YN OSPHEN ODON HUIZACHALENSIS. sp. nov.Fig. 2

Holotype-Cat. No. 10M 665 2. Dental port ion of the rightdent ary with the co mp lete tooth row.

Hypodigm-IOM 6653. poster ior right dentary; 10M 6654,anterior portion of a right maxill a; 10M 6655, midd le portio nof a right maxill a; 10M 6656. anter ior part of a right dentary ;10M 6657, anterio r port ion of a rig ht dentary; 10M 6658 sym­physial region of a right dentary; 10M 6659; anterior part of aleft dentary; 10M 6660. posterior part of a left dentary withcoronoid process.

Referred Material-I 6 incomplete lower j aws.Distribution-Lower part of La Boca Formation ; Hui zachal

Canyon , Municipio de Ciudad Victori a, Tamaulipas, Mex ico.Age-Middle Jurassic (Fastov sky et al., 1995) .Etymology-From huizachal, (= place of mesqu ites) na­

huat l. In reference to the name of the canyon where the taxonwas found.

Diagnosis-The lower jaw is simila r to that of Sphenodonbut much smalle r (Fig. 2). Un ique characters are : a sma llgroove dorsal but convergent to the Meckeli an Canal on theanterom ed ial surface of the dentar y; more posterior additio nalteeth become pro gressively sma ller than the middle ones andabo ut the same size as the anterior teeth ; a sma ll denticle in­tercalated within the posterior part of the chee k tooth series ;and a deep posteromedial wear face t on the ca niniform too thprob ably made by a large vomerine tooth or by the first toothof an anteri orly enlarged palat al tooth series.

DESCRIPTION

Dentary-Th e dentary is smaller and more slender than thatof Sphenodo n. On the basis of the holotype and IGM 6660 thelength of the dentary is es timated to have been 23 mm . Nospeci me n in the hypod igm is larger than the holo type. Me asuredbeneath the coronoid process the depth of the dentary fro m theventra l edge of the jaw to the base of the teeth ranges from 3.1mm to 1.1 mm in young indivi dua ls. In the holotype the lengthof the dental ma rgin anterior to the coronoid process is 14.9mm (tw o-th irds of the total length of the dentary). The depthof the jaw attai ns its maximum below the tenth tooth and isapproximate ly one- fifth of the total dentary length. Th e heightof the dentar y decreases anter iorly, reach ing its minimum sizej ust before the symphysial region (before the canin iform tooth ).At this po int it is almost half its maximum height (Fig. 3) .

Dor sally , the coronoid process of the dentary is about one­six th of the dentary length. The process is well developed andanteriorly is almost perpendicul ar to the dental series . In theposterior marg in an invag inatio n for the mandibular foramen ispre sent. Medi ally , the angular, articular, coro noid, pre articul ar,sura ngular, and dentary facets are present (Fig. 4). Apparentl ythe suture between both dentaries was not ossified as in othersphenodo ntians, so a ligamentou s connec tio n was probablypresent. Th e dorsal edge of the dentary is straight, but , posteri orto the corono id pro cess, it run s slightly oblique in relation tothe dental edge . The posteri or end of the dentar y tapers pos­teri orly as in Sphenodon . The artic ular, prearticular, and sur­angular are unknown.

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212 JOURNAL OF VERTEBRATE PALEONTOLOGY, VOL. 16, NO .2, 1996

me .f

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p.pr.d

,~6'/'

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~~""'---7--~=~~~-------pa B

FIGURE 2. Cynosphenodon huizachalensis, gen . et sp . nov . Reconstruction of the lower jaw and maxilla. A, lateral view; 8, medial view . ad.t,additional teeth ; ad.fos, adductor fossa; an, angular; c.t, caniniform tooth; d , dentary bone; dtc, denticle; e.n, external nares; e.r, edentulous ridge;c.pr, coronoid process; me.f, mental foramina; h.t , hatchling teeth ; m , maxilla; m.c, Meckelian Canal ; 0, orbit; pa, prearticular; pm, premaxilla;p.pr.d, posterior process of dcntary ; sa , surangular; s.m.g, secondary medial groove; sy.f, symphysial facet. Premaxilla based upon materialtentatively referred to this species. Scale bar equals 5 mm .

In lateral view, the ventral border of the dentary is nearlystraight (Fig. 3), but beneath the coronoid process it curvesgently upward and extends straight to the posterior end of thedentary. Close to the mandibular symphysis, the ventral edgeof the dentary projects strongly downward. The dorsal marginof the jaw projects further forward than the ventral margin.

In dorsal view, the dentary is straight except where its an­terior part turns medially at the symphysis (Fig. 3A). The oc­clusal margin bears an edentulous ridge in the symphysial re­gion followed by the caniniform tooth. Between the caniniformand the cheek tooth series another edentulous ridge is aboutone-tenth the dental margin length. Immediately before the pos­terior edentulous ridge begins, the dorsal edge of the dentarybecomes very thin . Both edentulous ridges functioned as tritu­ration surfaces and are heavily worn. In the smaller specimens,hatchling dentitions are present in these areas. Posterior to theedentulous ridge, the tooth row extends posteriorly to about thecoronoid process.

The external surface of the dentary is smooth (Fig. 3B) andfour to five mental foramina aligned close to the anterior endare present on 10M 6657 and 10M 6658, but are not evidentin the holotype. A slight ridge extends from below the posteriorpart of the caniniform tooth to slightly forward of the coronoidprocess . The surface above that flange is well polished by tooth­on-bone wear; no scorings or bite marks are evident.

