But a new study says it isn't so.
Most of the modern groups -- or "orders" -- of mammals apparently began before the dinosaurs met their doom 65 million years ago, some researchers now conclude.
The work suggests at least five major lineages, which today include such creatures as rodents, elephants, and armadillos, might have appeared more than 100 million years ago.
The results, based on analysis of genes, are presented in Thursday's issue of the journal Nature by S. Blair Hedges and a colleague at Pennsylvania State University. Hedges and co-authors published a similar study in 1996, but their new study includes many more genes.
They 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.
Michael Novacek, curator of vertebrate paleontology at the American Museum of Natural History in New York, said he is skeptical of that approach.
"I think the fact that the fossil record doesn't support this is a serious issue," he said.
By Malcolm Ritter