EXTINCTION AND SURVIVAL OF FROG CRABS (CRUSTACEA: BRACHYURA: RANINOIDA) FROM THE EARLY CRETACEOUS TO THE PRESENT

Understanding diversity and extinction patterns in fossil crustaceans is critical to our understanding of the environments of the past and will allow us to better predict the futures of our extant species. Several taxa within Decapoda show strong patterns of faunal progression, with groups such as the clawed lobsters, podotrematous crabs, and heterotrematous crabs diversifying and facing high rates of extinction in succession. As the heterotremes are the most recent group to diversify and have not yet faced a downturn, they contribute to the majority of extant crabs. Meanwhile, the more primitive podotremes have been in decline since the Late Cretaceous, and therefore offer a complete picture of a ‘diversify then decline’ trend. Commonly called ‘frog crabs’, Raninoida is a monophyletic clade and constitutes a significant portion of the podotremes. Studying Raninoida offers a unique opportunity to examine these trends, as well as the potential patterns present between taxa that survive and taxa that go extinct.

A variety of data has been collected in order to explain patterns of extinction and taxon longevity within raninoid crabs, including carapace morphology, environment type, and stratigraphic and paleogeographical occurrences. These factors are compared across raninoidan taxa though time, ranging from the Early Cretaceous to the present. Preliminary data reveal that while most families survived the end-Cretaceous, all but three were extinct by the end of the Eocene, and only two of them survive to the present day. These two families, Raninidae and Lyreididae, are known for elongated carapaces and other specializations indicative of back-burrowing. This seems counter to the idea that generalist taxa are more likely to survive ongoing changes to their environment. It is possible that back-burrowing conferred some advantage that helped these two families through the shifting environments of these time periods. However, other factors could be at play and are still being tested. The reasons behind the counterintuitive survival of specialist raninoids may hold implications for our understanding of the environments they inhabited, as well as the evolutionary patterns of other decapod taxa.