On bright mornings in the tropics, crocodiles haul themselves out of the water to bask in the sun. Like other reptiles, they rely on its warmth to help fuel their metabolisms.

This might seem to be a vision from a prehistoric world: sluggish creatures unchanged for millenniums. But evidence is building that modern crocodiles differ substantially from their earliest ancestors.

In a recent study published in Paleobiology, a pair of paleontologists at the Sorbonne University in Paris examined bone from a 237-million-year-old reptile and found signs that the ancestors of the crocodile family had remarkably active metabolisms — and that the creatures evolved them far earlier than expected.

“After the enthusiasm for dinosaurs provoked by the ‘Jurassic Park’ films, I thought that the time had come to look at closely related groups,” said Jorge Cubo, lead author of the study. “The central question was whether these organisms were cold or warmblooded.”