Tarsometatarsus

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Pigeon skeleton; number 8 indicates the right tarsometatarsus

The tarsometatarsus is a bone that is found in the lower leg of certain tetrapods, namely birds.

It is formed from the fusion of several bones found in other types of animals, and homologous to the mammalian tarsal (ankle) and metatarsal (foot) bones.

There are several ways and extents in which the tarsometatarsal fusion is accomplished throughout bird evolution. Namely, in Neornithes (modern birds) the fusion is most thorough at the distal (metatarsal) end though present along the entire length of the bone. In the Enantiornithes, a group of Mesozoic birds, the fusion was complete at the proximal (tarsal) end but the distal metatarsi still were somewhat separate.

Cast of the type specimen of Heterodontosaurus tucki. Right tarsometatarsus is clearly visible.

While birds are the most well-known animals featuring this fused bone, they were not the only and not even the first to have it: in a remarkable case of parallel evolution, a tarsometatarsus was also present in the Heterodontosauridae, a group of tiny cerapod dinosaurs quite unrelated to birds. Their oldest remains date from the Late Triassic more than 200 million years ago, and predate the first birds with a tarsometatarsus by nearly 100 million years.