Halved fossil of the aquatic vertebrate Tullimonstrum gregarium, known as the Tully monster. This creature, which grew up to 35 centimetres in length, had a shovel-like tail (at right in top fossil) and a trunk-like snout with keratin teeth. It's eyes were at either end of a horizontal bar far back on its head. Fossils, dating to 300 million years ago, have only been found in the Mazon Creek fossil beds of Illinois, USA. X-ray studies of the fossils have shown that T. gregarium was closely related to modern lampreys.