Suminia lived at a time when all other known vertebrates lived on the ground and tree leaves were an 'untapped resource,' said University of Toronto paleontologist Robert Reisz. (Diane Scott/Field Museum)An ancestor of mammals first took to the trees before the age of dinosaurs, evolving the first known opposable thumbs on the way, a group of Toronto researchers reports.

The animal, dubbed Suminia getmanovi, lived at a time when other known vertebrates, including both predators and plant-eaters larger than itself, spent all their time on the ground, said University of Toronto paleontologist Robert Reisz, who studied its complete fossil remains with his PhD student Jörg Fröbisch.

"This is a small plant eater that was not only able to get away from the predators by climbing into the trees," Reisz said in an interview Tuesday, "but also to access an untapped food resource — that is, the leaves of the trees."

Reisz and Fröbisch, who has since graduated to become a postdoctoral fellow at the Field Museum in Chicago, publish their findings in Wednesday's issue of Proceedings of the Royal Society B.

Suminia lived 260 million years ago in the Late Paleozoic era, just before the Mesozoic era, dominated by the dinosaurs. At the time, the Earth was an ice-free world with a single continent, and Suminia lived in the temperate and sub-tropical forests of its north — a climate similar to modern-day Florida — in what is now central Russia, Reisz said.

Not a reptile

Suminia's opposable thumb was well adapted for climbing trees and may have allowed it to bring food to its mouth, Reisz said. (Christina Stoppa/Field Museum)While Suminia may have looked lizard-like, he added, the anatomy of its skull and skeleton show that it actually was an ancestor of modern mammals, and not part of the evolutionary branch that led to reptiles and birds.

The first Suminia fossil was reported in the 1990s, and the description of its teeth — indicating it was a plant eater — caught Reisz's eye.

He decided to do some more digging near where the fossil was found in Russia's Kirov region, about 800 kilometres east of Moscow. As part of the project, his research group trained local villagers and provided them with equipment to help in the excavation. A little museum was even set up in the village. The excavation yielded more than 20 Suminia skeletons.

"These large blocks came out with a whole slew of these little animals that basically died together," Reisz said.

He added that the most exciting parts of the fossils were the hands.

"What really grabbed my attention was the first digit, the thumb-like structure. It does look opposable," he said, adding that Suminia was "obviously beautifully adapted to climb trees."

It likely could also bring food to its mouth using its hands. The researchers also found fossil dung indicating the animal ate plants.

Suminia's thumb isn't built the same way as human opposable thumbs and those of other primates, which evolved separately, Reisz said.

Because Fröbisch is German, the research was funded by both the Canadian and German governments, as well as the National Geographic Society and the Field Museum.