LONGRICH, N. R.; University of Calgary: Aquatic marsupials from the Late Cretaceous of North America

Following the end of the Cretaceous, a wide range of mammals became adapted to life in water, but no strong evidence is available for aquatic mammals during the Mesozoic. Recently, a series of unusual, large (1.4-2.6 cm) mammalian caudal vertebrae were identified from the Campanian Judith River group of Alberta. These vertebrae are flat, broad and bear large transverse processes. Their resemblance to the vertebrae of the platypus (Ornithorhynchus) and beaver(Castor) suggests an oarlike tail used for steering underwater. The vertebrae do not seem to pertain to multituberculates; of the remaining mammals in the fauna, only the stagodont marsupials Eodelphis browni and E. cutleri are large enough to account for them. As in river otters (Lutra canadensis) and mink (Mustela vison), Eodelphis exhibits unusually heavy tooth wear, perhaps indicating a diet of fish and shellfish. The Maastrichtian Didelphodon (including D. vorax, D. padanicus, and D. coyi) is a larger, more specialized stagodont, and possesses robust mandibles and bulbous premolars resembling those of the sea otter Enhydra. The teeth possess large, planar wear facets and are sometimes worn down to the roots, providing evidence of resistant prey items. It may have preyed on the diverse gastropods and bivalves of the Hell Creek and Lance formations. A semiaquatic lifestyle also predicts heavy bones to decrease buoyancy. As predicted, the jaws of stagodonts have greatly thickened cortices. The large size of stagodonts and their occurrence alongside a diverse freshwater biota are also consistent with semiaquatic habits. Stagodonts therefore represent the earliest known aquatic mammals. The largest mammals in Late Cretaceous North America were aquatic, underscoring the extent to which dinosaurs constrained mammalian diversification, yet stagodonts also hint at an underappreciated ecological diversity among Mesozoic mammals.