Mammals May Have Diversified Before the Dinosaurs Died Out

Studies of fossils have long indicated that mammals waited until the dinosaurs died out before they started diversifying into the variety of major groups seen today. A new study in today’s Nature, however, indicates that most of the modern groups, or orders, of mammals apparently began before the dinosaurs met their doom 65 million years ago.

Analyzing gene sequences, molecular biologist S. Blair Hedges and a colleague at Pennsylvania State University looked at 658 genes from 207 kinds of modern animals. They chose genes that accumulate changes over time, apparently at a constant rate, and used them like tiny clocks to estimate when various species arose. They found that at least five major lineages, which today include such creatures as rodents, elephants and armadillos, might have appeared more than 100 million years ago.

Compiled by Times medical writer Thomas H. Maugh II