Snakes, with their sleek bodies and kaleidoscopic diversity, have long entranced humans. But we know very little about the evolutionary past of these legless lizards because of a scarcity of fossils left by snake ancestors that shared the earth with dinosaurs.

That’s why recently excavated snake fossils from Argentina, described in a study published Wednesday in Science Advances, are such a big deal for serpent fans. The intricate fossils, mostly skulls, are nearly 100 million years old and belong to the extinct snake group Najash, which still retained hind legs. The fossils suggest that snakes lost their front legs much earlier than had previously been believed but also held onto their hind legs for millions of years. The find will also help to resolve mysteries over when snakes began their transition to their modern form.

Fernando Garberoglio, who led the research, discovered the most spectacular of these new skull specimens, called MPCA 500, in 2013 when he was an undergraduate student.