Fossil crabs (Crustacea, Decapoda) from the Maastrichtian Difunta Group, northeastern Mexico

Two species of fossil crabs have been collected from Maastrichtian deltaic deposits referred to the Potrerillos Formation of northeastern Mexico.Mascaranada difuntaensis is a new genus and species of carcineretid natatory crab.Morphological and paleoecological analysis suggest that the Carcineretidae is a heterogeneous group of reptant and natatory crabs whose taxonomy must be reconsidered.The fifth pereiopods of the natatory group are modified as in portunids, a family which may have descended from the carcineretids.The reptant forms may be more closely related to the Xanthidae.The youngest and southernmost occurrence of Dakoticancer australis Rathbun is recorded from two localities in the lower siltstone member of the formation.Twenty-four carapaces were found in two penecontemporaneous but paleoenvironmentally different localities.Most of the carapace remains are molts which, at one locality, were found associated with burrows, suggesting that this crab molted in burrows for protection.Some specimens are larger than previously reported for this species.Although this species is abundant in Mississippi Embayment D. australis Assemblages, it is an uncommon taxon in molluscan fossil assemblages at these Mexican localities.Biostratigrapnic and paleobiogeographic ranges of D. australis are extended; the lower siltstone member or the Potrerillos Formation (Difunta Group) seems to be among the youngest Cretaceous crustacean-bearing units in North America.