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The Skull and Neck of the Basal Theropod Herrerasaurus Ischigualastensis Author(S): Paul C

The Skull and Neck of the Basal Theropod Herrerasaurus Ischigualastensis Author(S): Paul C

The and Neck of the Theropod Ischigualastensis Author(s): Paul C. Sereno and Fernando E. Novas Reviewed work(s): Source: Journal of , Vol. 13, No. 4 (Jan. 14, 1994), pp. 451-476 Published by: Taylor & Francis, Ltd. on behalf of The Society of Stable URL: http://www.jstor.org/stable/4523532 . Accessed: 27/04/2012 22:07

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452 JOURNAL OF VERTEBRATE PALEONTOLOGY, VOL. 13, NO. 4, 1993

TABLE 1. Measurements (mm) of the skull of Herrerasau- Dorsal Skull Roof--The premaxilla has two prin- rus ischigualastensis (PVSJ 407). Measurements of paired cipal processes, a broad triangular posterolateral pro- structures are from the left side. () = estimated. cess and a narrow internarial process. The posterolat- eral process extends between the and nasal, Abbreviation: excluding the former from the external naris by a broad Maximum skull length 300 margin as in ornithischians. The slender internarial Preorbital length 171 process inserts between the nasals in the midline, but Maximum height of posterior skull (95) the distal end of the premaxilla-nasal cannot be Maximum transverse width of occiput 75 discerned. Whether the tips of the internarial processes Vertical diameter of orbit 60 are separated in the midline by the premaxillae, as Maximum of 70 length occurs in most theropods, cannot be determined. Al- Length of upper row 180 of lower tooth row though the ventral aspect of the premaxilla is not ex- Length (122) the narrow width of the snout that the Depression of jaw articulation posed, suggests from maxillary alveolar margin (21) premaxillary was narrow (Figs. 7D, 8D). The maxilla contacts the premaxilla anteriorly, nasal dorsally, lacrimal and jugal posteriorly, and palatine based principally on one skeleton (PVSJ 407; see also medially. The maxilla forms the posterior border of Sereno, 1993) that includes a well-preserved skull and an oval fenestra, located posterior to the external naris neck. The skeleton was collected in 1988 during an (Figs. 1A, B, 7A, 8A). This large oval fenestra, a unique expedition to the in northwest feature of the , is positioned posterior to the (Ischigualasto-Villa Uni6n Basin; Stipani- external naris and opens internally above the premax- cic, 1983). The skull is preserved with articulated lower illary palate. As in most saurischians (e.g., , jaws, a partial sclerotic ring in the right orbit, and the Madsen, 1976), a small subnarial foramen is located left stapes with the footplate lodged in the fenestra along the premaxilla-maxilla suture and opens inter- ovalis. Postmortem distortion of the skull is limited nally at the posterior end of the premaxillary palate to some transverse compression of the snout and man- (Figs. 1A, B, 7A, 8A). Several larger foramina exit dibular rami and some elevation of the right side of above the tooth row and pass anterioventrally in a the skull. Checkered bone surfaces suggest that the skull shallow grooves. may have been exposed subaerially before final burial. The antorbital fossa (the depression bordering the PVSJ 407 was buried within a medium-grained sand- antorbital fenestra) is very narrow in H. ischigualas- stone lens, and most of the skull and postcranial skel- tensis. The maxillary portion of the fossa forms a nar- eton was enveloped in a hematite precipitate. row band along the anterior and dorsal margins of the The cervical column was preserved in articulation antorbital fenestra (Figs. 1A, B, 2, 3, 7A, 8A). The with the skull. The anterior are bet- anterior margin of the fossa is invaginated, as occurs ter preserved than the posterior cervical vertebrae, and in many other (Fig. 10A, B). nearly all of the ribs are lacking. Parts of the , atlas, The medial aspect of the maxilla is exposed in and several cervical centra are also preserved in other PVSJ 53 and shows distinct interdental plates. Contact individuals (MACN 18.060, PVSJ 53, 373; MCZ 7064, with the palatine along the lateral margin of the palate Brinkman and Sues, 1987:fig. lA). is visible through the antorbital fenestra in PVSJ 407 (Figs. 1A, 2). DESCRIPTION The nasal forms the narrow snout roof, spanning two-thirds of the of the skull. Skull approximately length Sutural contacts of the nasal include the premaxilla, The skull is nearly four times as long as deep as seen maxilla, lacrimal, prefrontal, and frontal. The anterior in lateral view, and the snout is particularly narrow two-thirds of each nasal is transversely arched, result- (Figs. 1-9). In posterior view, the skull is deeper than ing in a shallow median depression. The lateral margin it is wide (Figs. 6, 7C, 8C; Table 1). of the nasal approaches the dorsal rim of the antorbital

FIGURE 1. A-C, F-G, skull of Herrerasaurusischigualastensis (PVSJ 407) in left lateral (A), right lateral (B), dorsal (C), ventral (F), and posterior (G) views; D, posterodorsal process of the right dentary in cross-section (lateral to the right); E, mid- section of left mandibularramus in ventral view. Abbreviations:a, angular;ar, articular;bo, basioccipital;bs, basisphenoid; d, dentary; emf, external mandibular fenestra; ec, ectopterygoid; eo, exoccipital; ept, epipterygoid; f, frontal; imf, internal mandibular fenestra; j, jugal; 1, lacrimal; Is, laterosphenoid; m, maxilla; n, nasal; op, opisthotic; p, parietal; pl, palatine; pm, premaxilla; po, postorbital; pra, prearticular; prf, prefrontal; ps, parasphenoid; pt, pterygoid; q, quadrate; qf, quadrate foramen; qj, quadratojugal;sa, surangular;saf, surangularforamen; scp, sclerotic plate; sf, subnarialforamen; so, supraoccipital;sp, splenial; sq, squamosal; st, stapes; stf, supratemporal fossa; tm, tooth mark; v, vomer. Double-bar pattern indicates matrix. Cross-hatching indicates broken bone surface. bar for A-C and F-G (below G) equals 5 cm; scale bars in D and E equal 1 cm. SERENO AND NO SKULL AND NECK 453 VAS--HERRERASAURUS

