B A S A L D I N O S A U R I F O R M R E M A I N S F R O M

B R I T A I N A N D T H E D I A G N O S I S O F T H E

D I N O S A U R I A

by N . C . F R A S E R , K . P A D I A N , G . M . W A L K D E N and A . L . M . D A V I S

ABSTRACT. A new genus and species of dinosauriform is described from the Triassic of south-west England. Thedescription is based on isolated elements recovered from an assemblage of other dissociated tetrapod remains thatinclude crocodylomorphs, rauisuchiforms and sphenodontians. The key elements in the new taxon are the ilium,astragalus, and the humerus, and these exhibit ®ve synapomorphies of Dinosauria. Three of these, namely a largely tofully perforate acetabulum, the presence of a brevis fossa, and a reduced astragalus with an ascending process, areconsidered to be particularly relevant. The de®nition and diagnosis of the Dinosauria are restated and the positions ofthe new form, herrerasaurs and Eoraptor relative to true dinosaurs are discussed.

KEY WORDS: dinosauriform, Dinosauria, Late Triassic, south-west England, ®ssure ®lls.

A P A R T I C U L A R feature of the Lower Carboniferous limestones below the Carboniferous/Mesozoicunconformity in the Mendip and Cotswold hills of south-west England and parts of South Wales is thepresence of early Mesozoic sediments lodged within palaeokarstic cavities. The sediments are Triassic toMid Jurassic in age, and result from numerous successive tectonic and emergent events that ®ssured andexposed the limestones. Quarrying operations over the past century have repeatedly encountered these`®ssure deposits,' which became particularly famous in the search for early mammals (e.g. Moore 1867,1881; Parrington 1941, 1946; KuÈhne 1956; Kermack et al. 1956, 1973; Kermack 1975). Perhaps the richestand certainly the most varied locality is Cromhall Quarry, Avon. A detailed study of this locality resultedin a stratigraphic framework that potentially places all the fossil assemblages from these Mesozoic ®llsinto context (Walkden and Fraser 1993). In 1990 a completely new ®ll was discovered, corresponding toan ancient cave deposit, with a great density of relatively large bones concentrated in a layer 50±100 mmthick, essentially forming a bone bed. Its assemblage is clearly different from any of the assemblagespreviously described from the quarry. Its lepidosauromorphs (mostly sphenodontians) are known inabundance in the other ®lls, but numerous new archosaurian elements include some with distinctdinosaurian characteristics. Their description and analysis form the principal subject of this paper.

T H E N E W A S S E M B L A G E

The principal component of the new assemblage is archosaurian, and the sphenosuchian crocodylomorphTerrestrisuchus is the predominant taxon. In this respect the new faunal assemblage more closelyresembles one described from Pant-y-ffynon Quarry (Crush 1980, 1984; Warrener 1983) than it doesany of the other assemblages from Cromhall. Three archosaurs dominate this new assemblage:Terrestrisuchus, a small rauisuchiform, and a dinosauriform. Recent work suggests the presence of twodistinct morphs of Terrestrisuchus in the Cromhall assemblages (Fraser in prep.), which in turn appear todiffer in some respects from the type species (Terrestrisuchus gracilis) from Pant-y-ffynon. The smallestarchosaurian so far identi®ed in the new deposit is tentatively assigned to the Rauisuchiformes (sensuParrish 1993). Several small rauisuchians have been reported from a wide range of ®ssure localities(e.g. Robinson et al. 1952; Robinson 1957; Whiteside 1983; Fraser 1988a, 1994). Although they seem tobe generally smaller than Terrestrisuchus, there is clearly an overlap in size between mature rauisuchiform

[Palaeontology, Vol. 45, Part 1, 2002, pp. 79±95] q The Palaeontological Association

individuals and the fairly abundant juvenile specimens of Terrestrisuchus. The third archosaur is acompletely new form that is not known from any of the other ®ssure deposits. It bears certain uniquelydinosaurian characteristics, yet in other respects it is unlike any basal members of the main dinosauriformclades (including Herrerasauridae).

D E S C R I P T I O N A N D C O M P A R I S O N S

The elements referred here to a single taxon were all dissociated, so it is an inference that they belong toone species. However, this is standard practice in vertebrate palaeontology (e.g. Long and Murry 1995),and potential association is best tested by recurrence of the same pattern. In this assemblage, the taxonomicassignment of most isolated elements to crocodylomorphs, rauisuchiforms, or dinosauriforms is clear.Furthermore, the ®rst two taxa are known from other localities, so their association is uncontroversial. Ofalmost 800 archosaurian elements recovered so far, approximately 85 per cent belong to Terrestrisuchus, 5per cent belong to rauisuchiforms, and the remaining 10 per cent are compatible with basal dinosauriformstructure. The apparent absence of additional archosaurs suggests that this last group of elementsrepresents a single taxon, as we document in the following comparisons.

