A crurotarsal joint is one that’s situated between the bones of ''

crus Crus can refer to: *''Crus'', a subgenus of the fly genus ''Metopochetus'' *Crus (lower leg) *Crus, a plural of Cru (wine) *CRUs, an abbreviation of Civil Resettlement Units * Rektorenkonferenz der Schweizer Universitäten (CRUS; English: Rectors' ...

'', i.e. shin (

tibia The tibia (; ), also known as the shinbone or shankbone, is the larger, stronger, and anterior (frontal) of the two bones in the leg below the knee in vertebrates (the other being the fibula, behind and to the outside of the tibia); it connects ...

and

fibula The fibula or calf bone is a leg bone on the lateral side of the tibia, to which it is connected above and below. It is the smaller of the two bones and, in proportion to its length, the most slender of all the long bones. Its upper extremity is ...

) and the proximal tarsal bones, i.e.

astragalus ''Astragalus'' is a large genus of over 3,000 species of herbs and small shrubs, belonging to the legume family Fabaceae and the subfamily Faboideae. It is the largest genus of plants in terms of described species. The genus is native to tempe ...

and

calcaneum In humans and many other primates, the calcaneus (; from the Latin ''calcaneus'' or ''calcaneum'', meaning heel) or heel bone is a bone of the tarsus of the foot which constitutes the heel. In some other animals, it is the point of the hock. St ...

. The ankle joint of

theria Theria (; Greek: , wild beast) is a subclass of mammals amongst the Theriiformes. Theria includes the eutherians (including the placental mammals) and the metatherians (including the marsupials) but excludes the egg-laying monotremes. Ch ...

mammal Mammals () are a group of vertebrate animals constituting the class Mammalia (), characterized by the presence of mammary glands which in females produce milk for feeding (nursing) their young, a neocortex (a region of the brain), fur or ...

s (

marsupials Marsupials are any members of the mammalian infraclass Marsupialia. All extant marsupials are endemic to Australasia, Wallacea and the Americas. A distinctive characteristic common to most of these species is that the young are carried in a po ...

and

placentals Placental mammals (infraclass Placentalia ) are one of the three extant subdivisions of the class Mammalia, the other two being Monotremata and Marsupialia. Placentalia contains the vast majority of extant mammals, which are partly distinguishe ...

) is a crurotarsal joint, with the main joint of ankle bending between the tibia and the astragalus; the calcaneum has no contact with the tibia but forms a heel to which muscles can attach. A group of archosauriform

diapsid Diapsids ("two arches") are a clade of sauropsids, distinguished from more primitive eureptiles by the presence of two holes, known as temporal fenestrae, in each side of their skulls. The group first appeared about three hundred million years ago ...

Crurotarsi Crurotarsi is a clade of archosauriform reptiles that includes crocodilians and stem-crocodilians and possibly bird-line archosaurs too if the extinct, crocodile-like phytosaurs are more distantly related to crocodiles than traditionally though ...

(including living

crocodilia Crocodilia (or Crocodylia, both ) is an order of mostly large, predatory, semiaquatic reptiles, known as crocodilians. They first appeared 95 million years ago in the Late Cretaceous period ( Cenomanian stage) and are the closest living ...

ns and their extinct relatives) is named after specialized crurotarsal joint in the skeletons of the members of this group, located between their fibula and calcaneum, with a hemicylindrical

condyle A condyle (;Entry "condyle"
in
suchia Suchia is a clade of archosaurs containing the majority of pseudosuchians (crocodilians and their extinct relatives). It was defined as the least inclusive clade containing '' Aetosaurus ferratus'', '' Rauisuchus tiradentes'', '' Prestosuchus ch ...

ns (including crocodilians) and

phytosaur Phytosaurs (Φυτόσαυροι in greek) are an extinct group of large, mostly semiaquatic Late Triassic archosauriform reptiles. Phytosaurs belong to the order Phytosauria. Phytosauria and Phytosauridae are often considered to be equivalent ...

s, and was cited as one of the characters supporting uniting these two groups in a clade to the exclusion of

avemetatarsalia Avemetatarsalia (meaning "bird metatarsals") is a clade of diapsid reptiles containing all archosaurs more closely related to birds than to crocodilians. The two most successful groups of avemetatarsalians were the dinosaurs and pterosaurs. Dinos ...

archosaur Archosauria () is a clade of diapsids, with birds and crocodilians as the only living representatives. Archosaurs are broadly classified as reptiles, in the cladistic sense of the term which includes birds. Extinct archosaurs include non-avian d ...

s (

bird Birds are a group of warm-blooded vertebrates constituting the class Aves (), characterised by feathers, toothless beaked jaws, the laying of hard-shelled eggs, a high metabolic rate, a four-chambered heart, and a strong yet lightweigh ...

s and their extinct relatives). However, according to a study published in 2011, suchians are more closely related to Avemetatarsalia than to phytosaurs; there is, however, not enough information to find out whether the aforementioned crurotarsal joint evolved independently in suchians and in phytosaurs, or whether it was already present in the skeleton of their most recent common ancestor (and secondarily lost in avemetatarsalians). CrocNormal01

The ankle joint of

pseudosuchia Pseudosuchia is one of two major divisions of Archosauria, including living crocodilians and all archosaurs more closely related to crocodilians than to birds. Pseudosuchians are also informally known as "crocodilian-line archosaurs". Prior to ...

ns (including crocodilians) and phytosaurs, passing between the astragalus and calcaneum, is also called crurotarsal joint in the literature. In the skeletons of the phytosaurs and most of the pseudosuchians this joint bends around a peg on the astragalus which fits into a socket in the calcaneum (the “crocodile normal” tarsus); only in the skeletons of the

ornithosuchid Ornithosuchidae is an extinct family of pseudosuchian archosaurs (distant relatives of modern crocodilians) from the Triassic period. Ornithosuchids were quadrupedal and facultatively bipedal (e.g. like chimpanzees), meaning that they had the a ...

pseudosuchians a peg on the calcaneum fits into a socket in the astragalus (the “crocodile reversed” tarsus). Strictly speaking this ankle is not a crurotarsal joint in the previously discussed sense, as it's situated between the two proximal tarsal bones. However, while calcaneum is not fixed to the fibula, the astragalus is fixed to the tibia by a suture and thus in practice it functions as an extension of the ''crus''.

References

{{Reflist Vertebrate anatomy