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Archosauromorpha ( Greek for "ruling lizard forms") is a clade of diapsid reptiles containing all reptiles more closely related to
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 ...
s (such as
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 dinosaurs, including birds) rather than
lepidosaurs The Lepidosauria (, from Greek meaning ''scaled lizards'') is a subclass or superorder of reptiles, containing the orders Squamata and Rhynchocephalia. Squamata includes snakes, lizards, and amphisbaenians. Squamata contains over 9,000 specie ...
(such as tuataras,
lizard Lizards are a widespread group of squamate reptiles, with over 7,000 species, ranging across all continents except Antarctica, as well as most oceanic island chains. The group is paraphyletic since it excludes the snakes and Amphisbaenia althou ...
s, and snakes). Archosauromorphs first appeared during the late
Middle Permian The Guadalupian is the second and middle series/ epoch of the Permian. The Guadalupian was preceded by the Cisuralian and followed by the Lopingian. It is named after the Guadalupe Mountains of New Mexico and Texas, and dates between 272.95 ± ...
or
Late Permian Late may refer to: * LATE, an acronym which could stand for: ** Limbic-predominant age-related TDP-43 encephalopathy, a proposed form of dementia ** Local-authority trading enterprise, a New Zealand business law ** Local average treatment effect, ...
, though they became much more common and diverse during the
Triassic period The Triassic ( ) is a geologic period and system which spans 50.6 million years from the end of the Permian Period 251.902 million years ago ( Mya), to the beginning of the Jurassic Period 201.36 Mya. The Triassic is the first and shortest period ...
. Although Archosauromorpha was first named in 1946, its membership did not become well-established until the 1980s. Currently Archosauromorpha encompasses four main groups of reptiles: the stocky, herbivorous allokotosaurs and
rhynchosaur Rhynchosaurs are a group of extinct herbivorous Triassic archosauromorph reptiles, belonging to the order Rhynchosauria. Members of the group are distinguished by their triangular skulls and elongated, beak like premaxillary bones. Rhynchosaurs ...
s, the hugely diverse
Archosauriformes Archosauriformes (Greek for 'ruling lizards', and Latin for 'form') is a clade of diapsid reptiles that developed from archosauromorph ancestors some time in the Latest Permian (roughly 252 million years ago). It was defined by Jacques Gauthie ...
, and a polyphyletic grouping of various long-necked reptiles including ''
Protorosaurus ''Protorosaurus'' ("first lizard") is a genus of lizard-like early reptiles. Members of the genus lived during the late Permian period in what is now Germany and Great Britain. Once believed to have been an ancestor to lizards, ''Protorosaurus'' ...
'', tanystropheids, and ''
Prolacerta ''Prolacerta'' is a genus of archosauromorph from the lower Triassic of South Africa and Antarctica. The only known species is ''Prolacerta broomi''. The generic name ''Prolacerta'' is derived from Latin meaning “before lizard” and its speci ...
''. Other groups including
pantestudines Pantestudines or Pan-Testudines is the group of all reptiles more closely related to turtles than to any other living animal. It includes both modern turtles (crown group turtles, also known as Testudines) and all of their extinct relatives (also ...
( turtles and their extinct relatives) and the semiaquatic choristoderes have also been placed in Archosauromorpha by some authors. Archosauromorpha is one of the most diverse groups of reptiles, but its members can be united by several shared skeletal characteristics. These include laminae on the vertebrae, a posterodorsal process of the premaxilla, a lack of notochordal canals, and the loss of the entepicondylar foramen of the humerus.