On the medial surface, below the dental series, a slight ridgeand a polished surface similar to, but less conspicuous than, thelateral surface is observed. Posterior to them, the jaw bearsthree small foramina. Two are placed one below the other andthe third is slightly forward. All the sutural facets are clearlypreserved. The symphysial facet is suboval and bears a smallgroove (Fig. 3C). The angular facet is extensive and indicates

that the angular bone covered at least one-third of the dentarysurface below the tooth row. It approaches the ventral edge butis not exposed on the lateral side of the jaw (Fig. 4B). Thecoronoid and prearticular facets are above and posterior to theMeckelian Canal, respectively; and there is a facet for the sur­angular at the posterior end of the coronoid process. The artic­ular facet is evident at the tip of the posterior process of thedentary.

The Meckelian Canal , as in all sphenodontians, is open me­dially and extends along the midline of the jaw. In Cynosphen­odon, however, the anterior end of the canal angles downwardsrunning close to the ventral edge of the dentary. Anteriorly, itreaches the symphysis and is continuous with a small grooveover the symphysial facet. Below the fifth tooth, the canal dou­bles in diameter due to the fusion of a second smaller grooveplaced dorsally and convergent to the anterior part of the Meek­elian Canal. Posteriorly, the canal becomes narrow again untilit reaches the adductor fossa. The transition between the Meek­elian Canal and the adductor fossa was presumably covered bythe coronoid and articular, as in Sphenodon.

Dentition-The teeth are acrodont and subconical. One suc­cessional tooth and an additional tooth series (Robinson, 1975)can be identified. The additional teeth are spaced similar tothose of Sphenodon, but, the spacing is greater in the anteriorteeth. The holotype is the only specimen with a complete toothrow, bearing 14 teeth; however, the maximum possible numberof additional teeth remains unknown. In the holotype the firstand second teeth are almost worn away. The third and fourthteeth are broken (Fig. 3) .

There are two different kinds of teeth in the additional series.The anterior ones are narrower transversely and triangular inlateral view (Fig. 5A). Their tips are close to the transverse axis

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REYNOSO-SPHENODON-LIKE SPHENODONTIAN 213

sy

A

B

c

s.m.gm.c

d.w.s C.t

an.f

FIGURE 3. Cynosphenodon huizachalensis, gen. et sp. nov. Holotype IGM 6652. dental portion of dentary. A. occlusal view. B. lateral view.and C, medial view. Abbreviations as in Figure 2 plus : a.ad.t, anterior additional teeth; an.f, angular facet ; p.ad.t, posterior additional teeth; sy,symphysis. Scale bar equals 2 mm .

of the tooth. In dorsal view, the base is rectangular, and noflanges are evident. The posterior teeth are more robust (Fig .5B) . In lateral view, they are also triangular, but their tips arelocated well forward of the transverse axis of the tooth. In oc­clusal view, their bases are rectangular, with each side slightlyconvex. The basal edge of the anterior face of each tooth, isshorter than the posterior, and the curvature of the posteriorface is stronger than the anterior. The medial face is morerounded than that of the labial, which is worn flat.

Tooth height increases from the first to the tenth tooth of theseries. The remaining teeth decrease gradually in height pos­teriorly. In the posterior part of the series there is a small den­ticle which is considerably smaller than any of the adjacentteeth. The presence of this tooth interrupts the gradual patternof increase in size among the preceding teeth. Although in theholotype the highest tooth of the series is the second of theposterior additional kind, and in 10M 6653 it is the fifth (the

size of the second and third is unknown), the denticle occursat the fourth position in both specimens (Figs. 3, 4). This den­ticle is also observed in another referred specimen. The peculiargrowth pattern of the additional dental series and the presenceof a denticle has not been reported previously in any spheno­dontian.

Although most of the anterior additional teeth are heavilyworn and their flanges have disappeared, there is a clear mor­phological gap between the anterior and posterior teeth. There­fore, the degree of wear is not distributed uniformly along theseries. The flanges of the posterior additional teeth are welldeveloped but not expanded forward as in Opisthias. Wear fac­ets are present on both sides. The last tooth of the row has verywell developed flanges and shows no signs of wear (Fig . 5C).This suggests that this last tooth was most recently added.

A single caniniform tooth is present anteriorly on the dentary.This tooth is present in all six specimens that preserve the syrn-

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Bad.fos

FIGURE 4. Cynosphenodon huizachalensis, gen. et sp . nov. Posterior part of the dentary in medial view . A, IGM 6653, B, IGM 6660. Abbre­viations as in Figure 2 plus: an.f, angular facet ; ar.f', articular facet ; c.f', coronoid facet ; ma.f', mandibular foramen ; pa.f' , prearticular facet ; sa.f,surangular facet. Scale bar equals 2 mm .

physis. The caniniform tooth is as tall as the tallest additionaltooth. In lateral view, it is triangular and has a truncated tip dueto wear (Fig. 5D). The lateral face of the tooth is flatter thanthe medial face . A deep wear facet is present on each side . Inlateral view, a vertical groove runs from the base to the tip ofthe tooth, where it fades away. A shallower wear facet is alsoobserved on the medial face , but it is placed posteriorly anddoes not extend to the base of the tooth .

Maxilla-Two gracile maxillae with dentition very similarto that of the dentary are assigned to this taxon (Fig. 6A , B) .These maxillae are quite distinct from other sphenodontianmaxillae in the Huizachal assemblage. Both elements are in­complete, so their overall shape cannot be determined. Only thelateral side is exposed in both elements, and the medial surfaceremains embedded in matrix .

The maxilla is high, but the extent of the anterior and sub­orbital processes are unknown. A small ridge extends parallelto the dental series, with several foramina above it. On thedorsal process of the maxilla, a small portion of the narial bor­der is preserved anteriorly, and on the posterior end of the dor­sal process are remains of the prefrontal facet.

