A Bygone Shrew

Last week, as if to reinforce Dougal Dixon's point, a team led by Harvard Paleontologist Parish Jenkins Jr. announced a rare discovery from northeastern Arizona: a fossil jaw from a tiny, shrewlike, insect-eating mammal that lived during the early Jurassic period, 180 million years ago. At that time the first small mammals evolved from a kind of mammalian reptile. In evolutionary terms, these creatures bided their time, for 115 million years, until the disappearance of dinosaurs and other reptiles allowed them to evolve thousands of different shapes and sizes. Significantly, the Arizona find adds a third major branch of mammals to the two that had previously been identified: one resulted in the egg-laying platypus and the spiny anteater of today; the other led to all modern animals that produce live young. The new branch may have been a zoological dead end, but its discovery illustrates the extraordinary precariousness and complexity of the evolutionary process.

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