History and definition

The term Archosauromorpha was first used by Friedrich von Huene in 1946 to refer to reptiles more closely related to archosaurs than to lepidosaurs. However, there was little consensus on ancient reptile relationships prior to the late 20th century, so the term Archosauromorpha was seldom used until many years after its creation. The advent of
cladistics Cladistics (; ) is an approach to biological classification in which organisms are categorized in groups ("clades") based on hypotheses of most recent common ancestry. The evidence for hypothesized relationships is typically shared derived cha ...
helped to sort out at least some of the relationships within Reptilia, and it became clear that there was a split between the archosaur lineage and the lepidosaur lineage somewhere within the Permian, with certain reptiles clearly closer to archosaurs and others allied with lepidosaurs. Jacques Gauthier reused the term Archosauromorpha for the archosaur lineage at the 1982 annual meeting of the
American Society of Zoologists The Society for Integrative and Comparative Biology is organized to integrate the many fields of specialization which occur in the broad field of biology.. The society was formed in 1902 as the American Society of Zoologists, through the merger of ...
, and later used it within his 1984 Ph.D. thesis. Archosauromorpha, as formulated by Gauthier, included four main groups of reptiles: Rhynchosauria, "Prolacertiformes", "Trilophosauria", and Archosauria (now equivalent to the group
Archosauriformes Archosauriformes (Greek for 'ruling lizards', and Latin for 'form') is a clade of diapsid reptiles that developed from archosauromorph ancestors some time in the Latest Permian (roughly 252 million years ago). It was defined by Jacques Gauthie ...
). Cladistic analyses created during the 1980s by Gauthier,
Michael J. Benton Michael James Benton One or more of the preceding sentences incorporates text from the royalsociety.org website where: (born 8 April 1956) is a British palaeontologist, and professor of vertebrate palaeontology in the School of Earth Sciences ...
, and Susan E. Evans implemented Gauthier's classification scheme within large studies of reptile relations.
Michel Laurin Michel Laurin is a Canadian-born French vertebrate paleontologist whose specialities include the emergence of a land-based lifestyle among vertebrates, the evolution of body size and the origin and phylogeny of lissamphibians. He has also made impo ...
(1991) defined Archosauromorpha as the clade containing the most recent common ancestor of ''
Prolacerta ''Prolacerta'' is a genus of archosauromorph from the lower Triassic of South Africa and Antarctica. The only known species is ''Prolacerta broomi''. The generic name ''Prolacerta'' is derived from Latin meaning “before lizard” and its speci ...
'', ''
Trilophosaurus ''Trilophosaurus'' (Greek for "lizard with three ridges") is a lizard-like trilophosaurid allokotosaur known from the Late Triassic of North America. It was a herbivore up to 2.5 m long. It had a short, unusually heavily built skull, equipped ...
'', '' Hyperodapedon'' and all of its descendants. David Dilkes (1998) formulated a more inclusive (and currently more popular) definition of Archosauromorpha, defining it as the clade containing ''
Protorosaurus ''Protorosaurus'' ("first lizard") is a genus of lizard-like early reptiles. Members of the genus lived during the late Permian period in what is now Germany and Great Britain. Once believed to have been an ancestor to lizards, ''Protorosaurus'' ...
'' and all other
sauria Sauria is the clade containing the most recent common ancestor of archosaurs (such as crocodilians, dinosaurs, etc.) and lepidosaurs (lizards and kin), and all its descendants. Since most molecular phylogenies recover turtles as more closely r ...
ns that are more closely related to ''Protorosaurus'' than to Lepidosauria. In 2016,
Martin Ezcurra Martin may refer to: Places * Martin City (disambiguation) * Martin County (disambiguation) * Martin Township (disambiguation) Antarctica * Martin Peninsula, Marie Byrd Land * Port Martin, Adelie Land * Point Martin, South Orkney Islands Aus ...
named a subgroup of Archosauromorpha, Crocopoda ("crocodile feet"). Crocopoda is defined as all archosauromorphs more closely related to allokotosaurs (specifically ''
Azendohsaurus ''Azendohsaurus'' is an extinct genus of herbivorous archosauromorph reptile from roughly the late Middle to early Late Triassic Period of Morocco and Madagascar. The type species, ''Azendohsaurus laaroussii'', was described and named by Jean-M ...
'' and ''
Trilophosaurus ''Trilophosaurus'' (Greek for "lizard with three ridges") is a lizard-like trilophosaurid allokotosaur known from the Late Triassic of North America. It was a herbivore up to 2.5 m long. It had a short, unusually heavily built skull, equipped ...
''), rhynchosaurs (specifically ''
Rhynchosaurus ''Rhynchosaurus'' (''beaked lizard'') is a genus of rhynchosaur that lived during the Middle Triassic period. It lived in Europe. It was related to the archosaurs, but not within that group. The type species of ''Rhynchosaurus'' is ''R. articep ...
''), or archosauriforms (specifically ''Proterosuchus'') rather than ''Protorosaurus'' or tanystropheids (specifically ''
Tanystropheus ''Tanystropheus'' (Greek ~ 'long' + 'hinged') is an extinct archosauromorph reptile from the Middle and Late Triassic epochs. It is recognisable by its extremely elongated neck, which measured long—longer than its body and tail combined. Th ...
''). This group roughly corresponds to Laurin's definition of Archosauromorpha.


