Tullimonstrum gregarium, better known as the ‘Tully Monster’ is a problematic fossil from the Late Carboniferous Mazon Creek lagerstätte, Illinois, USA. The identity of this fossil has been the subject of much debate, due to its peculiar form. Several competing hypotheses have placed it within the arthropods, fish, worms or even molluscs. Joining us in this interview is Dr Victoria McCoy whose work at Yale University (recently published in Nature) was able to demonstrate that the Tully Monster belonged to a different group entirely.