MAMMALIAN COMMUNITY CHANGE AFTER THE K/T EXTINCTION IN NORTH AMERICA

The extinction event at the K/T boundary was responsible for the demise of most animals larger than a kilogram. The subsequent radiation of mammals into herbivorous niches recently vacated by the dinosaurs is a well-known phenomenon, reflected in a general body size increase and the adaptation of dentitions to the shearing forces that the breakup of tough plant matter requires. Based on tooth morphology alone it has long been assumed that the first large-bodied mammals of the Paleocene subsisted on fruit fall rather than leaf browsing. However, mammals had by then attained the body sizes necessary to subsist solely on browse. The absence of high fiber herbivory from a fauna of such large mammals would be unprecedented and without any modern analog. The timing and nature of mammalian dietary evolution in the Paleocene was investigated using low-magnification microwear techniques, which allows for the evaluation of an animal’s herbivorous niche (grazer, browser, hard-object feeder) more precisely than is usually possible by analyzing tooth shape alone. Although the Paleocene saw a huge radiation of mammalian diversity in terms of taxonomy, results show that this trend did not extend to their diets. There was an initial transition from predominantly insectivorous communities in the Cretaceous to hard-object-feeding dominance in the Puercan. Communities remained hard-object dominant through the Paleocene--even as both size and taxonomic diversity increased. Evidence of prominent leaf-browsing does not appear until the Wasatchian, when a general drying trend in the Western interior triggered a change from closed-canopy Paleocene rainforests to more open vegetation. These results suggest that Paleocene mammalian communities were strongly influenced by their environment, not resorting to browse consumption until the Paleocene rainforest fruit-fall became unavailable.