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Heterodontosaurus cranial anatomy described



From: Ben Creisler
bh480@scn.org

In advance papers for Zoological Journal of the Linnean Society: 

DAVID B. NORMAN, ALFRED W. CROMPTON, RICHARD J. BUTLER, LAURA B. PORRO and
ALAN J. CHARIG (2011)
The Lower Jurassic ornithischian dinosaur Heterodontosaurus tucki Crompton
& Charig, 1962: cranial anatomy, functional morphology, taxonomy, and
relationships.
Zoological Journal of the Linnean Society (advance online publication)
DOI: 10.1111/j.1096-3642.2011.00697.x
http://onlinelibrary.wiley.com/doi/10.1111/j.1096-3642.2011.00697.x/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 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-supported hypothesis is the one which places Heterodontosauridae
in a basal (non-genasaurian) position; however, the evidence is not fully
conclusive and further information is still needed in respect of the
anatomy of proximate outgroups, as well as more complete anatomical details
for other heterodontosaurids. Heterodontosaurids were not such rare
components of the late Karoo ecosystem as previously thought; evidence also
suggests that from a phylogenetic perspective they occupied a potentially
crucial position during the earliest phases of ornithischian dinosaur
evolution.


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