During the Carboniferous Period—a geologic period that lasted from about 359.2 to 299 million years ago, during the late Paleozoic Era—the world was filled with frightening, foreign, and funny-looking creatures. One of the most puzzling of these prehistoric animals may have been Tullimonstrum, better known as the Tully monster.

The Tully monster was a sea creature that resembled a giant worm crossed with a small squid. Out of its head jutted a long, skinny appendage with a hook that may have been a mouth, and its eyes were attached to fleshy stalks.

As bizarre as these features might appear to the modern eye, scientists in recent years have focused more on whether or not the Tully monster had a backbone. This defining feature could help them figure out the creature's evolutionary relationship to its ancestors and descendants. But so far, as PBS Eons explains below, experts are still at a loss for where the Tully monster fits into the tree of life.

Learn more about the mysterious Tully monster below: