Fraser et al, 2002 by Felipe Elias

BASAL DINOSAURIFORM REMAINS FROM BRITAIN AND THE DIAGNOSIS OF THE DINOSAURIA by

N. C. FRASER, K. PADIAN, G. M. WALKDEN

and A .

L. M. DAVIS

ABSTRACT. A new genus and species of dinosauriform is described from the Triassic of south-west England. The description is based on isolated elements recovered from an assemblage of other dissociated tetrapod remains that include 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 to fully perforate acetabulum, the presence of a brevis fossa, and a reduced astragalus with an ascending process, are considered to be particularly relevant. The de®nition and diagnosis of the Dinosauria are restated and the positions of the 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/Mesozoic unconformity in the Mendip and Cotswold hills of south-west England and parts of South Wales is the presence of early Mesozoic sediments lodged within palaeokarstic cavities. The sediments are Triassic to Mid Jurassic in age, and result from numerous successive tectonic and emergent events that ®ssured and exposed 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 richest and certainly the most varied locality is Cromhall Quarry, Avon. A detailed study of this locality resulted in a stratigraphic framework that potentially places all the fossil assemblages from these Mesozoic ®lls into context (Walkden and Fraser 1993). In 1990 a completely new ®ll was discovered, corresponding to an ancient cave deposit, with a great density of relatively large bones concentrated in a layer 50±100 mm thick, essentially forming a bone bed. Its assemblage is clearly different from any of the assemblages previously described from the quarry. Its lepidosauromorphs (mostly sphenodontians) are known in abundance in the other ®lls, but numerous new archosaurian elements include some with distinct dinosaurian characteristics. Their description and analysis form the principal subject of this paper. THE NEW ASSEMBLAGE

The principal component of the new assemblage is archosaurian, and the sphenosuchian crocodylomorph Terrestrisuchus is the predominant taxon. In this respect the new faunal assemblage more closely resembles one described from Pant-y-ffynon Quarry (Crush 1980, 1984; Warrener 1983) than it does any 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 two distinct morphs of Terrestrisuchus in the Cromhall assemblages (Fraser in prep.), which in turn appear to differ in some respects from the type species (Terrestrisuchus gracilis) from Pant-y-ffynon. The smallest archosaurian so far identi®ed in the new deposit is tentatively assigned to the Rauisuchiformes (sensu Parrish 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 to be 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

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individuals and the fairly abundant juvenile specimens of Terrestrisuchus. The third archosaur is a completely new form that is not known from any of the other ®ssure deposits. It bears certain uniquely dinosaurian characteristics, yet in other respects it is unlike any basal members of the main dinosauriform clades (including Herrerasauridae). DESCRIPTION AND COMPARISONS

The elements referred here to a single taxon were all dissociated, so it is an inference that they belong to one 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 taxonomic assignment 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. Of almost 800 archosaurian elements recovered so far, approximately 85 per cent belong to Terrestrisuchus, 5 per cent belong to rauisuchiforms, and the remaining 10 per cent are compatible with basal dinosauriform structure. The apparent absence of additional archosaurs suggests that this last group of elements represents 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 May 1993; Padian and Currie 1997). This implies a diagnosis of characters, which in turn may be used to assess whether newly discovered material pertains to the taxon. We examine the following features in this light. SYSTEMATIC PALAEONTOLOGY

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 the Dinosauria. 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 distinct ascending process and a prominent depression immediately posterior to the ascending process; in dorsal aspect an acute anteromedial corner on the astragalus. Description Maxilla An incomplete maxilla (Text-®g.1) bearing laterally compressed serrated teeth is basically similar in shape to that of a theropod or herrerasaurid dinosaur, but there is no indication of a subnarial foramen, and interdental plates are absent.

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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卒0 mm.

In other respects there is nothing particularly remarkable about this element. Much of the anterior two-thirds of the element is present, including the dorsal process that separated the external naris from the antorbital fenestra. Ten alveoli are preserved, with the remains of four teeth still in position. The teeth are recurved, bear serrated keels, and broadly 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-速g. 2). The narrow dorsal process slopes dorsoposteriorly and has a straight anterior edge that shows no indication of a subnarial foramen. A shallow groove running along the anterior margin was for the reception of the nasal. Humerus The humerus (Text-速g. 3) is represented by a number of fragments and two almost complete specimens. Together they offer some of the most convincing evidence for the dinosauriform af速nities of the new form. Sereno (1991), in an

TEXT-FIG.

