LATE CRETACEOUS AND EARLY PALEOCENE MAMMALIAN FAUNAL EXCHANGE BETWEEN ASIA AND NORTH AMERICA

Early Paleocene faunal recovery in the North American Western Interior includes the appearance of mammals whose lineages are unknown from North America in the preceding Maastrichtian stage. Possible sources of Early Paleocene diversity are the southern Western Interior, from which few mammal-producing faunas are known; the Cordillera, from which paleoenvironments were not preserved; and Asia. Because biogeographic sampling in the Late Cretaceous Western Interior is restricted to the foreland basins East of the rising Cordillera, it can be difficult to identify the geographic origins of new taxa.

Both new discoveries and phylogenetic analyses have shed light on this question in recent years, and there is some evidence of faunal exchange between North America and Asia. Elements of a North American mammalian fauna from about 100 MYA resemble Asian taxa, suggesting a dispersal event at about that time. Some Late Campanian taxa, such as Gallolestes, are hypothesized to have Asian affinities, but must have diverged from Asian lineages previous to 78 MYA. However, while there is considerable evidence for faunal interchange of dinosaurian taxa between North America and Asia in the Campanian and Maastrichtian, mammals seem not to have dispersed as easily, perhaps because of their relatively small size. There is no direct evidence supporting mammalian faunal exchange between 78 and 65 MYA.

There was, however, substantial endemic North American diversity. One example of an endemic radiation may be the multituberculate family Taeniolabididae. The earliest Paleocene taeniolabidid Catopsalis was once thought to occur in Mongolia as well as in North America, but subsequent study has shown this not to be the case. It has also been hypothesized that Taeniolabididae originated in North America and dispersed to Asia in the Paleocene. Recognition of a taeniolabidid from the Maastrichtian of North America that is more derived than some Paleocene species suggests that the family may have been diverse in Late Cretaceous North America, although they may have lived in environments that were not often preserved. It thus seems likely that, although mammalian faunal exchange might not have occurred in the latest Cretaceous, corridors of dispersal between North America and Asia existed in the Early Paleocene.