We accept that the concept of Dinosauria must be based on a phylogenetic de®nition (Padian and May1993; Padian and Currie 1997). This implies a diagnosis of characters, which in turn may be used to assesswhether newly discovered material pertains to the taxon. We examine the following features in this light.

S Y S T E M A T I C P A L A E O N T O L O G Y

Class REPTILIA

Infraclass ARCHOSAUROMORPHA

Superorder ARCHOSAURIA

Order DINOSAUROMORPHA

AGNOSPHITYS gen. nov.

Agnostiphys cromhallensis sp. nov.

Text-®gures 1, 2B, 3±4, 5A, 6B, 7, 8K±N

Derivation of name. Greek, unknown or uncertain, with reference to the position of the new form relative to theDinosauria. The speci®c name refers to the locality.

Holotype. VMNH 1745, left ilium.

Referred material. VMNH 1751, left maxilla; 1748, left astragalus, 1749, right astragalus; and 1750, right humerus.AUP 11095A, isolated tooth.

Locality. Cromhall Quarry, Avon, England; Upper Triassic ®ssure deposits.

Diagnosis. Dinosauromorph with a well-de®ned brevis fossa on the ilium; semi-perforate acetabulum;`kidney-shaped' antitrochanter; well-developed posterior portion of the iliac blade; two sacral vertebrae;subrectangular deltopectoral crest that is 33 per cent of the length of the humerus; astragalus with a distinctascending process and a prominent depression immediately posterior to the ascending process; in dorsalaspect an acute anteromedial corner on the astragalus.

Description

MaxillaAn incomplete maxilla (Text-®g.1) bearing laterally compressed serrated teeth is basically similar in shape to that of atheropod or herrerasaurid dinosaur, but there is no indication of a subnarial foramen, and interdental plates are absent.

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In other respects there is nothing particularly remarkable about this element. Much of the anterior two-thirds of theelement is present, including the dorsal process that separated the external naris from the antorbital fenestra. Tenalveoli are preserved, with the remains of four teeth still in position. The teeth are recurved, bear serrated keels, andbroadly resemble the teeth of Terrestrisuchus from the same deposit. However, they are not as laterally compressed,and they bear a shallow longitudinal furrow along the length of the crown that is absent in Terrestrisuchus (Text-®g. 2).The narrow dorsal process slopes dorsoposteriorly and has a straight anterior edge that shows no indication of asubnarial foramen. A shallow groove running along the anterior margin was for the reception of the nasal.

HumerusThe humerus (Text-®g. 3) is represented by a number of fragments and two almost complete specimens. Together theyoffer some of the most convincing evidence for the dinosauriform af®nities of the new form. Sereno (1991), in an

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TEXT-FIG. 1. Agnosphitys cromhallensis gen. et sp. nov., left maxilla, VMNH 1751, in A, lateral and B, medial views.Scale bar represents 5´0 mm.

analysis of basal archosaurs, recorded a basic difference in the shape of the humerus between ornithodirans(pterosauromorphs and dinosauromorphs) and pseudosuchians. He noted that in pseudosuchians (crurotarsans ofSereno 1991) the proximal head had a prominent medial tuberosity, and that distal to the tuberosity the medial marginwas strongly arched. In basal archosauriforms and ornithodirans, by contrast, the medial margin of the humerus isnearly straight and lacks the distinct tuberosity. Furthermore, Sereno recorded a difference in the shape of thedeltopectoral crest. In Ornithodira the deltopectoral crest is subrectangular in shape and its distal corner, or apex,extends down the shaft for a distance in excess of 25 per cent of the total length of the humerus. Pseudosuchians, on theother hand, typically have a crescentic deltopectoral crest that is positioned within the proximal 25 per cent of thelength of the humerus. Parrish (1993) questioned the precise de®nition of these characters because certain features aregradational, particularly the degree of curvature of the medial margin of the proximal head. More recently, Sereno(1994) clari®ed his description of the deltopectoral crest character, stating that in Dinosauria the deltopectoral crestwas at least 35 per cent of the length of the humerus. Even so, the new humeri are suf®ciently distinct to dispel anysuggestion that they are pseudosuchian. They have a very distinct square-ended proximal head, and the markeddeltopectoral ridge extends fully one-third along the length of the shaft. They lack any pterosaurian features, such asthe greatly enlarged deltopectoral crest and exceptionally thin-walled bones.