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2. Teeth of A, ?Terrestrisuchus sp. and B, Agnosphitys cromhallensis gen. et sp. nov. from Cromhall Quarry. Scale bar represents 1´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 of Sereno 1991) the proximal head had a prominent medial tuberosity, and that distal to the tuberosity the medial margin was strongly arched. In basal archosauriforms and ornithodirans, by contrast, the medial margin of the humerus is nearly straight and lacks the distinct tuberosity. Furthermore, Sereno recorded a difference in the shape of the deltopectoral 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 the other hand, typically have a crescentic deltopectoral crest that is positioned within the proximal 25 per cent of the length of the humerus. Parrish (1993) questioned the precise de®nition of these characters because certain features are gradational, 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 crest was at least 35 per cent of the length of the humerus. Even so, the new humeri are suf®ciently distinct to dispel any suggestion that they are pseudosuchian. They have a very distinct square-ended proximal head, and the marked deltopectoral ridge extends fully one-third along the length of the shaft. They lack any pterosaurian features, such as the greatly enlarged deltopectoral crest and exceptionally thin-walled bones. Ilium The most intriguing element is the ilium (Text-®g. 4). To date at least ten partial or almost complete specimens have been recovered, permitting a complete restoration (Text-®g. 5A). A well-developed supra-acetabular ridge deeply overhangs the semi-perforate acetabulum, and it has a prominent antitrochanter. The portion of the antitrochanter preserved 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 and Dinosauria. The iliac blade is generally rather robust, but the posterior portion of the iliac blade is more developed than in 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 and somewhat indistinct, and so it is dif®cult to ascertain the number of sacral vertebrae, though there seems to have been only two. Basal dinosaurs characteristically had three sacral vertebrae, so the new form does not have the dinosaurian

FRASER ET AL.: DINOSAURIA 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, a left ilium in lateral view.

condition. Nor do the dinosauriforms Staurikosaurus, Herrerasaurus, or Eoraptor have more than two, though some non-dinosaurian archosaurs do (e.g. the poposaurid Postosuchus). Basal dinosauromorphs such as Lagosuchus/Marasuchus show some similarities to the new form, particularly in the overall 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 the specimen (PVL 3870) on which Sereno and Arcucci based their restoration was incomplete, and that the ventral margin of the ilium had a triangular projection (like those of pseudosuchians with an imperforate acetabulum), so in reality the acetabulum in Marasuchus was practically imperforate. There is no question that the new form lacks the triangular process on the ventral margin, and that the acetabulum was therefore perforate. This suggests a placement closer to dinosaurs 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 of the 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 did not call it a brevis fossa until very recently. It might be more accurate to refer to this as a brevis shelf, lacking any real development of the brevis fossa. Although Sereno and Arcucci denied the presence of a brevis fossa in the dinosauromorph Marasuchus, there is certainly a small indentation in this region (Text-®g. 5C) that is not seen in other archosaurian ilia (including that of Terrestrisuchus) from the same ®ssure deposit (Text-®g. 5B). Once again this could be interpreted as an incipient brevis shelf that is not present in phytosaurs such as Rutiodon, nor in rauisuchids such as Postosuchus.

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TEXT-FIG. 5. A, Agnosphitys B, ?Terrestrisuchus sp., right

cromhallensis gen. et sp. nov., restoration of the right ilium in lateral view. ilium, AUP 11320, in lateral view. C, Marasuchus/Lagosuchus, right ilium in lateral view (after Sereno and Arcucci, 1994). Scale bars represent 5Â´0 mm.

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TEXT-FIG.

