LATE CRETACEOUS DINOSAURS OF EASTERN NORTH AMERICA-HOW AND WHEN DID THEY CROSS THE WESTERN INTERIOR SEAWAY?

The cross-continental relationships of Late Cretaceous terrestrial fauna in North America are uncertain, but may be constrained by new data. The confounding matter is the presence of the Western Interior Seaway (WIS) from the early Albian to the late Maastrichtian, which is assumed to present an absolute barrier to terrestrial migration. Yet, four dinosaur clades (tyrannosaurs, hadrosaurs, ankylosaurs, dromaeosaurs), two crocodyliformes (Deinosuchus, Leidyosuchus), and trionychid turtles, appear on both sides of the WIS penecontemporaneously, and appear to have at least familial relationships across the continent. Either eastern and western terrestrial populations evolved in parallel on both sides of the WIS from pre-Albian common ancestors, or managed to cross the WIS during Santonian time (when the first eastern Late Cretaceous dinosaurs appear) by unknown means. New information on two theropod clades indicates both were present in the East before the WIS formed, and the parallel evolution model is supported. Two tyrannosauroid theropods, one from the mid-Campanian of central Alabama, and the other from the late Maastrichtian of New Jersey, are closer to the base of the tyrannosaur clade than all Late Cretaceous western American taxa. Since the deeper ancestry of tyrannosauroids is almost certainly Asian, these eastern specimens must represent relicts of a pre-Albian, whole-continental population. Another recent discovery, a single velociraptorine dromaeosaurid tooth in the lower Campanian of western Alabama, is also interpreted to represent a whole-continental population, since similar teeth are in the Lower Cretaceous of Montana and Maryland. Less compelling but positive paleogeographic data suggest that eastern hadrosaurs also derive from common pre-Albian ancestry on both sides of the WIS. The relationships and timing of migrations of crocodylians and turtles is ambiguous, because these paralic clades conceivably crossed the WIS aided by volcanic archipelagos, which may have spanned much of the southern ocean during the Campanian.