Avisauridae leg formed by fused metatarsals (the bones which comprise the foot in humans). As a result, members of this family are distinguished from other enantiornitheans exclusively by features of the tarsometatarsal and pedal phalanges (toe bones). Unlike in some prehistoric birds, avisaurid tarsometatarsals were not completely fused, with the distal (outer) parts of the metatarsals being separate from each other. The proximal (near) half of metatarsal III (the long bone in the middle of the tarsometatarsus) is convex from the front. The inside edge of this bone's trochlea (toe joint) has a bony tab which points downward, known as a
Phalanx bone terminal phalanges and fused metacarpal bones able to absorb the stress from running. The sloth spends its life hanging upside-down from branches, and has highly specialized third and fourth digits for the purpose. They have short and squat proximal phalanges with much longer terminal phalanges. They have vestigial second and fifth metacarpals, and their palm extends to the distal interphalangeal joints. The arboreal specialization of these terminal phalanges makes it impossible for the sloth to walk on the ground where the animal has to drag its body with its claws. Phalanx bone The phalanges (singular: "phalanx" ) are digital bones