THE IMPACTS OF CENOZOIC CLIMATE AND HABITAT CHANGES ON SMALL MAMMAL DIVERSITY OF NORTH AMERICA

Through the Cenozoic, paleoclimate records show general trends of global cooling and increased aridity, and environments shifted from predominantly forests to more open habitats. Paleosol and phytolith studies suggest some sites began to be dominated by open habitats in the Oligocene. Studies of large mammalian herbivores have documented changes in ecomorphology and community structure through time, but shifts occurred millions of years after the environmental changes thought to have triggered them. Smaller mammals, like rodents and rabbits, should more closely track climate and habitat changes due to their shorter generation times and smaller ranges, but these animals have received much less study.

To examine changes in smaller mammals through time, we have assembled and analyzed a database of all 1135 North American rodent and lagomorph species. For each species, first and last appearance data were compiled from databases and literature sources. Tooth crown height and locomotion were categorized based on published and unpublished data, along with phylogenetic inferences. Crown heights are expected to have increased through the Cenozoic as habitats became more open and arid. Burrowing, jumping, and cursorial adaptations are also expected to become more prevalent as environments changed.

Analyses of these data found that small mammal community structure changed dramatically through the Cenozoic. In the Eocene, most species had brachydont dentition and arboreal taxa were abundant. In the Oligocene, there were dramatic increases in the diversity and proportion of higher crowned taxa, as well as adaptations for burrowing, jumping, and cursoriality. From the late Miocene through Pleistocene the proportion of hypselodont taxa steadily increased, to ultimately represent nearly half of all species.

Our results suggest that rodents and lagomorphs responded quickly to Cenozoic environmental changes. Over time, a large proportion of species have adapted to feed and move in open environments. Communities shifted in the Oligocene from predominantly low crowned to higher crowned taxa. Similarly, a large proportion of species have had adaptations for locomotion in open habitats since the late Oligocene. Adaptations for open habitats appear in rodents and lagomorphs about 10 million years earlier than ungulates.