The maxillary dentition is gracile, with little spacing betweenteeth. IGM 6654 represents a mature individual with all addi­tional teeth completely worn away except for two anterior suc­cessional teeth . The taller tooth has a caniniform shape. It istriangular with the tip displaced anterior to the mid-transverseaxis. It has a lateral crest slightly anterior to the transverse axis.In the smaller specimen, IGM 6655, some hatchling teeth arepresent posterior to the caniniform tooth. In IGM 6654, how­ever, the jaw margin forms an edentulous trituration surface.

DISCUSSION

The upper and lower jaw elements of Cynosphenodon hui­zachalensis are similar to those of Sphenodon. It also shares anumber of characters with Opisthias and Theretairus. Althoughthe presence of a propalinal jaw movement is also shared withEilenodon and Toxolophosaurus, these taxa differ considerablyin their laterally expanded teeth and very deep jaw. Cynos­phenodon is not directly comparable with European and Asiaticsphenodontians since it lacks a precision bite, has a slenderlower jaw and an anterior caniniform tooth. Therefore the dis­cussion and comparisons primarily focus on Opisthias, There­tairus, and Sphenodon : genera with slender jaws and a propal­inal jaw action.

The dentition of Cynosphenodon is similar to that of Sphen­odon as described by Robinson (1976) and Throckmorton et al.(1981) . Acrodont triangular teeth with a rounded shape in cross­section are present in both species and shared with most sphen­odontians. The short anteroposterior length of the teeth in Cy­nosphenodon and Sphenodon, however, is not known in anyother sphenodontian. In lateral view, the triangular shape ismore of a straight triangle than an equilateral. In other sphen­odontians (e .g., Clevosaurus, Planocephalosaurus. Kallimodon,and Sigmala) the posterior teeth are obtusely triangular and innone of them does the tip of each tooth extend anteriorly be­yond its medial transverse axis. In unworn teeth of Opisthias,the anteriorly projected tip is stronger than in Sphenodon; how­ever, it does not project as far as in Cynosphenodon. In Cynos­phenodon the anterior face of almost every tooth is orientedperpendicular to the dorsal jaw margin as seen in lateral view.

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A

B

c

D

I

w.f

_ _------ ...i- /

.:,...•,.."" .. .. .•, .. . . ...' - " ..,..•... .

II

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I.w.f

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ap

FIGURE 5. Dental wear variation as shown in the holotype of Cynosphenodon huizachalensis. A, eighth additional tooth, B, tenth additionaltooth , C , last additional tooth, D, caniniform tooth. I , medial view, II , occlusal view, III, lateral view . ap, tooth apex ; fl. tooth flange; I, lateral;me, medial; w.f, wear facet.

In Sphenodon and in the anterior teeth of Opisthias this marginis somewhat oblique, but not as much as in Sigmala, Plano­cephalosaurus, Clevosaurus, and other sphenodontians (Fraser,1982, 1986, 1988) .

A distinctive characteristic of Cynosphenodon is the smallsize of recently added cheek teeth. Typically in most spheno­dontians, new teeth added at the posterior end of the tooth rowincrease in size caudad. This character is extreme in Planoce­phalosaurus where the last tooth is considerably larger than theanterior ones (Fraser, 1982). In Diphydontosaurus, Clevosaurus,Kallimodon, Brachyrhinodon, Homoeosaurus and Toxolopho­saurus (Cocude-Michel , 1963; Throckmorton et al., 1981; White­side , 1986 ; Fraser, 1988; Fraser and Benton, 1989), the lasttooth of the series is the largest. In Sphenodon and Sigmala,the last teeth are almost equal in size to the middle ones. InSphenodon, these teeth can be smaller, but they are never asreduced as the posterior teeth of Cynosphenodon. In Palaeo­pleurosaurus and in some Clevosaurus species, the posterior-

most tooth is smaller than the penultimate tooth. However thereis no progressive reduction in size along the series as in Cy­nosphenodon (Fig. 4A). The presence of the small denticleamong the posterior teeth is also an unique character of Cy­nosphenodon.

Throckmorton et al. (1981) stated that the spacing betweenteeth is greater in Sphenodon than in Opisthias. The tooth spac­ing of Cynosphenodon, is similar to that of Sphenodon. How­ever, the pattern of distribution of the teeth along the dentalseries resembles that of Theretairus. In Cynosphenodon andTheretairus, the anterior teeth are widely separated while theposterior teeth are closer to each other. In other sphenodontians,the teeth are equally distributed along the dental series.

The principal character shared by Sphenodon and Cynos­phenodon is the presence of a single caniniform tooth at theanterior end of the jaw. This tooth was reported in Opisthiasrarus (as described and figured by Gilmore, 1909 and Simpson,1926) and in the Kirtlington sphenodontian (Evans, 1992) . In

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m.i.pr

A

rn.d.pr

C.t

pm

pm.f

m.f

FIGURE 6. Cynosphenodon huizachalensis, gen . et sp. nov. A, IGM 6654. lateral view of a right maxilla of an old specimen showing alladditional teeth completely worn . The two anterior teeth seem to be of the successional kind . B. IGM 6655. young right maxilla with apparenthatchling dentition posterior to the caniniform tooth. C, IGM 6661. anterior view of the right premaxilla. Abbreviations as in Figure 2 plus : m.f,maxillary foramina; m.d.pr, maxillary dorsal process; m.i.pr, maxillary infraorbital process; pm.f, premaxillary facet; pm.t, premaxillary teeth.Scale bars equal I mm.

