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SYSTEMATIC PALAEONTOLOGY

CROCODYLOTARSI SUCHIA

UNNAMED TAXON (Ruthin form + Crocodylomorpha)

INTRODUCTION

The most easily recognisable Ruthin archosauriform is known from osteoderms and cranial bones sharing a similar sculpture pattem. Although no physical association has been observed, comparison with crocodylomorphs suggests that the recurved, laterally-flattened, serrated teeth found at Ruthin may also be attributable to the same form.

Ruthin assemblage: Archosauriforms 192

Suchia include the large-bodied, exclusively Triassic, Rauisuchia, Poposauria, and heavily armoured herbivorous Aetosauria; as well as the more diverse and long lasting Crocodylomorpha. Basal crocodylomorphs may be divided into two groups, the Sphenosuchia and Crocodyliformes. Sphenosuchia are gracile, erect, terrestrial crocodylomorphs with a digitigrade pes, known from Upper Triassic and Early Jurassic sediments worldwide (Sereno & Wild, 1992; figure 3.4.3A). Crocodyliformes range from the Triassic to the recent. They include early, probably terrestrial Protosuchus-likQ animals, more heavily built than sphenosuchians, with extensive dermal armour and a plantigrade pes (Colbert & Mook, 1951; figure 3.4.3B). More derived crocodyliforms appear as progressively more ‘crocodile-like’ aquatic forms and survive to the present day. The monophyly of Sphenosuchia has been disputed by the analyses of Clark (1986 & in Benton & Clark, 1988) and Parrish (1991) who consider Sphenosuchia to be a grade-group with some members more closely related to the Crocodyliformes. However, the more recent analysis of Sereno & Wild (1992) has supported sphenosuchian monophyly.

B

Figure 3.4.3 Triassic crocodylomorphs. A) The sphenosuchian Terrestrisuchus (from Sereno &

Wild, 1992). B) The crocodyliform Protosuchus (dermal armour omitted; from Colbert & Mook,

Ruthin assemblage: Archosauriforms 193

Based on a key-character definition (loss of postffontal), the Ruthin suchian cannot be considered a crocodylomorph. However, it shares several derived characters previously used to define Sphenosuchia within the Crocodylomorpha. The character complex of the Ruthin form shows that at least two supposed sphenosuchian synapomorphies are in fact primitive for all Crocodylomorpha, characterising the group (Ruthin form (Crocodylomorpha)) within the Suchia.

DESCRIPTION

TOOTH-BEARING ELEMENTS

Premaxillae:

A fragment of a single small premaxilla has been found which shows some faint sculpture on its surface (figure 3.4.4A). The specimen is 1.5mm long and preserves three thecodont tooth positions (no teeth are implanted) and part of a process curving away dorsally from the tooth row. This is probably a posterior, maxillary process, forming the posterior border of the external nares as in most other archosaurs.

Two premaxillae (figure 3.4.4B,C), similar to the sculptured specimen, but with smooth labial surfaces, have also been found. Both specimens are small, BM(NH)R16072 is approximately 1mm long (three tooth positions), BM(NH)R16073 is 1.5mm long (two tooth positions). They have posterodorsally directed maxillary processes, at the base of which both specimens show a foramen for the premaxillary continuation of the supra-alveolar canal. The specimens show at least three tooth positions to have been present. The sockets are largely open on their medial surface. These premaxillae are tentatively attributed to the suchian.

Ruthin assemblage: Archosauriforms 194 ,?mx pr _ii) scu 0 1mm

B

ma fo mx pr 1mm mx pr 0 1mm fo

Figure 3,4.4 Premaxillae attributed to the Ruthin suchian. A) ?right sided element (BM(NH) R16078) in i) medial and ii) lateral views. B) left-sided element (BM(NH) R16072) in i) medial and ii) lateral views. C) right-sided element (BM(NH) R16073) in i) lateral and ii) medial views.

Ruthin assemblage: Archosauriforms 195

Maxillae:

Three maxillae may also be attributable to the suchian (figure 3.4.5): NMW88.12G.55 (figure 3.4.5A, 4mm long, seven tooth positions), shows light sculpturing; UMZC.2000.X.21 (figure 3.4.5B, 6mm long, five tooth positions), shows very faint sculpturing; AUP 13 (figure 3.4.5C, 4mm long, five tooth positions) does not show any evidence of sculpturing. The tooth sockets are sub-circular in cross-section with labial walls higher than the lingual walls.

In lateral view the anterior end of the maxilla is quite deep and rectangular (figure 3.4.5C), unlike the other thecodont maxillae found at Ruthin (section 3.4.3b, figure 3.4.22), and no large alveolar foramen is present. Posterior to this region the bone rises steeply to form a triangular facial, or nasal process, seen in all three specimens. The anterior edge of this process carries a nasal facet (figure 3.4.5B,C), excluding or limiting the maxilla’s contribution to the posterior border of the external nares. The posterior edge of the facial process is not facetted, and formed the anterior border of the antorbital fenestra. No antorbital fossa was present.

In medial view a depression on the facial process marks the anterior extent of the antorbital sinus (figure 3.4.5A,B). Below this the supra-alveolar canal opens into a groove running backwards across the bone.

