Tarsometatarsus

The tarsometatarsus is a bone that is only found in the lower leg of birds and some non-avian dinosaurs.

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 avialans, the fusion was complete at the proximal (tarsal) end, but the distal metatarsi were still partially distinct.

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 distantly related to birds.