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

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 and possess a distinct posterior process of the iliac blade (Text-®g. 6A±B). The preacetabular process of the new form lacks the great extension typical of theropods such as Coelophysis and Dilophosaurus (Text-®g. 6F), and does not readily conform to the overall morphology of basal ornithischians such as Lesothosaurus (Text-®g. 6E), or basal sauropodomorphs such as Thecodontosaurus or Plateosaurus (Text-®g. 6D). Astragalus Three complete astragali of basal dinosaurian con®guration (Text-®g. 7) are known from the deposit. In anterior and posterior views they have a distinct ascending process, but in all three specimens their proximal parts are broken off and their full extent is unclear. It would appear to have been more pronounced than in Marasuchus and was probably comparable 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, immediately adjacent to the ascending process there is a marked basin visible in posterior view that is also pierced by a small foramen. Novas (1996) regarded this basin as a fundamental character of the dinosaurian astragalus, giving the ascending process its characteristic pyramidal shape. This is markedly different from the condition in Lagerpeton and Marasuchus, in which there is no depression behind the ascending process, which is essentially a longitudinal ridge separating the facets for the tibia and ®bula. Viewed dorsally, these astragali have an acute anteromedial corner, which Sereno (1991) cited as a synapomorphy of Dinosauromorpha. Chindesaurus, described as a herrerasaur by Long and Murry (1995), does not have quite such an acute antero-medial angle. Furthermore, in Chindesaurus the ascending process is much closer to the lateral edge of the astragalus than in the Cromhall form, Herrerasaurus, and theropods

TEXT-FIG.

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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.

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

Elements of Agnosphitys clearly exhibit dinosaurian af®nities, but before its phylogenetic position can be properly 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 common ancestry of all the members of the taxon. This name cannot be altered by changes in diagnosis or placement of 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) is unstable, and a name erected upon a speci®ed list of taxa (taxon-based de®nition) may not allow the inclusion of forms still to be found in the future, or the rearrangement of known forms (Padian and May 1993; Padian 1997a). In the interests of taxonomic stability, we support the premise that the de®nition of a taxon should be based on ancestry rather than a list of characters or taxa (Padian and May 1993; Padian 1997a, b; Sereno 1997; Padian et al. 1999). Although it was not until 1974 that Owen's Dinosauria were ®rst explicitly stated to be monophyletic by Bakker and Galton, Owen constituted his group and diagnosed it by a unique combination of features (Desmond 1979), and incorporated Iguanodon, Megalosaurus and Hylaeosaurus 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®nition of Dinosauria based on ancestry, not on a list of characters or taxa (Gauthier 1986; Padian and May 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 50 postcranial synapomorphies to support the monophyly of Dinosauria. The principal characters discussed here are listed in the Appendix. Disputes concerning these characters only serve to emphasize the importance of basing taxa on de®nitions of ancestry, not on the characters themselves. There are two principal problems that underlie these disputes, which must be recognized before the Cromhall form or many others can be fully assessed.

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Selective diagnoses Sereno and Novas (1994; also Sereno 1994, and Novas 1994) disputed many of the synapomorphies listed in the Appendix because they are not found in Herrerasaurus. They contended that because Herrerasaurus 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 within Dinosauria. The fact that herrerasaurs only share a few of the previously considered dinosaur synapomorphies does not automatically reveal a problem with the previous diagnosis of Dinosauria: it could equally indicate that herrerasaurs lie outside Dinosauria. The supposed saurischian and theropod synapomorphies of herrerasaurs might be convergences, or characters that apply to a more general level than Dinosauria. This is not surprising, inasmuch as herrerasaurs and theropods retain the basic carnivorous habits of ornithodirans generally, so would be expected to have shared primitive and convergent features related to that way of life. We know that saurischians and ornithischians were dinosaurs; 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 trees based on nearly 100 characters situated them either as saurischians outside Sauropodomorpha and Theropoda, 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 basal dinosauromorphs, such as Lagerpeton, Lagosuchus/Marasuchus, Pseudolagosuchus, and even basal ornithischians. Many of these taxa are unrepresented by skull and hand characters, or by anything but hindlimbs. And it is important to remember that we have no good material of ornithischians until the Early Jurassic, so we are missing some 20Âą25 million years of early ornithischian evolution. The Early Jurassic ornithischians are so transformed that it is difÂŽcult to obtain a picture of their basal form (the poorly preserved and incomplete Pisanosaurus provides our only tantalizing glimpse of Late Triassic ornithischians). As these basal dinosauromorphs become better known, we should have a much better idea of character distributions, polarities, and the degree of convergence and retained primitive features that are now problematic. Distribution of synapomorphies within basal Dinosauriformes The distribution and interpretation of some characters that have been central to recent discussions of basal dinosaur systematics require a brief review. Three sacral vertebrae. The presence of three sacral vertebrae is frequently cited as an apomorphy of Dinosauria (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, according to Novas (1996), there are clearly only two, as in Lagosuchus/Marasuchus and Lagerpeton, and he regarded 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 for Scleromochlus, which he considered to be the sister taxon to pterosauromorphs and dinosauromorphs. But all true dinosaurs have at least three. Brevis shelf and fossa. Novas (1996) was among the ÂŽrst to propose the brevis fossa as a derived character of Dinosauria, and to distinguish between the brevis shelf and the brevis fossa. We follow Novas in regarding the brevis shelf as a posterolateral margin of the iliac blade. The brevis fossa is the ventral trough that develops between the brevis shelf and the posteromedial wall of the iliac blade. In Herrerasaurus the shelf 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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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 and J, posterior views. Agnosphitys cromhallensis gen. et sp. nov. in K, anterior, L, proximal, M, distal and N, posterior views. 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. TEXT-FIG.

