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
Ostromia addition, the ungual (claw) of the first digit of the hand is smaller than the corresponding first metacarpal, while in "Archaeopteryx" the claw is larger. The Haarlem specimen's metatarsals are also estimated to be proportionally longer than those of "Archaeopteryx" specimens. In addition, the Haarlem specimen shares several features with "Anchiornis". Most notably, they both have longitudinal furrows on the top and bottom sides of their manual phalanges (finger bones). While such structures can be a result of collapsed or broken bones (as is the case in several "Archaeopteryx" specimens), the straight, smooth edges of the furrows in "Ostromia" and