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The tarsometatarsus is a bone that is only found in the lower leg of 
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. The oldest remains of this taxon date from the
The tarsometatarsus is a bone that is only found in the lower leg of bird
Birds are a group of warm-blooded vertebrates constituting the class (biology), class Aves (), characterised by feathers, toothless beaked jaws, the Oviparity, laying of Eggshell, hard-shelled eggs, a high Metabolism, metabolic rate, a fou ...
s and some non-avian dinosaurs. It is formed from the fusion of several bird bones found in other types of animals, and homologous to the mammalian tarsus (ankle bones) and metatarsal bones (foot). Despite this, the tarsometatarsus of flying types is often referred to as just the shank, tarsus or metatarsus.
Tarsometatarsal fusion occurred in several ways and extents throughout bird evolution
Evolution is the change in the heritable Phenotypic trait, characteristics of biological populations over successive generations. It occurs when evolutionary processes such as natural selection and genetic drift act on genetic variation, re ...
. Paticularly, in Neornithes (modern bird
Birds are a group of warm-blooded vertebrates constituting the class (biology), class Aves (), characterised by feathers, toothless beaked jaws, the Oviparity, laying of Eggshell, hard-shelled eggs, a high Metabolism, metabolic rate, a fou ...
s), 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
The Mesozoic Era is the Era (geology), era of Earth's Geologic time scale, geological history, lasting from about , comprising the Triassic, Jurassic and Cretaceous Period (geology), Periods. It is characterized by the dominance of archosaurian r ...
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. The oldest remains of this taxon date from the Late Triassic
The Late Triassic is the third and final epoch (geology), epoch of the Triassic geologic time scale, Period in the geologic time scale, spanning the time between annum, Ma and Ma (million years ago). It is preceded by the Middle Triassic Epoch a ...
more than 200 million years ago, and predate the first birds with tarsometatarsi by nearly 100 million years.
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References
*Proctor NS, Peterson RJ and Lynch PJ (1993) Manual of Ornithology: Avian Structure and Function. New Haven, CT: Yale University Press. Bird anatomy Dinosaur anatomy Bones of the lower limb