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SERENO AND NO VAS--HERRERASAURUS SKULL AND NECK 459 and contacts the parietal and exoccipital-opisthotic. Its sus XII, and the more anterior descends from the par- ventrolateral apex reaches the small post-temporal fo- occipital process anterior to the fenestra ovalis. None ramen (Figs. 7C, 8C). A strong median ridge is present of the cranial foramina are exposed, except the foramen in the dorsal one-half of the supraoccipital for attach- for the internal carotid artery. ment of nuchal ligaments. The wedge is lodged in a The prootic is partially exposed in the right supra- posterior notch between the parietals and is visible in temporal fossa. The right laterosphenoid is also ex- dorsal view of the skull (Figs. 7B, 8B). posed anterior to the prootic and ventral to the parietal The exposed contacts of the basioccipital include the in the center of the right supratemporal fossa (Fig. IC). exoccipital-opisthotic and basisphenoid. The basioc- The transversely expanded articular head of the latero- cipital forms the ventral two-thirds of the occipital sphenoid is smooth and rounded and articulated in a condyle. The basal tubera project ventrally a few mm socket on the ventral side of the frontal and postorbital, below the condyle in posterior view (Figs. 7C, E, 8C, as in most ornithischians and sauropodomorphs. E). The tubera, which are separated by a median cleft, Sclerotic Ossicles--Several disarticulated sclerotic are each composed of a medial columnar thickening plates are preserved in the anteroventral one-half of and a lateral plate-like tab. The lateral margin of the the right orbit (Fig. IB). One of the better exposed tab continues dorsally as a ridge on the lateral side of plates is subrectangular, the long axis of which is ap- the braincase posterior to the fenestra ovalis (Figs. 7E, proximately 1 cm in length. Little else can be said about 8E). the shape of the individual plates or the arrangement The basisphenoid contacts the basioccipital poste- of the ring. riorly and the pterygoids anteriorly at the basipterygoid Lower Jaw - The lower jaws are preserved in several articulation. In ventral view, a shallow central depres- individuals (PVSJ 53, 407; MACN 18.060). In PVSJ sion is present in the center of the basisphenoid; the 407, the anterior half of the lower jaws are pressed basipterygoid processes and posterolateral buttresses together, such that only the posterior half of the lower to the basal tubera radiate away from the central de- jaw is exposed in medial view (Figs. IF, 5). As a result, pression, forming a symmetrical cross in ventral view the presence or absence of a coronoid bone cannot be (Figs. 7D, 8D). The short basipterygoid processes are determined. A striking feature of the lower jaw is the slightly flattened dorsoventrally and oval in cross-sec- well-developed intra-mandibular joint. tion. In lateral view, they project anteroventrally at The dentary, the longest and most robust element about 450 from the vertical (Figs. 7E, 8E). In ventral of the lower jaw, is slightly upturned anteriorly. Several view, they project anterolaterally at about 450 from the vascular foramina open along its length, the largest midline (Figs. 7D, 8D), and, in posterior view, they positioned posteriorly and connected to more anterior project at about 450 ventrolaterally from the sagittal foramina by a groove. Several smaller foramina open plane, extending slightly below the level of the occipital near the symphysis, as in small theropods such as Dei- condyle (Figs. 7C, 8C). The transversely compressed nonychus or . Medially, the dentary is parasphenoid rostrum projects anteriorly from the body overlapped by the splenial, and the symphysis is re- of the basisphenoid toward the converging palatal rami stricted to a bevelled surface at the distal end of the of the pterygoids (Figs. 1A, 2, 7A, 8A). dentary (Fig. IF). As in all dinosaurs, the exoccipital-opisthotic is com- Posteriorly, the dentary is forked. The arched margin pletely co-ossified. It contacts the basioccipital ven- between the posterior processes forms the anterior trally, the quadrate, squamosal, and prootic anteriorly, margin of the mandibular fenestra (Figs. 1B, 3, 7A, and the parietal dorsally. It forms the lateral border of 8A). An unusually elongate and slender posterodorsal the and has an articular facet on process, which reaches a maximum depth of only 7 each side for the anterior end of the proatlas (Fig. 8C, mm, inserts into a slot on the surangular. It slides apro). A notch in the dorsal margin of the paroccipital against smooth surfaces on the lateral side of the dorsal process forms the ventral border of the post-temporal prong of the surangular and on the medial side of the foramen, which is enclosed dorsally by the parietal. ventral prong. The slender dentary process is rein- On the side wall of the braincase, two crests, or flanges forced by a prominent lateral rib (4 mm in depth), of bone, are present on the exoccipital-opisthotic (Figs. which extends along the length of the dentary process. 7E, 8E). The more posterior crest separates the otic The posterodorsal process of the dentary thus is openings from the posterior foramina for N. hypoglos- T-shaped in cross-section (Fig. 1D), a condition unique

FIGURE 7. A-D, Skull reconstructionof Herrerasaurusischigualastensis (based on PVSJ 407) in left lateral (A), dorsal (B), posterior (C), and ventral (D) views. E, left lateral view of disarticulated braincase. See Figure 8 for bone identifications. FIGURE 8. A-D, Skull reconstruction of Herrerasaurus ischigualastensis (based on PVSJ 407) in left lateral (A), dorsal (B), posterior (C), and ventral (D) views. E, left lateral view of disarticulated braincase. Abbreviations: antfo, antorbital fossa; apro, articular surface for the proatlas; fica, foramen for the internal carotid artery; ppf, postpalatine fenestra; pra, prearticular; prf, prefrontal; ps, parasphenoid; pt, pterygoid; ptf, post-temporal foramen. Other abbreviations as in Figure 1.

SERENO AND NO VAS--HERRERASAURUS SKULL AND NECK 461

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eo-op bo\ apro 462 JOURNAL OF VERTEBRATEPALEONTOLOGY, VOL. 13, NO. 4, 1993

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FIGURE 9. Skull reconstructionof Herrerasaurusischigualastensis showing the location of autapomorphies1-8 and 10.

to H. ischigualastensis. During flexion of the intra- transversely convex, which is opposite the condition mandibular joint, it abuts against the ventral prong of in other theropods. This tongue-shaped process of the the surangular, resisting further ventral rotation of the splenial is visible in lateral view ventral to the dentary dentary. The thin, subtriangular posteroventral process (Figs. 7A, 8A) and articulates on the ventral aspect of of the dentary slides against a polished articular surface the angular, as in dromaeosaurid theropods (Colbert on the lateral side of the angular. and Russell, 1969; Ostrom, 1969). The remainder of The surangular forms the lateral portion of the jaw the posterior end of the splenial is exposed between articulation where it contacts the lateral condyle of the the posterior processes of the right dentary (Fig. 1B). quadrate (Figs. 1A, B, F, G, 7A, 8A). It curls dorsally Judging from this exposure, the splenial appears to behind the jaw articulation, overlapping the lateral as- have contacted the prearticular on the dorsal margin pect of the articular. Just below the jaw articulation, a of the internal mandibular fenestra. rounded ridge is developed and disappears anteriorly. The prearticular, a thin plate-like bone on the medial A small surangular foramen is present under the ridge. aspect of the lower jaw, extends from the coronoid The ventral margin of the surangular is overlapped by region anteriorly to the distal end of the retroarticular the angular posteriorly and forms most of the dorsal process posteriorly (Fig. 1B, F). Anteriorly, the prear- margin of the external mandibular fenestra. Anteriorly, ticular is broad, as seen through the external mandib- the surangular is deeply cleft to receive the postero- ular fenestra (Figs. 1B, 3). Dorsally, it borders the ad- dorsal process of the dentary. The entire lateral surface ductor fossa and, ventrally, it forms the margin of the of the dorsal prong of the surangular is bevelled for internal mandibular fenestra. The internal mandibular smooth articulation with the posterodorsal process of fenestra appears to be at least 25 mm in anteroposterior the dentary. length, or more than one-third the length of the exter- The angular extends from the intra-mandibularjoint nal mandibular fenestra. Posteriorly, the prearticular to the posterior extremity of the retroarticular process. twists onto the ventral side of the retroarticular pro- Anteriorly, the angular curves dorsally to form a pol- cess. Here it overlaps the articular and, in turn, is over- ished hook-shaped process that articulates laterally with lapped by the angular. the dentary and ventrally with the splenial (Figs. 1B, The articular, best exposed in posterior view (Fig. 3). Posteriorly, the angular continues as a narrow strip 1G), is overlapped by the surangular, angular, and to the distal end of the retroarticular process, where it prearticular. The medial surface of the articular is cres- overlaps the articular, surangular, and prearticular (Fig. centic and extends posterior to the jaw articulation, IF). curving dorsally as a vertical process. A second vertical The splenial extends anteriorly at least as far as the process, formed by the articular and surangular, is equal seventh dentary tooth, where it tapers in width to a in height and positioned laterally (Figs. 1G, 6). thin plate. Posteriorly, the splenial wraps around the Stapes--The left stapes is preserved with the prox- ventral margin of the dentary as a tongue-shaped pro- imal end in place in the fenestra ovalis and the shaft cess (Figs. 1E, 5), with a trough-shaped articular surface hanging ventrolaterally alongside the left basal tuber for the angular. The tongue-shaped process of the (Figs. 1G, 6). The foot-plate is not exposed. Judging splenial is transversely concave and the angular is from the position of the proximal shaft, the foot-plate SERENO AND NO SKULL AND NECK 463 VAS--HERRERASAURUS

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FIGURE 10. Comparative skull reconstructions of the basal dinosaurs (A) diagnosticus (after Sereno, 1991 a), (B) engelhardti (based on SMNS 13200), and (C) nasicornis (based on USNM 4735). 464 JOURNAL OF VERTEBRATE PALEONTOLOGY, VOL. 13, NO. 4, 1993