Opisthias, however, this tooth is not present in any of the para­types, and its presence in the holotype is uncertain (Evans andFraser, 1992; pers. obs .). If the caniniform tooth is present inOpisthias, it would differ from that of Sphenodon in its posi­tion . In Opisthias, it would be located in the anteriormost partof the jaw, and in Cynosphenodon, Sphenodon, and the Kir­t1ington sphenodontian (Evans, 1992), the caniniform is pre­ceded by an edentulous ridge, and displaced further back in thejaw. According to Robinson (1976), in Sphenodon, hatchlingteeth are present on the anterior edentulous ridge early in on­togeny. This same condition is observed in the juveniles ofCynosphenodon.

A comparison of the distribution of the dental wear facets inthe caniniform tooth of Sphenodon and Cynosphenodon showsthem to be quite different. As seen in a very mature specimenof Sphenodon (RM 1135) , no lateral wear facets are evident,but a major facet is present along the anterior face of the tooth(Fig . 7) . This facet was made by the occlusion between theanterior part of the tooth and the posterior face of the lateralpremaxillary tooth. Such a facet, however, is not present in ayoung specimen of Sphenodon (KU 98454). As described be­fore , the wear facets of the caniniform tooth of Cynosphenodonare on the lateral and medial surfaces. Though the lateral facetcan be explained by inferring occlusion with an anterior max ­illary tooth, the medial facet is more difficult to explain. In allsphenodontians, the palatine tooth row that runs parallel or sub­parallel to the maxillary tooth series is located posterior to theposition of the caniniform tooth of the dentary. In Sphenodon,the area anterior to the palatine tooth series, just above the

caniniform tooth position, is occupied by the internal nares . Thetwo possible explanations for the origin of the caniniform me­dial facet in Cynosphenodon is either the presence of huge vo­merine teeth parallel to those of the premaxilla or the extensionof the palatine tooth series further anteriorly. Although, vomer­ine teeth are present in several sphenodontians such as Plano­cephalosaurus, Diphydontosaurus, Clevosaurus, and Brachyr­hinodon (Fraser, 1982, 1988 ; Whiteside, 1986; Fraser and Ben­ton, 1989), these teeth are not big enough to produce such alarge groove; on the other hand, no known sphenodontians havetheir palatine tooth series extending into the region of the in­ternal nares. Consequently it is postulated that Cynosphenodon,had a rather different arrangement of the anterior palatal region.

While the teeth of Cynosphenodon resemble those of Sphen­odon, the overall shape of the dentary is closer to that of Op­isthias (Gilmore, 1909) . In Cynosphenodon and Opisthias, theventral edge of the dentary is straight in lateral view, exceptfor the symphysial region. In contrast, the ventral edge of thedentary in Sphenodon and all other sphenodontians describes agentle curve. The straight shape of the ventral edge of the den­tary in Cynosphenodon makes the anteroventral expansion ofthe mandibular symphysis more exaggerated. This condition isshared by Opisthias, Cynosphenodon, and probably Sigmala,but not Sphenodon.

The Meckelian Canal of all sphenodontians usually extendsanteroposteriorly along the midline of the medial side of thedentary. The canal of Cynosphenodon is quite distinct in twoways. It angles downwards reaching the ventral edge of thedentary, and contacts a small secondary dorsal groove at the

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REYNOSO-SPHENODON-LIKE SPHENODONTIAN 217

Additional sphenodontian elements collected in the La BocaFormation are only tentatively assigned to Cynosphenodon hui­zachalensis because they were never found directly associatedwith lower jaw material. Reynoso (1992) referred them only toSphenodontia, but their similar relative size, locality distribu­tion, and relative abundance suggests that they belong to thesame taxon.

Material-IGM 6661, distal part of the right premaxilla;IGM 6662 mid-caudal vertebra; IGM 6663, lateral fragment ofa mid-caudal vertebra; IGM 6664, anterior caudal vertebra;IGM 6665, proximal portion of a mid-caudal vertebra.

Premaxilla-There is only one poorly preserved anteriorportion of a right premaxilla (Fig. 6C) . The medial premaxillaryfacet indicates that both premaxillae remained separate. Twolarge teeth are present, the medial tooth being one-third talIerthan the lateral. Both teeth are triangular and are partly fusedto form a chisel-like structure.

The premaxilla shows the derived condition within spheno­dontians. It has only two teeth as in adult premaxillae of Sphen­odon , and it forms a chisel-like structure. In IGM 6661 and inSphenodon, the lateral tooth is larger than the medial. This con­dition is unknown in any other sphenodontian. Clevosaurus hasthree teeth, two of equal size or none at all; Polysphenodon andBrachyrhinodon bear two equal-sized teeth; and Palaeopleu­rosaurus has a single shovel-like tooth. The premaxilla is notknown in Opisthias and eilenodontines, so no further compar­ison can be made.

Vertebrae-Some isolated caudal vertebrae were found (Fig .8). They are amphicoelous with the notochord ossified, slender,and with the centrum somewhat elongated. The diapophysesand zygapophyses are welI developed, but the zygosphene/zyg­antrum accessory articulation is not preserved in any specimen.

Ventrally, the centra are concave with a ridge extendingalong the ventral surface. DorsalIy, the neural spines are wideand extend almost the entire length of the neural arch. Theirshape and height are not known. There is no evidence of a

Possible Additional Material ofCynosphenodon huizachalensis

and backwards within the palato-maxillary groove during mas­tication.

