FROM CRAYFISH TO DINOSAURS: EARLY CRETACEOUS TRACE FOSSILS OF VICTORIA, AUSTRALIA AND THEIR USES IN INTERPRETING CIRCUMPOLAR PALEOENVIRONMENTS

Lower Cretaceous strata of Victoria, Australia contain perhaps the best-documented body fossil record of freshwater-terrestrial flora, invertebrates, and vertebrates of any formerly circumpolar (about 80° S paleolatitude) Mesozoic deposits. Alluvial, fluvial, and lacustrine lithofacies of the Otway and Strzelecki Groups (Albian and Aptian, respectively) were deposited in rift basins associated with the separation of Australia from Antarctica, with deposition largely a result of high seasonal discharge. Consequently, body fossils are a mixture of allochthonous and autochthonous elements, and may be more representative of upland environments. In contrast, most trace fossils are definitely in-situ indicators of depositional environments, yet these have only recently augmented body fossils in the paleontological database for this area. Nonetheless, trace fossils found since 2006 have expanded our understanding of organismal behavior in Mesozoic polar environments in ways that cannot always be supplied by body fossils. For example, fossil crayfish burrows (Thalassinoides), the oldest in Australia, indicate adaptations of burrowing freshwater decapods to cold-water environments. Included with these are enormous burrow systems that may be linked to selection of large decapod body sizes in cold freshwater environments, a possible reflection of Bergmann’s rule. Other invertebrate burrows (Arenicolites, Ancornichnus) likely demonstrate infaunal activity during polar Cretaceous springs and summers, hinting at their use for interpreting seasonality. Although relatively rare, dinosaur tracks also point toward a presence of small ornithopods and large theropods in the same places where their bones were deposited. Lastly, the most recent interpretation of possible dinosaur burrows in the Otway Group suggests burrowing as a dinosaurian strategy for overwintering in circumpolar environments. We anticipate that further documentation of invertebrate and vertebrate trace fossils in this area will reveal much more about organismal interactions with Mesozoic polar environments, including adaptations to fluctuating global temperatures.