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FIGURE 11. Anterior cervical column of Herrerasaurus ischigualatensis (PVSJ 407). A-C, "exploded" views of the proatlas, atlas, axis, and third cervical vertebrae in left lateral (A), dorsal (B), and ventral (C) views; D, right atlantal neural arch in medial view; E-F, axis in anterior (E) and posterior (F) views. Abbreviations: aoc, articular surface for the occipital condyle; aod, articular surface for the odontoid; axi, axial intercentrum; di, diapophysis; epi, epipophysis; nc, neural canal; od, odontoid; SERENO AND NO VAS- HERRERASAURUS SKULL AND NECK 465 is probably seated in the fenestra ovalis in natural ar- TABLE 2. Measurements (mm) of cervical vertebrae in ticulation. The slender columelliform shaft has a di- Herrerasaurus ischigualastensis (PVSJ 407, MACN 18.060). ameter somewhat less than 2 mm and a preserved Measurements of paired structures are averages of both sides. length of 28 mm. The stapes with its foot-plate, thus, Abbreviations: c = calculation based on proportions of third cervical r = measurement from side is approximately 30 to 32 mm long, which nearly spans ; right only. the distance from the side wall of the braincase to the 407 posterior margin of the quadrate shaft, where the tym- PVSJ panum must have been located. The cartilagenous ex- Proatlas trastapes could not have been very long. Maximum length 26 Teeth--There are four premaxillary teeth. Because Maximum depth 10 of poor preservation, some structural characteristics Atlas and virtually all surface details are lacking. The ante- Maximum length of neural arch 34r riormost tooth is positioned very near the midline and Maximum length of ventral is the least transversely compressed of the premaxillary surface of intercentrum 10 teeth. In lateral view, the crown is gently recurved with Maximum length of odontoid 10 a rounded anterior margin. Because the posterior mar- Maximum width of odontoid 14 gin is not exposed, it is impossible to determine wheth- Axis er the crown was elliptical or had a D-shaped cross- Maximum length of intercentrum 11 section. The second, third, and fourth premaxillary Maximum width of intercentrum 25 teeth are transversely compressed, with rounded an- Maximum length of centrum 33 terior and sharp posterior margins and increase in size Maximum depth of anterior and from the second to the fourth tooth. The transition posterior centrum faces 15 Maximum width across 13 between premaxillary and maxillary tooth rows is con- prezygapophyses Maximum width across postzygapophyses 27 tinuous, without an intervening diastema or noticeable change in crown structure. Postaxial cervical vertebrae There are 17 or 18 maxillary teeth. Despite some Maximum length variation in crown height due to tooth replacement, between pre- and maximum tooth size (measured from the crown base Maximum postzygapophyses or alveolus) occurs in the third or fourth maxillary centrum (excluding tooth position, posterior to which tooth size gradually length ) decreases. The posterior maxillary teeth under the or- Cervical 3 40 53 bit are relatively short and posteriorly directed. The 4 43c 56 serrations are not particularly well-preserved. The 5 44c 58 proximal half of the anterior margin is rounded and 6 43c 57 - appears to lack serrations. The distal halves of anterior 7 - 8 31 43 and posterior margins have approximately five serra- - mm. 9 46 tions per 10 -- There are approximately 16 dentary teeth, which are somewhat shorter than the opposing maxillary teeth. MACN 18.060 The short first dentary tooth is slightly procumbent Maximum Maximum and only gently recurved. The anterior margin of this centrum centrum height tooth is rounded and lacks serrations. The posterior length (posterior face) margin forms a sharp edge a short distance above the Cervical 4? 37 19 base of the crown and has approximately six serrations 5? 35 19 per mm. Dentary tooth size increases to a maximum in the third and fourth tooth positions, slightly in ad- vance of the tooth. The anterior end largest maxillary Cervical Column of the dentary ramus is slightly swollen to accommo- date the roots of these large dentary teeth. Posterior to Proatlas - Both proatlantal elements were preserved the fourth tooth, the lateral alveolar margin is gently near the foramen magnum, but neither was in natural concave, and tooth size decreases posteriorly. Serra- articulation with the skull or atlas (Fig. 11A-C; Table tions are exposed on the posterior margins of several 2). Each element is subtriangular and transversely flat- dentary crowns and are equal in size and form to those tened. The dorsal margin and apex are thickened, in on maxillary crowns. contrast to the thin ventral margin. The anterior end

pa, parapophysis;poz, postzygapophysis;prz, prezygapophysis.Cross-hatching indicates broken bone surface.Scale bar equals 1 cm. 466 JOURNAL OF VERTEBRATE PALEONTOLOGY, VOL. 13, NO. 4, 1993 is narrower than the tablike posterior end and has a Axis--The axial centrum is almost twice as long as ridge on its external surface near the ventral margin deep (Fig. 11A-C, E, F; Table 2). A strong ventral keel (Fig. 11A). The tip of the anterior end articulates against is present in PVSJ 407 (Fig. 11C), but the keel is less a shallow facet lateral to the foramen magnum (Figs. pronounced in other specimens (PVSJ 53, MACN 1G, 6, 8C, apro). The posterior end is rounded laterally 18.060). The parapophysis is a raised rugosity posi- and flat medially (Fig. I1C). The flat medial surface of tioned low on the anterior end of the centrum (Fig. the posterior end overlaps the prezygapophysis of the 11A). The weakly developed diapophysis indicates that atlantal neural arch. In natural articulation, the proat- the axial rib is double-headed. The side of the centrum lantal elements would converge slightly toward their between the parapophysis and diapophysis is deeply cranial articulation (Fig. 11B, C). excavated, and the posterior face of the centrum is Atlas--The atlas is composed of a centrum, inter- concave. centrum, and paired neural arches (Fig. 11A-E; Table The axial intercentrum is fused to the anterior end 2). The centrum, or odontoid, is completely co-ossified of the axis. As measured in the midline, it is approx- with the axis (Fig. 11A, E; Brinkman and Sues, 1987: imately one-third the length of the axial centrum (Fig. fig. 1A). The odontoid is subcircular in dorsal view 11C; Table 2). It is very broad, exceeding the width of with a distinct anteromedian projection. This rounded both the anterior and posterior faces of the axial cen- projection fits into a pit on the occipital condyle. Ven- trum. Anteriorly, it is deeply cupped to receive the tral to this projection, the anterior surface of the odon- atlantal intercentrum (Fig. 11E). In lateral view, it is toid forms a broad, U-shaped trough, which is concave subtriangular and projects anteriorly under the atlantal dorsoventrally and convex transversely (Fig. 11A, E). intercentrum (Fig. 1IA). The occipital condyle articulates posteriorly against The neural arch is completely co-ossified with the this trough-shaped surface during lateral rotation of centrum on each side of a spacious neural canal that the skull. Ventral to the trough-shaped surface, the has a diameter one-half that of the posterior face of odontoid is slightly concave dorsoventrally and strong- the centrum (Fig. 11E-F). Raised, elliptical prezyg- ly convex transversely and articulates with the rounded apophyses face dorsolaterally with the long axis of the posterodorsal rim and posterior surface of the atlantal facet angling anterolaterally (Fig. 11A). The articular intercentrum. surface is gently convex along its long axis, accom- The atlantal intercentrum is U-shaped in anterior modating the concave atlantal postzygapophyseal facet view (PVSJ 407; MACN 18.060). The articular surface during rotation of the atlas against the axis. The neural for the occipital condyle is cup-shaped and faces an- spine projects anteriorly as a short prong between the terodorsally (Fig. 11B). The articular surface for the prezygapophyses and angles posterodorsally at ap- odontoid faces posterodorsally and has rounded dorsal proximately 250. At its posterodorsal extremity, the and ventral margins (Fig. 11B). The broad, U-shaped neural spine divides into two ventrolaterally inclined trough between these margins accommodates the ven- laminae. These laminae terminate ventrally as strong tral crest on the odontoid. The articular surface for the epipophyses that project posterior to the postzyg- axial intercentrum is also U-shaped and faces postero- apophyses. In posterior view, a deep triangular fossa ventrally (Fig. I IC). It is dorsoventrally convex, in is located between the apex of the neural spine and the contrast to the articular surfaces for the occipital con- postzygapophyses (Fig. 11F). The subtriangular post- dyle and odontoid, and is separated from the nonar- zygapophyseal facets are flat and angle ventrolaterally ticular ventral surface of the intercentrum by a deep at 20 to 300 from the horizontal (Fig. 1 lC, F). A thin groove which receives the anterior rim of the axial lamina joins the medial edges of the postzygapophyses intercentrum. The elliptical surface for the pedicels of and floors the fossa at the base of the neural spine (Fig. the neural arches is exposed in dorsal view and angles 11C). somewhat ventrolaterally (Fig. 11 B). The single-head- Postaxial Cervical Vertebrae--The postaxial cervi- ed atlantal rib articulates against a circular depression cal vertebrae are best preserved in PVSJ 407, although on the lateral side of the intercentrum (Fig. I lA). a complete cervical series is not available (Figs. 11A- The atlantal neural arch is approximately twice as C, 12; Table 2). Because the cervical-dorsal transition long as deep in lateral view (Fig. 11A). The medial in vertebrae or ribs is not preserved, we regard the first surface of the tab-shaped pedicel is dorsoventrally con- ten presacral vertebrae as cervical vertebrae, based on cave and forms the wall of the neural canal. The an- the condition in other basal dinosaurians. Disarticu- teroventral margin of the pedicel is not preserved well lated postaxial cervical vertebrae (MACN 18.060, PVSJ enough to determine if the pedicel participated in the 373) have been numbered by comparison to PVSJ 407. articular cup for the occipital condyle. The postzyg- The third cervical vertebra is longer than the axis, apophyseal articular surface is elliptical and slightly and centrum length increases slightly in the fourth to concave and faces ventromedially (Fig. 11D). A well- the sixth cervical vertebrae (Figs. 11, 12; Table 2). A developed, arched epipophysis is present and projects portion of the seventh cervical centrum (PVSJ 407) posteriorly. The short, tab-shaped prezygapophysis suggests that it was shorter than the sixth centrum, and curves dorsomedially. A subtriangular depression on the eighth centrum is substantially shorter than the its lateral side marks the articular surface for the proat- mid-cervical vertebrae. The postaxial cervical centra las (Fig. 11A). are parallelogram-shaped in lateral view, with eleva- SERENO AND NO SKULL AND NECK 467 VAS--HERRERASAURUS A B

