Describer

Chatterjee, 1983

Time

Triassic Late Carnian

Classification

Saurischia Theropoda Tetanurae Coelurosauria Shuvosauridae

Diet

Omnivore

Fossilsite

Dockum Group, Texas, US

Info

Genus - Typespecies - Skull

N.B. The dinosaurian status of the genus Shuvosaurus is still being debated. As example according to Nesbitt, S.J., Irmis, R.B. and Parker, W.G. (2007) Shuvosaurus is a suchian and not a dinosaur.

Shuvosaurus looks less like an ornithomimid than some kind of lotosaurid. The front of the skull shares some characters with that of [Lotosaurus] (beak, narial opening caudally placed from tip, shelf at dentary symphysis), but the back of the skull is different.

Long and Murry (1995) have suggested that the skull may belong instead to a small bipedal rauisuchian they have named Chatterjee, based on post-cranial material originally attributed to the rauisuchian [Postosuchus], and found at the same fossil site.

Genus and species established on a nearly complete small skull (17cm long) with lower jaw and a dorsal vertebra; referred material consists i.a. of a second partial skull with atlas, anterior part of two dentaries, a vertebral arch and a right scapula.

All material was recovered from the Upper Dockum, near the town of Post in Texas, together with remains of metoposaurs, aetosaurs, prosauropods, phytosaurs, and (importantly,see further) rauisuchians and poposaurs.

[Translation of: Rauhut, O. W. M. 1997: Zur Schädelanatomie von Shuvosaurus inexpectatus (Dinosauria; Theropoda): p. 17-21 in: Sachs, S., Rauhut, O. W. M. & Weigert, A. (eds.): 1. Treffen der deutschsprachigen Palaeoherpetologen, Düsseldorf, 21.-23.02.1997; Extended Abstracts. - Terra Nostra 7/97. Translated by O. Rauhut.] [On the cranial anatomy of Shuvosaurus inexpectatus (Dinosauria; Theropoda)]

Oliver W. M. Rauhut

Abstract

The cranial anatomy of Shuvosaurus inexpectatus Chatterjee, 1993, is briefly reviewed. Shuvosaurus is an unusual edentulous theropod from the Late Triassic of Texas. The holotype represents a juvenile individual. It is characterised by a strong reduction of the maxilla, an anteriorly elongated jugal, the antorbital opening being dorsally bordered by the nasals, and the presence of a secondary anterior platform in the dentaries. Shuvosaurus can be referred to the Theropoda with some certainty. Within this group, it probably represents a highly derived basal theropod, and not an ornithomimosaur, as originally suggested.

Introduction

In 1993, S. Chatterjee described the skull of an unusual new theropod from the upper Triassic (Norian) [Dockum Formation] Dockum Group of Texas and referred it to a new genus and species, Shuvosaurus inexpectatus. He tentatively referred the new genus to the Ornithomimosauria, a group formerly only known from the Cretaceous.

Long & Murry (1995) argued that Shuvosaurus represents an edentulous pseudosuchian, rather than a theropod, stating that: \\\\\\\"...Chatterjee...failed to demonstrate why Shuvosaurus is not a highly derived predinosaurian archosaur, rather than a theropod.\\\\\\\" (Long & Murry 1995: p. 162).

In this abstract, the cranial anatomy of Shuvosaurus is reviewed briefly and some comments on its possible systematic position are given.

Abbreviation

TTU - Texas Tech University , Lubbock , Texas .

Description

The holotype of Shuvosaurus inexpectatus (TTU P 9280) represents the complete left and parts of the right side of an articulated skull, including a fragment of the braincase. Further skull elements are found in the paratypes (TTU P 9281 and P 9282), including an almost complete braincase (TTU P 9282). Chatterjee (1993) furthermore referred a vertebra, a scapular fragment and a tibia to the same species; however, since the association of this material with the cranial elements is unclear, it is not taken into consideration here.

The skull of Shuvosaurus is low and elongate. The external nares are relatively small, in contrast to the large antorbital fenestra and orbit. The infratemporal fenestra is slit-like and subdivided ventrally by an anterior process of the quadratojugal. The supratemporal fenestra is oval in outline, as it is usual in archosaurs. Judged by the well visible and sometimes even open sutures, the holotype represents a juvenile individual, despite its rather large size of c. 170 mm. Therefore, some changes might occur in an adult skull, like a relative reduction of the size of the orbit, an elongation of the preorbital part of the skull and a rise of the occipital region (see e.g. Carpenter, 1994).

The premaxilla is elongate and relatively massive. It forms the anterior, ventral and, with a narrow posterior process, parts of the posterior rim of the external nares. The edentulous ventral margin of the bone is slightly concave.

The maxilla also is edentulous, but its ventral margin is slightly convex. The small size of this bone is especially striking: it is shorter than the premaxilla and lacks the dorsal process that usually borders the antorbital fenestra anteriorly and dorsally in other archosaurs. The presence of a secondary antorbital opening, as reconstructed by Chatterjee (1993: Fig. 4, 5A), is questionable, since no evidence for the presence of a pila interfenestralis can be found in the specimen. Furthermore, this opening is usually completely enclosed by the maxilla in other theropods, while it would have been bordered dorsally and posteriorly by the nasals in Shuvosaurus.

The nasal is long and slender. Its anterior end is dorso-ventrally expanded and forked, to form the dorsal and parts of the posterior rim of the external nares. Posteriorly, the nasal forms a large part of the dorsal margin of the antorbital fenestra.