within Theropoda. In the Cromhall form there is a very distinct brevis shelf and, between it and the posteromedial 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 is best considered primitive, not secondarily derived, especially given the functional correlates of the brevis fossa for theropod hindlimb evolution (Gatesy and Dial 1996). Antitrochanter. The antitrochanter at the posterior border of the acetabulum is a raised area that articulated with 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 Dinosauriformes. 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 the antitrochanter 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, including Marasuchus and Dinosauria. Nevertheless, Novas (1996) then went on to question the utility of the character even in Dinosauromorpha, citing inconsistency within a single taxon. Thus in Herrerasaurus the antitrochanter was apparently present in only one of three available specimens of the pelvic girdle.

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TEXT-FIG.

8. Continued.

Furthermore, according to Novas, the antitrochanter is absent in Argentinian collections of sauropodomorph material. The elliptical shape of the iliac portion of the antitrochanter in the Cromhall ilium is strongly suggestive of a kidney-shaped antitrochanter. Astragalus. The astragalus has been one of the most critical elements in discussions of the diagnosis of Dinosauria, and its ascending process has assumed particular importance. Because of subtleties in the descriptions 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 the development of a prominent basin posterior to it (Text-®g. 8G±J). Novas (1989) considered this basin a derived feature and apomorphic for Dinosauria. The anterior hollow and its associated foramen are prominent 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 condition for Dinosauria, from which those of Theropoda, Sauropodomorpha and Ornithischia were derived. In basal Sauropodomorpha, Novas cited the transversely broad astragalus as a synapomorphy. Typically in Theropoda the ascending process is medially prolonged to form a `wall' separating the anterior and dorsal surfaces as seen in Dilophosaurus (Text-®g. 8O±R). In Ornithischia, such as Pisanosaurus and

