A phylogeny of fossil and extant mammalian taxa, combined with the known ages of fossils. The earliest fossils are found in Asia, with more modern fossils found in North America, and then spreading to South America and Australia. This pattern is consistent with known land connections between the continents at the times of apparent interchange. Zhe-Xi Luo, Qiang Ji, John R. Wible, Chong-Xi Yuan (2003) "An Early Cretaceous Tribosphenic Mammal and Metatherian Evolution", Science, 302(5652):1934-1940.