The Tully monster remains an evolutionary enigma

The affinities of the Tully monster (Tullimonstrum gregarium), from the late Carboniferous Mazon Creek Lagerstätte (~309 Ma) of Illinois, have been debated since its discovery. Tullimonstrum is up to ~35 cm long with a long proboscis ending in pincers and an elongate tapering body with caudal dorsal and ventral fins. This iconic evolutionary enigma or ‘weird wonder’ has been suggested to relate to arthropods, various ‘worms’, tunicates, conodonts, lancelets, vetulicolians and even vertebrates, but it may be a mollusc (Caenogastropoda) such as a pterotracheid (heteropod) pelagic gastropod, similar to Pterotrachea coronata (the ‘sea elephant’).