This fossil looks like a lumpy, rounded rock with a couple of exposed teeth that are a little worse for wear, having been submerged and tumbled along the floor of the Gulf of Mexico for thousands of years before washing up on a beach. But when it was X-rayed a doctoral student saw there was more to the fossil that met the eye: a hidden canine tooth that had not yet erupted from the jaw bone. It was just what researchers needed to identify the fossil as belonging to a Homotherium, a genus of large cat that roamed much of the Earth for millions of years.