Members


Unambiguous members

Since the seminal studies of the 1980s, Archosauromorpha has consistently been found to contain four specific reptile groups, although the definitions and validity of the groups themselves have been questioned. The least controversial group is Rhynchosauria ("beak reptiles"), a monophyletic clade of stocky herbivores. Many rhynchosaurs had highly modified skulls, with beak-like premaxillary bones and wide heads. Another group of archosauromorphs has traditionally been represented by ''Trilophosaurus'', an unusual iguana-like herbivorous reptile quite different from the rhynchosaurs. Gauthier used the name "Trilophosauria" for this group, but a 2015 study offered an alternative name. This study found that
Azendohsauridae Azendohsauridae is a family of allokotosaurian archosauromorphs that lived during the Middle to Late Triassic period, around 242-216 million years ago. The family was originally named solely for the eponymous ''Azendohsaurus'', marking out its ...
, Triassic reptiles previously mistaken for " prosauropod" dinosaurs, were in fact close relatives of ''Trilophosaurus'' and the rest of
Trilophosauridae Trilophosaurs are lizard-like Triassic allokotosaur reptiles related to the archosaurs. The best known genus is '' Trilophosaurus'', a herbivore up to long. It had a short, unusually heavily built skull, equipped with massive, broad flattened c ...
. Trilophosaurids and azendohsaurids are now united under the group Allokotosauria ("strange reptiles"). These two groups did not survive the end of the Triassic period, but the most famous group of archosauromorphs not only survived, but have continued to diversify and dominate beyond the Triassic-Jurassic extinction. These were the Archosauriformes, a diverse assortment of animals including the famous dinosaurs and
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. Two subclades of Archosauriformes survive to the present day: the semiaquatic crocodilians and the last of the feathered dinosaurs: birds. Gauthier used the name Archosauria to refer to what is now called the Archosauriformes; in modern studies, the name Archosauria has a more restricted definition that only includes the ancestors of crocodilians (i.e.
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 ...
) and birds (i.e.
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 ...
). The final unambiguous members of Archosauromorpha represent the most controversial group. These were the first archosauromorphs to appear, and can be characterized by their long necks, sprawling posture, and carnivorous habits. One name for the group,
Protorosauria Protorosauria is an extinct polyphyletic group of archosauromorph reptiles from the latest Middle Permian (Capitanian stage) to the end of the Late Triassic (Rhaetian stage) of Asia, Europe and North America. It was named by the English anato ...
, is named after ''Protorosaurus'', the oldest archosauromorph known from good remains. Another name, Prolacertiformes, is in reference to a different member, ''Prolacerta''. Protorosauria/Prolacertiformes has had a complicated history, and many taxa have entered and left the group as paleontologists discover and re-evaluate reptiles of the Triassic. By far the most famous of these are tanystropheids such as ''Tanystropheus'', known for having necks longer than their entire body. Other notable genera include ''
Boreopricea ''Boreopricea'' is an extinct genus of archosauromorph reptile from the Early Triassic of arctic Russia. It is known from a fairly complete skeleton discovered in a borehole on Kolguyev Island, though damage to the specimen and loss of certain b ...
'', ''
Pamelaria ''Pamelaria'' is an extinct genus of allokotosaurian archosauromorph reptile known from a single species, ''Pamelaria dolichotrachela'', from the Middle Triassic of India. ''Pamelaria'' has sprawling legs, a long neck, and a pointed skull with no ...
'', and ''
Macrocnemus ''Macrocnemus'' is an extinct genus of archosauromorph reptile known from the Middle Triassic (Late Anisian to Ladinian) of Europe and China. ''Macrocnemus'' is a member of the Tanystropheidae family and includes three species''. Macrocnemus ba ...
,'' as well as strange gliding reptiles such as ''
Sharovipteryx ''Sharovipteryx'' ("Sharov's wing", known until 1981 as ''Podopteryx'', "foot wing"), is a genus of early gliding reptiles containing the single species ''Sharovipteryx mirabilis''. It is known from a single fossil and is the only glider with a m ...
'' and ''
Mecistotrachelos ''Mecistotrachelos'' is an extinct genus of gliding reptile believed to be an archosauromorph, distantly related to crocodylians and dinosaurs. The type and only known species is ''M. apeoros''. This specific name translates to "soaring longest ...
''. A landmark 1998 study by David Dilkes completely deconstructed the concept of Prolacertiformes as a traditional monophyletic group (i.e. one whose members have a single common ancestor). He argued that ''Prolacerta'' was much closer to Archosauriformes than to other "prolacertiforms", invalidating the name. Likewise, ''Pamelaria'' is now considered an allokotosaur, ''Macrocnemus'' is a tanystropheid, and ''Protorosaurus'' may be too basal ("primitive") to form a clade with any of its supposed close relatives. As such, this final group of Archosauromorpha is generally considered paraphyletic or polyphyletic, and few modern studies use it.