epi i ana iJ poz

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9 FIGURE 12. Reconstructionof the cervical vertebraeof Herrerasaurusischigualastensis (based on PVSJ 407 and MACN 18.060). A, cervical vertebraein lateral view. B, cervical 5 in dorsal and ventral views. C, right proatlasand cervicals 1-3 in dorsal view. Abbreviations:1-9, cervicalvertebrae 1-9; ai, atlantalintercentrum; ana, atlantalneural arch; k, keel;pro, proatlas. Other abbreviationsas in Figure 11.

tion of the anterior centrum face (Fig. 12A). Excava- vertebra but are well-developed by the eighth cervical tion of the sides of the centrum is marked in the axis vertebra. The transverse processes appear broadly tri- and third cervical but gradually diminishes in posterior angular in dorsal view (Fig. 12B), because the lamina cervical vertebrae. Both the anterior and posterior fac- between the prezygapophysis and transverse process is es of the centrum appear to be weakly concave in all particularly broad. postaxial vertebrae. As on the axis, a strong ventral The prezygapophyses project beyond the anterior keel is present on the third cervical centrum but is face of the centrum in the third cervical vertebra, a reduced in depth in more posterior cervical vertebrae. condition which is progressively reduced in mid- and The parapophyses of the posterior cervical vertebrae posterior cervical vertebrae (Figs. 11, 12). The oval are positioned posterior to the centrum rim and are articular surfaces in the third vertebra are flat and face elevated to mid-height on the centrum (Fig. 12A; PVSJ dorsomedially, permitting transverse movement 407, MACN 18.060). The parapophyses are larger in against the postzygapophyses of the axis. A narrow posterior cervical vertebrae but reduced in promi- fossa is present at the base of the neural spine between nence. The diapophyses, which are developed as crests the prezygapophyses. In contrast to the axis, the post- in the axis and third cervical vertebra, project as ven- zygapophyses of the postaxial cervical vertebrae do not trolaterally directed flanges in the fourth and fifth cer- project behind the posterior centrum face. Arched epi- vical vertebrae (PVSJ 407, MACN 18.060). In the sev- pophyses are present in the third cervical vertebra and enth and eighth cervical vertebrae, the diapophyses are remain prominent as far posteriorly as the eighth cer- anteroposteriorly shortened, and in the ninth and tenth vical vertebra. cervical vertebrae, they project dorsolaterally. Four The plate-like neural spine of the third cervical ver- laminae extend from the transverse process to pre- and tebra is subquadrate in lateral view and posterodorsally postzygapophyses and to the anteroventral and pos- inclined (Fig. 11A). In more posterior vertebrae, the teroventral corners of each neural arch. These cross- neural spines decrease in length and thicken trans- shaped laminae are rudimentary in the fifth cervical versely, particularly in the sixth to the eighth cervical 468 JOURNAL OF VERTEBRATEPALEONTOLOGY, VOL. 13, NO. 4, 1993 vertebrae. Posterodorsal inclination of the neural spines ternal naris (e.g., Ceratosaurus, Fig. 10C; Allosaurus, decreases along the cervical series such that by the fifth Madsen, 1976). also lack a broad premax- cervical vertebra the spine axis is vertical. As in the illary posterolateral process, as do nearly all axis, a deep fossa is present at the base of the neural with rare exceptions among suchians (e.g., spine between the postzygapophyses. The fossa is di- , Sill, 1974; ). amond-shaped, rather than triangular, due to the steep- In summary, a broad posterolateral process of the er inclination of the laminae between the postzyg- premaxilla is present among dinosaurs in ornithischi- apophyses. ans and in a few advanced theropod subgroups. This condition does not seem to occur in theropods or sau- The broad DISCUSSION rischians, plesiomorphically. process ap- pears to have arisen independently in H. ischigualas- The new skull and neck materials of H. ischigualas- tensis and . tensis are compared below to other basal dinosaurs and (2) Premaxilla-maxilla fenestra (Figs. 7A, 8A, 9). successive dinosaurian outgroups. First, we describe An oval fenestra is present between the premaxilla and diagnostic characters in the skull and neck of H. is- maxilla posterior to the external nares in H. ischigu- chigualastensis. Second, we discuss cranial characters alastensis. A similar fenestra has not been reported in that are unique to dinosaurs and that are relevant to other dinosaurs or immediate dinosaurian outgroups. the phylogenetic position of H. ischigualastensis within Of course, no data are available for , Dinosauria. Finally, we consider the function of the "," and . As mentioned above, well-developed intra-mandibular joint and the occur- this fenestra should not be confused with the sauris- rence of tooth injuries on the skull. chian subnarial foramen, which is located ventral Unfortunately, comparative skull materials among (rather than posterior) to the external naris, with an immediate outgroups to H. ischigualastensis are lim- internal opening ventral (rather than dorsal) to the pre- ited. Only the lower jaws are known in the herrara- maxillary palate. saurid Staurikosaurus pricei (Colbert, 1970), and only (3) Narrow, U-shaped anterior antorbitalfossa (Figs. the maxilla and braincase are preserved in the dino- 7A, 8A, 9). The anterior portion of the antorbital fossa sauriform"Lagosuchus" lilloensis(Bonaparte, 1975; Ser- in H. ischigualastensis is narrow and U-shaped. The eno and Arcucci, 1994). No part of the skull is known external rim of the fossa and margin of the fenestra in Lagerpeton chanarensis (Sereno and Arcucci, 1993). closely parallel one another. In other dinosaurs, the Cranial comparisons to non-dinosaurian archosaurs external margin of the fossa is asymmetrical. In basal thus are often limited to pterosaurs and various cru- ornithischians, for example, the fossa is subtriangular rotarsal archosaurs. (Fig. 10A). In saurischians, the fossa has a rounded corner anteroventrally and a more open curve dorsally where it approaches the skull roof (e.g., Syntarsus, Autapomorphies of the Skull and Neck of Rowe, 1989; Allosaurus, Madsen, 1976). In Cerato- Herrerasaurus ischigualastensis saurus (Fig. 10C), the external margin of the fossa is Several characters of the skull and neck of H. is- semicircular as in H. ischigualastensis. The margin of chigualastensis are not present in other basal dinosaurs the fenestra, however, does not closely parallel the ex- or immediate dinosaurian outgroups and are regarded ternal rim of the fossa as in H. ischigualastensis. In here as autapomorphies (see Appendix for character- "Lagosuchus," the antorbital fossa probably had a sub- state distributions). It should be kept in mind, how- triangular shape, judging from the shape of the maxilla ever, that because the cranium is not known in the and antorbital fenestra (Bonaparte, 1975:fig. 2). In close relative Staurikosaurus, autapomorphies in the pterosaurs, the antorbital fossa is reduced or absent cranium (Fig. 8) may eventually characterize Herrera- anteriorly (Witmer, 1987); the anterior margin of the sauridae. antorbital fenestra, however, is asymmetrical (Welln- (1) Broad posterolateral process ofpremaxilla (Figs. hofer, 1978:figs. 2, 4). 7A, 8A, 9). In H. ischigualastensis, a broad triangular (4) Broad supratemporal depression (Figs. 1C, 4, 7B, process of the premaxilla extends between the maxilla 8B, 9). In H. ischigualastensis, the supratemporal fossa and nasal posterior to the external naris, separating the is inset sharply into the skull roof as a broad, flat surface maxilla from the border of the external naris by a anterior to the supratemporal opening. This depressed significant distance. A similar process is present in or- surface exceeds the area occupied by the supratemporal nithischians but generally is narrow or short in sauris- opening. No other or dinosaurian outgroup chians (Fig. 10B, C). Notable exceptions among sauris- exhibits the same proportionately broad depression chians include ornithomimids (Barsbold and Osm6lska, anterior to the supratemporal opening. 1990), oviraptorids (Barsbold et al., 1990), and dro- (5) Jugal ridge (Figs. 7B, 8B, 9). In H. ischigualas- maeosaurids (Ostrom, 1990), although only ornithom- tensis, a rugose ridge passes across the center of the imids have a broad, triangular premaxillary process jugal beneath the orbit. A similar jugal ridge is not similar to that in H. ischigualastensis. The premaxil- known in other basal dinosaurs or in pterosaurs. lary process in ceratosaurs and tetanurans, in contrast, (6) Narrow dorsal portion oflaterotemporalfenestra is very short and does not extend posterior to the ex- (Figs. 7A, 8A, 9). In H. ischigualastensis, the latero- SERENO AND NO SKULL AND NECK 469 VAS--HERRERASAURUS temporal fenestra is trapezoidal, with the narrow dorsal TABLE 3. Skull and neck characters cited by previous au- portion of the fenestra less than one-half the antero- thors as synapomorphies of Dinosauria. Abbreviations: = = = posterior width of the ventral margin. The propor- B Benton, 1984; G Gauthier, 1986; P1 Paul, 1984a; P2 = 1984b. tionately narrow dorsal extent of the fenestra is related Paul, to the short postorbital-squamosal bar and the antero- dorsal tilt of the quadrate shaft. Ceratosaurus (Fig. Skull 10C) and Abelisaurus (Bonaparte and Novas, 1985) 1. Anteroventrally expanded antorbital fossa (P1). with short anterior are the other dinosaurs with strong constriction 2. Tall lacrimal prong (P1). only 3. of the dorsal portion of the laterotemporal fenestra Three-pronged jugal (P1). 4. Postfrontal absent (B). (Fig. 10C). The condition is not known in Stauriko- 5. Lightly built palate (P1). saurus, "Lagosuchus," or Lagerpeton. 