You Are What You Eat ... With
Today two types of mammals dominate the world: marsupials and placentals. Together they account for the vast majority of species diversity seen in mammals.
While they differ vastly in body shape and lifestyle, the niche they fill in nature boils down to their teeth.
"It has long been supposed that the adaptation that allowed them to be so successful was a multifunction molar that can both cut and grind," Cifelli explained.
"For mammals you are what you eat with," added Zhe-Xi Luo, a paleontologist at the Carnegie Museum of Natural History in Pittsburgh, Pennsylvania, and one of the study's authors.
"Essentially all mammalian diversity is broken down pretty much by the different dental design that we each have."
Whether it be hyenas that can grind bone or horses that eat hay, all teeth in today's mammal species are derived from this basic dental template.
The finding also suggests that early mammals were beginning to diversify much earlier than previously thought.
The first two-thirds of mammalian history takes part in the age of the dinosaurs. During that time it has been thought that mammals remained in much the same form: small, furry, nocturnal, insect-eating animals that skirted around dinosaurs. (Related: "Dinosaur Extinction Spurred Rise of Modern Mammals, Study Says" [June 20, 2007].)
"Our general view of what happened is that they didn't really go into any extravagant ecological niches until dinosaurs became extinct," Cifelli said. "Now we're finding that wasn't the case."
Previous fossil finds at the same lakebed show that mammals were already gliding around and swimming as far back as about 165 million years ago.
Along with the new find, the discoveries suggest that mammals underwent tremendous diversification during the middle of the Jurassic period, Cifelli said.
"We're seeing a host of skeletal adaptations that say, hey, mammals were doing these wild and crazy things—they weren't just lying around in their little hidey holes," he added.
"[Pseudotribos robustus] helps to show that the earliest mammals coexisted with the dinosaurs are far more diverse than we ever have imagined," study author Luo said.
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