current diversity pattern of north american mammals a recent trend, study finds /

It's called the latitudinal diversity gradient,a phenomenon seen nowadays in most plant and animal species around the world: Biodiversity decreases from the equator to higher latitudes. A unusual study of fossils representing 63 million of the past 65 million years reveals that—for North American mammals, at least—the modern LDG is the exception rather than the rule.

Source: phys.org