Kinematic model of tyrannosaurid (Dinosauria: Theropoda) arctometatarsus function.

We present a hypothesis of tyrannosaurid foot function termed the "tensile keystone model," in which the triangular central metatarsal and elastic ligaments dynamically strengthened the foot. The tyrannosaurid arctometatarsus, in which the central metatarsal is proximally constricted, displays osteological correlates of distal intermetatarsal ligaments. The distal wedge-like imbrication of tyrannosaurid metatarsals indicates that rebounding ligaments drew the outer elements towards the middle digit early in the stance phase, unifying the arctometatarsus under high loadings. This suggests increased stability and resistance to dissociation and implies, but does not demonstrate, greater agility than in large theropods without an arctometatarsus.