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Lesothosaurus, the astragalus has a vertical lateral surface that is continuous with that of the ascending process, so it lacks a dorsal facet for articulation with the ®bula. In the ®ssure material the ascending process is incomplete, and its full extent is consequently unclear. However, in common with Dinosauriformes, the astragalus has an acute anteromedial corner. In addition the anterior hollow and foramen are present. More importantly, the posterior basin is well developed, and the position and shape of the ascending process are very similar to those of Herrerasaurus, so it is consistent with Novas' concept of the primitive condition for Dinosauria. Other potential synapomorphies of Dinosauria, including the absence of the postfrontal, lateral exposure of the articular head of the quadrate, and reduction in size of the post-temporal opening, may also eventually be shown to occur in forms such as Marasuchus. Thus Sereno and Novas (1994) were rightly somewhat cautious on these points. They were also correct to be critical of some other so-called dinosaurian synapomorphies of the skull because they are dif®cult to support in many clearly acceptable dinosaurs. For example, the `slenderness' of the pterygoid and the `lightly built' palate are vague and may vary 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 nesting Herrerasaurus 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 the lacrimal by the jugal appears to be the most common condition in crurotarsal archosaurs', but it is probably more correct to say that the lacrimal overlaps the jugal in most crurotarsans, and Sereno and Novas cited the examples of Saurosuchus and Ornithosuchus. However, as they also pointed out, sphenosuchians share the saurischian condition, in which the jugal overlaps the lacrimal. This is also true for an undescribed small `crocodylomorph' from the ®ssures (NCF, pers. obs.). The condition is variable in ornithischians. In Lesothosaurus and Psittacosaurus, the lacrimal apparently weakly overlaps the jugal (Sereno and Novas 1994). In hadrosaurs the jugal clearly overlaps the lacrimal, whereas in still other ornithischians the jugallacrimal articulation is non-overlapping (this is also, incidentally, the condition in Postosuchus). Because the condition in basal dinosaurs and their outgroups is variable or unknown, its value is questionable as an unequivocal apomorphy for Saurischia. Posterior process of jugal forked. Sereno and Novas (1994) suggested that a forked posterior process of the jugal is a saurischian characteristic. Although in Sauropoda the jugal is broadly overlapped by the quadratojugal and is not forked, in the basal sauropodomorph Plateosaurus and most theropods the forked process is clearly pronounced. However, in other basal sauropodomorphs, e.g. Anchisaurus, Massospondylus (at least as an adult), and Lufengosaurus, the forked process is absent or poorly de®ned. Furthermore a forked process is present in the ornithischians Psittacosaurus and Lesothosaurus. The condition in the dinosauromorphs Lagerpeton and Marasuchus is unknown, but even so, it is not a synapomorphy for Saurischia. Subnarial foramen. Whereas a subnarial foramen is known to occur in some suchians (e.g. rauisuchians such as 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 the same 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 and Theropoda. 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 different from the joint in Theropoda. Rieppel and Zaher (in press) have shown that detailed relationships of the individual 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 uniting herrerasaurs and theropods. The condition in Lagerpeton and basal ornithischians is not known, but Marasuchus apparently lacks epipophyses. Other possible synapomorphies of herrerasaurs and theropods cited by Sereno and Novas (Novas, 1994; Sereno 1994; Sereno and Novas 1994) include the strap-shaped scapula (present in pterosaurs and Terrestrisuchus), the distally enlarged pubis (present also in rauisuchians), the elongate manus, strongly reduced metacarpals IV and V, and the reduction in the length of the humerus. The latter three characters may be of particular signi®cance but they are not known in the immediate dinosaurian outgroups. Again, our record of basal sauropodomorphs and ornithischians is quite 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, the scapular blade in herrerasaurs is thin and greatly reduced, and does not ¯are dorsally as in basal theropods such as ceratosaurs, so this is clearly homoplastic resemblance. The pubis is slightly retroverted and basal theropods lack the triangular posterior expansion of the distal end seen in herrerasaurs, so this is also convergent. The manus is only `elongate' and the humerus `reduced' compared to some neutral metric that has not been speci®ed. And the reduced condition of metacarpals and phalanges of digits IV and V are clearly convergent, because they are not so reduced in basal theropods. This questionable support for inclusion of Herrerasaurus within Theropoda is offset by the absence in Herrerasaurus 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 Herrerasaurus or Eoraptor. In addition, the long preacetabular process on the ilium and a pronounced brevis fossa are not found in Herrerasaurus. (The term `pronounced' is admittedly subjective.) Equally important is the retention 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 not the longest in Herrerasauridae, as it is in all saurischians, and the thumb is not even slightly offset from the other 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 best equivocal (Holtz and Padian 1995). Consequently its characteristics should not be used as a basis for diagnosing Dinosauria, Saurischia or Theropoda. Herrerasaurs are clearly dinosauriforms close to dinosaurs, but we contend that they lack many of the unique features shared by the animals that Owen united within the Dinosauria. Using dinosauromorphs (a more inclusive outgroup) to test the validity of dinosaurian synapomorphies will inevitably lead to a change in composition not mandated by its nodebased de®nition. By de®ning Dinosauria on the basis of a chosen set of apomorphies, it is possible to ¯ip the position of Herrerasauridae. If the elongate manus, subnarial fenestra and pubic foot are used to the exclusion 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 complete material, the dissociated elements from the ®ssure ®lls prove even more problematic. Our incomplete knowledge of the anatomy of crucial Late Triassic taxa impedes us from recognizing a sequence of acquisition of characters and functional changes that marks the inception of the dinosaur lineage. We could weight various characters, depending upon their distribution in non-dinosaurian lineages and their distribution in basal members of the Saurischia and Ornithischia. For example, the number of sacral vertebrae may be of crucial functional importance. Unfortunately `more than two sacrals' is not restricted entirely 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 somewhat subjective because it could be expected to vary with size and function (see, e.g., Mononykus). The form and distribution among archosaurs of the brevis fossa also has some merit, because it is conspicuous in all basal dinosaurs such as Coelophysis, Lesothosaurus and basal sauropodomorphs, and absent in other archosaurs. 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

