Most of the Paleo Profiles I’ve written so far have highlighted new species, either found in the field or rediscovered in museum collections. But this week is a little different. The animal at the center of this story isn’t new to science, but a historic fossil whose identity has just changed.

In 1854, during the early days of American paleontology, the Philadelphia naturalist Joseph Leidy described a new fossil species found on Prince Edward Island, Canada. The piece included several elements from the animal’s upper jaw and, given its general form, Leidy named it Bathygnathus borealis, or “the deep jaw from the north.”

Nothing like Bathygnathus had been seen before. In fact, it was the first fossil of a non-mammalian synapsid –