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 (; : tibiae or tibias), also known as the shinbone or shankbone, is the larger, stronger, and anterior (frontal) of the two Leg bones, bones in the leg below the knee in vertebrates (the other being the fibula, behind and to the outsi ...

and

fibula The fibula (: fibulae or fibulas) 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. ...

) and the proximal tarsal bones, i.e.

astragalus Astragalus may refer to: * ''Astragalus'' (plant), a large genus of herbs and small shrubs *Astragalus (bone) The talus (; Latin for ankle or ankle bone; : tali), talus bone, astragalus (), or ankle bone is one of the group of foot bones known ...

and

calcaneum In humans and many other primates, the calcaneus (; from the Latin ''calcaneus'' or ''calcaneum'', meaning heel; : calcanei or calcanea) or heel bone is a bone of the tarsus of the foot which constitutes the heel. In some other animals, it is t ...

. The ankle joint of

theria Theria ( or ; ) is a scientific classification, subclass of mammals amongst the Theriiformes. Theria includes the eutherians (including the Placentalia, placental mammals) and the metatherians (including the marsupials) but excludes the egg-lay ...

mammal A mammal () is a vertebrate animal of the Class (biology), class Mammalia (). Mammals are characterised by the presence of milk-producing mammary glands for feeding their young, a broad neocortex region of the brain, fur or hair, and three ...

s (

marsupials Marsupials are a diverse group of mammals belonging to the infraclass Marsupialia. They are natively found in Australasia, Wallacea, and the Americas. One of marsupials' unique features is their reproductive strategy: the young are born in a ...

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 Archosauriformes (Greek for 'ruling lizards', and Latin for 'form') is a clade of diapsid reptiles encompassing archosaurs and some of their close relatives. It was defined by Jacques Gauthier (1994) as the clade stemming from the last common anc ...

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 earliest traditionally identified diapsids, the araeosc ...

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 () is an order of semiaquatic, predatory reptiles that are known as crocodilians. They first appeared during the Late Cretaceous and are the closest living relatives of birds. Crocodilians are a type of crocodylomorph pseudosuchia ...

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

on the calcaneum articulating against fibula. This joint is present in the skeletons of

suchia Suchia (from Ancient Greek σούχος (''soúkhos''), meaning "crocodile") is a clade of archosaurs containing the majority of pseudosuchians (crocodilians and their extinct relatives). It was defined as the least inclusive clade containing ''A ...

ns (including crocodilians) and

phytosaur Phytosaurs (Φυτόσαυροι in Greek, meaning 'plant lizard') are an extinct group of large, mostly semiaquatic Late Triassic archosauriform or basal archosaurian reptiles. Phytosaurs belong to the order Phytosauria and are sometimes ref ...

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 Reptile, reptiles containing all archosaurs more closely related to birds than to crocodilians. The two most successful groups of avemetatarsalians were the dinosaurs and pterosau ...

archosaur Archosauria () or archosaurs () is a clade of diapsid sauropsid tetrapods, with birds and crocodilians being the only extant taxon, extant representatives. Although broadly classified as reptiles, which traditionally exclude birds, the cladistics ...

s (

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 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, from Ancient Greek ψεύδος (''pseúdos)'', meaning "false", and σούχος (''soúkhos''), meaning "crocodile" is one of two major divisions of Archosauria, including living crocodilians and all archosaurs more closely relat ...

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 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