Tullimonstrum

Tullimonstrum, popularly known as the Tully monster, is an extinct genus of soft-bodied bilaterian marine animal that lived in shallow tropical coastal waters of muddy estuaries during the Pennsylvanian period, roughly 310 million years ago. The genus contains a single known species, Tullimonstrum gregarium. Fossils of Tullimonstrum are known only from the Mazon Creek fossil beds of Illinois in the United States, preserved in sediments deposited far from the ancient coastline. Its true classification within the animal kingdom has long been disputed among paleontologists.