The mysterious, soft-bodied fossil known as ‘Tully monster’ (Tullimonstrum gregarium), from the 307-309-million-year-old Mazon Creek fauna of Illinois, USA, is a vertebrate with similarities to lampreys, reports a new study published in Nature this week. The zoological affinity of this enigmatic fossil - which has been designated the official state fossil of Illinois - has puzzled scientists for over 50 years. Past interpretations based on morphological features of the fossil have variously likened the animal to a worm, a mollusc, an arthropod and a conodont.
Victoria McCoy and colleagues analysed more than 1,200 museum specimens of Tullimonstrum in order to elucidate its affinity. They reinterpret the two-dimensional, light-coloured, medial structure present in the fossils - generally thought to be a gut trace - as a notochord, a skeletal rod which is found in chordates. They describe the animal as being large (about 10 cm long), possessing a slender, segmented body, with eyes at each end of a long rigid bar, jaws at the end of a proboscis, and a caudal fin. This combination of features, coupled with phylogenetic analysis, suggest the animal is a vertebrate related to lampreys. These findings increase the range of forms of extinct lampreys.