In Cynosphenodon, the dental wear surfaces and the impli­cations for tooth size are comparable with Sphenodon, Opis­thias, and Toxolophosaurus (Throckmorton et al., 1981) . In allthese genera, the anterior teeth are smalIer than the posteriorbecause a) they were added earlier in ontogeny when the animalwas smalIer; and b) because of wear. In Cynosphenodon theanterior teeth are extremely worn, hence the transverse diameterdecreases as the lateral crests are eroded. The spacing betweenteeth is also directly affected by wear. Once the lateral crestsdisappear, the tooth crown begins to be used like a chewingsurface, and eventualIy it is worn away. The tip of each toothis also eroded so the teeth acquire an equilateral shape. Theposterior teeth, in contrast, have only superficial wear marksand the flanges are stilI visible, with the lateral surfaces moreeroded than those on the medial side. The lack of wear facetson the posteriormost tooth indicates the addition of teeth at theposterior end of the dental edge of the jaw as in all spheno­dontians.

In Cynosphenodon, the anteriormost teeth are completelyworn away, indicating that both anterior edentulous dorsal mar­gins were used as chewing surfaces. This pattern is also presentin Sphenodon, though in this genus, the hatchling dentition iscommonly preserved in mature animals. In the holotype of C.huizachalensis, the early hatchling teeth were completely wornaway, suggesting that it was a mature adult.

e.r

C.t

w.f

B

cFIGURE 7. Symphysial region of an old specimen of Sphenodon (RM1135) . A, occlusal view, B, medial view. The wear facet of the cani­niform tooth is on the anterior face of the tooth and no lateral wearfacets are evident. C, KU 98454, a younger specimen lacking wearfacets in medial view. Abbreviations as in previous figures .

point of flexure . The presence of a secondary dorsal groove hasnever been reported in any other sphenodontian. However theanterior part of the Meckelian Canal extending close to theventral edge of the bone was also reported in the Kirtlingtonsphenodontian (Evans, 1992). This character, plus the presenceof a caniniform tooth, may be indicative of a close relationshipbetween Cynosphenodon and the Kirtlington sphenodontian.

Chewing Apparatus and Dental Variation-One interest­ing feature that is shared by Cynosphenodon, Sphenodon, Op­isthias, and eilenodontines, is the very similar disposition andshape of the wear facets on the additional teeth and dentary. InSphenodon, these wear facets are produced by a distinctive pro­palinal jaw action, contrasting with the precision bite markspresent in Clevosaurus, Brachyrhinodon, Palaeopleurosaurus,and most other sphenodontians. Cynosphenodon exhibitssmoothly polished lateral and medial surfaces of the jaw andteeth. Medial wear facets on the teeth and dentary suggest thepresence of a palatal tooth series extending paralIel to the max­illary series, together forming a groove into which the lowerjaw fits. A masticatory system similar to that described by Rob­inson (1976) for Sphenodon is therefore suggested. As in thisspecies, it seems that Cynosphenodon moved its jaw forward

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2 18 JOURNAL OF VERTEBRATE PALEONTOLOGY, VOL. 16, NO.2, 1996

FIG URE 8. M idd le an terior ca udal vertebrae of? Cyn ospheno do n hui­zachalens is. A, dorsal view , B, poste rior view, C , latera l view, D, an­terior view , and E, ventral view. a. zy anterior zygapophysis; ce , cen­trum; ne.arc , neu ra l arch; ne.c, neural ca nal; ne.sp, neural spine; n.c ,notochordal canal ; p. zy posterior zygapop hysis ; tr.p , transverse process .Scale bar eq uals 1 mm .

SYSTE MAT IC AN D TAXO NOMIC REL ATIONSHIPS

Th e verte brae do not show any spec ial fea tures distingu ishingthem fro m any othe r sp henodo ntian vertebrae. Th ey are simi larto the anterio r ca uda l vertebrae of Sp henodon and Gephyrosau­rus, although their neural arc hes not as wide dorsovent rall y asfigured by Evans ( 1981 :fig. 9c ) for Gep hyrosaurus, possiblybecause they co rrespond to a more posterior vertebra. The ver­tebrae referred to Cynosphenodon, however, are more graci lethan those anterior to the autoto mo us vertebrae of Sp henodon.

Th e phylogeneti c rel ation sh ips of the sphe nodo ntians havebee n studied by Whiteside ( 1986 ), Evans ( 1988) , Ga uthier etal. ( 1988), Fraser ( 1988), Fraser and Benton ( 1989) , Wu ( 199 1,1994) and Sues et al. (1994) . However, taxa closely re lated toSp heno don have often been left out of such studies becausemuch of the mat eri al is very inco mplete. Cynosp heno don is alsopoorly known, but several characters ca n be ob served or de­duced fro m the ava ilable eleme nts , and these characters permita hypoth esis of the interre lationships of this taxon to be pre­se nted.

PAUP 3. 1.1 (Swofford, 1992) Bran ch and Bound search wasmade using Wu 's (1994) data matri x, mostly coded as in Sueset al. (1994) . So me change s were made (Table I and Appendix)Character I I and 12, and characters 22 , 23 and 24 were mergedand recoded as suggested by Sues et al ( 1994) . Character 13wa s coded aga in according to the index supratemporal fenestralength/ skull lengt h. Although Wu's index is easier to est imate,it does not reflect the relative size of the tem poral region , asproposed by Fraser and Ben ton ( 1989 :character 18). The indexsupratemporal fenes tra/orbit does not properly describe anyskull character. Th e derived condition of character 29, "pres­ence of ex tens ive posterolingual flanges on so me maxillaryteeth," de pends upon the presence of flanges (character 26) , sothat there are two di ffe ren t derived states of the sa me character[charac ter 34 here]. Th e charac ter is newly coded as suggestedby Ga uthier et al. ( 1988 ). Th e cha racter state " all teeth acro­dont " (Benton, 1985 ; Eva ns , 1988) is a clear sy napo mo rphydeterminabl e in most sphenodo ntians . It was incl uded as anex tra state of the cha rac ter " de nta l implantation" [character26 ]; the state " lateral tooth attac hme nt reduced (Gauthier et al.,1988) showing some degree of acrodo nty" (Fraser and Bent on ,1989) is also incl uded.