IliumThe most intriguing element is the ilium (Text-®g. 4). To date at least ten partial or almost complete specimens havebeen recovered, permitting a complete restoration (Text-®g. 5A). A well-developed supra-acetabular ridge deeplyoverhangs the semi-perforate acetabulum, and it has a prominent antitrochanter. The portion of the antitrochanterpreserved on the ilium is elliptical (not circular as in Terrestrisuchus; Text-®g. 5B), like the `kidney-shaped'antitrochanter (combined contribution from the ilium and ischium) that Novas (1996) described for Herrerasaurus andDinosauria. The iliac blade is generally rather robust, but the posterior portion of the iliac blade is more developed thanin herrerasaurs, and it is more like that of a theropod in this respect. On the other hand, in sharp contrast to theropods,there is no exaggerated anterior development of the iliac blade. The facets for the sacral ribs are partly con¯uent andsomewhat indistinct, and so it is dif®cult to ascertain the number of sacral vertebrae, though there seems to have beenonly two. Basal dinosaurs characteristically had three sacral vertebrae, so the new form does not have the dinosaurian

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TEXT-FIG. 2. Teeth of A, ?Terrestrisuchus sp. and B, Agnosphitys cromhallensis gen. et sp. nov. from Cromhall Quarry.Scale bar represents 1´0 mm.

condition. Nor do the dinosauriforms Staurikosaurus, Herrerasaurus, or Eoraptor have more than two, though somenon-dinosaurian archosaurs do (e.g. the poposaurid Postosuchus).

Basal dinosauromorphs such as Lagosuchus/Marasuchus show some similarities to the new form, particularly in theoverall shape of the iliac blade and the deep overhang of the acetabulum by the supracetabular ridge. However,Lagerpeton has a completely closed acetabulum, whereas the new form is semiperforate. Sereno and Arcucci (1994)reconstructed Marasuchus with a semicircular opening in the acetabulum. But Novas (1996) contended that thespecimen (PVL 3870) on which Sereno and Arcucci based their restoration was incomplete, and that the ventral marginof the ilium had a triangular projection (like those of pseudosuchians with an imperforate acetabulum), so in reality theacetabulum in Marasuchus was practically imperforate. There is no question that the new form lacks the triangularprocess on the ventral margin, and that the acetabulum was therefore perforate. This suggests a placement closer todinosaurs than Marasuchus.

Perhaps the most signi®cant feature of the new ilium is the presence of a clearly de®ned brevis fossa at the base ofthe iliac blade immediately behind the acetabulum. To our knowledge, this character occurs only within Dinosauria,where it is particularly well pronounced in basal taxa, including basal theropods, sauropodomorphs and ornithopods.Within herrerasaurs there is only an incipient fossa: Novas initially described a horizontal furrow in this region, but didnot call it a brevis fossa until very recently. It might be more accurate to refer to this as a brevis shelf, lacking any realdevelopment of the brevis fossa. Although Sereno and Arcucci denied the presence of a brevis fossa in thedinosauromorph Marasuchus, there is certainly a small indentation in this region (Text-®g. 5C) that is not seen inother archosaurian ilia (including that of Terrestrisuchus) from the same ®ssure deposit (Text-®g. 5B). Once again thiscould be interpreted as an incipient brevis shelf that is not present in phytosaurs such as Rutiodon, nor in rauisuchidssuch as Postosuchus.

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TEXT-FIG. 3. Right humerus of Agnosphitys cromhallensis gen. et sp. nov., VMNH 1750,in A, lateral and B, medial views. Scale bar represents 5´0 mm.

TEXT-FIG. 4. Agnosphitys cromhallensis gen. et sp. nov., the holotype, VMNH 1745, aleft ilium in lateral view.

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TEXT-FIG. 5. A, Agnosphitys cromhallensis gen. et sp. nov., restoration of the right ilium in lateral view.B, ?Terrestrisuchus sp., right ilium, AUP 11320, in lateral view. C, Marasuchus/Lagosuchus, right ilium in lateral

view (after Sereno and Arcucci, 1994). Scale bars represent 5´0 mm.

In some respects the ilium resembles that of the early dinosauriforms Herrerasaurus (Text-®g. 6C) and Eoraptor,but in outline it is more like that of Marasuchus/Lagosuchus in that both are approximately rhomboidal in outline andpossess a distinct posterior process of the iliac blade (Text-®g. 6A±B). The preacetabular process of the new form lacksthe great extension typical of theropods such as Coelophysis and Dilophosaurus (Text-®g. 6F), and does not readilyconform to the overall morphology of basal ornithischians such as Lesothosaurus (Text-®g. 6E), or basal sauropo-domorphs such as Thecodontosaurus or Plateosaurus (Text-®g. 6D).