Disputed members

Apart from these four groups, Archosauromorpha is sometimes considered to encompass several additional groups of reptiles. One of the most common additions is
Choristodera Choristodera (from the Greek χωριστός ''chōristos'' + δέρη ''dérē'', 'separated neck') is an extinct order of semiaquatic diapsid reptiles that ranged from the Middle Jurassic, or possibly Triassic, to the late Miocene (168 to ...
, a group of semiaquatic reptiles with mysterious origins. Although choristodere fossils are only known from the Jurassic through the Miocene, it is theorized that they first appeared during the Permian alongside the earliest archosauromorphs. Choristoderes share numerous otherwise unique traits with archosauromorphs, but they share an equal or greater number of unique traits with lepidosauromorphs as well, so there is still some debate over their inclusion within either group. The
chameleon Chameleons or chamaeleons (family Chamaeleonidae) are a distinctive and highly specialized clade of Old World lizards with 202 species described as of June 2015. The members of this family are best known for their distinct range of colors, bei ...
- or
tamandua ''Tamandua'' is a genus of anteaters with two species: the southern tamandua (''T. tetradactyla'') and the northern tamandua (''T. mexicana''). They live in forests and grasslands, are semiarboreal, and possess partially prehensile tails. They ...
-like
drepanosaur Drepanosaurs (members of the clade Drepanosauromorpha) are a group of extinct reptiles that lived between the Carnian and Rhaetian stages of the late Triassic Period, approximately between 230 and 210 million years ago. The various species of dr ...
s are also semi-regularly placed within Archosauromorpha, although some studies have considered them to be part of a much more basal lineage of reptiles. The aquatic
thalattosaur Thalattosauria (Greek for "sea lizards") is an extinct order of prehistoric marine reptiles that lived in the middle to late Triassic period. Thalattosaurs were diverse in size and shape, and are divided into two superfamilies: Askeptosauroidea a ...
s and gliding kuehneosaurids are also irregularly considered archosauromorphs. Genetic studies have found evidence that modern testudines ( turtles and tortoises) are more closely related to crocodilians than to lizards. If this evidence is accurate, then turtles are part of basal Archosauromorpha. Likewise, extinct turtle relatives known as
Pantestudines Pantestudines or Pan-Testudines is the group of all reptiles more closely related to turtles than to any other living animal. It includes both modern turtles (crown group turtles, also known as Testudines) and all of their extinct relatives (also ...
would also fall within Archosauromorpha. Some geneticists have proposed a name to refer to reptiles within the group formed by relatives of turtles and archosaurs. This name is the clade
Archelosauria Archelosauria is a clade grouping turtles and archosaurs (birds and crocodilians) and their fossil relatives, to the exclusion of lepidosaurs (the clade containing lizards, snakes and the tuatara). The majority of phylogenetic analyses based on ...
. Since Pantestudines may encompass the entire aquatic reptile order Sauropterygia, this means that Archosauromorpha (as Archelosauria) may be a much wider group than commonly believed. However, anatomical data disagrees with this genetic evidence, instead placing Pantestudines within Lepidosauromorpha but many modern studies have supported Archelosauria.


