Dinosaur burrows in the Otway Group (Albian) of Victoria, Australia, and their relation to Cretaceous polar environments

Three enigmatic structures in an outcrop of the Otway Group (Albian) of Victoria, Australia, compose the first known evidence suggestive of dinosaur burrows outside of North America and the oldest from the fossil record. The most complete of the Otway structures nearly matches the size and morphology of a burrow attributed to the only known burrowing dinosaur, Oryctodromeus cubicularis from the Upper Cretaceous (Cenomanian) of Montana (USA). The suspected burrows cross-cut alluvial facies and overlie nearby strata containing dinosaur tracks. The structures contain identical sand fills in their upper portions, implying a near-synchronous origin and filling; graded bedding in the most complete structure also indicates passive filling of an originally open structure. This probable burrow is a 2.1 m long, gently descending, semi-helical tunnel, with a near-constant diameter (about 30 cm) that connects with an enlarged terminal chamber. The structures are unlikely to have been caused by physical or chemical sedimentary processes, and hence are considered as biogenic structures; moreover, their size and morphology imply tetrapod tracemakers. Burrow allometry indicates tracemakers with a mass of 10–20 kg, matching size estimates for small ornithopods from the Otway Group. Burrowing behavior in hypsilophodontid-grade dinosaurs, which compose most of the dinosaurian assemblage in the Lower Cretaceous of Victoria, was proposed previously as an adaptation for surviving formerly polar conditions in southeastern Australia. This paradigm is explored in detail, particularly through actualistic examples of tetrapod burrowing in cold climates. These structures may provide the first clues of ornithopod burrowing in these extreme environments, while also establishing search images for similar structures in other Lower Cretaceous outcrops in Victoria.