Tullimonstrum, colloquially known as the Tully Monster, is an extinct bilaterian animal that lived in shallow tropical coastal waters of muddy estuaries during the Pennsylvanian geological period, about 300 million years ago. A single species, T. gregarium, is known. Its classification has been the subject of controversy, and interpretations of the fossil likened it to molluscs, arthropods, conodonts, worms, and vertebrates. It is unknown weather "Tullimonstrum" was a vertebrate or an invertebrate.