Several character s used by Benton ( 1985), Fraser ( 1988),Ga uthie r et al. (1988), and (or) Fraser and Bent on (1989) wererei ntroduced to the ana lysis. C haracters [20], [21 ], [22], [27 ],[29 ], [30], [31], [37 ], [40], [41] and [42] were ignore d by Wu

ne.c

p.zy

a.zy

ne.sp

A

c

suture be tween the neural arc h and the ce ntrum nor of subcen­tral fora mina .

Th e most anterior verte bra is co mpressed dorsoventrall y andthe tran sverse processes are present. In the mid-anterior caudalverte brae, the transverse processes are well devel oped and per­pendicul ar to the longitudinal axis . Th eir bases are broad and,a lthough their total length is unknown, they see m to be qu itesho rt. Th e transverse processes gradually becom e sho rte r inmore poster ior vertebrae. No ne of the vertebrae show a septumfor autotomy, perh aps becau se no posterior ca uda l ve rtebrae arerepresented .

E

TABLE I. Character state ma trix used to estab lish phy logenetic re lationshi ps o f rhy nchocepha lia ns. Abbreviations: 0 = primitive ; I , 2, and 3 =derived states; ? = unknown; - = not applicable ; V = variable (0 & I ); X = variable ( I & 2) .

1 2 3 41 2 3 4 56 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 56 7 8 9 0 1 2

? ? ?

? ? ?

21 0

? 1 ?1 11 1

o 0 0 0 02 0 01 0000 ? ?o 01 1 021 01 ? ? ? 11 ? ? ? 01 1 1 1 02 1 1 2 12 1 1 2 0

1 2 ?

1 1 2

o 0o 0o 01 01 ?1 ?1 11 ?1 ??

000

? ? ? 1 1 ? ? ? 11 1 ? ? ? 1 ? ? ?? 1 ? ? ? 11 1 1

o ? ?1 2 11 2 1

1 ?1 1o 1o 1

21121 12 1 1211

1 11 11 11 11 1? 11 11 ?1 11 1 1 0 1 1 100 1 1 1 1 1 0 1 1 1

YounginaSqu amataGephyrosaurusDiphydontosaurusP/anoceph a/osaurusPalaeopleurosaurusPolysphenodonBrachyrhinodonClevos aurusHomoeosau rusKallimodonSaph eosaurusEilenodo nToxo lophosaurusCyno sp henodonSp henodon

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REYNOSO-SPHENODON-UKE SPHENODONTIAN 219

lJ) lJ)

lJ) i: i:i: :::. :::. lJ)

('(I ('(I c i:~

lJ) :::. lJ) lJ) c: .g 2 lJ) :::.2 ('(I Q e .glJ) lJ) 2 ('(I:::. s ('(I :::. a a 2 :::. c :::. lJ) a('(I .c:

~ c: .S ('(I a ('(I c: a c: cQ. lJ) c g. Ql -e :::.

~ "tl .c: Ql .g::;, e .g §- ('(I lJ) .g~

.c:.c: a a~ E u g. E lJ) Ql

.§ Ql lil- aE a Ql ~ a a c

.S' c: ~ ~u E .c: c: ~ a Ql

2- .9-~ ~ Ql

~lij- Ql s .c:::;, e & <3 ~ in ~ ~0 (!) o CO CJ)

UG

clevosaurs

Sphenodontidae

A

Sphenodontia

Rhynchocephalia

HGVRE 9. Most parsimonious tree obtained by the Branch andBound search function of PAVP. All characters are unordered and Op­isthias was deleted. Tree length = 79, Consistency Index = 0.722, andRetention Index = 0.798 . Ambiguous characters are in brackets. Rhyn­chocephalia: 8, 14,23, [35]. Sphenodontia: 6,26,27, [32] . Node A: 21,24, 30, 39, [32(2), 40] . Sphenodontidae : 2, 8(0) , 26(2), 28, 33, 36 [22 ,34(2)]. Node B: 5, 18, 37, [15(0)]. Clevosaurs (sensu Wu 1994) : 1(2),3,4,7, [38] . Node C: 34. Node 0: 13, 16,41(2), [38(2)]. Node E: 10,II, [12]. Node F: I. Node G: 25, [19, 34]. Sphenodontinae: 31. NodeH = Eilenodontinae: 20, 29. Gephyrosaurus: [9] . Diphydontosaurus: I,[22]. Planocephalosaurus: [9, 15(0), 34, 38] . Palaeopleurosaurus: 10,II, 12(2) , 14(0), 17, [9,38(2)]. Polysphenodon: 2(0) ,42. Homoeosau­rus: 2(0) ,25,42. Sphenodon: [3,7, 17,40(2),41(1)].

(1994). In character [40] the state " prominent posterior processof ischium" (Evans 1988: character K14), was also reconsid­ered. Although some of these characters may be dubious (e.g.,limb proportions), this study is attempting to incorporate allavailable evidence (Kluge 1989). Characters 21, 22 , and 23 ofFraser and Benton (1989), character 20 of Wu (1994) and char­acters 1,4, 9, 33, and 31 of Sues et al (1994) are autapomorphicand so were excluded.

Several outgroup states were miscoded by Wu (1994: char­acters 30, 31, and 32: tables I and 2). Squamates do not havea palatine tooth row, and they may have one pterygoid toothrow or none at all. These characters were coded as 'not appli­cable' in squamates, adding the state " none or one pterygoidtooth rows " (state 2) splitting and modifying Wu's state (1) ofthe character " number of pterygoid tooth rows" [38 here]. Topolarize conflicting and variable states in squamates, Younginawas included as a second outgroup. The inclusion of Young inareversed the polarity of character [IS] .