AstragalusThree complete astragali of basal dinosaurian con®guration (Text-®g. 7) are known from the deposit. In anterior andposterior views they have a distinct ascending process, but in all three specimens their proximal parts are broken offand their full extent is unclear. It would appear to have been more pronounced than in Marasuchus and was probablycomparable in height to those of Lagerpeton (Sereno and Arcucci 1994a, ®g. 4) and Herrerasaurus (Text-®g. 8G±J)(Novas 1989, 1992, 1994). There is a prominent vascular depression on the anterior surface. In addition, immediatelyadjacent to the ascending process there is a marked basin visible in posterior view that is also pierced by a smallforamen. Novas (1996) regarded this basin as a fundamental character of the dinosaurian astragalus, giving theascending process its characteristic pyramidal shape. This is markedly different from the condition in Lagerpeton andMarasuchus, in which there is no depression behind the ascending process, which is essentially a longitudinal ridgeseparating the facets for the tibia and ®bula. Viewed dorsally, these astragali have an acute anteromedial corner, whichSereno (1991) cited as a synapomorphy of Dinosauromorpha. Chindesaurus, described as a herrerasaur by Long andMurry (1995), does not have quite such an acute antero-medial angle. Furthermore, in Chindesaurus the ascendingprocess is much closer to the lateral edge of the astragalus than in the Cromhall form, Herrerasaurus, and theropods

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TEXT-FIG. 6. Left ilia of dinosauromorphs in lateral aspect. A, Marasuchus/Lagosuchus. B, Agnosphitys cromhallensisgen. et sp. nov. C, Herrerasaurus. D, Plateosaurus. E, Lesothosaurus. F, Dilophosaurus.

such as Dilophosaurus (Novas 1989). Its mediolateral width and lateral surface suggest that the calcaneum was greatlyreduced and that the astragalus bore most of the contact with the ®bula. These features suggest that the Cromhall formis at least as close to dinosaurs as Herrerasaurus is.

D I S C U S S I O N

Elements of Agnosphitys clearly exhibit dinosaurian af®nities, but before its phylogenetic position can beproperly evaluated the question of the constitution of the Dinosauria needs to be brie¯y reviewed. In the`phylogenetic system', the name of a taxon presumes a de®nition, which is based on a principle of commonancestry of all the members of the taxon. This name cannot be altered by changes in diagnosis or placementof individual constituent taxa (de Queiroz and Gauthier 1990, 1992, 1994; Padian et al. 1999). In contrast,de®ning a group on the basis of the presence or absence of characters (apomorphy-based de®nition) isunstable, and a name erected upon a speci®ed list of taxa (taxon-based de®nition) may not allow theinclusion of forms still to be found in the future, or the rearrangement of known forms (Padian and May1993; Padian 1997a). In the interests of taxonomic stability, we support the premise that the de®nition of ataxon should be based on ancestry rather than a list of characters or taxa (Padian and May 1993; Padian1997a, b; Sereno 1997; Padian et al. 1999).

Although it was not until 1974 that Owen's Dinosauria were ®rst explicitly stated to bemonophyletic by Bakker and Galton, Owen constituted his group and diagnosed it by a uniquecombination of features (Desmond 1979), and incorporated Iguanodon, Megalosaurus and Hylaeo-saurus into it. Today, Dinosauria is de®ned as Triceratops (representing Ornithischia) plus Aves(representing Saurischia) and all the descendants of their last common ancestor. This is a de®nitionof Dinosauria based on ancestry, not on a list of characters or taxa (Gauthier 1986; Padian andMay 1993).

Having de®ned Dinosauria, it is then possible to evaluate its diagnostic characteristics. Various authors(e.g. Gauthier 1986; Benton 1994; Sereno 1991, 1997) have used as many as nine cranial and 50postcranial synapomorphies to support the monophyly of Dinosauria. The principal characters discussedhere are listed in the Appendix. Disputes concerning these characters only serve to emphasize theimportance of basing taxa on de®nitions of ancestry, not on the characters themselves. There are twoprincipal problems that underlie these disputes, which must be recognized before the Cromhall form ormany others can be fully assessed.

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TEXT-FIG. 7. Left astragalus of Agnosphitys cromhallensis gen. et sp. nov., VMNH 1745, in A, proximal B, distal, C,posterior and D, anterior views. Scale bar represents 5´0 mm.

Selective diagnoses

Sereno and Novas (1994; also Sereno 1994, and Novas 1994) disputed many of the synapomorphieslisted in the Appendix because they are not found in Herrerasaurus. They contended that becauseHerrerasaurus shared other synapomorphies of saurischians and theropods (e.g. a subnarial foramen,elongate manus, epipophyses on the cervical vertebrae, and a pubic foot), it was nested well withinDinosauria. The fact that herrerasaurs only share a few of the previously considered dinosaursynapomorphies does not automatically reveal a problem with the previous diagnosis of Dinosauria: itcould equally indicate that herrerasaurs lie outside Dinosauria. The supposed saurischian and theropodsynapomorphies of herrerasaurs might be convergences, or characters that apply to a more general levelthan Dinosauria. This is not surprising, inasmuch as herrerasaurs and theropods retain the basiccarnivorous habits of ornithodirans generally, so would be expected to have shared primitive andconvergent features related to that way of life. We know that saurischians and ornithischians weredinosaurs; the question is whether herrerasaurs also belong in this group. Herrerasaurs may be dinosaurs,but this needs to be tested against a matrix of characters that true dinosaurs share. Holtz and Padian(1995) were unable to place herrerasaurs and Eoraptor as theropods, and equally parsimonious treesbased on nearly 100 characters situated them either as saurischians outside Sauropodomorpha andTheropoda, or as dinosauromorphs outside true Dinosauria.

