First forelimb reconstruction and range of motion assessment of the Late Cretaceous dinosaur Troodon formosus

Troodon formosus ( T. formosus ) is a theropod dinosaur from the Late Cretaceous of North America. T. formosus , like many theropods, are speculated to have used their forelimbs to hunt, and the complexity of their preserved egg clutches suggests T. formosus may have also used its forelimbs to move its eggs. Understanding the morphology and range of movement of T. formosus ’s forelimb could help shed light onto these hypotheses. However, no complete forelimb material has yet been found for T. formosus , and a 3D reconstruction and range of motion (ROM) estimate has not been attempted. This study aims to address this gap by leveraging digital modelling technology to create the first forelimb reconstruction and ROM for T. formosus . Surface scans from multiple T. formosus fossils housed in the Museum of the Rockies (Bozeman, Montana) were digitally combined to reconstruct a nearly complete forelimb. Digital articulations based on this assembled model were compared with physical ROM using 3D printed copies. Results show higher ranges of flexion than extension in T. formosus ’s joints, consistent with closely related species. However, T. formosus shows higher manual extension than close relatives. The humerus also shows anatomy convergent with more basal theropod species. These differences may imply a divergent morphology and function of the manus, as well as a deviation from avian ancestor forelimb morphology. ROM results cannot confirm whether T. formosus was able to grasp objects single-handed, but two-handed apprehension of objects, including eggs, remains feasible.