A finite element approach to the biomechanics of dromaeosaurid dinosaur claws

Dromaeosaurid theropod dinosaurs possessed strongly recurved, hypertrophied and hyperextensible ungual claws on both the pes (digit II) and manus. The morphology of these unguals has been linked to the capture and dispatching of prey. The effectiveness of the enlarged pedal digit II ungual as a disemboweling implement has been challenged by recent experiments using a hydraulic reconstruction of a dromaeosaurid hind limb. However, the mechanical properties or, more importantly, the mechanical potential of these structures have not been explored. The generation of a 3D finite element (FE) stress/strain contour map of a Velociraptor manual ungual has for the first time allowed quantitative evaluation of the mechanical behavior of a dromaeosaurid terminal ungual phalanx. The role of the finite element analysis was to test the relationship of form and function, from an engineering point of view. X-ray microtomography scans have allowed the construction of an accurate 3D finite element mesh. Analogue biomaterials from an extant avian theropod, the pedal digit and claw of an eagle owl (Bubo bubo), were analysed to provide input data for the Velociraptor claw FE model. The resultant finite element model confirms that claws of dromaeosaurid dinosaurs were well-adapted for climbing as they would have been resistant to forces acting in a single (longitudinal) plane, in this case due to gravity. However, the strength of an ungual claw was limited with respect to forces acting tangential to the longaxis of the structure. The tip of the claw functioned as the puncturing and gripping element of the structure, while the expanded proximal portion transferred the load stress through the cortical and cancellous bone. Claw form and function varies widely among vertebrates however, claw sheath composition does not. The possession of a keratinous sheath also played an integral role in the distribution of load stress into the bone core of the claw. The proposed enhanced climbing abilities of dromaeosaurid dinosaurs, a function of pedal morphology, support a scansorial phase in the evolution of flight.