6. Vomera elongate G). ventral with (P1, (7) Subquadrate process of squamosal 7. Slender pterygoid (P1). lateral depression (Figs. 7A, 8A, 9). In H. ischigualas- 8. Vertical quadrate (P2). tensis, the ventral process of the squamosal has an 9. Narrow, elongate opisthotic wings (P2). unusual subquadrate shape with a shallow depression Neck on its lateral surface. The only other dinosaur that 10. Curved neck (P1). approaches this condition is (Welles, 1984). The squamosal is not preserved in Stauriko- saurus, "Lagosuchus," or Lagerpeton. It has a slender ventral process in pterosaurs. (8) Quadratojugal overlapping posterior side of Galton and Bakker, 1985; , Colbert, 1989; quadrate shaft (Figs. IG, 7C, 8C, 9). The posterior Allosaurus, Madsen, 1976). Because the skull is prac- portion of the quadratojugal in H. ischigualastensis is tically unknown in basal dinosauromorphs such as deflected medially, overlapping the posterior side of "Lagosuchus," nevertheless, it is effectively impossible the quadrate shaft. This condition has not been re- to identify unequivocal cranial synapomorphies for ported elsewhere among dinosaurs or dinosaurian out- Dinosauria. groups. Recent cladistic surveys have listed several cranial (9) Basal tuber and occipital condyle subequal in and cervical synapomorphies for Dinosauria (Table 3, width (Figs. 6, 7C, 8C, 9). In H. ischigualastensis, the synapomorphies 1-10). Except for the absence of the basal tubera form a pair of broad plates, with each postfrontal, however, we cannot support these as di- tuber equal to the occipital condyle in width. In other nosaurian synapomorphies. Most of the characters are dinosaurs and in "Lagosuchus" (PVL 3870), the width absent in H. ischigualastensis and do not describe vari- of each tuber is usually considerably less than the width ation that is distinctive for dinosaurs. In H. ischigu- of the occipital condyle (e.g., Allosaurus; Madsen, 1976). alastensis and other basal dinosaurs, for example, the A few dinosaurs have relatively broad tubera (e.g., Ty- antorbital fossa is not expanded anteroventrally (char- rannosaurus; Osborn, 1912), but none have achieved acter 1; Figs. 7, 10), as compared to "Lagosuchus" the proportions described above in H. ischigualasten- (Bonaparte, 1975:fig. 2), pterosaurs (Wellnhofer, 1978), sis. or various crurotarsal archosaurs (e.g., ornithosuchids; (10) Slender, ribbed posterodorsal dentary process Walker, 1964; Bonaparte, 1972; Sereno, 199 lb). Com- and slotted surangular (Figs. lD, 7A, 8A, 9). In H. pared to other archosaurs, the lacrimal is not especially ischigualastensis, the posterodorsal process of the den- tall relative to snout height (character 2) in H. ischigua- tary is very elongate and slender and has a strong lateral lastensis or in many other dinosaurs (Figs. 7A, 10A, rib along its length. It articulates in a deep slot on the B). The jugal is not strictly "three-pronged" (character anterior end of the surangular. This configuration of 3) in H. ischigualastensis. Rather, the anterior end of the dentary and surangular is unknown elsewhere the bone has a rudimentary dorsal (fourth) process that among archosaurs. As described below, Staurikosaurus overlaps the lacrimal on the orbital margin (Fig. 7A). has an intra-mandibular sliding joint as in H. ischigua- Although ornithischians and sauropodomorphs lack lastensis, but the posterodorsal process of the dentary any development of this jugal process (Fig. 9A, B), is not preserved. several basal theropods show a condition similar to H. ischigualastensis (e.g., , Bonaparte et al., 1990; Dilophosaurus, Welles, 1984). The supposedly Dinosaurian Synapomorpies plesiomorphic "four-pronged" condition of the jugal Although the skull is quite distinctive within each is well-developed in pterosaurs and ornithosuchids of the three major dinosaurian (Ornithischia, (Wellnhofer, 1978; Walker, 1964; Bonaparte, 1972) , and ), cranial charac- but is poorly developed or absent in many crurotarsal ters that unite all dinosaurs are not obvious. Recent archosaurs such as (Chatterjee, 1978), Gra- information on the skull in several early dinosaurs al- cilisuchus (Romer, 1972), basal suchians (e.g., Spheno- lows a more thorough comparison with H. ischigu- suchus; Walker, 1990), proterochampsids (Romer, alastensis (Lesothosaurus; Thulborn, 1970; Sereno, 1971), and (Ewer, 1965). The palate is not 1991a; Sellosaurus, Plateosaurus; Galton, 1973, 1985; especially "lightly-built" (character 5) and the ptery- 470 JOURNAL OF VERTEBRATE PALEONTOLOGY, VOL. 13, NO. 4, 1993 goid is not particularly "slender" (character 7) in H. don, Galton, 1974). Sauropods appear to represent the ischigualastensis (Figs. 7D, 8D) or in other basal di- exception among dinosaurs, with the transverse flange nosaurs (e.g., Lesothosaurus; Sereno, 1991 a:fig. 11D) of the pterygoid positioned dorsal to the ectopterygoid as compared to basal archosaurs. Although the anterior (e.g., , CM 11338; , Ja- end of the vomer is not exposed in H. ischigualastensis, nensch, 1935-1936:fig. 60). The condition in diplo- the length of the vomer (character 6) does not appear docids is less certain, with the ectopterygoid apparently to be more than 25 percent of skull length, a proportion positioned on the "anterior border of the transverse that is not unusual among various short-snouted ar- process of the pterygoid" (McIntosh and Berman, chosaurs. In H. ischigualastensis, the shaft of the quad- 1975). rate slopes anterodorsally, as in several theropods (Fig. Among dinosaurian outgroups, the relation between 10C), and is not erect (character 8). Finally, the length the pterygoid and ectopterygoid generally resembles of the paroccipital processes (character 9) is not rela- that in sauropods, with the pterygoid overlapping the tively greater than in many crurotarsal archosaurs (e.g., dorsal side of the ectopterygoid. In pterosaurs, the ec- , ; Walker, 1964; Chatterjee, topterygoid appears to be positioned on the ventral 1978; Sereno, 1991b) or proterochampsids (Chana- aspect of the pterygoid (Campylognathoides, Scaphog- resuchus; Romer, 1971). nathus, Gnathosaurus, Nyctosaurus; Wellnhofer, 1978: We outline below several potential cranial synapo- figs. 3, 6). On the other hand, the the ectopterygoid morphies for Dinosauria (see Appendix for character- passes above the pterygoid in at least some pterodac- state distributions). tyloids (, Eaton, 1910; Araripesaurus, San- (11) Postfrontal absent (Figs. 7, 8, 10). There is no tanadactylus, Wellnhofer, 1985:figs. 3b, 32a). The con- postfrontal bone on the skull roof between the orbit dition in pterodactyloids, however, is difficult to assess. and supratemporal fossa in H. ischigualastensis and In other dinosaurian outgroups, the ectopterygoid other dinosaurs. Most basal archosaurs, in contrast, passes ventral to the transverse flange of the pterygoid, retain the bone (e.g., parasuchians, ornithosuchids, as observed in sphenosuchians (Wu, 1986), ornithosu- Gracilisuchus, rauisuchians), albeit reduced in some chids (, PVL 3827; Ornithosuchus, Walker, groups (e.g., , Walker, 1961; Saurosuchus, 1964), phytosaurs (Parasuchus, Chatterjee, 1978), and PVSJ 32). Reduction or loss of the postfrontal has long proterochampsids (, PVL 4586). Al- been noticed as a general pattern among archosaurian though the ectopterygoid is shown with a dinosaurian subgroups (Romer, 1956:130). Crocodylomorphs have position in passing dorsal to the transverse independently lost the postfrontal (Gauthier, 1986; flange of the pterygoid (Chatterjee, 1985:fig. 5b), the Sereno, 1991 b), and the condition in pterosaurs is not identification of the ectopterygoid is questionable obvious. The posterior orbital margin is not well-pre- (shown as larger than the palatine) and its relation with served in the basal pterosaurs Eudimorphodon (Wild, the pterygoid apparently has been reconstructed from 1978) and Campylognathoides (Wellnhofer, 1974), and disarticulated materials. the presence or absence of this bone cannot be deter- (13) Quadrate head laterally exposed (Figs. 7A, 8A, mined. Other pterosaurs show one or two slender os- 10). In H. ischigualastensis and other dinosaurs, the sifications along the dorsal and posterior orbital margin articular head of the quadrate is exposed in lateral (Wellnhofer, 1978). These bones have been interpreted view. The head of the quadrate is smooth, suggesting either as neomorphic ossifications (Gauthier, 1984:175) that a synovial joint existed between the quadrate and or (at least one of them) as the postfrontal. In a well- squamosal. In pterosaurs and other basal archosaurs, preserved pterodactyloid cranium, a slender, curved the quadrate head is hidden in lateral view by the element is present along the posterodorsal margin of squamosal. the orbit, articulating with the frontal and postorbital (14) Post-temporalforamen (Figs. 7C, 8C). In H. is- (Wellnhofer, 1985:figs. 31, 32). These relations are chigualastensis, the post-temporal opening is repre- consistent with the position and sutural contacts of the sented by a foramen between the paroccipital process postfrontal in other archosaurs, and we consider this and parietal near the lateral corner of the supraoccip- element to represent the postfrontal. ital. The opening in H. ischigualastensis and other di- (12) Ectopterygoid dorsal to transverse flange of nosaurs is much smaller than the foramen magnum. pterygoid (Figs. 7A, D, 8A, D). In H. ischigualastensis, In some thyreophoran ornithischians, the foramen is the ectopterygoid arches from the infraorbital bar to closed. In dinosaurian outgroups, the post-temporal the dorsal aspect of the pterygoid. It curves posteriorly opening has a maximum diameter more than one-half and extends to the tip of the transverse flange of the that of the foramen magnum, as seen in pterosaurs pterygoid, maintaining its dorsal position relative to (Wellnhofer, 1978; see also Araripesaurus, Wellnhofer, the pterygoid. The same relation between the ectopter- 1985:fig. 34) and crurotarsal archosaurs (Saurosuchus, ygoid and pterygoid occurs in early sauropodomorphs Gracilisuchus, aetosaurs, ornithosuchids, and phyto- (e.g., Plateosaurus, Galton, 1985), theropods (e.g., Al- saurs). Proterochampsids (Proterochampsa, Sill, 1967; losaurus, Gilmore, 1920:24, contra Madsen, 1976; Chanaresuchus, PVL 4575, 4586) and crocodylo- , Colbert and Russell, 1969:fig. 10OB; morphs (e.g., , Walker, 1990) have in- , Osborn, 1912:fig. 6), and ornithischi- dependently reduced the post-temporal opening to a ans (e.g., Lesothosaurus, Sereno, 1991la; Hypsilopho- foramen. SERENO AND NO VAS- HERRERASAURUSSKULL AND NECK 471

