研究发现陆地食草动物的祖先

New research from the University of Toronto Mississauga demonstrates how carnivores（食肉动物） transitioned into herbivores for the first time on land. "The evolution of herbivory（食草性） was revolutionary to life on land because it meant terrestrial vertebrates could directly access the vast resources provided by terrestrial plants," says paleontologist Robert Reisz, a professor in the Department of Biology. "These herbivores in turn became a major food resource for large land predators."

Previously unknown, the 300-million-year old fossilized juvenile skeleton of Eocasea martini is less than 20 cm long. Found in Kansas, it consists of a partial skull, most of the vertebral column, the pelvis and a hind limb.

By comparing the skeletal anatomy（解剖学） of related animals, Reisz and colleague Jörg Fröbisch of the Museum für Naturkunde and Humboldt-University in Berlin, discovered that Eocasea martini belonged to the caseid branch of the group Synapsid. This group, which includes early terrestrial herbivores and large top predators, ultimately evolved into modern living mammals.

Eocasea lived nearly 80 million years before the age of dinosaurs. "Eocasea is one of the oldest relatives of modern mammals and closes a gap of about 20 million years to the next youngest members of the caseid family," says Fröbisch. "This shows that caseid synapsids（下孔类） were much more ancient than previously documented in the fossil record."

It's also the most primitive member and was carnivorous, feeding on insects and other small animals. Younger members were herbivorous, says Reisz, clear evidence that large terrestrial herbivores evolved from the group's small, non-herbivorous members, such as Eocasea.

"Eocasea is the first animal to start the process that has resulted in a terrestrial ecosystem with many plant eaters supporting fewer and fewer top predators," he says.