The resulting single most parsimonious tree (Fig. 9) has thesame basic topology as that published by Wu (1994:fig. 21)showing a single minor change in the sister-group relationshipsof " clevosaurs" (sensu Wu, 1994). According to this hypothesis

the sister relationship of Polysphenodon and Brachyrhinodon issupported by two characters: antorbital region/skull length lessthan one-fourth (not shared with any other taxa) and maxillaryteeth with a small medial flange. In Wu's hypothesis, the sister­grouping of Clevosaurus and Brachyrhinodon is supported bya single character: inferior temporal fenestra/skull length morethan one-fourth. If the sister-grouping of these taxa is forced,the tree length increases by one step.

According to the most parsimonious hypothesis, Cynosphen­odon is the sister group of Sphenodon. It shares several syna­pomorphies with the Rhynchocephalia as defined by Gauthieret al. (1988). The presence of the posterior process of the den ­tary projecting beyond the coronoid process is unique to rhyn­chocephalians and is also present in Cynosphenodon. The pre s­ence of tooth replacement by adding teeth at the back of thejaw, places this genus within the Sphenodontia as defined byBenton (1985). The presence of a high coronoid process, abroad mandibular symphysis, anterior hatchling teeth, and an­terolabial flanges on at least one dentary tooth, strongly sup­ports the placement of Cynosphenodon within a clade of sphen­odontians that excludes Diphydontosaurus (Node A, Fig. 9). Alarge mandibular foramen, fully acrodont dentition, well devel­oped wear facets, and three or less premaxillary teeth forminga chi sel-like structure are characters shared by Cynosphenodonand all the sphenodontians except Diph ydontosaurus and Plan ­ocephalosaurus. In summary, derived states of characters [21] ,[22]. [23], [24] , [26], [27] , [28], [30], [32] , [33] , and [39]strongly support that Cyno sphenodon is a crown member of theSphenodontia. However additional resolution is only providedby a few characters.

One of the most studied features in sphenodontian morphol­ogy is the form and function of the jaw (Farlow, 1975 ; Rob­inson, 1976 ; Throckmorton et al., 1981; Rasmussen and Calli­son , 1981; Gorniak et al., 1982 ; Whiteside, 1986) . Of specialinterest is the propalinal jaw action present in several spheno­dontians. Propalinal jaw action is related to other modificationsof the mandible: antero-posterior enlargement of the mandibulararticular surface, and the presence of smooth wear facets on thedentary and teeth (Gauthier et al., 1988) . The presence of thesecharacters supports the monophyly of Node G (Fig . 9). Thesefeatures are shared by Toxolophosaurus, Eilenodon, and Sphen­odon. Smooth wear facets on the dentary and teeth are alsoundoubtedly present in Cynosphenodon. The reduction of theretroarticular process is also synapomorphic at this node. How­ever, the character is not known in Toxolophosaurus and Cy­nosphenodon. Toxolophosaurus and Eilenodon are sister-taxasharing the presence of mediolateral expansion of the additionalteeth and a deep jaw.

The incorporation of Cynosphenodon leads to the addition ofthe new character "presence of a caniniform tooth posterior tothe anterior edentulous ridge in adults", once autapomorphic ofSphenodon. The presence of a caniniform tooth was previouslysuggested by Gauthier et al. (1988) and Evans (1988) as a syn­apomorphy of Opisthias and Sphenodon. This tooth, however,is not clearly present in Opisthias (Evans and Fraser, 1992 ; pers .obs. ) and no clear relation of this taxon to Sphenodon is sup­ported. A caniniform tooth posterior to an edentulous marginis present in Sphenodon and Cynosphenodon, as well as inTheretairus (which has two caniniform teeth) and in an unna­med sphenodontian from England (Evans, 1992). This syna­pomorphy diagnoses the Sphenodontinae. The presence of can­iniform teeth is shared by four taxa, and although the numberof taxa that share a character is not an argument to support thequality of the character and the group it defines, the hypothesisof it being acquired separately is much less parsimonious. TheSphenodontinae here described is not the same as Gauthier etal.'s (1988) informal group " sphenodonts" (Opisthias + Sphen­odon). The possible lack of a caniniform tooth in Opisthias

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220 JOURNAL OF VERTEBRATE PALEONTOLOGY, VOL. 16, NO.2, 1996

restricts it from the clade as here defined. Further studies onOpisthias will clarify its phylogenetic status (Fraser and Evans1992).

CONCLUSIONS

The morphotype of Sphenodon was considered a highlyprimitive reptile by early workers, and it has been used as amodel to establish polarity in squamate phylogeny (e.g ., Esteset al ., 1988). As result of recent studies (Carroll, 1977; Benton,1985; Whiteside, 1986; Evans, 1988; Fraser and Benton, 1989),the view of Sphenodon as a primitive lepidosaur must be mod­ified . Within the rhynchocephalian lineage, Sphenodon is ahighly derived genus.

In spite of this new understanding, the morphology and ageof Cynosphenodon shows that derived features such as the pres­ence of a caniniform tooth, shared with Sphenodon, are notrecent among sphenodontians. This structure arose at least inthe Middle Jurassic. On the other hand, Robinson (1976) sug­gested that propalinal movements arose relatively late in sphen­odontian evolution. Throckmorton et al. (1981), however, sug­gested that such movements were present as early as the LateJurassic. The discovery of Cynosphenodon indicates that thisfunction was developed even earlier among sphenodontians.