PALAEONTOLOGY, VOLUME 45

9. The relationships of dinosauromorphs proposed in this study. Some of the critical characters for each branch are as follows: (1) deltopectoral crest extending far down (at least one third) the length of the humerus; largely to fully perforate acetabulum; reduced or absent calcaneum; reduced astragalus with an ascending process; (2) presence of a distinct brevis fossa; (3) presence of three or more sacral vertebrae.

TEXT-FIG.

(1992, 1994)? Judgment of such features cannot be made simply in the context of minimalist assumptions about changes in characters and taxa. At the risk of sounding recidivist, to move beyond the protocol of cladograms requires an integrative analysis of functional complexes of characters and the assembly of 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 is in the discovery of new material that ®lls the gaps in our present knowledge. CONCLUSIONS

The new dinosauriform from Cromhall cannot be unequivocally regarded as dinosaurian, but in many regards it is closer to true dinosaurs than Eoraptor and herrerasaurs appear to be. In our view, Eoraptor and herrerasaurs are dinosauriforms, closer to true dinosaurs than other ornithodiran taxa, but are probably not true dinosaurs because they lack many synapomorphies that have been proposed to unite Saurischia and Ornithischia. To accept the view that they are true dinosaurs requires the reversal of several characteristics that appear dif®cult or unusual to reverse because the characters are often strongly involved in functional complexes, and reversals of such features are not known in other taxa. Of the 17 potential synapomorphies of 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 or not 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 made with 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, and will improve understanding of how dinosaurian adaptations were sequentially assembled. But the de®nition of Dinosauria, which is based on its ancestry, cannot change. Acknowledgements. We thank Amey Roadstone Corporation for kindly providing unlimited access to Cromhall Quarry. John Hutchison read an early draft of the manuscript and provided us with many insightful comments. We are grateful to the reviewers, Eric Buffetaut and David Norman, who provided a number of helpful suggestions. Field work was supported by a grant from the National Geographic Society. REFERENCES BAKKER, R. T.

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Virginia Museum of Natural History 1001 Douglas Avenue Martinsville, VA 24112, USA K. PADIAN

Museum of Paleontology University of California 1101 Valley Life Sciences Building Berkeley, CA 94720, USA G. M. WALKDEN

Department of Geology University of Aberdeen Meston Walk Aberdeen AB9 1AS, UK A. L. M. DAVIS

Typescript received 23 November 1999 Revised typescript received 9 April 2001

Department of Geological Sciences 4044 Derring Hall Virginia Tech Blacksburg, VA 24061, USA

FRASER ET AL.: DINOSAURIA

APPENDIX Principal diagnostic characters of Dinosauria discussed in the text. 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17.

The loss of the postfrontal. Elongate vomers reaching posteriorly at least to the level of the antorbital fenestra. Glenoid facing fully backwards. Deltopectoral crest extending far down (at least one third) the length of the humerus. Largely to fully perforate acetabulum. Brevis fossa. Reduced pubis/ischium contact. Lesser trochanter forming a spike or a crest on the femur. Prominent fourth trochanter situated far down the femur. Proximal head of the femur set off from the shaft. At least three sacral vertebrae. `Twisted' tibia (distal end broadened mediolaterally). Reduced or absent calcaneum. Reduced astragalus with an ascending process. Increased asymmetry of hand with small outer two digits bearing fewer phalanges. Three or fewer phalanges in the fourth manual digit. Pedal digit V shorter than metatarsal I (foot tridactyl in typical dinosaur condition).