Evolutionary morphology of the coelurosaurian arctometatarsus: Descriptive, morphometric and phylogenetic approaches

Descriptive, principal component (PCA), and thin-plate spline (TPS) analyses of theropod third metatarsals (MT III) definitively segregate the arctometatarsus from other theropod pedal morphologies and reveal variation within phylogenetic and functional subgroups of metatarsi. PCA indicates that the arctometatarsalian MT III differs in shape from the nonarctometatarsalian condition independently of size, indicating that allometric differences among taxa produced this divergence in MT III shape. TPS indicates substantial transfer of footfall force from MT II to MT III in ornithomimids and tyrannosaurids and from MT IV to MT III in troodontids. The study suggests different modes of ligament-damped sagittal rotation of MT III in tyrannosaurids, ornithomimids, and troodontids. Deinonychus had a large MT II-MT III articulation consistent with resisting forces of predatory strikes, while MT III of some large carnosaurs are less robust than expected. Phylogenetic bracketing suggests that proximal intermetatarsal ligaments in theropods were a key innovation preceding arctometatarsus evolution. A Bayesian phylogenetic analysis indicates that an arctometatarsus evolved in the common ancestor of the Tyrannosauridae + (Ornithomimosauria + Troodontidae) clade, but other optimizations are plausible. The most likely selective benefit of the structure was increased agility; if so, homoplasy indicates multiple exaptive and adaptive pathways towards predation and escape roles.