Synapomorphies within Dinosauria stitute an exception in which the jugal is not forked posteriorly and is broadly overlapped by the quadra- Several derived characters are present only in H. tojugal (e.g., Camarasaurus, Gilmore, 1925). Among ischigualastensis and saurischians or saurischian sub- ornithischians, the articular relation between the jugal groups. and quadratojugal is the opposite of that in sauropods, (15) Subnarialforamen (Figs. 7A, 8A, 10). A small with the jugal overlapping the quadratojugal. A forked subnarial foramen is present along the premaxilla- posterior process occurs in only a few forms (e.g., Psit- maxilla suture in H. ischigualastensis. Nearly all sauris- tacosaurus; Sereno et al., 1988b), and in these cases chians have a subnarial foramen, which is usually the jugal processes are broad, unlike the condition in slightly larger than in H. ischigualastensis (e.g., Cera- H. ischigualastensis and other saurischians. The broad- tosaurus, USNM 4735; Camarasaurus, CM 11338). ly overlapping or interdigitating jugal-quadratojugal The foramen is absent in some advanced ceratosaurs suture in many ornithischians may be too transformed that have acquired a substantial premaxilla-maxilla for comparison, for it does not resemble the outgroup diastema (e.g., Dilophosaurus, Welles, 1984; Coelo- condition described below. physis, Colbert, 1989). A subnarial foramen is absent A forked posterior process of the jugal is absent in ornithischians, pterosaurs, and nearly all crurotarsal among dinosaurian outgroups. The jugal-quadrato- archosaurs. Some suchians have evolved a similar fo- jugal articulation is narrow and apparently planar in ramen (e.g., Saurosuchus, Sill, 1974). pterosaurs (Wellnhofer, 1978, 1985). In other basal (16) Lateral overlap of lacrimal by jugal (Figs. 7, archosaurs, the jugal overlaps the quadratojugal with 10). In H. ischigualastensis, the anterior end of the jugal an approximately planar articular surface (Sphenosu- broadly overlaps the ventral end of the lacrimal at the chus, Walker, 1990; , Barbarena, 1978; posteroventral corner of the antorbital fossa. Broad Ornithosuchus, Walker, 1964:fig. 3d; Riojasuchus, PVL jugal overlap is also present in saurischians (Fig. 10B, 3827; Parasuchus, Chatterjee, 1978; Chanaresuchus, C; Plateosaurus, Galton, 1985; Camarasaurus, Gil- PVL 4586, 4575). more, 1925; Carnotaurus, Bonaparte et al., 1990; Al- Finally, one cranial and one cervical synapomorphy losaurus, Madsen, 1976). The lacrimal-jugal articula- unite H. ischigualastensis and Theropoda. tion among ornithischians is usually rugose or (18) Well-developed intra- mandibularjoint between interdigitating, with minimal overlap. In the basal or- the dentary and surangular and between the splenial nithischian Lesothosaurus, the ventral edge of the lac- and angular (Figs. 7A, 8A). In H. ischigualastensis, a rimal extends somewhat lateral to the jugal (Sereno, well-developed intra-mandibular joint separates the 1991a:fig. 2B). Likewise, in the ceratopsian Psittaco- lower jaw into anterior and posterior functional units. saurus, the lacrimal appears to extend slightly further Broad sliding articulations are present between the on the lateral side, although the articulation is grooved dentary and surangular above the external mandibular (Sereno, 1987). In other ornithischians, however, the fenestra and between the splenial and angular below. anterior end of the jugal clearly overlaps the lacrimal The splenial-angular articulation is developed between (e.g., hadrosaurs, Ostrom, 1961) as in H. ischigualas- a tongue-shaped posterior process of the splenial and tensis and other dinosaurs. Many ornithischians do not a hook-shaped anterior process of the angular. The have a broadly overlapping lacrimal-jugal articulation, splenial process wraps around the ventral margin of and thus this character is too transformed for inter- the angular, sliding against its smooth, convex ventral pretation of this character. margin. The same articular configuration between the In pterosaurs and crurotarsal archosaurs, the ventral splenial and angular is preserved in the right lower jaw end of the lacrimal broadly overlaps the jugal in ptero- of Staurikosaurus (MCZ 1669). saurs (Wellnhofer, 1985:figs. 31, 32), opposite to the Kinetic dentary-surangular and splenial-angular ar- condition in H. ischigualastensis and most other di- ticulations are also present in theropods (e.g., Cera- nosaurs. In Ornithosuchus, the lacrimal articulates on tosaurus, Fig. 10C; Carnotaurus, Bonaparte et al., 1990). the posterolateral side of the jugal, such that when the In theropods, however, the articular surfaces of the jugal is disarticulated, the articular surface for the lac- splenial-angular sliding joint are the reverse of that in rimal is only exposed in lateral view (Walker, 1964: Herrerasaurus and Staurikosaurus; the tongue-shaped fig. 3c, d). In Saurosuchus (PVSJ 32), the lacrimal process of the splenial has a convex dorsal articular broadly overlaps the jugal. Thus, lateral overlap of the surface that slides against a concave depression on the lacrimal by the jugal appears to be the most common angular. The dentary-surangular joint is also present condition in crurotarsal archosaurs, with a notable ex- in theropods, but the posterodorsal process of the den- ception among sphenosuchian crocodylomorphs (Wu, tary is not elongated as in H. ischigualastensis. The 1986; Walker, 1990). unusual intra-mandibular joint described above is (17) Posterior process of jugalforked (Figs. 7, 8, 10). found only in herrerasaurids and theropods among di- In H. ischigualastensis, the posterior process of the nosaurs. Dinosaurian outgroups (pterosaurs, crurotar- jugal is forked for the reception of the anterior process sal archosaurs) also lack an intra-mandibular joint. of the quadratojugal. Most saurischians exhibit a sim- (19) Prominent postaxial cervical epipophyses (Figs. ilar condition (e.g., Plateosaurus, Ceratosaurus, Fig. 11, 12). In H. ischigualastensis, prominent, arched epi- 10B, C; Allosaurus, Madsen, 1976). Sauropods con- pophyses are present in the cervical series from the 472 JOURNAL OF VERTEBRATE PALEONTOLOGY, VOL. 13, NO. 4, 1993 atlas to at least the eighth cervical vertebra. The tips tation of the intra-mandibular articular surfaces sug- of these epipophyses extend posterior to the articular gest that movement between anterior and posterior facet of the postzygapophysis and decrease in strength mandibular segments was primarily in the plane of the in the posterior cervical vertebrae. Axial and, especial- mandibular ramus. In the dorsal articulation, the ar- ly, atlantal epipophyses are common among dinosaurs. ticular surfaces between the dentary and surangular are Prominent epipophyses in postaxial cervical vertebrae anteroposteriorly elongate with a near vertical orien- occur principally among theropods. In Staurikosaurus tation; lateral deviation at this joint does not seem (MCZ 1669), cervical epipophyses have been reported possible. Similarly, the ventral articulation appears to as absent (Sues, 1990). The cervical vertebrae and post- be designed for flexion in the plane of the mandibular zygapophyses, however, are poorly preserved. Among ramus; the plate-shaped posteroventral dentary pro- theropods, postatlantal cervical epipophyses are pres- cess overlaps the lateral surface of the angular hook. ent in ceratosaurs and most tetanurans (Ceratosaurus, Significant lateral deviation of these joints would result Gilmore, 1920; Dilophosaurus, Welles, 1984; Pro- in disarticulation. Thus, the intra-mandibular joint , Sereno and Wild, 1992; Allosaurus, seems to be designed for flexion and extension of the Madsen, 1976; , Ostrom, 1969). mandibular ramus in a vertical, or near vertical, plane, Although some basal dinosaurs have short epipoph- with approximately 150 of rotation. yses on the third cervical, they are poorly developed In anguimorph lepidosaurs, a hinge-like intra-man- or absent in other cervical vertebrae. In the ornithis- dibular joint is present (McDowell and Bogert, 1954: chians Lesothosaurus (Thulborn, 1972:fig. 3A, C; Ser- fig. 32). In several regards, it is functionally analogous eno, 1991a:fig. 8A) and (SAM to the sliding joint in H. ischigualastensis and thero- 1332), low epipophyses are present on the third cer- pods. Except for the enlarged coronoid bone, the joint vical vertebra but are absent in the axis and in mid- in anguimorphs is developed between the same ele- and posterior cervical vertebrae. In basal sauropodo- ments and divides the lower jaw into the same anterior morphs, epipophyses are present on the axis but are and posterior segments. Mandibular flexion in lepi- either absent or very weak in postaxial cervical ver- dosaurs appears to be an adaptation for a grasping, as tebrae (e.g., Plateosaurus, , Camarasau- opposed to a crushing, bite (McDowell and Bogert, rus). Gauthier (1986:16) cited the presence ofepipoph- 1954:106). The toothed anterior segment of the man- yses on anterior cervical vertebrae as a saurischian dible flexes around struggling prey, preventing escape synapomorphy, but postaxial epipophyses are usually from the open anterior ends of the jaws. The dentary absent in basal sauropodomorphs. Except for low epi- tooth row is proportionately shorter in anguimorphs pophyses in the third cervical vertebra of some basal with a flexible intra-mandibular hinge, a condition also ornithischians and sauropodomorphs, prominent present in H. ischigualastensis, where the dentary tooth postaxial epipophyses occur elsewhere in Dinosauria row is only two-thirds as long as the opposing maxillary only among theropods. tooth row (Figs. 7A). Thus, a mandibular adaptation Among dinosaurian outgroups, postaxial cervical for securing live prey in anguimorph lepidosaurs may epipophyses are absent in "Lagosuchus" (Bonaparte, be the best functional hypothesis for the sliding joint 1975; Sereno and Arcucci, 1994) and apparently in in the lower jaw of H. ischigualastensis. An alternative most pterosaurs (Wild, 1978; Wellnhofer, 1978). Sev- explanation, that the intra-mandibular joint allowed eral advanced short-necked pterodactyloids have well- lateral flexion for increasing gape, is not tenable given developed postaxial epipophyses (Howse, 1986), which the vertical articular surfaces between the dentary, an- is regarded here as an independent acquisition. gular, and surangular. In H. ischigualastensis, as in extant lepidosaurs, flexion of the intra-mandibular joint The Intra-Mandibular Joint must have been accomplished largely by elastic liga- ments that spanned the joint, because adductor mus- A sliding intra-mandibular joint is well-developed culature could not have extended far enough anteriorly in H. ischigualastensis (Figs. 1, 3, 7), which closely to insert on any part of the anterior segment of the resembles that in theropods. Two articulations are de- lower jaw. The elastic connecting ligaments may have veloped between anterior and posterior segments of also functioned to absorb, or soften, impact during the lower jaw dorsal and ventral to the external man- biting. dibular fenestra. In the dorsal articulation, the slender, finger-shaped posterodorsal ramus of the dentary ar- Tooth Injuries ticulates in a deep slot in the surangular. The slender dentary process is strengthened by an elongate, per- Three healed puncture wounds are present on the pendicular strut (Fig. 1D), which appears to limit ex- new skull of H. ischigualastensis (Figs. 1B, C, E, 5; tm). tension of the joint. In the ventral articulation, a flat One is located on the posterior margin of the left pa- subtriangular process of the dentary and a tongue- rietal, and two occur on the left splenial. These punc- shaped posterior process of the splenial articulate ture wounds were inflicted independently, as evi- against the lateral and ventral aspects, respectively, of denced by their disparate locations and orientations. the polished, hook-shaped anterior end of the angular. The parietal puncture passes through the free pos- The shape of the articular processes and the orien- terior margin of the left parietal. Natural lineations in SERENO AND NO SKULL AND NECK 473 VAS--HERRERASAURUS the surface texture of the parietal are deflected around In summary, H. ischigualastensis cannot be posi- the wound, suggesting that some bone remodeling had tioned as the to other dinosaurs on the occurred after the injury. The splenial wounds are lo- basis of the skull and neck (Sereno and Novas, 1992). cated on the distal one-half of the bone along the ven- Cranial and cervical evidence suggests, to the contrary, tral margin of the left lower jaw. The anterior wound that H. ischigualastensis is a basal theropod. was sustained from a ventral direction; the posterior wound is located 4 cm away near the distal end of the ACKNOWLEDGMENTS tongue-shaped process and was sustained from a me- We thank C. Abraczinskas and C. Vanderslice for dial direction, nearly perpendicular to the axis of the skilled execution of the finished illustrations. We also first wound (Figs. 1E, 5). The porous swollen rim around thank A. Monetta, J. Powell, and C. Schaff for access each of the wounds suggests that the injury was fol- to in their care and H.-D. Sues and D. B. Brink- lowed by an initial that later healed with some man for access to fossils under their study. For critical deposition of new bone (Rothschild and Tanke, in review of the manuscript, we are indebted to P. Currie, press). No additional puncture marks are evident on C. Forster, and H.-D. Sues. This research was sup- the dorsal aspect of the snout or elsewhere on the skull. ported by National Science Foundation Research Grant The large rauisuchian Saurosuchus is the only other BSR-8722586, The David and Lucile Packard Foun- known contemporary predator in the Ischigualasto dation, and the National Geographic Society (to P. S.). Formation with a gape large enough to produce these Acknowledgment is also made to the Donors of The puncture wounds. However, the moderate size of the Petroleum Research Fund, administered by the Amer- puncture wounds, and the fact that the wounds were ican Chemical Society (grant ACS-PRF 22637-G8 to not responsible for the death of the , suggest P. S.), for partial support of this research. that they may have been sustained during intraspecific aggression. LITERATURE CITED CONCLUSIONS Barbarena,M. C. 1978. A huge thecodont skull from the of . Pesquisas 9:62-75. The skull and neck of Herrerasaurus ischigualasten- Barsbold,R., T. Maryafiska,and H. Osm61ska. 1990. Ovi- sis confirm that herrerasaurid dinosaurs were active raptorosauria;pp. 249-258 in D. B. Weishampel, P. predators. The skull is long and narrow, with diagnostic Dodson, and H. Osmolska (eds.), The Dinosauria.Uni- features that include the semicircular anterior margin versity of California Press, Berkeley. of the antorbital opening, the narrow proportions of Barsbold,R., and H. Osm61ska. 1990. ; the dorsal half of the laterotemporal opening, and the pp. 225-244 in D. B. Weishampel, P. Dodson, and H. broad depression anterior to the supratemporal open- Osmolska (eds.), The Dinosauria. University of Cali- ing. The neck is relatively slender with prominent pro- fornia Press, Berkeley. M. J. 1984. of the German Late cesses (epipophyses) for attachment of cervical mus- Benton, Triassic and the origin of dinosaurs;pp. 13-18 in W. E. culature. Reif and F. Third on Me- in the skull include the Westphal (eds.), Symposium Dinosaurian synapomorphies sozoic Terrestrial Ecosystems. Attempto Verlag, Tii- absence of the postfrontal, lateral exposure of the ar- bingen. ticular head of the quadrate, and reduction in the size 1990. Origin and interrelationshipsof dinosaurs; of the post-temporal opening. The functional signifi- pp. 11-30 in D. B. Weishampel, P. Dodson, and H. cance of these characters remains obscure. All of these Osm61ska(eds.), The Dinosauria. University of Cali- potential dinosaurian synapomorphies may eventually fornia Press, Berkeley. be shown to occur in immediate dinosaurian out- Bonaparte,J. F. 1972. Los tetraipodosdel sector superior groups, such as "Lagosuchus," when more complete de la Formaci6n Los Colorados, La Rioja, Argentina material is discovered. (TriaisicoSuperior). Parte I. Opera Lilloana 22:1-183. . 1975. Nuevos materiales de Lagosuchus talam- Within dinosaurs, one is shared only apomorphy payensis Romer (-) y su sig- between H. ischigualastensis and ornithischians, the nificado en el origen de los .Chafiarense in- broad, triangular posterolateral process on the pre- ferior, Triasico medio de Argentina. Acta Geologica maxilla. This feature appears to have arisen indepen- Lilloana 13:1-90. dently in H. ischigualastensis and ornithischians, in and F. E. Novas. 1985. Abelisauruscomahuensis, the light of other character evidence. Three apomor- n. g., n. sp., Carnosauriadel Cretficicotardio de Pata- phies are shared with saurischians, the most notable gonia. Ameghiniana 21:259-265. being the presence of a subnarial foramen. Finally, two --, F. E. Novas, and R. A. Coria. 1990. Carnotaurus apomorphies are shared with theropods, an intra-man- sastrei Bonaparte, the horned, lightly built carnosaur from the Middle Cretaceousof Patagonia.Contributions dibular joint and prominent postaxial epipophyses. The in Natural Museum of Los intra-mandibular in H. allowed Science, History Angeles joint ischigualastensis County 416:1-42. the toothed anterior segment (composed of dentary Brinkman,D. B., and H.-D. Sues. 1987. A staurikosaurid and splenial) to rotate about 150 and is interpreted here dinosaur from the Upper Triassic Ischigualasto For- as a mechanism enhancing prey capture, as appears to mation of Argentina and the relationshipsof the Stau- be the case in some extant anguimorph with rikosauridae.Palaeontology 30:493-503. analogous intra-mandibular flexibility. Chatterjee, S. 1978. A primitive parasuchid () 474 JOURNAL OF VERTEBRATE PALEONTOLOGY, VOL. 13, NO. 4, 1993