Missing data and character state polarity

A further problem is that data on many of these key characters are missing for other basaldinosauromorphs, such as Lagerpeton, Lagosuchus/Marasuchus, Pseudolagosuchus, and even basalornithischians. Many of these taxa are unrepresented by skull and hand characters, or by anything buthindlimbs. And it is important to remember that we have no good material of ornithischians until theEarly Jurassic, so we are missing some 20±25 million years of early ornithischian evolution. The EarlyJurassic ornithischians are so transformed that it is dif®cult to obtain a picture of their basal form (thepoorly preserved and incomplete Pisanosaurus provides our only tantalizing glimpse of Late Triassicornithischians). As these basal dinosauromorphs become better known, we should have a much betteridea of character distributions, polarities, and the degree of convergence and retained primitive featuresthat are now problematic.

Distribution of synapomorphies within basal Dinosauriformes

The distribution and interpretation of some characters that have been central to recent discussions of basaldinosaur systematics require a brief review.

Three sacral vertebrae. The presence of three sacral vertebrae is frequently cited as an apomorphy ofDinosauria (e.g. Benton 1990; Gauthier 1986). Sereno and Novas have variously stated that there are two(Sereno and Novas 1992) or three sacrals in Herrerasaurus (Sereno et al. 1993). Most recently, accordingto Novas (1996), there are clearly only two, as in Lagosuchus/Marasuchus and Lagerpeton, and heregarded the condition as an apomorphic reversal in herrerasaurs in order to include it within Dinosauria.This view is less parsimonious than the conclusion that two sacrals is a retained primitive feature.Pterosaurs, convergently, have more than two sacrals. Benton (1999) reported as many as four forScleromochlus, which he considered to be the sister taxon to pterosauromorphs and dinosauromorphs. Butall true dinosaurs have at least three.

Brevis shelf and fossa. Novas (1996) was among the ®rst to propose the brevis fossa as a derived characterof Dinosauria, and to distinguish between the brevis shelf and the brevis fossa. We follow Novas inregarding the brevis shelf as a posterolateral margin of the iliac blade. The brevis fossa is the ventral troughthat develops between the brevis shelf and the posteromedial wall of the iliac blade. In Herrerasaurus theshelf is represented by a rudimentary ridge and the fossa is little more than an incipient furrow at best.Novas again postulated that this is a reversal in Herrerasaurus, because he regarded it as nested well

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within Theropoda. In the Cromhall form there is a very distinct brevis shelf and, between it and theposteromedial wall of the ilium, the brevis fossa takes the form of a distinct trough. In basal theropods (e.g.Coelophysis), basal sauropodomorphs (e.g. Ammosaurus, Plateosaurus), and basal ornithischians (e.g.Lesothosaurus), the brevis fossa is even more dorsoventrally deepened. The condition in Herrerasaurus isbest considered primitive, not secondarily derived, especially given the functional correlates of the brevisfossa for theropod hindlimb evolution (Gatesy and Dial 1996).

Antitrochanter. The antitrochanter at the posterior border of the acetabulum is a raised area that articulatedwith the greater trochanter on the proximal head of the femur. Sereno and Novas (1990), Novas (1992),and Sereno and Arcucci (1994) considered the presence of an antitrochanter diagnostic of Dinosauri-formes. However, an antitrochanter is known in Caiman as well as in Terrestrisuchus (Crush 1984; NCF,pers. obs.) (Text-®g. 5B), and its shape may be more critical than its presence. In Lagerpeton theantitrochanter is a raised lip that is similar to that of Terrestrisuchus. On the other hand, Novas (1996)described a kidney-shaped antitrochanter as a more speci®c character for Dinosauromorpha, includingMarasuchus and Dinosauria. Nevertheless, Novas (1996) then went on to question the utility of thecharacter even in Dinosauromorpha, citing inconsistency within a single taxon. Thus in Herrerasaurus theantitrochanter was apparently present in only one of three available specimens of the pelvic girdle.

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TEXT-FIG. 8. Left astragali of dinosauromorphs. Marasuchus/Lagosuchus in A, anterior, B, proximal and C, distal views.Pseudolagosuchus in D, anterior, E, proximal and F, distal views. Herrerasaurus in G, anterior, H, proximal, I, distal andJ, posterior views. Agnosphitys cromhallensis gen. et sp. nov. in K, anterior, L, proximal, M, distal and N, posteriorviews. Dilophosaurus in O, anterior, P, proximal, Q, distal and R, posterior views. Riojasaurus in S, anterior, T, proximal

and U, posterior views. A±C after Sereno, D±J, O±U after Novas, 1989. Not to scale.

