Tullimonstrum
(Tullimonstrum gregarium)

Living during the Carboniferous period, Tullimonstrum — the Tully Monster — is one of paleontology's most enduring mysteries. Found only in the Mazon Creek fossil beds of Illinois, this soft-bodied creature had a long, flexible body, a claw-tipped proboscis extending from its front end, and eyes on rigid stalks projecting from the sides of its body. Decades of debate have failed to definitively classify it — proposed variously as a vertebrate, a mollusk, or something else entirely — making it one of the most genuinely puzzling animals ever discovered!