ACKNOWLEDGMENTS

I especially would like to thank J. Clark who invited me toparticipate in the description of the Huizachal sphenodontians.I also want to thank M. Montellano, 1. Ferrusqufa, L. Espinosa,and O. Flores for reading the original Spanish version of thispaper, and to J. Clark, H. D. Sues and X. Wu for reading theEnglish version. I thank D. Fastovsky for introducing me to thegeology of the Huizachal Canyon and for providing me withaccess to unpublished information. I am indebted to R. Her­nandez and J. Clark who collected many of the fossils describedhere and to R. Carroll, for permitting me to use his copy ofPAUP. Finally I deeply appreciate the hospitality of Sr. F. Mo­reno and all the people of the Ejido EI Huizachal.

The research and field work was supported by the DireccionGeneral de Asuntos del Personal Acadernico of the UniversidadNacional Autonorna de Mexico and the National Science Foun­dation, Grants EAR 8917368, 81917389, 9218871, 9218971,and a Grant of the National Geographic Society.

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Received 13 July 1994; accepted 9 February 1995.

APPENDIX

Characters and character states used for the rhynchocephalian phy­logenetic analysis . Character numbers in parentheses correspond tothose of Wu (1994).

Skull

1 (11 , 12 modified). Antorbital region/skull length: one-third or more(0) ; between one-fourth and one-third (I); one fourth or less (2) .

2 (13 modified). Supratemporal fenestra/skull length: one-fourth or less(0); more than one-fourth (I).

3 (14). Lower temporal fene stra/skull length: one-fourth or less (0) ; morethan one-fourth (I).

4 (I). Premaxillary process of maxilla: elongate (0); reduced (I).5 (2). Posterior portion of maxilla: gradually tapering off or very narrow

(0); broad (I).6 (3) . Lacrimal: present (0) ; absent (1).7 (9) . Dorsal process jugal: broad and short (0); narrow and elongate (I).8 (4) . Frontals: separated (0) ; fused (I).9 (5) . Parietals: separated (0); fu sed (1).

10 (6). Parietal width between supratemporal passages: broader thaninterorbital width (0); narrower (I) .

II (7) . Parietal crest: absent (0); present (I).12 (8) . Posterior edge of parietal: greatly incurved (0) ; slightly incurved

(I); or convex (2).13 (10). Parietal foramen position: posterior to the anterior margin of

supratemporal fenestra (0); level or anterior (I).14 (17). Lower temporal bar: aligned with the maxillary tooth row (0);

bowed away beyond the limit of the abductor chamber (I).15 (18 polarity reversed) . Lower temporal bar: complete (0); incom­

plete (I).

Palate

16 (33). Palatine: tapers posteriorly (0) ; becomes relatively wide (I).17 (34). Central region of pterygoid between three rami: short (0) ; elon-

gate (1).18 (35). Pterygoid: borders suborbital fenestra (0); precluded from the

suborbital fenestra (I).

Mandible

19 (36). Jaw motion: orthal (0) ; propalinal (I) .20. Mandible: narrow (0) ; deep (1), (Rasmussen and Callison, 1981;

see also Benton, 1985)21. Mandibular symphysis breadth: slender (0) ; broad (1), (Benton,

1985: character Z13)] .22. Mandibular foramen: small (0) ; big (I), (Benton, 1985: character

ZI5).23 (15). Posterior process of dentary: ends anteriorly to coronoid pro­

cess (0); ends posteriorly (I).24 (16). Coronoid process : absent or weak (0); pronounced (1) .25 (19). Retroarticular process size: pronounced (0); reduced (1).

Teeth

26 (21 modified) . Dental implantation: pleurodont (0) ; a degree of ac­rodonty (I); fully acrodont (2) .

27 . Tooth replacement: alternate (0) ; addition of teeth at the back of thejaw (I), (Benton, 1985: character Z8)] .

28 (27). Lateral and medial wear facets on marginal teeth: absent orpoorly developed (0); well established (I) .

29. Marginal teeth breadth: equal to length (0); mediolaterally expanded(I), (Fraser and Benton, 1989: character 27).

30. Dental regionalization with small juvenile teeth at anterior end ofmaxilla and dentary: absent (0) ; present (I), [Benton, 1985 : characterZ7)].

31. Anterior caniniform tooth on dentary and maxilla: absent (0) ; pres­ent (I), (Gauthier et al ., 1988: character 51).

32 (22 , 23, 24 merged). Number of premaxillary teeth: more than seven(0); seven to four (I); three or less (2) .

33 (25). Premaxillary teeth: separated (0) ; forming a chisel-like struc­ture in mature individuals (1).

34 (26, 29 merged). Posterior maxillary tooth shape: simple cones (0);with posteromedial flanges in at least one tooth (I); with extensiveflanges (2) .

35 (30). Lateral palatine tooth row: small (0) ; enlarged (1) .36 (31). Number of palatine tooth rows: more than one (0) ; a single

lateral tooth row (1) .37. Flanges on palatal tooth row: absent (0); present on some (I), (Fra­

ser and Benton, 1989: character 15) .38 (32 modified). Number of pterygoid tooth rows: three or more (0) ;

two rows (I); one or ab sent (2).39 (28). Anterolateral flanges on dentary teeth: absent (0); present on

at least one tooth (1) .

Postcranial

40. Posterior process of second sacral vertebra: small (0); prominent(1) ; absent (2), (Gauthier et al., 1988: character 88, modified).

41. Ischium: uninterrupted posterior edge extending from the acetabu­lum to the median symphysis (0); posterior process on posteriormargin (1); prominent posterior process present (2), (Evans, 1988 :character K14; Fraser and Benton, 1989: character 4, modified).

42. Limb proportions with respect to the presacral column: humerus:=:; 0.20, tibia es 0 .25, femur < 0 .30, radius < 0.15 (0) ; all measuresgreater than these values (1), (Fraser and Benton, 1989: character29).

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