 The earliest described chelydrid is "Emarginachelys cretacea", known from well-preserved fossils from the Maastrichtian stage of the Late Cretaceous of Montana.Another well-preserved fossil chelydrid is the Late Paleocene "Protochelydra zangerli" from North Dakota.The carapace of "P. zangerli" is higher domed than that of the recent "Chelydra", a trait conjectured to be associated with the coexistence of large, chelonivorous (i.e., turtle-eating) crocodilians.