The new study, published in Wednesday’s online edition of Nature, focuses on remains of the snake species Coniophis precedens, which lived roughly 70 million years ago during the late cretaceous period. While the snake had been described in the scientific literature and remains had previously been collected, those studies had only looked at vertebrae.

In the new report, researchers from Yale and Harvard analyzed the upper and lower jawbones of the snake as well as its vertebrae, and came to the conclusion that the snake was transitional because its head was more similar to a lizard’s than a snake. In particular, this snake lacks the ability to open its jaw wide and swallow prey whole, one of the hallmarks of modern snakes. The ancient snake likely ate small vertebrate animals.

Because the snake’s fossils are from the plains of Montana, the scientists argue that the remains provide strong evidence that modern snakes evolved on land. They suggest that the lizard-like head and long body indicate that early snakes evolved as burrowers.