Two nearly complete sets of fossilized animal remains from 130-million-year-old rocks in China are revealing fresh details about the size and dietary habits of ancient mammals. The newly described finds counter the common presumption that such creatures remained small and ecologically suppressed until dinosaurs became extinct about 65 million years ago.
The fossils were unearthed from volcanic ash laid down by an ancient eruption. Many of the entombed animals have been preserved in their original three-dimensional forms, says Jin Meng, a paleontologist at the American Museum of Natural History in New York.
The larger of the two finds, a 1-meter-long mammal dubbed Repenomamus giganticus, had a stout build and squat posture like that of modern-day badgers. The wear pattern on the creature’s teeth hints that the mammal was a young adult when the lethal eruption occurred, says Meng.
He and his colleagues, who describe the fossils in the Jan. 13 Nature, estimate that R. giganticus weighed between 12 and 14 kilograms, which makes the species the largest mammal known to have lived during the 170-million-year reign of dinosaurs.
The other set of remains—in some ways, the more astounding one—is a nearly complete specimen of Repenomamus robustus, a smaller cousin of R. giganticus. This opossum-size species was first described from fragmentary remains about 5 years ago.
Within the well-preserved rib cage of the new specimen, the researchers found a compact wad of broken bones and teeth. Some of those remains, including a skull, spinal column, and limb bones, still had some joints intact, suggesting that the bones derive from prey that had been dismembered and swallowed in chunks. Comparisons with known fossils suggest that those fragments belong to a hand-length hatchling of a Psittacosaurus, a common plant-eating dinosaur that grew to a length of around 2 meters.
The newfound Repenomamus fossils are challenging the popular notion that the mammals that lived side by side with dinosaurs generally were as small as shrews and ecologically marginalized, says Anne Weil, a paleontologist at Duke University in Durham, N.C. A few previous finds had shown signs that some ancient mammals were substantially larger than shrews and rats, but the fossils were so fragmentary that the size estimates were imprecise.
The dinosaur-as-prey scenario upsets another long-held paleontological contention by suggesting that predatory mammals may have influenced dinosaur evolution. For example, says Weil, some feathered dinosaur contemporaries of Repenomamus over time became smaller—and thus presumably better equipped to fly or escape predation. At the same time, some mid-size dinosaurs were becoming larger in perhaps another evolutionary response to mammalian predation.