tibiatarsus

The tibiatarsus, often called the tibiotarsus in birds, is the leg bone formed by the fusion of the tibia with the proximal tarsal bones (the astragalus and calcaneum). It is the primary weight-bearing segment of the avian lower leg and contributes most of the leg length.

Proximally it articulates with the distal end of the femur at the knee, and distally it forms a joint with the tarsometatarsus at the ankle. The tibiatarsus thus functions as a single, elongated lever that anchors leg muscles and supports propulsion during locomotion.

Developmentally, the tibia fuses with the proximal tarsals to form the tibiotarsus during ontogeny in birds. This fusion is a characteristic avian feature and reflects their evolutionary history within the theropod lineage.

Variation among species is notable: the relative length and robustness of the tibiatarsus vary, with long, stout bones in large flightless birds such as ostriches and emus, and shorter, more slender bones in small passerines. Functionally, the tibiatarsus bears body weight during standing and locomotion and contributes to stride and propulsion. In avian medicine, fractures of the tibiatarsus are encountered and require appropriate veterinary management.