Over 50 years ago, researchers unearthed 300-million-year-old fossils of a mysterious, soft-bodied animal from a coal mining pit in Illinois. They called it the "Tully monster," after fossil collector Francis Tully who discovered it, and in 1989, it was designated the official state fossil. Ideas about what it could be ranged from a slug-like critter to a fish relative. Now, according to researchers studying more than a thousand of these specimens, this mysterious little monster is definitely a vertebrate. It’s closely related to jawless fish called lampreys. The findings are published in Nature this week. 

"I was really surprised when I realized the Tully monster was a vertebrate!" Yale’s Victoria McCoy tells IFLScience. "It's really rare in the fossil record to have a vertebrate without bones or other hard tissues, so it was definitely unexpected." Based on its shape and structure, the fossil has previously been linked to worms, mollusks (like snails and clams), arthropods (which includes insects and spiders), and conodonts, which is an extinct chordate. Chordata is the phylum that all vertebrates belong to, and all chordates have a skeletal rod called a notochord. In humans, this goes on to become part of our spine.

One of the best-supported hypotheses was that it was a conodont, and with that in mind, McCoy and colleagues analyzed more than 1,200 Tullimonstrum gregarium museum specimens uncovered from late Carboniferous Mazon Creek sediments dating back 307 to 309 million years.

^{Holotype (species-defining) fossil of Tullimonstrum gregarium, the "Tully monster." This specimen has the best preservation of morphological features, including muscle segments in the body, the eye bar, the tail fin, and the proboscis and jaw folded back over the body. Paul Mayer/Field Museum of Natural History}

They reinterpreted the two-dimensional, light-colored structure present length-wise in the fossils as a notochord. It was previously thought to be the trace of a gut. The 10-centimeter-long (4-inch-long) animal also has gills, a segmented body, eyes projecting from the ends of a long rigid bar (like in hammerhead sharks), a jaw apparatus containing small pointy teeth at the end of a trunk-like proboscis, and a tail fin. This combination of features – together with an analysis of its evolutionary relationships – suggest that the animal is a vertebrate. 

Furthermore, the Tully monster is closely related to today's lampreys, but it’s not quite their ancestor. "If you think of the lamprey family tree as an actual tree, all living lampreys are up near the top, in the crown where all the leaves are," McCoy explains. "The ancestors of modern lampreys are on the trunk, right near the top where it starts branching out into the leafy part. However, there are little branches coming out of the trunk, below the ancestor of all modern lampreys, and these are the stem lampreys, including the Tully monster." 

^{Reconstruction of the Tully monster. Notice the jointed proboscis, the multiple rows of teeth, and the dorsal eye bar. Sean McMahon/Yale University}