Modified distal caudal vertebrae (the handle) and large terminal osteoderms (the knob) form the tail club of ankylosaurid dinosaurs. It has been assumed that the club was used as a weapon, but the biological feasibility of this behaviour has not been tested. Computed tomographic scans of tail clubs referred to Euoplocephalus, and measurements of free caudal vertebrae, were used to estimate the impact force of clubs of various sizes. Tails are modeled as segments for which mass, muscle cross-sectional area, torque, and angular acceleration are calculated. Large knobs could generate sufficient force to break bone during impacts, but average and small knobs could not. Finite element analyses showed that stress is distributed evenly along the handle, preventing fracture of the club during impacts. Tail clubbing behaviour is feasible in ankylosaurids, but it remains unknown whether the tail was used for interspecific defense, intraspecific combat, or both.
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