Ruthin assemblage: Archosauriforms 196

A

scu ii) _depr 2mm

B

nas fct depr ma il) nas fct _scu 2mm

C i ) nas fct _ii) nas fct

2mm

Figure 3.4.5 Maxillae of the Ruthin suchian. A) Right-sided element (NMW 88.12G.55) in i) lateral and ii) medial view. B) Left-sided element (UMZC.2000.X.21) in i) medial and ii) lateral view. C) Left-sided element (AUP 13) in i) medial and ii) lateral view.

Ruthin assemblage: Archosauriforms 197

Serrated teeth tentatively attributed to the suchian:

Serrated thecodont teeth are a quite common element of the Ruthin assemblage (figure 3.4.6A-E). No serrated teeth have been found articulated in jaws or jaw fragments. Although some symmetrical serrated teeth have been found in the assemblage the vast majority are recurved. Frequently only crowns are recovered, but many teeth with roots attached have been found, so the assemblage does not only represent shed teeth. The teeth range in size from crowns 0.5mm long to the largest specimen yet recovered (figure 3.4.6E), which is 7mm long (3mm of which is root). The teeth are laterally-flattened, and, while they bear serrations on both their anterior (convex) and posterior (concave) edges, those on the posterior edge extend a greater distance down the tooth and appear more prominent. Scanning electron micrographs show the serrations to be spaced at c.l8 per mm in small teeth, decreasing to only c.5 per mm in the largest specimens.

It has been suggested that these teeth are very similar to a tooth found in the Middle Triassic, Sherwood Sandstone Formation of Warwick (D.J. Gower pers. comm. 1997). This tooth (BGS(GSM) 4873, figure 3.4.6F) is laterally-flattened, slightly recurved and bears fine serrations on both carinae, spaced at 26 per 5mm; it is of a comparable size to the Ruthin forms, with a crown 5mm long (Benton and Gower, 1997). This tooth was attributed to the prosauropod dinosaur Thecodontosaurus by Walker (1969 p473), but since the tooth does not show the basal swelling and constriction of the characteristic, leaf-shaped, prosauropod tooth, this is probably unlikely (PM. Barrett, pers. comm. 1997).

The Warwick tooth and the Ruthin teeth both conform to a fairly standard archosaur pattem, characteristic of some crocodylomorphs {Pseudohesperosuchus, Terrestrisuchus, Pedeticosaurus and Saltoposuchus among sphenosuchians (Sereno & Wild, 1992)), early theropods (e.g. Coelophysis Colbert, 1989), and many crocodylomorph outgroups, so the similarity may not indicate closely related taxa are present in each locality.

A few symmetrical, serrated thecodont teeth have been found in the Ruthin assemblage (figure 3.4.22). These may be attributable to the suchian, since some

Ruthin assemblage: Archosauriforms 198

2mm

Figure 3.4.6 A-E) Serrated teeth tentatively attributed to the Ruthin suchian. A/B, C/D) unnumbered NMW specimens. E) UMZC.2000.X.18. F) Serrated tooth from the Middle Triassic Sherwood Sandstone Formation of Warwick (BGS (GSM) 4873; from Benton & Gower, 1997).

Ruthin assemblage: Archosauriforms 199

sphenosuchian crocodylomorphs show a leaf-shaped tooth pattem, “bulbous maxillary and dentary tooth crown bases” (equivalent to ‘leaf-shaped’) being a pattem common to Sphenosuchus, Hesperosuchus, Dibothrosuchus (Sereno & Wild, 1992, p.450 character 19) and a new form from China (Harris et a l, 1998). Leaf-shaped teeth are also shown at some point along the tooth row by Pedeticosaurus (Gow and Kitching, 1988). However, their relative rarity, and the co-occurence of a prosauropod ilium in the locality, suggests the teeth might be attributable to the prosauropod (prosauropods also have very similar teeth). The leaf-shaped teeth are therefore described in section 3.4.3b below.

OTHER CRANIAL BONES

Frontal:

A number of fragmentary frontals have been found in the Ruthin assemblage, all sharing a similar sculpture pattem. The most complete is NMW88.12G.62 (figure 3.4.7), a right-sided element.

The specimen is 8mm long and 4mm wide. The dorsal surface is marked by a reticulate pattem of fine ridges. Where these ridges meet they demarcate a series of anteroposteriorly elongate pits. The ventral surface carries a prominent crista cranii, and the bone lateral to this is lightly pitted and is pierced by many small nutrient foramina. Most of the orbital margin is preserved, from back to front, it angles shghtly towards the midline and then about two thirds along the length there is a sharp change of angle and the remainder of the bone angles outwards.

The posteromedial edge of the bone, where the frontal thinned towards the contact with the parietal, is damaged. Comparison with smaller specimens, e.g. BM(NH)R16046 (figure 3.4.9), suggests this may be close to the tme shape of the bone and, when articulated, anterior processes of the parietals may have separated the backwardly directed processes of the frontals. The medial border of the bone is straight where it contacted the adjacent frontal. In medial view (figure 3.4.7) the frontal facet is lens-shaped, thinning posteriad, further supporting the view that the parietal may have extended along the midline in this region. The anterior edge of the frontal is almost

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