Illinois’ Tully monster reinterpreted

Since its discovery in 1958 by Francis Tully in the approximately 300-million-year-old Mazon Creek fossil beds in Illinois, the zoological affinities of the soft-bodied animal popularly known as the Tully monster (Tullimonstrum gregarium) have remained mysterious. It is a remarkable looking creature, with a fish-like body featuring prominent eyes at each end of a horizontal bar, and jaws on the end of a long, jointed proboscis. Its origins have been compared variously with nemertean and polychaete worms, molluscs, conodonts and even stem-group arthropods. Now two papers in this issue identify Tullimonstrum securely as a vertebrate. Thomas Clements et al. studied the eyes and found ultrastructural details indicating homology with those of vertebrates. Victoria McCoy et al. examined more than 1,200 specimens. They re-interpret many known features, and describe and interpret many new ones: all are consistent with Tullimonstrum being a vertebrate, akin to lampreys and increasing the morphological disparity of that group.