What makes a snake a snake? The oldest snake fossils on record are forcing researchers to reconsider the question.

When it comes to separating snakes from their lizard ancestors, paleontologists have long considered legs to be the deciding factor: lizards have them, snakes do not.

In a report published in the journal Nature Communications, scientists describe four newly discovered fossils 140 million to 167 million years old. Two fossils were originally thought to be lizards, but they have now been classified as snakes because of the structure of their skulls and jawbones. Those features evolved before limblessness, the researchers said.

“Skull evolution and feeding mechanics is most likely what drove the evolution of snakes,” said Michael Caldwell, a paleontologist at the University of Alberta and lead author of the study. “The limbless thing comes later.”