ASSESSING PATTERNS OF GEOGRAPHIC VARIATION IN LATEST CRETACECOUS MAMMALIAN ASSEMBLAGES FROM NORTH AMERICA

Virtually all of what is known of the pattern of latest Cretaceous mammalian evolution (from approximately 68 to 66 million years ago) is documented in stratigraphic sequences from the Western Interior of North America; yet, significant latitudinal differences are present among these latest Cretaceous assemblages, including a number of eutherian mammals that were apparently confined to more northerly latitudes. This has led some to suggest that the northern part of the Western Interior of North America represented a distinct biogeographic region during the latest Cretaceous, separate from that in the south, and that discrete evolutionary processes were operating in these different regions. We compiled information on the relative abundances of mammals from nine latest Cretaceous assemblages, extending from Alberta to Wyoming, and used a series of bootstrapping analyses to generate assemblage-specific abundance estimates of each taxon based on the composition of the Type Lance local fauna of Wyoming. Using these estimates, we quantitatively assess the faunal similarity between assemblages and identify taxa that were significantly more abundant or rare. Our results suggest that the composition of latest Cretaceous mammalian assemblages across the Western Interior of North America was not homogeneous. Several eutherians (e.g., Cimolestes, Gypsonictops) are more abundant in the northern assemblages relative to the faunal composition of the Type Lance and other more southerly sites, whereas some multituberculates (e.g., Meniscoessus) and marsupials (e.g., Pediomys) are unusually rare in the northern assemblages. However, rather than suggesting a latitudinal gradient, the pattern reflects similarity between geographically close spaced assemblages. Although discriminating regional geographic patterns from temporal or local ecological effects remains difficult, the faunal composition of the more northerly assemblages suggests that eutherians were undergoing an evolutionary radiation in the northern part of the Western Interior of North American during the latest Cretaceous and that this region may have been the source for mammalian immigrants into more southern localities during the earliest Paleogene.