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."

Tomiya worked with Tseng, an anatomist at the University at Buffalo, to put the fossils in a micro-CT scan and create 3D reconstructions of the skulls. Previously, scientists had only been able to analyze the teeth and outside features of the rock-encrusted fossils, but Tseng's micro-CT scan gave them a look inside at the fine structures of the ears. Tseng immediately identified a telltale shape inside the ear that also appears in later beardogs. In a Royal Society Open Science paper, Tomiya and Tseng reclassify the unknown skulls as the early beardog ancestors Angelarctocyon australis and Gustafsonia cognita.

What's remarkable about this discovery is that it gives us a new understanding of where these incredibly successful hunters evolved. Though previous research has suggested they evolved in Europe and crossed over to the Americas during a period of climate change during the Eocene, Tomiya and Tseng's findings are making the story more complicated. There is now ample evidence from these newly analyzed skulls, along with many other fossils, that the beardog arose in North America. However, the researchers are careful to say that all the evidence is not in, and a Eurasian origin is still possible.

"Our research pinpoints the southwestern US as a key region in understanding the diversification and proliferation of this once successful group of predators prior to their extinction millions of years ago," Tseng said. "We're not saying we've solved where they fit on the tree of life, but it's the most progress that's been made in quite a while. Our work provides a clearer connection between the rest of the beardog family and their evolutionary roots."

We also have more evidence that the ferocious beardog, a lone hunter like bears, had pretty humble origins. Though the enormous beardogs of the Miocene probably looked and acted like modern bears, the predator's earliest ancestors looked like tiny dogs or foxes. Ultimately, the beardog became an ancestor of today's dogs, too; it's less closely related to bears. Its large size is probably what consigned it to extinction in the end. Though a hefty frame makes predators tough opponents, their build also means they need more food to survive. Three million years ago, as the icy mid-Pliocene made hunting tougher, it's likely the beardog was unable to compete with early wolves and dogs that hunted in packs. Still, beardogs had a good reign for more than 30 million years across the northern continents, ruling with all the mass of bears and all the cunning of canines.

Royal Society Open Science, 2016. DOI: 10.1098/rsos.160518