Abstract
Early Cretaceous (late Aptian–early Albian) fossil localities in the south coastal region of Victoria, southeastern Australia, provide a snapshot of the rich terrestrial biota that lived on the floodplain within the extensional rift system that existed between the eastern Gondwanan landmasses of Australia and Antarctica. Amongst the terrestrial vertebrate assemblage, agile, bipedal, small-bodied (‘turkey’- to ‘ostrich’-sized) ornithopod dinosaurs have been considered abundant and arguably diverse. Although the Victorian ornithopods potentially hold significant information on Gondwanan dinosaur phylogeography, their taxonomy and phylogenetic systematics have been poorly understood.This research provides a sedimentological and taphonomic reassessment of the Leaellynasaura amicagraphica holotype locality at Dinosaur Cove in the Eumeralla Formation (Otway Group). The osteology and systematics of the three previously named Victorian ornithopods (Atlascopcosaurus loadsi, Leaellynasaura amicagraphica and Qantassaurus intrepidus) are revised, and potential new taxa recognised. The postulated diversity of the Victorian ornithopods is reassessed and their phylogenetic and phylogeographic relationships are investigated.Contrary to the original interpretation of a quiet-water depositional setting (such as a billabong), the holotype locality of L. amicagraphica is reinterpreted to be a point-bar deposit. Fossil accumulation in the heterolithic cross-strata of the point-bar is considered to be allochthonous. As a result, the referral of additional specimens to L. amicagraphica, other than materials directly attributable to the holotype, cannot be adequately supported. In addition, newly identified material of the L. amicagraphica holotypic cranial fragment (MV P185991) provides important new information on this taxon, and facilitates anatomical and taxonomic differentiation of the cranial table (MV P185990), originally considered to be from the holotype individual.The validity of L. amicagraphica (Eumeralla Formation) and Q. intrepidus from the Wonthaggi Formation (Strzelecki Group) is upheld. However, Atlascopcosaurus (Eumeralla Formation) is considered to be a junior subjective synonym of Leaellynasaura (L. loadsi nov. comb. [Rich & Rich, 1989]). A third species of Leaellynasaura (Leaellynasaura sp. nov.) extends the range of this genus to the Wonthaggi Formation. Two premaxillary, one maxillary (Leaellynasaura sp. nov.) and two dentary morphotypes (including Q. intrepidus) are recognised from the Wonthaggi Formation, whilst four maxillary morphotypes (including two species of Leaellynasaura), two dentary and four postcranial morphotypes are recognised from the Eumeralla Formation. Although taxonomic differentiation of these morphotypes has not been ascertained, it is possible that some pertain to the same taxon. The five maxillary morphotypes, provide an overall measure of the diversity of theVictorian ornithopod assemblage.Of the postcranial morphotypes, three partial postcranial skeletons are recognised from the Eumeralla Formation: two from Dinosaur Cove (Victorian Ornithopod Postcranial morphotypes [VOPC] I and II), both previously attributed to L. amicagraphica, and the third (VOPC III) from the locality of Eric the Red West. VOPC I and II are differentiated from each other on the basis of their haemal arch morphology. VOPC I differs from all other ornithischians on account of its extreme taillength (~13 times femoral length), while the hind limb proportions of VOPC II suggest a highly cursorial ornithischian. VOPC III appears to be a more robust morphotype than both VOPC I and II. The femur of the fourth postcranial morphotype, Victorian ‘Hypsilophodontid’ Femur Type 2 of Rich & Vickers-Rich (1989), differs morphologically from the femora of VOPC I and II, and represents a larger more robust body-type. The morphological differences evident in both the maxillary and the postcranial morphotypes suggest differing habitat preferences among the ornithopods from the region.Cladistic analysis using implied weighting methodology suggests that Leaellynasaura and the postcranial morphotypes VOPC I and II are most closely related to Australia’s Muttaburrasaurus and Patagonia’s Anabisetia and Gasparinisaura. It is hypothesized that these taxa form a Gondwanan non-iguanodontian ornithopod clade that diverged from a Pangaean progenitor of Iguanodontia through vicariance during Pangaean fragmentation in the Late Jurassic. Interestingly, the Laurentian Parksosaurus and Thescelosaurus may be more closely related to Muttaburrasaurus and Patagonia’s Talenkauen than to basal ornithopod taxa from Asia. The hypothesised relationship between these non-iguanodontian ornithopods from Laurentia and Gondwana may stem from the opening of an Antillean dispersal route between North America and South America during the Late Cretaceous. Although Qantassaurus was not included in the cladistic analysis, its dental morphology resembles that of Kangnasaurus from the Late Cretaceous of South Africa. In addition, the proportions of metatarsal II of VOPC II and III more closely resemble those of dryosaurids (Dryosaurus, Dysalotosaurus and Kangnasaurus), Anabisetia and Gasparinisaura. These similarities either suggest convergence between these taxa, or a closer relationship between dryosaurids and Gondwanan non-iguanodontian ornithopods than suggested by the results of the cladistic analysis.The Victorian ornithopods currently represent the oldest body-fossils of non-iguanodontian ornithopods from Gondwana. It is speculated that the high diversity of this key ornithopod fauna from Victoria is linked to the geologically dynamic, high-latitude floodplain palaeoenvironment of the Australian-Antarctic Rift System, and, tantalizingly, the early radiation of angiosperms in eastern Gondwana.
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