How is it that a museum can have over twelve hundred fossils of a particular species in its collection and not even know what it is? For decades, it was thought the Tully Monster, a bizarre animal that lived 307 million years ago, was an invertebrate. However, as host Emily Graslie reveals in this episode, scientists at The Field Museum finally cracked the mystery of the monster. It is actually related to lamprey fish. Part of "The Brain Scoop" series.