The fossil of a shrewlike animal uncovered a decade ago in Mongolia's Gobi Desert set off one of the most extensive probes ever into the origins of placental mammals, the vast majority of all living mammals (which excludes marsupials and egg-layers, like the platypus).

"Of course you're excited when you find something well preserved from the Cretaceous [period 145 million to 65 million years ago]," says John Wible, curator of mammals at the Carnegie Museum of Natural History in Pittsburgh and senior author of a new report that concludes placental mammals originated around 65 million years ago, between the Cretaceous and Tertiary periods when dinosaurs disappeared. "This is one those instances where the fossil really is not all that interesting in a sense, but rather, it's the study that the fossil led us to."

To properly age and classify the Mongolian fossil Maelestes gobiensis, estimated to be between 71 million and 75 million years old, Wible and his team compared it with 409 features culled from the skulls, teeth and skeletal remains of other animals ranging in age from present-day mammals to those estimated to have lived over 100 million years ago. In an attempt to determine whether it was a placental mammal, the scientists constructed a tree charting the evolution of placental mammals beginning well in the Cretaceous. "We wanted to find out what our fossil was," Wible says, "and we wanted to test whether any of [the other] Cretaceous fossils could be placentals."

Wible and his colleagues report in Nature that when they finished analyzing and classifying the specimens, they discovered that none dating back to the Cretaceous appeared to be placental mammals; it seemed such mammals more likely evolved some 65 million years ago, which would support the long held "explosive model'' theory that a dino die-off made way for them to spring up.

"The dinosaurs die out and all these niches open up. Then placental mammals go into these niches and…. Wham!" Wible says. "They just explode into these new niches opened up by dinosaurs."

J. David Archibald, an evolutionary biologist at San Diego State University, praised the new study as being the most comprehensive analysis yet into the evolution of placental mammals based on the shapes and forms of fossils. But he quibbled with the conclusion, noting that previous studies indicated that some placental mammals were present in the late Cretaceous.

Wible acknowledges there are still many questions to be answered. "One of the problems is that we have to deal with the fossil record and the fossil record is imperfect," he says. "We don't have Cretaceous life perfectly distributed over every continent." Still, he says, age is relative, especially when you're talking about millions of years. He cites a recent study, for instance, conducted by an international team of molecular biologists that used the DNA of living placental mammals to estimate that their ancestors originated more than 100 million years ago. "Tomorrow, I would not be surprised if I found an 80 million-year-old Cretaceous placental—what's another 10 [million] or 15 million years?" he says. "On the other hand, I would be surprised to find a 120 million-year-old cretaceous placental."