Tarsus (skeleton) - in Other Animals

In Other Animals

In primitive tetrapods, such as Trematops, the tarsus consists of three rows of bones. There are three proximal tarsals, the tibiale, intermedium, and fibulare, named for their points of articulation with the bones of the lower limb. These are followed by a second row of four bones, referred to as the centralia (singular: centrale), and then a row of five distal tarsals, each articulating with a single metatarsal. In the great majority of tetrapods, including all of those alive today, this simple pattern is modified by the loss and fusion of some of the bones.

In reptiles and mammals, there are normally just two proximal tarsals, the calcaneus (equivalent to the amphibian fibulare) and the talus (probably derived from a fusion of multiple bones). In mammals, including humans, the talus forms a hinge joint with the tibia, a feature especially well developed in the artiodactyls. The calcaneus is also modified, forming a heel for the attachment of the Achilles tendon. Neither of these adaptations is found in reptiles, which have a relatively simple structure to both bones.

The fifth distal tarsal disappears relatively early in evolution, with the remainder becoming the cuneiform and cuboid bones. Reptiles usually retain two centralia, while mammals typically have only one (the navicular).

In birds, the tarsus has disappeared, with the proximal tarsals having fused with the tibia, the centralia having disappeared, and the distal bones having fused with the metatarsals to form a single tarsometatarsus bone, effectively giving the leg a third segment.

See also: Carpus and tarsus of land vertebrates