Anatomy

Although the most diverse clade of living archosauromorphs are birds, early members of the group were evidently reptilian, superficially similar to modern lizards. When archosauromorphs first appeared in the fossil record in the Permian, they were represented by long-necked, lightly built sprawling reptiles with moderately long, tapering snouts. This body plan, similar to that of modern
monitor lizard Monitor lizards are lizards in the genus ''Varanus,'' the only extant genus in the family Varanidae. They are native to Africa, Asia, and Oceania, and one species is also found in the Americas as an invasive species. About 80 species are recogn ...
s, is also shared by Triassic archosauromorphs such as tanystropheids and ''
Prolacerta ''Prolacerta'' is a genus of archosauromorph from the lower Triassic of South Africa and Antarctica. The only known species is ''Prolacerta broomi''. The generic name ''Prolacerta'' is derived from Latin meaning “before lizard” and its speci ...
.'' Other early groups such as trilohpsaurids, azendohsaurids, and rhynchosaurs deviate from this body plan by evolving into stockier forms with semi-erect postures and higher metabolisms. The archosauriforms went to further extremes of diversity, encompassing giant sauropod dinosaurs, flying
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 and birds, semiaquatic
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, phytosaurs, and
proterochampsia Proterochampsia is a clade of early archosauriform reptiles from the Triassic period. It includes the Proterochampsidae (e.g. '' Proterochampsa'', '' Chanaresuchus'' and '' Tropidosuchus'') and probably also the Doswelliidae. Nesbitt (2011) ...
ns, and apex predators such as erythrosuchids,
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, and theropod dinosaurs. Despite the staggering diversity of archosauromorphs, they can still be united as a clade thanks to several subtle skeletal features.


Skull

Most archosauromorphs more "advanced" than ''Protorosaurus'' possessed an adaptation of the
premaxilla The premaxilla (or praemaxilla) is one of a pair of small cranial bones at the very tip of the upper jaw of many animals, usually, but not always, bearing teeth. In humans, they are fused with the maxilla. The "premaxilla" of therian mammal has ...
(tooth-bearing bone at the tip of the snout) known as a posterodorsal or postnarial process. This was a rear-facing branch of bone that stretched up below and behind the external nares (nostril holes) to contact the nasal bones on the upper edge of the snout. A few advanced archosauriforms reacquired the
plesiomorphic In phylogenetics, a plesiomorphy ("near form") and symplesiomorphy are synonyms for an ancestral character shared by all members of a clade, which does not distinguish the clade from other clades. Plesiomorphy, symplesiomorphy, apomorphy, and ...
("primitive") state present in other reptiles, that being a short or absent posterodorsal process of the premaxilla, with the rear edge of the nares formed primarily by the maxilla bones instead. As for the nares themselves, they were generally large and oval-shaped, positioned high and close to the midline of the skull. Many early archosauromorphs, including ''Protorosaurus'', tanystropheids, ''Trilophosaurus'', and derived rhynchosaurs, have a blade-like
sagittal crest A sagittal crest is a ridge of bone running lengthwise along the midline of the top of the skull (at the sagittal suture) of many mammalian and reptilian skulls, among others. The presence of this ridge of bone indicates that there are exceptiona ...
on the parietal bones at the rear part of the
skull roof The skull roof, or the roofing bones of the skull, are a set of bones covering the brain, eyes and nostrils in bony fishes and all land-living vertebrates. The bones are derived from dermal bone and are part of the dermatocranium. In comparat ...
, between a pair of holes known as the supratemporal (or upper temporal) fenestrae. However, in other allokotosaurs, the basal rhynchosaur ''
Mesosuchus ''Mesosuchus'' ("middle crocodile") is an extinct genus of basal Rhynchosaur from early Middle Triassic (early Anisian stage) deposits of Eastern Cape, South Africa. It is known from the holotype SAM 5882, a partial skeleton, and from the para ...
'', and more crownward archosauromorphs, the sagittal crest is weakly differentiated, although the inner edge of each supratemporal fenestra still possessed a depressed basin of bone known as a supratemporal fossa. Ezcurra (2016) argued that presence of supratemporal fossae and an absence or poor development of the sagittal crest could be used to characterize Crocopoda. He also noted that in almost all early archosauromorphs (and some choristoderes), the parietal bones have an additional lowered area which extends transversely (from left to right) behind the supratemporal fenestrae and sagittal crest (when applicable). The lower temporal fenestra is not fully enclosed in early archosauromorphs (and choristoderes) due to alterations to the structure of the
quadratojugal bone The quadratojugal is a skull bone present in many vertebrates, including some living reptiles and amphibians. Anatomy and function In animals with a quadratojugal bone, it is typically found connected to the jugal (cheek) bone from the front and ...
at the rear lower corner of the skull. This bone is roughly L-shaped in these taxa, with a tall dorsal process (vertical branch), a short anterior process (forward branch), and a tiny or absent posterior process (rear branch). The bones surrounding the quadratojugal also reconfigure to offset the changes to the quadratojugal. For example, the lower branch of the squamosal bone is shortened to offset the tall dorsal process of quadratojugal which connects to it. On the other hand, the rear branch
jugal bone The jugal is a skull bone found in most reptiles, amphibians and birds. In mammals, the jugal is often called the malar or zygomatic. It is connected to the quadratojugal and maxilla, as well as other bones, which may vary by species. Ana ...
lengthens to fill some of the space left by the shortening of the anterior process of the quadratojugal. In archosauriforms, the jugal even re-encloses the lower temporal fenestra. The stapes are long, thin, and solid, without a perforating hole (stapedial foramen) present in the more robust stapes of other reptiles.


