Arctometatarsalia

Arctometatarsalia is a largely abandoned clade of theropod dinosaurs originally proposed to unite all forms possessing an arctometatarsalian foot — a condition in which the upper bones of the foot are pinched tightly together, interpreted as an adaptation for fast running. As first conceived in the early 1990s, it grouped the troodontids, tyrannosauroids, and ornithomimosaurs under the assumption that this distinctive foot structure marked a single natural lineage. Later phylogenetic studies undermined that idea, showing that tyrannosaurids and troodontids were more closely related to other coelurosaurs than to the ostrich-like ornithomimosaurs. Because its strictest definition was anchored on Ornithomimus, the name became redundant with Ornithomimosauria and fell out of general use.