It's called the latitudinal diversity gradient, a phenomenon seen today in most plant and animal species around the world: Biodiversity decreases from the equator to higher latitudes. A new 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. The findings, reported in the Proceedings of the National Academy of Sciences, point to the importance of not assuming that the way things are today is the way they've always been, the researchers say... It may seem obvious that...