Tarsometatarsus

Pigeon skeleton. Number 8 indicates both left and right tarsometatarsus

The tarsometatarsus is a bone that is only found in the lower leg of birds and certain non-avian dinosaurs. 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. Despite this, the tarsometatarsus of birds is often referred to as just the tarsus or metatarsus.

There are several ways and extents in which tarsometatarsal fusion was accomplished throughout bird evolution. Specifically, in Neornithes (modern birds) although the bones are joined along their entire length, the fusion is most thorough at the distal (metatarsal) end. In the Enantiornithes, a group of Mesozoic birds, the fusion was complete at the proximal (tarsal) end but the distal metatarsi were still partially distinct.

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

While these fused bones are best known from birds and their relatives, avians are neither the only group nor the first to possess tarsometatarsi. In a remarkable case of parallel evolution, they were also present in the Heterodontosauridae, a group of tiny ornithischian dinosaurs quite unrelated to birds. The oldest remains of this taxon date from the Late Triassic more than 200 million years ago, and predate the first birds with tarsometatarsi by nearly 100 million years.

References

• Proctor NS, Peterson RJ and Lynch PJ (1993) Manual of Ornithology: Avian Structure and Function. New Haven, CT: Yale University Press.