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

Abstract

Troodon formosus (Troodon) is a theropod dinosaur from the Late Cretaceous of North America primarily known from fragmentary fossil material. Hypothesized to hunt small game, Troodon, like many theropods, may have actively engaged its forelimbs to aid hunting behavior. Furthermore, the complexity of preserved nests associated with Troodon suggest it had sufficient range of motion and dexterity to use its forelimbs to manipulate its eggs. Additionally, Troodon's evolutionary position as one of the closest groups to living birds could indicate important information about the evolution of the avian wing. However, no complete forelimb material has yet been found for Troodon, and as such a complete reconstruction and range of motion (ROM) estimate of its forelimbs has yet to be attempted. This study aims to address these hypotheses and shortcomings by leveraging recent digital modelling technology to create the first forelimb reconstruction and ROM for Troodon. To overcome preservation limitations, surface scans from multiple Troodon fossils housed in the Museum of the Rockies (Bozeman, Montana) were digitally combined to reconstruct most of a forelimb. From this model, digital articulation in the software package Maya was compared with physical articulation ROM methods using 3D printed copies of the forelimb. The maximum angle of allowed motion for each forelimb joint until bone-on-bone contact was assessed. Results show increased flexion over extension in Troodon's joints, consistent with closely related species. However, Troodon also shows higher manual extension than close relatives. The humerus is also shorter and stockier, showing mechanical specialization for strength vs speed, and convergent features with primitive theropod species. These ROM and morphological differences imply a deviation in hunting strategy or forelimb utility and a deviation from avian forelimb morphology. Total manual ROM results cannot confirm whether Troodon was able to grasp its eggs and small prey single-handed, but two-handed apprehension remains feasible. This study utilized digital and physical ROM methods, analyzing for the first-time specimens that would have been previously overlooked. These results shed light on both the foraging and reproductive behavior of Troodon and have implications for understanding the evolution of modern avian reproductive behavior and the dinosaur-bird transition.