NEW AZHDARCHOID (PTEROSAURIA: PTERODACTYLOIDEA) REMAINS FROM THE LANCE FORMATION (MAASTRICHTIAN), WYOMING

The Hanson Ranch Bonebed, located in the Lance Formation of eastern Wyoming has been excavated for over 20 years, yielding over 40,000 fossils from many creatures including dinosaurs such as Edmontosaurus, Tyrannosaurus, and Triceratops; crocodilians such as Brachychampsa; and many other creatures. As a whole, the uppermost Cretaceous layers of western North America have been investigated for over a century. Yet, pterosaur fossils are rarely found in the Lance Formation with only two published discoveries. The first pterosaur fossil identified from the Hanson Ranch Bonebed was excavated in 2012 and was described and identified by the authors in 2022. Since its identification, the collections from this site have been reviewed. Special attention was given to fossils that remained unidentified.

A small, pointed bone fragment (HRS13086) that was excavated from the site in 2011, caught the attention of the curators. The specimen is 15mm in length and 3mm in width at its widest point. Although somewhat conical, the long axis of the bone is not straight but gently curved toward the tip. Additionally, HRS13086 is roughly triangular in cross section, with two convex surfaces and one concave surface. We interpret this bone to be the anterior tip of a mandible, with the two convex sides being the lateral surfaces and the concave portion forming the oral surface. A single, rounded foramen is located ventrally on the left side of the mandible. The right side of the mandible lacks any foramina. The presence of foramina excludes an avian identification, and instead suggests an edentulous pterosaur. The overall morphology of HRS13086 agrees very well with mandibles seen in azhdarchoid pterosaurs, however, azhdarchoids typically possess elongated and paired foramina on their mandibles. Nevertheless, at least one azhdarchoid (Mistralazhdarcho) possesses rounded and unpaired foramina, comparing well with the condition in HRS13086.

Our hope is that discoveries such as these will cause the continued review of collections from the Lance Formation in search of any misidentified or overlooked pterosaur specimens. These kinds of finds indicate that even well-studied fossil-bearing formations can still yield surprises, providing us with a richer understanding of ancient ecosystems.