It's not what David Weishampel was looking for, but the tiny skeleton found this summer during his annual search for dinosaur bones in northwest Montana may be the prize of his career.
Barely three inches long, embedded in a small chunk omudstone, the fossilized bones are the ribs, backbone and tail of a rodent-like mammal that lived 72 million years ago side-by-side with duckbill dinosaurs.
And it's a rare find, one of only three of the early mammal skeletons unearthed in North America, said Dr. Weishampel, 37, an assistant professor of anatomy at the Johns Hopkins University School of Medicine.
"There's a gap in our mammalian history which this will help fill in," he said last week. "These guys may have been ancestors of modern mammals, the ones who lived on" after the dinosaurs became extinct 64 million years ago.
Dr. Weishampel has traveled for four summers to the Montana site, just east of the Rockies on the Blackfoot Indian Reservation, recruiting a crew of first-year medical students, largely from Johns Hopkins, and camping in tepees and tents along the scenic Two Medicine River.
Today, it's an arid area of grass-topped buttes and sharp-sloped ravines. Seventy million years ago, it was a subtropical coastal plain of a huge inland sea that periodically grew and retreated over the center of North America.
Dr. Weishampel's research, funded by the National Science Foundation, has focused on uncovering the remains of duckbill dinosaurs, or hadrosaurs, one of the most highly developed species of plant-eaters that evolved late in the 160-million-year reign of dinosaurs.
Hadrosaurs were abundant on the coastal plain, and more than their fossilized bones have come to light on the Weishampel expeditions, which this year featured a crew of 10 medical students, his wife, Judy, and their two daughters, Sarah and Amy.
At a cliff-side site dubbed Shell Hell, the diggers found several nests of fossilized dinosaur eggs, one of the most significant finds since Dr. Weishampel's colleague John Horner of the Museum of the Rockies discovered the first North American eggs in Montana in the early 1980s.
"They appear intact, and there may be embryonic material inside," Dr. Weishampel said, adding that further examination of the eggs could include X-ray-like CAT (computerized axial tomography) scans of the interiors.
University of Maryland medical student Joel Max found the partial mammal skeleton adjacent to a nest this summer. It was a treasure that would have been invisible to an untrained observer.
"Surface collecting for mammal bones is a bit harder than looking for dinosaur bones," said Dr. Weishampel. "You can stare at the ground for a half hour and not see anything if you don't know
what to look for."
The picture is sketchy of the mammals that first appeared during the Mesozoic Era, from about 225 million years to 64 million years ago, during the long domination of Earth by the dinosaurs.
Species diversity by predation and competition for food, kept them "mostly shrew and mouse-sized, only occasionally cat-sized, probably insect-eaters, night-creatures, tree-dwellers," Dr. Weishampel said.
It's thought there were three early mammal groups, said J. David Archibald, an expert on mammals of the late Cretaceous Period (90 million to 64 million years ago) at San Diego State University.
One group, the multituberculates, became extinct. But two others survived -- marsupials, represented now by opossums and kangaroos, and placental mammals, the ancestors of all modern mammals.
Unlike large dinosaur bones, the mammal skeletons "were small and fragile; they are not well-represented in the fossil record," Dr. Weishampel said. "The best record has been teeth and jaw fragments."
One of the most exciting recent discoveries was a dime-sized mammalian skull found in August near Lubbock, Texas, by a team from the New Mexico Museum of Natural History. The skull is believed to be 220 million years old, 10 million years older than the oldest known mammal.
Although Dr. Weishampel and his crew found a wealth of "fantastic" mammal teeth at Shell Hell, it's the skeleton that may provide information about "what they looked like, how they walked and ran, how they breathed and reproduced, if they climbed."
"Just the fact that another skeleton has been found is reason for rejoicing," Dr. Archibald said. "I hope it can tell us a whole lot
about the functional anatomy of these mammals."
The catastrophe that killed the little mammal and entombed the unhatched dinosaur eggs 72 million years ago was "most likely a local flood," Dr. Weishampel speculated. "Perhaps he was waiting there for the chance to slip into the nest for an egg dinner when Mama took a walk."
Some of the larger fossils collected this summer were removed from the field in bulk by surrounding them in plaster. They rest at the Museum of the Rockies in Bozeman, Mont., where preparators will painstakingly work this winter to uncover the secretswithin.
Dr. Weishampel can hardly wait, for among those large pieces is one taken from almost the exact spot where the mammal skeleton was found.
"The rest of this critter may be in a plaster jacket in Bozeman," he said. "Wouldn't that be something?"