The extinct ocean creature Tullimonstrum gregarium, popularly known as the "Tully monster," is one of those outsized weirdos that defies easy categorization. A soft-bodied swimmer that lived 307 million years ago in what is now Illinois, the Tully monster measured around four inches long from its bizarre, pincer jaws to its posterior caudal fin. Its eyes were connected dumbbell-style by a lateral bar, its elongated snout dangled loosely from its head, and it sported porthole-like gills along its flank.
The result is an animal that looks like the cross between a child's bath toy, a disembodied crab leg, and Star Wars bit character Sy Snootles. Indeed, because the Tully monster is such an evolutionary misfit, paleontologists have struggled to pin down its place in the tree of life ever since its remains were first discovered in the Mazon Creek fossil beds in 1958.
But now, an exhaustive new study involving over 1,200 Tully monster specimens has finally shed light on this unique creature's heritage, revealing that it was a vertebrate related to modern lampreys. The research, led by Yale paleontologist Victoria McCoy, is published today in Nature.
"In addition to increasing the known morphological disparity of extinct lampreys, a chordate affinity [meaning vertebrate features] for T. gregarium resolves the nature of a soft-bodied fossil which has been debated for more than 50 years," McCoy's team wrote in the study.
One of the biggest breakthroughs is the reinterpretation of a distinct band of material that runs horizontally along the bellies of numerous Tully monster fossils.
Traditionally, these markings were interpreted as so-called "gut traces," or the fossilized remains of the digestive system. But the new study's precision approach—which included synchrotron analysis—revealed that the structure was much more reminiscent of skeletal spinal cords found in the ancient hagfish Gilpichthys greenei, a contemporary of the Tully monster in Carboniferous Illinois.
The reinterpretation of this distinct feature has settled decades of speculation over the taxonomical group to which Tullimonstrum belongs. Now we know that this animal was not, as previously suggested, an invertebrate worm, mollusk, or arthropod, but a vertebrate swimmer supported by a slim backbone.
That said, it's not as if the Tully monster has given up all of its secrets. Though it left behind an abundant trove of remains and has been enshrined as the state fossil of Illinois, the animal's behaviors and ecological roles are still shrouded in mystery. Indeed, even its new status as a bonafide lamprey relative raises as many questions as it answers.
"With the very bizarre nature of Tullimonstrum's morphology it seems likely that it was doing something very different to modern lampreys," paleobiologist James Lamsdell, a co-author of the study, told me via email.
"The short body and high aspect ratio of the tail suggests that it was an able swimmer capable of fast bursts of motion," he said. "Unlike modern lampreys Tullimonstrum would likely have been an active predator, grabbing prey with its proboscis and rasping flesh off with its tongue. The eyes on their stalks would have provided a broader field of view allowing them to easily spot prey, and allowed them to focus on the tip of the proboscis which would have let them accurately grab at prey."
In other words, the Tully monster capitalized on each of its exceedingly unusual adaptations to eke out a living in the Carboniferous oceans that once covered Illinois. But while it is wonderful to celebrate its updated placement on the phylogenetic tree, by no means does that mean that there is nothing left to discover about this gnarly little carnivore that lived so long ago.