Trends in mammal diversity

Analysis of more than 27,000 Cenozoic terrestrial mammal fossils from western North America reveals that the latitudinal diversity gradient (LDG), the trend across almost all clades of life for the number of species to decline from the equator to the poles, was considerably weaker for much of the past 65 million years than during present times, that the LDG may weaken during globally warm periods, and that the modern LDG of North American mammals became established over the last 4 million years. - Read at

Article #15-24750: “Late Cenozoic onset of the latitudinal diversity gradient of North American mammals,” by Jonathan D. Marcot, David L. Fox, and Spencer R. Niebuhr.