PECULIAR FOSSIL IN LANCE FORMATION (CRETACEOUS) BONEBED IN WYOMING DETERMINED TO BELONG TO A PTEROSAUR

The uppermost Cretaceous dinosaur-fossil-bearing layers of western North America have been investigated for over a century. One site in particular, located in the Lance Formation of eastern Wyoming has been excavated for over 20 years, yielding over 35,000 fossils from dinosaurs such as Edmontosaurus, Tyrannosaurus, and Triceratops; crocodilians such as Brachychampsa; and other creatures. However, while pterosaurs are known in the coeval Hell Creek Formation, there have been no published reports of pterosaurs in the Lance Formation.

In 2012, a very thin and hollow fossil fragment (HRS17126) with well-defined curvature was excavated. In an attempt to identify the fossil, we compared its shape, and R/t ratio (the ratio of the bone wall thickness to the diameter of the bone), to the shape and ratio of fossils found in the literature. HRS17126 fell within the expected shape and R/t ratio range for pterosaurs. The specimen also contains needle-like, bone structures known as trabeculae. These structures are very common in pterosaur bones, as they provide structural stability in the hollow interior. Other fossils of hollow boned creatures such as theropods and avians have been discovered in the Lance Formation. However, these fossils either lacked the extremely thin bone walls, or the greater R/t ratios which are found in pterosaurs, and which we find in HRS17126. For these reasons, it was determined that this fossil is that of a pterosaur, and the size and elliptical curvature lead us to believe that it may be a portion of a long bone.

This discovery is not surprising as the Lance Formation’s coeval formations do possess pterosaur fossils. However, it does confirm that pterosaurs existed in Lance Formation dinosaur communities. Hopefully, this encourages the search for other pterosaur fossils that may have been misidentified in museum collections due to their fragmentary nature.