Tully Monsters were soft-bodied, marine invertebrates that lived during the Pennsylvanian Period, which is the younger of the two subdivisions of the Carboniferous Period. In short, they lived about 300 million years ago. They had long, segmented bodies that grew up to 14in (35.5cm), and possessed eight small teeth and a horizontal tail and dorsal fin. They were most likely carnivorous, based upon their mouth shape. They also had a bar that transected the body horizontally about half way down. It is thought that these served as sensory organs. Based on their body shape, they were most likely active swimmers.
The Tully Monster was first discovered by amateur fossil collector Francis Tully in 1958. Fossilized imprints were found in the Mazon Creek deposits, and he brought them to the Field Museum in Chicago. It was eventually determined that this was a whole new species and was officially named after its nickname, Tullymonstrum, with the genus, gregarium, meaning "common."