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Crurotarsi is a clade of archosauriform reptiles that includes
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 stem-crocodilians and possibly bird-line
archosaurs 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 ...
too if the extinct, crocodile-like phytosaurs are more distantly related to crocodiles than traditionally thought. Prior to 2011, the group had invariably included only archosaurs closer to crocodilians than to birds and other
dinosaurs Dinosaurs are a diverse group of reptiles of the clade Dinosauria. They first appeared during the Triassic period, between 243 and 233.23 million years ago (mya), although the exact origin and timing of the evolution of dinosaurs is the ...
. An equivalent term for the crocodilian side of the archosaur family tree is
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 ...
. This traditional definition of Crurotarsi assumed that phytosaurs were
crown-group In phylogenetics, the crown group or crown assemblage is a collection of species composed of the living representatives of the collection, the most recent common ancestor of the collection, and all descendants of the most recent common ancestor. ...
archosaurs and more closely related to crocodilians than to birds. However, a 2011 study argued that the phytosaur lineage evolved prior to the split between birds and crocodilians. This would mean that phytosaurs were not true archosaurs, and therefore could not be considered representatives of croc-line archosaurs. The name Crurotarsi is derived from the Latin word ''
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: Recto ...
'' (lower leg) and the Greek word ''
tarsos Tarsus ( Hittite: 𒋫𒅈𒊭 ; grc, Ταρσός, label=Greek ; xcl, Տարսոն, label=Armenian ; ar, طَرسُوس ) is a historic city in south-central Turkey, inland from the Mediterranean. It is part of the Adana-Mersin metropolitan ...
'' (ankle). It refers to the specialized articulation (a
crurotarsal 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 therian mammals (marsupials and placentals) is a crurotarsa ...
joint) between the lower leg (specifically the fibula) and the ankle (specifically the
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. S ...
) which is present in the skeletons of reptiles such as
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 c ...
ns and phytosaurs. In their ankle joint, a hemicylindrical
condyle A condyle (;Entry "condyle"
in


Taxonomic history

The name Crurotarsi was erected as a
node-based clade by
Paul Sereno Paul Callistus Sereno (born October 11, 1957) is a professor of paleontology at the University of Chicago and a National Geographic "explorer-in-residence" who has discovered several new dinosaur species on several continents, including at si ...
and A. B. Arcucci in 1990 to supplant the old term
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 ...
, but with a different definition. Crurotarsi includes, by most published definitions, all descendants of the common ancestor of modern
crocodile Crocodiles (family Crocodylidae) or true crocodiles are large semiaquatic reptiles that live throughout the tropics in Africa, Asia, the Americas and Australia. The term crocodile is sometimes used even more loosely to include all extant mem ...
s,
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 ...
s,
aetosaur Aetosaurs () are heavily armored reptiles belonging to the extinct order Aetosauria (; from Greek, (aetos, "eagle") and (, "lizard")). They were medium- to large-sized omnivorous or herbivorous pseudosuchians, part of the branch of archosaurs ...
s, and phytosaurs; Nesbitt (2011) provided a shorter definition, defining Crurotarsi as "the least inclusive clade containing ''
Rutiodon carolinensis ''Rutiodon'' ("Wrinkle tooth") is an extinct genus of phytosaur belonging to the family Parasuchidae. It lived during the Late Triassic period, and was about in length. ''Rutiodon'' is known from the eastern United States ( North Carolina, ...
'' Emmons, 1856, and ''
Crocodylus niloticus ''Crocodylus'' is a genus of true crocodiles in the family Crocodylidae. Taxonomy The generic name, ''Crocodylus'', was proposed by Josephus Nicolaus Laurenti in 1768. ''Crocodylus'' contains 13–14 extant (living) species and 5 extinct species ...
'' Laurenti, 1768". According to two studies published in 2011 by Nesbitt and coworkers, using either of these definitions leads to the inclusion of all other true
archosaurs 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 ...
in Crurotarsi, due to the possibly basal phylogenetic position of the phytosaurs. This means that grouping the phytosaurs and crocodilians into a clade while excluding the
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 ...
ns (
pterosaur Pterosaurs (; from Greek ''pteron'' and ''sauros'', meaning "wing lizard") is an extinct clade of flying reptiles in the order, Pterosauria. They existed during most of the Mesozoic: from the Late Triassic to the end of the Cretaceous (228 to ...
s, dinosaurs, and birds) would result in a paraphyletic grouping. A more definitive group is
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 ...
, which is defined as all archosaurs closer to crocodiles than to birds (matching the traditional content of Crurotarsi).


