BIOSTRATIGRAPHY, PALEOBIOGEOGRAPHY, AND EVOLUTION OF DROMAEOSAURIDS (DINOSAURIA: DROMAEOSAURIDAE) IN NORTH AMERICA

Dromaeosaurid dinosaurs (Dinosauria: Theropoda: Dromaeosauridae) first appear in North America during the Early Cretaceous. The earliest North American member, Yurgovuchia is from the Cedar Mountain Formation (lower Yellow Cat Member, ?Barremian, ~129-125 Ma) in Utah, quickly followed by Utahraptor (upper Yellow Cat Member, Barremian, ~127-123 Ma). Several million years later Deinonychus is known from the Cloverly and Antlers formations (Aptian-Albian, ~115-108 Ma) in Montana, Wyoming, and Oklahoma. The next oldest North American dromaeosaurids do not appear until about 77 Ma. Indeed of the 10 known North American taxa, five are from the late Campanian, ranging from New Mexico to Alberta, Canada. Dromaeosaurus is known from the Dinosaur Park Formation (Judithian, ~76.6-74.8 Ma) in Alberta, along with Hesperonychus (lower part, ~76.5 Ma) and Saurornitholestes langstoni (upper part, ~75.5 Ma). Those from farther south are slightly younger, with Saurornitholestes sullivani (Kirtlandian, ~73 Ma) from New Mexico and Bambiraptor (Edmontonian, ~72 Ma) from Montana. Atrociraptor, from the Horseshoe Canyon Formation (unit 4) in Alberta, is late early Maastrichtian in age (Edmontonian, ~70-68 Ma). Acheroraptor, from the upper Hell Creek Formation (Lancian, ~66 Ma) of Montana, represents the youngest dromaeosaurid known. While numerous isolated dinosaur teeth have been referred to various dromaeosaurid taxa from multiple localities in North America, their taxonomic identifications should be conservatively considered indeterminate dromaeosaurids instead. It is possible that North American dromaeosaurids gained their foothold in southern Laramidia, and subsequently migrated to northern Laramidia. Although whether this migration occurred from southern Laramidian members or from Asian members is yet unknown. Similarities between the northern and southern taxa during the Campanian (e.g., two species of Saurornitholestes), shows migration occurred between the two regions at some point prior to the late Campanian, when North American dromaeosaurids were at their most diverse. More complete finds (i.e., non-tooth fossils), particularly in places less well studied thus far (e.g., Mexico, Texas, eastern North America, Alaska), may shed further light on the evolution of dromaeosaurids in North America.