Furthermore, according to Novas, the antitrochanter is absent in Argentinian collections of sauro-podomorph material. The elliptical shape of the iliac portion of the antitrochanter in the Cromhall iliumis strongly suggestive of a kidney-shaped antitrochanter.

Astragalus. The astragalus has been one of the most critical elements in discussions of the diagnosis ofDinosauria, and its ascending process has assumed particular importance. Because of subtleties in thedescriptions of the ascending process, it might be easily miscoded. The ascending process in Marasuchus/Lagosuchus and Lagerpeton is anteroposteriorly elongate and separates the facets for the tibia and ®bula(Text-®g. 8A±C). In Herrerasaurus the ascending process assumes a more pyramidal form with thedevelopment of a prominent basin posterior to it (Text-®g. 8G±J). Novas (1989) considered this basin aderived feature and apomorphic for Dinosauria. The anterior hollow and its associated foramen areprominent features of Herrerasaurus and Marasuchus (Text-®g. 8) that are absent in Lagerpeton. Novas(1989) regarded the form of the tibiotarsus of Herrerasaurus as representative of the primitive conditionfor Dinosauria, from which those of Theropoda, Sauropodomorpha and Ornithischia were derived. In basalSauropodomorpha, Novas cited the transversely broad astragalus as a synapomorphy. Typically inTheropoda the ascending process is medially prolonged to form a `wall' separating the anterior anddorsal surfaces as seen in Dilophosaurus (Text-®g. 8O±R). In Ornithischia, such as Pisanosaurus and

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TEXT-FIG. 8. Continued.

Lesothosaurus, the astragalus has a vertical lateral surface that is continuous with that of the ascendingprocess, so it lacks a dorsal facet for articulation with the ®bula. In the ®ssure material the ascendingprocess is incomplete, and its full extent is consequently unclear. However, in common with Dinosauri-formes, the astragalus has an acute anteromedial corner. In addition the anterior hollow and foramen arepresent. More importantly, the posterior basin is well developed, and the position and shape of theascending process are very similar to those of Herrerasaurus, so it is consistent with Novas' concept of theprimitive condition for Dinosauria.

Other potential synapomorphies of Dinosauria, including the absence of the postfrontal, lateral exposureof the articular head of the quadrate, and reduction in size of the post-temporal opening, may alsoeventually be shown to occur in forms such as Marasuchus. Thus Sereno and Novas (1994) were rightlysomewhat cautious on these points. They were also correct to be critical of some other so-calleddinosaurian synapomorphies of the skull because they are dif®cult to support in many clearly acceptabledinosaurs. For example, the `slenderness' of the pterygoid and the `lightly built' palate are vague and mayvary with size or habitus. More precise descriptions of characters are needed.

Distribution of characters potentially uniting Herrerasaurus and Saurischia

Sereno and Novas (1994) discussed a number of characters that they argued as strong evidence for nestingHerrerasaurus well within Saurischia, and even within Theropoda, and these are re-examined here.

Lateral overlap of lacrimal by jugal. Sereno and Novas (1994, p. 471) stated that `lateral overlap of thelacrimal by the jugal appears to be the most common condition in crurotarsal archosaurs', but it is probablymore correct to say that the lacrimal overlaps the jugal in most crurotarsans, and Sereno and Novas citedthe examples of Saurosuchus and Ornithosuchus. However, as they also pointed out, sphenosuchians sharethe saurischian condition, in which the jugal overlaps the lacrimal. This is also true for an undescribedsmall `crocodylomorph' from the ®ssures (NCF, pers. obs.). The condition is variable in ornithischians. InLesothosaurus and Psittacosaurus, the lacrimal apparently weakly overlaps the jugal (Sereno and Novas1994). In hadrosaurs the jugal clearly overlaps the lacrimal, whereas in still other ornithischians the jugal-lacrimal articulation is non-overlapping (this is also, incidentally, the condition in Postosuchus). Becausethe condition in basal dinosaurs and their outgroups is variable or unknown, its value is questionable as anunequivocal apomorphy for Saurischia.

Posterior process of jugal forked. Sereno and Novas (1994) suggested that a forked posterior process of thejugal is a saurischian characteristic. Although in Sauropoda the jugal is broadly overlapped by thequadratojugal and is not forked, in the basal sauropodomorph Plateosaurus and most theropods the forkedprocess is clearly pronounced. However, in other basal sauropodomorphs, e.g. Anchisaurus, Massospon-dylus (at least as an adult), and Lufengosaurus, the forked process is absent or poorly de®ned. Furthermorea forked process is present in the ornithischians Psittacosaurus and Lesothosaurus. The condition in thedinosauromorphs Lagerpeton and Marasuchus is unknown, but even so, it is not a synapomorphy forSaurischia.