The lacrimal is inverted L-shaped, as it is typical for theropods. A dorsal anterior process contributes to the dorsal border of the antorbital fenestra, and the main body separates the former opening from the orbit.

A small, triangular prefrontal also takes part in the anterior rim of the orbit dorsally.

The jugal is long and rod-like. It forms most of the ventral margin of the antorbital fenestra and the ventral rim of the orbit. Posteriorly, it is bent dorsally to meet the postorbital in front of the supratemporal fenestra; a posterior process for the quadratojugal seems to be absent.

The frontal is also long and slender and borders the orbit dorsally. Posteriorly, the two frontals are divided by an anterior medial process of the parietals.

The postfrontal is absent.

The postorbital shows the typical triradiate shape and borders the orbit posteriorly and the two temporal fenestra anteriorly. An unusual character of this bone is the dorso-ventrally expanded posterior process, which overlaps the anterior part of the squamosal.

The parietal is only represented by fragments. It borders the supratemporal fenestra medially and forms a low supraoccipital crest posteriorly.

The squamosal is shaped like a cross, with a long process towards the postorbital. The quadrate articulation is found on the ventral side of the main body, underneath the meeting point of the four processes.

The quadratojugal contacts the squamosal dorsally and the jugal anteriorly and thus forms the entire ventral margin and the ventral part of the posterior rim of the infratemporal fenestra. An unusual feature of this bone is a small triangular anterior process that subdivides the infratemporal fenestra into a small ventral and a much larger dorsal part.

The dorsal part of the quadrate is strongly bent posteriorly. The pterygoid wing of the quadrate is large and arises from almost the complete height of the quadrate body. Only the anterior part of the quadrate body is covered laterally by the quadratojugal and the ventral process of the squamosal.

Of the palate, only little is preserved. Only a fragment of the palatine is present. It was apparently rather small and contacted the elongate anterior process of the jugal.

The ectopterygoid is hook-shaped, with a significantly expanded medial part. A deep fossa is found ventrally on the medial expansion.

The braincase is well preserved in the paratype TTU P 9282. It was described in detail by Chatterjee (1993), and thus, only a few characters are highlighted here. The paroccipital process are large and directed significantly ventro-laterally. The basisphenoid is elongate and shows a well developed basisphenoidal recess ventrally. The basipterygoid processes are long and slender, and the cultriform process of the parasphenoid is developed as a long, slender rod. A well developed pneumatic recess is present above the basipterygoid processes.

The dentary is edentulous. The articulated dentaries meet in a smooth arch, and the symphysis is rather well developed. A secondary platform is present above and behind the symphysis; it is restricted to the anterior part of the dentaries.

A large, elongate mandibular fenestra is present in the posterior part of the mandible. Both the surangular and the angular are rather poorly preserved, so little can be said about their morphology. The surangular foramen seems to have been slightly enlarged. The articular, which overlies the posterior end of the angular, is expanded transversely and, together with the posterior part of the surangular, forms the mandibular joint. The retroarticular process is short and slightly inclined dorsally.

Discussion

Shuvosaurus inexpectatus shows many apomorphic characters, which clearly justify the reference to a new genus, and probably even family (Chatterjee, 1993). The most important of these characters are the strong reduction of the maxillary and the resulting reconfiguration of the bones surrounding the antorbital fenestra, the expansion of the squamosal process of the postorbital, the secondary anterior process of the quadratojugal, and the anterior platform in the dentaries.

Shuvosaurus differs from rauisuchians and other basal crurotarsans in the following characters: loss of the postfrontal, paroccipital process directed ventro-laterally, lacrimal dorso-ventrally elongated, inverted L-shaped and exposed on the skull roof, presence of a deep basisphenoid recess, and ectopterygoid with expanded medial part and deep ventral fossa. All of these characters are found in theropods, and the latter three probably represent synapomorphies for this group (Gauthier 1986); therefore, Shuvosaurus can be referred to the Theropoda.

The phylogenetic position of Shuvosaurus within theropods is problematical. The assignment to the Ornithomimosauria, as proposed by Chatterjee (1993) cannot be confirmed on the basis of osteological characters, apart from the temporal gap between this taxon (Carnian-Norian) and the oldest known ornithomimosaur (Barremian; Pérez-Moreno et al., 1994).

Shuvosaurus lacks several ornithomimosaur synapomorphies, such as the presence of an expanded parasphenoid capsule, and the short and broad basipterygoid processes (e.g. Barsbold & Osmólska, 1990). Differences between Shuvosaurus and coelurosaurs in general are the relatively small endocranial cavity, the lack of two accessory antorbital openings in the maxillary, and the presence of a ventral groove, rather than a deep ventral pocket, as it is found in coelurosaurs (e.g. Barsbold & Osmólska 1990, Currie, 1995), on the ectopterygoid. Furthermore, Shuvosaurus differs from all tetanurans in that the prefrontal is largely exposed on the anterior rim of the orbit, while it is displaced posteriorly and often medially in all tetanurans.

These characters indicate that Shuvosaurus is a basal theropod; however, more information is needed to assess its phylogenetic position.

Shuvosaurus , with its highly derived skull morphology, is a good example for a significant early differentiation within theropods, and it also shows us, how little is still known about the early evolution of this group.

Acknowledgements

I thank Sankar Chatterjee, Lubbock , for access to the Shuvosaurus material and David Unwin and Axel Hungerbühler, both Bristol , for useful discussions. Axel Hungerbühler is furthermore thanked for a critical review of the manuscript. I am supported by the EU under TMR grant ERBFMBICT 961013, which is gratefully acknowledged.

_{Source: Polyglot Paleontologist}

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