Tully Monster Solved
A team of scientists with varied backgrounds have suggested that the enigmatic “Tully Monster” fossil organism (late Carboniferous Maxon Creek biota of Illinois) is a proto-lamprey. This is based on analysis of over 1,200 fossils and a weight of evidence approach identifying vertebrate features, including: presence of 1) notochord, 2) gill pouches, 3) myomeres (muscle striations), 4) fins, 5) optic stalks and lobes, and 6) a toothed buccal cavity. In the past this organism was thought to be anything from a worm, gastropod, conodont to arthropod (invertebrates).
Above: Modern reconstruction of Tullimonstrum gregarium, compared to a modern lamprey, Below:
Here are some other historical reconstructions and fossil images.
Field Museum Link
Scientific American Article