Subnarial foramen. Whereas a subnarial foramen is known to occur in some suchians (e.g. rauisuchians suchas Postosuchus), it is absent in most pseudosuchians, and it is not present in any ornithischians or pterosaurs.In herrerasaurs the feature identi®ed as a subnarial foramen is neither of the same size and shape nor in thesame position as in Saurischia. Again, the condition in basal dinosaurian outgroups is unknown.

Intramandibular joint. There is a well-developed intramandibular joint in both herrerasaurs andTheropoda. But, as Sereno and Novas (1994) mentioned, such a joint is also found in anguimorph lizards,and could be associated with a feeding adaptation. In any case the joint in herrerasaurs is very differentfrom the joint in Theropoda. Rieppel and Zaher (in press) have shown that detailed relationships of theindividual mandibular elements to one another are of primary importance in phylogenetic considerations;the intramandibular joint itself is only secondary.

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Other features. Sereno (1994) cited prominent postaxial cervical epipophyses as a character unitingherrerasaurs and theropods. The condition in Lagerpeton and basal ornithischians is not known, butMarasuchus apparently lacks epipophyses. Other possible synapomorphies of herrerasaurs and theropodscited by Sereno and Novas (Novas, 1994; Sereno 1994; Sereno and Novas 1994) include the strap-shapedscapula (present in pterosaurs and Terrestrisuchus), the distally enlarged pubis (present also inrauisuchians), the elongate manus, strongly reduced metacarpals IV and V, and the reduction in thelength of the humerus. The latter three characters may be of particular signi®cance but they are not knownin the immediate dinosaurian outgroups. Again, our record of basal sauropodomorphs and ornithischians isquite poor, and the manus is not known in other ornithodirans except the highly modi®ed pterosaurs.Whereas this is not evidence against the hypothesis of synapomorphy, it suggests caution. For example, thescapular blade in herrerasaurs is thin and greatly reduced, and does not ¯are dorsally as in basal theropodssuch as ceratosaurs, so this is clearly homoplastic resemblance. The pubis is slightly retroverted and basaltheropods lack the triangular posterior expansion of the distal end seen in herrerasaurs, so this is alsoconvergent. The manus is only `elongate' and the humerus `reduced' compared to some neutral metric thathas not been speci®ed. And the reduced condition of metacarpals and phalanges of digits IV and V areclearly convergent, because they are not so reduced in basal theropods.

This questionable support for inclusion of Herrerasaurus within Theropoda is offset by the absence inHerrerasaurus of other characters previously used to diagnose Theropoda and Saurischia. Benton (1990)gave at least ®ve sacral vertebrae as a character of Theropoda, which are clearly not present in Herrerasaurusor Eoraptor. In addition, the long preacetabular process on the ilium and a pronounced brevis fossa are notfound in Herrerasaurus. (The term `pronounced' is admittedly subjective.) Equally important is theretention in Herrerasaurus of what Novas (1989) termed the primitive tibiotarsal condition for dinosaurs,and not the more derived condition of Theropoda or Sauropodomorpha. Furthermore, the second ®nger is notthe longest in Herrerasauridae, as it is in all saurischians, and the thumb is not even slightly offset from theother digits, nor does it bear an enlarged claw as in all Saurischia basally (Gauthier 1986).

In view of these con¯icting data, we argue that the taxonomic position of Herrerasaurus is at bestequivocal (Holtz and Padian 1995). Consequently its characteristics should not be used as a basis fordiagnosing Dinosauria, Saurischia or Theropoda. Herrerasaurs are clearly dinosauriforms close todinosaurs, but we contend that they lack many of the unique features shared by the animals that Owenunited within the Dinosauria. Using dinosauromorphs (a more inclusive outgroup) to test the validity ofdinosaurian synapomorphies will inevitably lead to a change in composition not mandated by its node-based de®nition. By de®ning Dinosauria on the basis of a chosen set of apomorphies, it is possible to ¯ipthe position of Herrerasauridae. If the elongate manus, subnarial fenestra and pubic foot are used to theexclusion of the number of sacral vertebrae, the long preacetabular process, the pronounced brevis fossa,and so on, then Herrerasauria will nest well within Saurischia and even within Theropoda.

