Sedimentological reappraisal of the Leaellynasaura amicagraphica (dinosauria, ornithopoda) holotype locality in the Lower Cretaceous of Victoria, Australia with taphonomic implications for the taxon

Abstract

<p>The holotype individual of the small-bodied ornithopod dinosaur, Leaellynasaura amicagraphica from Dinosaur Cove in the Lower Cretaceous of Victoria, southeastern Australia, traditionally comprises the holotype, a left-side cheek fragment of a juvenile (MV P185991), and three other specimens: a cranial table (P185990) and a partial postcranium (P185992, P185993), discovered at the same site and at about the same time as the holotype. The latter three specimens have significantly contributed to the systematics of Leaellynasauria amicagraphica and anatomical arguments for its status as a "dinosaur of darkness," pre-adapted to existence in the Antarctic polar circle.</p><p>The original attribution of the scattered material (cranial table and partial postcranium) to the Leaellynasaura amicagraphica holotype was based on the assumption that the sizes of the specimens were comparable, and the interpretation of the facies in which these associated fossils accumulated as a quiet-water deposit, such as an oxbow lake, billabong or pond. The inferred low-energy depositional conditions were used to suggest that associated material, other than that attributable to the holotype, was unlikely to be present in the facies hosting the holotype individual. However, a detailed sedimentological study supporting the interpretation of a quiet-water deposit hosting the Dinosaur Cove material is lacking, and the presence of a larger second partial ornithopod postcranium (P186047) in the same deposit, seems contradictory to arguments that all of the scattered associated skeletal specimens from this site are attributable to the Leaellynasaura amicagraphica holotype.</p><p>Our revised sedimentological investigation indicates that all vertebrate remains from the Leaellynasaura amicagraphica holotype locality were deposited under active hydraulic flow on a migrating point bar in a meandering river. We term the host deposit the "Tunnel Sandstone." As a result of this new interpretation, we regard the total vertebrate fossil assemblage from this site as time-averaged, and interpret the associated ornithopodremains as an allochthonous accumulation of up to four separate individuals, some potentially with unknown taxonomic affinities. Without unequivocal anatomical evidence of skeletal association, we regard the traditional attribution of the scattered cranial table and partial postcranium to the Leaellynasaura amicagraphica holotype as inadequately supported. We consider the referral of any specimen to Leaellynasaura amicagraphica should contain features that are compliant with those features on the holotype cheek fragment or other conclusively referred specimens.</p>