Snakes are in the same reptilian order that includes lizards, but just how and where they split off to live their legless lives has been a bit of a mystery. Transitional fossils showing the move from four-legged lizard to belly-crawling snake have remained scarce. However, that the jawbones of a Cretaceous snake from North America suggest that it might be the earliest snake on record. And this serpent was terrestrial, clearing up decades of debate about whether snakes evolved for swimming or slithering, researchers reported online July 25 in Nature (Scientific American is part of Nature Publishing Group).

The first known example of the snake in question, Coniophis precedens, was collected in eastern Wyoming and described from a single vertebra in 1892. In the 120 years since, additional vertebrae and jaw bones from the species ” have been collected but never described,” wrote the researchers, led by Nicholas Longrich, of Yale University’s geology and geophysics department. Their analysis shows that the 70 million-year-old specimen is probably the most primitive known snake—and a sister line to the nearly 3,000 snake species that exist today.