Madagascar’s aye-aye lemur is an endearing aberration of an animal.

It has enormous ears, a bushy tail, mammary glands between its legs and white hairs that bristle when it’s agitated. Above a vampiric nose and mouth of incisors that never stop growing, candy-corn-colored eyes glow like full moons. Gangly fingers capped in curled claws punctuate its hands. The middle finger, a spindly stick, rotates on a ball-and-socket joint. Sometimes it folds over an even longer fourth finger.

If these weren’t enough to meet the quota for eccentric adaptations, scientists just found another — a secret spare thumb. This pseudo thumb, as it is called, even has a fingerprint.

“We’ve studied the function of those weird, spindly middle fingers so long that nobody ever looked at this kind of lowly structure on the wrist,” said Adam Hartstone-Rose, an anatomist at North Carolina State University who led the team that completed the research. “But that structure has arguably more evolutionary significance than that weird, little spindly finger.”

Other animals also have extra appendages. But the aye-aye’s spare thumb, described on Monday in The American Journal of Physical Anthropology, may have developed under circumstances not yet observed in any other animals. The study demonstrates that there’s still much more to learn about how different lineages of vertebrates evolved — even by looking at anatomy we thought we understood.