Two mysterious skulls sat in storage at The Field Museum of Natural History in Chicago for 30 years. And then, thanks to one curious researcher, they changed our understanding of how one of America’s fiercest predators evolved.

Beardogs, or Amphicyonids, are a diverse group of mammals who emerged on the tree of life during the mid-Eocene period roughly 40 million years ago. By 20 million years ago, they had spread to most northern continents, grown in stature, and become a bone-crushing apex predator. Their signature bulky jaw and flat teeth (for crushing) are a common sight for paleontologists excavating fossils from the Eocene and Miocene, but their origins remained murky.

Paleontologist Susuma Tomiya was working on a postdoc at The Field Museum when he came across two oddly labeled fossils, about the size of Chihuahuas. Discovered in the mid-1980s in Texas, nobody was sure what they were. Based on their skulls and age (38-37 million years old), the fossils had been assigned to the genus Miacis, which Tomiya’s colleague Jack Tseng described in a release as “a kind of ‘trashbin’ genus” for unidentified carnivores. Tomiya added, “I thought it looked odd and too advanced for what it had been claimed to be—a more primitive carnivore. It reminded me of some much larger beardogs, so I decided to take a closer look.”