Courtesy of Martin Lockey, University of Colorado

It's a slab of rock, two-by-two feet square, which tells a 75-million-year-old story. Imprinted in one part is a footprint indentation (below the coin) of a tiny mammal and the impressions of a leaf; on another part are marks typically left by raindrops.

"It looks like the animal took cover underneath some type of leaf that protected the surface from raindrops," says Martin Lockley, University of Colorado, Denver, who discovered the fossilized traces of the cretaceous mammal in western Colorado. Along with similar footprints found from a site just outside Denver, they are the "first well-authenticated examples of mammal tracks from the age of dinosaurs," he and his coauthor claim.1

The prints, less than an inch long, exhibit the typical 2/3/3/3/3 formula of a mammalian hand: two joints in the thumb, three in each of the other fingers. "The animals were hardly...

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