tibiotarsus

The tibiotarsus is the principal leg bone of birds, formed by the fusion of the tibia with the proximal tarsal bones (the astragalus and calcaneum). It runs from the knee joint to the distal end where the tarsometatarsus begins, and it bears most of the body's weight during locomotion.

In modern birds, the fibula is greatly reduced and typically does not contribute significantly to the ankle joint; it runs alongside the tibiotarsus rather than forming a major articulation. The tibiotarsus articulates proximally with the femur at the knee and distally with the tarsometatarsus, which is the result of the fusion of distal tarsals with metatarsals.

Developmentally, birds form the tibiotarsus through the fusion of the tibia with the proximal tarsal elements during embryogenesis. This fusion is a defining feature of avian leg anatomy, distinguishing it from many non-avian theropods, where the tibia and tarsals remained separate.

Functionally, the tibiotarsus plays a central role in locomotion by transmitting forces from the body to the foot and providing leverage for propulsion. Its length and robustness vary with ecological lifestyle: birds adapted for running or perching often show a longer, more powerful tibiotarsus, while divers and wading birds may exhibit different proportions that suit their gait.

See also: tibia, tarsus, tarsometatarsus, fibula.