After nearly 6 decades in paleontological limbo, a weirdly shaped, soft-bodied sea creature that lived more than 300 million years ago has gained a spot on life’s family tree. Tullimonstrum gregarium, informally dubbed the “Tully Monster” after the amateur fossil hunter who discovered the first specimens in 1958, is monstrous only in terms of its appearance (artist’s concept above), with the largest individuals measuring in at about 15 centimeters long. Although unearthed by the thousands in the coal pits at one site in northeastern Illinois, where rocks were laid down between 307 million and 309 million years ago, its fossils have been found nowhere else in the world—one reason the Tully Monster was named Illinois’s state fossil in 1989. Now, a detailed analysis of more than 1200 fossils that provide views of the creature’s remains from many angles confirm its relation to the eellike creatures known as lampreys, the researchers report online today in Nature. Even though the Tully Monster was shaped like no other lamprey living or extinct, the new study reveals for the first time that the creature had gills and a flexible backbonelike rod called a notochord—anatomical features that make the creature, like today’s lampreys, a vertebrate. Tullimonstrum’s eyes sat far from the body at the ends of another flexible rod, which helped it get a 3D view of prey near the end of its tooth-sporting proboscis, the researchers propose. Rather than swimming with a snakelike motion, as modern-day lampreys typically do, the Tully Monster likely propelled itself with undulations of a muscular fringe of tissue on its tail, similar to the way that cuttlefish swim today.