Phylogeny

Paul Sereno and A. B. Arcucci named Crurotarsi in 1990, defining it as " Parasuchia hytosaurs
Ornithosuchidae 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 ...
, ''
Prestosuchus ''Prestosuchus'' (meaning "Prestes crocodile") is an extinct genus of pseudosuchian in the group Loricata, which also includes '' Saurosuchus'' and ''Postosuchus''. It has historically been referred to as a "rauisuchian", and was the defining m ...
'', Suchia, and all descendants of their common ancestor". The groups in this definition were considered crocodile-line archosaurs, as opposed to the bird-line archosaurs. Ornithosuchids were once considered bird-line archosaurs (as implied by their name, which means "bird crocodiles" in Greek), but were later recognized as crocodile-line archosaurs. This reclassification may have inspired Sereno's Crurotarsi, a node-based clade defined by the inclusion of ornithosuchids and other early archosaurs. Two names were proposed for crocodile-line archosaurs before Crurotarsi was erected. The first, Pseudosuchia, was established as a stem-based clade in 1985. It includes crocodiles and all archosaurs more closely related to crocodiles than to birds. The second, Crocodylotarsi, was named in 1988, possibly as a replacement for Pseudosuchia. The name Pseudosuchia, meaning "false crocodiles", has been used for over a century, and traditionally included aetosaurs. As a clade, Pseudosuchia includes the group
Eusuchia Eusuchia is a clade of crocodylomorphs that first appeared in the Early Cretaceous with ''Hylaeochampsa''. Along with Dyrosauridae and Sebecosuchia, they were the only crocodyliformes who survived the K-T extinction. Since the other two clades ...
, or "true crocodiles". Crocodylotarsi may have been named to remove confusion, but as a stem-based clade it is synonymous with Pseudosuchia. Because Pseudosuchia was named first, it has precedence. Crurotarsi traditionally contains the same archosaurs as Pseudosuchia, but as a node-based clade it is not synonymous. Below is a cladogram after Nesbitt & Norell (2006) and Nesbitt (2007) with Crurotarsi in its traditional sense encompassing just crocodile-line archosaurs: Cladogram after Brusatte, Benton, Desojo and Langer (2010): In 2011, Sterling J. Nesbitt found phytosaurs to be the sister taxon of Archosauria, and therefore not crocodile-line archosaurs. Because phytosaurs are included in the definition of Crurotarsi, this change in their
phylogenetic In biology, phylogenetics (; from Greek φυλή/ φῦλον [] "tribe, clan, race", and wikt:γενετικός, γενετικός [] "origin, source, birth") is the study of the evolutionary history and relationships among or within groups o ...
placement expanded the scope of Crurotarsi, which therefore now includes phytosaurs, crocodiles, pterosaurs and dinosaurs. However, Pseudosuchia still contains only crocodile-line archosaurs. Below is a
cladogram A cladogram (from Greek ''clados'' "branch" and ''gramma'' "character") is a diagram used in cladistics to show relations among organisms. A cladogram is not, however, an evolutionary tree because it does not show how ancestors are related to ...
modified from Nesbitt (2011) showing the new changes:


References

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External links


Palaeos
{{Taxonbar, from=Q131341 Tetrapod unranked clades Triassic reptiles Extant Early Triassic first appearances Taxa named by Paul Sereno