Incomplete material and isolated elements

Given the problems associated with character state polarities in taxa known from relatively completematerial, the dissociated elements from the ®ssure ®lls prove even more problematic. Our incompleteknowledge of the anatomy of crucial Late Triassic taxa impedes us from recognizing a sequence ofacquisition of characters and functional changes that marks the inception of the dinosaur lineage. We couldweight various characters, depending upon their distribution in non-dinosaurian lineages and theirdistribution in basal members of the Saurischia and Ornithischia. For example, the number of sacralvertebrae may be of crucial functional importance. Unfortunately `more than two sacrals' is not restrictedentirely to dinosaurs among Archosauria. But within true dinosaurs there is no reversion to two sacrals.The long deltopectoral crest on the humerus perhaps provides a consistent feature, but this is somewhatsubjective because it could be expected to vary with size and function (see, e.g., Mononykus). The formand distribution among archosaurs of the brevis fossa also has some merit, because it is conspicuous in allbasal dinosaurs such as Coelophysis, Lesothosaurus and basal sauropodomorphs, and absent in otherarchosaurs. Yet how does one interpret notches just posterior to the acetabulum in Marasuchus(Text-®g. 6A)? Is this an `incipient' brevis fossa or is it an apomorphic reversal as suggested by Novas

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(1992, 1994)? Judgment of such features cannot be made simply in the context of minimalist assumptionsabout changes in characters and taxa. At the risk of sounding recidivist, to move beyond the protocolof cladograms requires an integrative analysis of functional complexes of characters and the assemblyof clear adaptations (e.g., Padian 1995). The reconstruction of early dinosaur evolution is a dimension of`telling the tree' that uses cladograms as a starting point, not as an end point. The test of such narratives isin the discovery of new material that ®lls the gaps in our present knowledge.

C O N C L U S I O N S

The new dinosauriform from Cromhall cannot be unequivocally regarded as dinosaurian, but in manyregards it is closer to true dinosaurs than Eoraptor and herrerasaurs appear to be. In our view, Eoraptor andherrerasaurs are dinosauriforms, closer to true dinosaurs than other ornithodiran taxa, but are probably nottrue dinosaurs because they lack many synapomorphies that have been proposed to unite Saurischia andOrnithischia. To accept the view that they are true dinosaurs requires the reversal of several characteristicsthat appear dif®cult or unusual to reverse because the characters are often strongly involved in functionalcomplexes, and reversals of such features are not known in other taxa. Of the 17 potential synapomorphiesof dinosaurs listed above, based on uncontroversial saurischians and ornithischians, ®ve (4, 5, 6, 13, 14)are present in Agnostiphys, and only one (three sacral vertebrae) does not seem to be present. Whether ornot Agnostiphys is a true dinosaur, it is at least as close to dinosaurs as are Eoraptor and herrerasaurs(Text-®g. 9). The test of the evolution of features that characterized the origin of dinosaurs must be madewith reference to a node-based evolution of Dinosauria and a diagnosis that follows from this de®nition.Future discoveries will test the diagnosis of Dinosauria and the relationship of other taxa to dinosaurs, andwill improve understanding of how dinosaurian adaptations were sequentially assembled. But thede®nition of Dinosauria, which is based on its ancestry, cannot change.

Acknowledgements. We thank Amey Roadstone Corporation for kindly providing unlimited access to CromhallQuarry. John Hutchison read an early draft of the manuscript and provided us with many insightful comments. We aregrateful to the reviewers, Eric Buffetaut and David Norman, who provided a number of helpful suggestions. Field workwas supported by a grant from the National Geographic Society.

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N. C. FRASER

Virginia Museum of Natural History1001 Douglas Avenue

Martinsville, VA 24112, USA

K. PADIAN

Museum of PaleontologyUniversity of California

1101 Valley Life Sciences BuildingBerkeley, CA 94720, USA

G. M. WALKDEN

Department of GeologyUniversity of Aberdeen

Meston WalkAberdeen AB9 1AS, UK

A. L. M. DAVIS

Department of Geological Sciences4044 Derring Hall

Virginia TechBlacksburg, VA 24061, USA

Typescript received 23 November 1999Revised typescript received 9 April 2001

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A P P E N D I X

Principal diagnostic characters of Dinosauria discussed in the text.

1. The loss of the postfrontal.2. Elongate vomers reaching posteriorly at least to the level of the antorbital fenestra.3. Glenoid facing fully backwards.4. Deltopectoral crest extending far down (at least one third) the length of the humerus.5. Largely to fully perforate acetabulum.6. Brevis fossa.7. Reduced pubis/ischium contact.8. Lesser trochanter forming a spike or a crest on the femur.9. Prominent fourth trochanter situated far down the femur.

10. Proximal head of the femur set off from the shaft.11. At least three sacral vertebrae.12. `Twisted' tibia (distal end broadened mediolaterally).13. Reduced or absent calcaneum.14. Reduced astragalus with an ascending process.15. Increased asymmetry of hand with small outer two digits bearing fewer phalanges.16. Three or fewer phalanges in the fourth manual digit.17. Pedal digit V shorter than metatarsal I (foot tridactyl in typical dinosaur condition).

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