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APPENDIX CHARACTERS AND TAXON-CHARACTER MATRIX We show below the coding and distribution of 19 binary 2. Premaxilla-maxilla fenestra: absent (0), present (1). characters for Herrerasaurus ischigualastensis, Staurikosau- 3. Anterior margin of antorbital fossa: asymmetrical (0); rus pricei, several basal dinosaurs (Ceratosaurus nasicornis, semicircular (1). Plateosaurus englehardti, Lesothosaurus diagnosticus), and 4. Supratemporal depression: less than (0), or equal to immediate dinosaurian outgroups ("Lagosuchus" lilloensis, or greater than (1) the area of supratemporal opening. Pterosauria, ). Characters 1-10 are interpreted as 5. Jugal ridge: absent (0), present (1). autapomorphies of Herrerasaurus ischigualastensis. Char- 6. Dorsal portion of laterotemporal fenestra: subequal acters 1 1-19 are interpreted as synapomorphies supporting to (0), or less than one-half of (1) ventral portion. Dinosauria or a dinosaurian subgroup. The equivocal status 7. Ventral process of squamosal, shape: tapered (0); of a number of the characters due to missing data is under- subquadrate with lateral depression (1). scored and treated in more detail elsewhere. 8. Posterior portion ofquadratojugal, orientation: lateral (0); deflected strongly medially (1). 9. Basal tuber width: less than (0), or equal to (1) width Herrerasaurus ischigualastensis of occipital condyle. 1. Posterolateral process of premaxilla: short (0); long 10. Slender, ribbed posterodorsal process of dentary and and broad (1). slotted surangular: absent (0), present (1). 476 JOURNAL OF VERTEBRATE PALEONTOLOGY, VOL. 13, NO. 4, 1993

