The Lower Jurassic ornithischian dinosaur Heterodontosaurus tucki Crompton & Charig, 1962: cranial anatomy, functional morphology, taxonomy, and relationships

Abstract

The cranial anatomy of the Lower Jurassic ornithischian dinosaur Heterodontosaurus tucki Crompton & Charig, 1962 is described in detail for the first time on the basis of two principal specimens: the holotype (SAM-PK-K337) and referred skull (SAM-PK-K1332). In addition several other specimens that have a bearing on the interpretation of the anatomy and biology of Heterodontosaurus are described. The skull and lower jaw of Heterodontosaurus are compact and robust but perhaps most notable for the heterodont dentition that merited the generic name. Details of the cranial anatomy are revealed and show that the skull is unexpectedly specialized in such an early representative of the Ornithischia, including: the closely packed, hypsodont crowns and 'warping' of the occlusal surfaces (created by progressive variation in the angulation of wear on successive crowns) seen in the cheek dentition; the unusual sutural relationships between the bones along the dorsal edge of the lower jaw; the very narrow, deeply vaulted palate and associated structures on the side wall of the braincase; and the indications of cranial pneumatism (more commonly seen in basal archosaurs and saurischian dinosaurs). Evidence for tooth replacement (which has long been recognized, despite frequent statements to the contrary) is suggestive of an episodic, rather than continuous, style of tooth replacement that is, yet again, unusual in diapsids generally and particularly so amongst ornithischian dinosaurs. Cranial musculature has been reconstructed and seems to conform to that typically seen in diapsids, with the exception of the encroachment of M. adductor mandibulae externus superficialis across the lateral surface of the temporal region and external surface of the lower jaw. Indications, taken from the unusual shape of the occlusal surfaces of the cheek dentition and jaw musculature, are suggestive of a novel form of jaw action in this dinosaur. The taxonomy of currently known late Karoo-aged heterodontosaurids from southern Africa is reviewed. Although complicated by the inadequate nature of much of the known material, it is concluded that two taxa may be readily recognized: H. tucki and Abrictosaurus consors. At least one additional taxon is recognized within the taxa presently named Lanasaurus and Lycorhinus; however, both remain taxonomically problematic and their status needs to be further tested and may only be resolved by future discoveries. The only other named taxon, Geranosaurus atavus, represents an invalid name. The recognition of at least four distinct taxa indicates that the heterodontosaurids were speciose within the late Karoo ecosystem. The systematics of Heterodontosaurus and its congeners has been analysed, using a restricted sample of taxa. A basal (nongenasaurian) position within Ornithischia is re-affirmed. There are at least four competing hypotheses 182 concerning the phylogenetic placement of the Heterodontosauridae, so the evidence in support of the various hypotheses is reviewed in some detail. At present the best-suppor...

Taxonomy, morphology, masticatory function and phylogeny of heterodontosaurid dinosaurs

“…4), was initially referred to L. angustidens (Thulborn 1970). Later authors have regarded the specimen either as taxonomically indeterminate (Charig and Crompton 1974;Porro et al 2011), referable to Abrictosaurus consors (Hopson 1975), or referable to the Lanasaurus scalpridens (Norman et al 2011) (Table 2). New information on the specimen provided later in this study supports referral to L. angustidens as originally proposed by Thulborn. Two additional maxillae with worn teeth were collected by C. Gow and J. Kitching from the Transkei (Herschel) District of South Africa.…”

“…Others, such as specimen NHMUK RU A100, were described with important bones misidentified (Thulborn 1970). Many specimens collected were not described or assessed until recently Norman et al 2011). And finally, the taxonomic basis of heterodontosaurid genera and species has been obscured by the inclusion of primitive character states in differential diagnoses rather than a shorter list of putative autapomorphies alone (Sereno 1990).…”

“…Heterodontosaurus tucki (Crompton and Charig 1962), which came to light in 1961, exactly one century after Owen's report on Echinodon, has long functioned as the sole or primary reference point for heterodontosaurids (Santa Luca 1980;Norman et al 2011), despite the fact that it remains one of the most derived members of the group. The diversity of form and function among these early small-bodied herbivores is only now gaining attention.…”

“…1B), but their taxonomic status has remained unclear for several reasons. Some, such as Geranosaurus, were described on the basis of fragmentary, non-diagnostic jaw material (Broom 1911;Norman et al 2011). Others, such as specimen NHMUK RU A100, were described with important bones misidentified (Thulborn 1970).…”

“…Despite considerable prospecting along the coast near the site of the initial find, no additional remains of this taxon have been unearthed, and its status as a heterodontosaurid was not recognized until recently (Sereno 1997;Norman and Barrett 2002). Current knowledge of the morphology of heterodontosaurids is based largely on the South African genus Heterodontosaurus (Crompton and Charig 1962), a complete skull and nearly complete postcranial skeleton of which were discovered in 1966 (SAM-PK-K1332; Charig and Crompton 1974;Santa Luca et al 1976;Santa Luca 1980;Weishampel 1984;Norman et al 2011). The majority of heterodontosaurid specimens have been discovered in southern Africa (Fig.…”