Crurotarsal
Encyclopedia
A crurotarsal joint is one that’s situated between the bones of crus, i.e. shin (tibia
and fibula) and the proximal tarsal
bones, i.e. astragalus
and calcaneum.
The ankle joint of theria
n mammal
s (marsupials and placentals) 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
s, Crurotarsi
(including living crocodilia
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 on the calcaneum articulating against fibula. This joint is present in the skeletons of suchia
ns (including crocodilians) and phytosaur
s, and was cited as one of the characters supporting uniting these two groups in a clade to the exclusion of avemetatarsalia
n archosaur
s (bird
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).
The ankle joint of pseudosuchia
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 isn’t a crurotarsal joint in the previously discussed sense, as it's situated between the two proximal tarsal bones. However, while calcaneum isn’t 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.
Tibia
The tibia , shinbone, or shankbone is the larger and stronger of the two bones in the leg below the knee in vertebrates , and connects the knee with the ankle bones....
and fibula) and the proximal tarsal
Tarsus (skeleton)
In tetrapods, the tarsus is a cluster of articulating bones in each foot situated between the lower end of tibia and fibula of the lower leg and the metatarsus. In the foot the tarsus articulates with the bones of the metatarsus, which in turn articulate with the bones of the individual toes...
bones, i.e. astragalus
Talus bone
-External links:* *...
and calcaneum.
Theria
Theria is a subclass of mammals that give birth to live young without using a shelled egg, including both eutherians and metatherians . The only omitted extant mammal group is the egg-laying monotremes....
n mammal
Mammal
Mammals are members of a class of air-breathing vertebrate animals characterised by the possession of endothermy, hair, three middle ear bones, and mammary glands functional in mothers with young...
s (marsupials and placentals) 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.
Archosauriformes
Archosauriformes is a clade of diapsid reptiles that developed from archosauromorph ancestors some time in the Late Permian...
diapsid
Diapsid
Diapsids are a group of reptiles that developed two holes in each side of their skulls, about 300 million years ago during the late Carboniferous period. Living diapsids are extremely diverse, and include all crocodiles, lizards, snakes, and tuatara...
s, Crurotarsi
Crurotarsi
The Crurotarsi are a group of archosauriformes, represented today by the crocodiles,...
(including living crocodilia
Crocodilia
Crocodilia is an order of large reptiles that appeared about 84 million years ago in the late Cretaceous Period . They are the closest living relatives of birds, as the two groups are the only known survivors of the Archosauria...
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 on the calcaneum articulating against fibula. This joint is present in the skeletons of suchia
Suchia
Suchia is a clade of archosaurs that includes all pseudosuchians except ornithosuchids. It includes crocodilians and their extinct relatives. Paul Sereno defined Suchia as the least inclusive clade containing Aetosaurus ferratus, Rauisuchus tiradentes, Prestosuchus chiniquensis, Gracilisuchus...
ns (including crocodilians) and phytosaur
Phytosaur
Phytosaurs are an extinct group of large semi-aquatic Late Triassic archosaurs. Phytosaurs belong to the family Phytosauridae and the order Phytosauria. They were long-snouted and heavily armoured, bearing a remarkable resemblance to modern crocodiles in size, appearance, and lifestyle, an example...
s, and was cited as one of the characters supporting uniting these two groups in a clade to the exclusion of avemetatarsalia
Avemetatarsalia
Avemetatarsalia is a clade name established by British palaeontologist Michael Benton in 1999 for all crown group archosaurs that are closer to birds than to crocodiles. It includes a similarly defined subgroup, Ornithodira...
n archosaur
Archosaur
Archosaurs are a group of diapsid amniotes whose living representatives consist of modern birds and crocodilians. This group also includes all extinct non-avian dinosaurs, many extinct crocodilian relatives, and pterosaurs. Archosauria, the archosaur clade, is a crown group that includes the most...
s (bird
Bird
Birds are feathered, winged, bipedal, endothermic , egg-laying, vertebrate animals. Around 10,000 living species and 188 families makes them the most speciose class of tetrapod vertebrates. They inhabit ecosystems across the globe, from the Arctic to the Antarctic. Extant birds range in size from...
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).
The ankle joint of pseudosuchia
Pseudosuchia
Pseudosuchia is the name originally given to a group of prehistoric reptiles from the Triassic period. The name has been variously interpreted, and it is still sometimes, if infrequently, used in scientific literature today. A more commonly used name, Crurotarsi, is often substituted for...
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
Ornithosuchidae is an extinct family of reptiles from the Triassic period that were distantly related to crocodilians. They are classified as crurotarsan archosaurs. Ornithosuchids were quadrupedal and facultatively bipedal, meaning they had the ability to walk on two legs for short periods of time...
pseudosuchians a peg on the calcaneum fits into a socket in the astragalus (the “crocodile reversed” tarsus). Strictly speaking this ankle isn’t a crurotarsal joint in the previously discussed sense, as it's situated between the two proximal tarsal bones. However, while calcaneum isn’t fixed to the fibula, the astragalus is fixed to the tibia by a suture
Suture (anatomical)
In anatomy, a suture is a fairly rigid joint between two or more hard elements of an animal, with or without significant overlap of the elements....
and thus in practice it functions as an extension of the crus.