The tarsometatarsus is a bone that is only found in the

lower leg The leg is the entire lower limb (anatomy), limb of the human body, including the foot, thigh or sometimes even the hip or Gluteal muscles, buttock region. The major bones of the leg are the femur (thigh bone), tibia (shin bone), and adjacent f ...

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 The metatarsal bones or metatarsus (: metatarsi) are a group of five long bones in the midfoot, located between the tarsal bones (which form the heel and the ankle) and the phalanges ( toes). Lacking individual names, the metatarsal bones are ...

(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

s), although the bones are joined along their entire length, the fusion is most thorough at the

distal Standard anatomical terms of location are used to describe unambiguously the anatomy of humans and other animals. The terms, typically derived from Latin or Greek roots, describe something in its standard anatomical position. This position provi ...

(metatarsal) end. In the

Enantiornithes The Enantiornithes, also known as enantiornithines or enantiornitheans in literature, are a group of extinct Avialae, avialans ("birds" in the broad sense), the most abundant and diverse group known from the Mesozoic era. Almost all retained teet ...

, 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 Standard anatomical terms of location are used to describe unambiguously the anatomy of humans and other animals. The terms, typically derived from Latin or Greek roots, describe something in its standard anatomical position. This position prov ...

(tarsal) end, but the distal metatarsi were still partially distinct. Heterodontosaurus skeleton

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 Parallel evolution is the similar development of a trait in distinct species that are not closely related, but share a similar original trait in response to similar evolutionary pressure.Zhang, J. and Kumar, S. 1997Detection of convergent and pa ...

, they were also present in the

Heterodontosauridae Heterodontosauridae is a family (biology), family of ornithischian dinosaurs that were likely among the most Basal (phylogenetics), basal (primitive) members of the group. Their phylogenetic placement is uncertain but they are most commonly fou ...

, a group of tiny

ornithischian Ornithischia () is an extinct clade of mainly herbivorous dinosaurs characterized by a pelvic structure superficially similar to that of birds. The name ''Ornithischia'', or "bird-hipped", reflects this similarity and is derived from the Greek st ...

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 A year is a unit of time based on how long it takes the Earth to orbit the Sun. In scientific use, the tropical year (approximately 365 solar days, 5 hours, 48 minutes, 45 seconds) and the sidereal year (about 20 minutes longer) are more exa ...

, and predate the first birds with tarsometatarsi by nearly 100 million years. {{Clear, left

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