THE LATITUDINAL DIVERSITY GRADIENT WITHIN LATEST CRETACEOUS AND EARLY PALEOGENE MAMMALIAN ASSEMBLAGES FROM THE WESTERN INTERIOR OF NORTH AMERICA

The latitudinal diversity gradient (i.e., the increase in taxonomic richness from the poles to the equator) is one of the most widely recognized patterns in modern ecology. Although the mechanisms for the establishment and maintenance of this phenomenon are poorly understood, the gradient is commonly suggested to have been in place throughout much of the Mesozoic and Cenozoic. Several recent studies, however, have indicated that certain taxonomic groups during select time intervals, including non-avian dinosaurs during the Mesozoic, do not conform to this pattern. Using a large, newly developed dataset, comprised of 188 fossil assemblages with information on nearly 500 genera, and a least-squares regression of generic richness on latitude, we assessed the latitudinal diversity gradient within latest Cretaceous and early Paleogene mammalian assemblages from the Western Interior of North America (from approximately 69 to 57 million years ago). Based on this approach, a latitudinal gradient is not present among latest Cretaceous and early Paleogene mammalian assemblages, with no statistical difference in richness across latitude. To further determine whether these results merely reflect differences in sampling intensity, we conducted a linear modeling approach employing the number of assemblages within each geological formation and the site-occupancy of genera within these formations. A strong association between richness and sampling intensity is recovered. Yet, even after mediating for the variance in sampling intensity, a latitudinal diversity gradient comparable to that established at the present is still not present. A shallow temperature gradient and more equable climates across North America during the latest Cretaceous and early Paleogene, as well as instability among mammalian communities near the boundary, are likely mechanisms for the absence of a latitudinal diversity gradient.