Dinosauria 19. Postaxial cervical epipophyses: rudimentary or absent 11. Postfrontal: present (0); absent (1). (0), prominent (1). 12. Pterygoid-ectopterygoid articular relation: ectopter- ygoid ventral (0); ectopterygoid dorsal (1). 5 10 15 13. Lateral of head: hidden exposure quadrate by squa- Herrerasaurus ischi- mosal (0); laterally exposed (1). gualastensis 11111 11111 11111 1111 14. Size of post-temporal fenestra (0); foramen opening: Staurikosaurus pricei ????? ????? ????? ??1? (1). Ceratosaurus nasicornis 00000 10000 11111 1111 Plateosaurus engel- Saurischia hardti 00000 00000 11111 1100 15. Subnarial foramen: absent (0); present (1). Lesothosaurus diagnos- 16. Jugal-lacrimal articular relation: lacrimal overlaps ju- ticus 10000 00000 11110 XX00 gal (0); jugal overlaps lacrimal (1). "Lagosuchus" lilloensis ?0??? ????? ????? ???0 17. Shape of posterior process ofjugal: tapering (0); forked PTEROSAURIA 00000 00000 00000 0000 (1). CRUROTARSI 00000 00000 00000 0000 Character-state abbreviations: 0 = plesiomorphic state; 0 = Theropoda plesiomorphic state for a with some ingroup variation; 18. Intra-mandibular joint: absent or poorly developed 1 = apomorphic state; ? = not preserved/unknown; X = (0); well-developed (1). unknown due to transformation.

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