Vertebrae

In conjunction with their long, S-shaped necks, early archosauromorphs had several adaptations of the
cervical In anatomy, cervical is an adjective that has two meanings: # of or pertaining to any neck. # of or pertaining to the female cervix: i.e., the ''neck'' of the uterus. *Commonly used medical phrases involving the neck are **cervical collar **cervic ...
(neck)
vertebra The spinal column, a defining synapomorphy shared by nearly all vertebrates,Hagfish are believed to have secondarily lost their spinal column is a moderately flexible series of vertebrae (singular vertebra), each constituting a characteristic ...
e, and usually the first few
dorsal Dorsal (from Latin ''dorsum'' ‘back’) may refer to: * Dorsal (anatomy), an anatomical term of location referring to the back or upper side of an organism or parts of an organism * Dorsal, positioned on top of an aircraft's fuselage * Dorsal co ...
(back) vertebrae as well. The centrum (main body) of each vertebra is parallelogram-shaped, with a front surface typically positioned higher than the rear surface. The transverse processes (rib facets) of these vertebrae extend outwards to a greater extent than in other early reptiles. In many long-necked archosauromorphs, the rib facets are slanted, connecting to
cervical rib A cervical rib in humans is an extra rib which arises from the seventh cervical vertebra. Their presence is a congenital abnormality located above the normal first rib. A cervical rib is estimated to occur in 0.2% to 0.5% (1 in 200 to 500) of the ...
s that are often long, thin, and dichocephalous (two-headed). Thin, plate-like ridges known as laminae develop to connect the vertebral components, sloping down from the elongated transverse processes to the centra. Laminae are practically unique to archosauromorphs, being present even in the earliest Permian genera such as ''
Aenigmastropheus ''Aenigmastropheus'' is an extinct genus of early archosauromorph reptiles known from the middle Late Permian Usili Formation of Songea District, southern Tanzania. It contains a single species, ''Aenigmastropheus parringtoni'', known solely fr ...
'' and ''
Eorasaurus ''Eorasaurus'' is an extinct genus of archosauromorph reptile known from the middle Late Permian (late Capitanian or early Wuchiapingian age) of Tatarstan, European Russia. It contains a single species, ''Eorasaurus olsoni''. When originally des ...
.'' However, they are also known to occur in the bizarre semiaquatic reptile '' Helveticosaurus'', as well as the
biarmosuchia Biarmosuchians are an extinct clade of non-mammalian synapsids from the Permian. They are the most basal group of the therapsids. All of them were moderately-sized, lightly-built carnivores, intermediate in form between basal sphenacodont "pelyc ...
n synapsid '' Hipposaurus''. In all adult archosauromorphs with the exception of ''Aenigmastropheus'', the vertebrae lack notochordal canals, holes which perforate the centra. This also sets the archosauromorphs apart from most other Permian and Triassic reptiles.


Forelimbs

The
humerus The humerus (; ) is a long bone in the arm that runs from the shoulder to the elbow. It connects the scapula and the two bones of the lower arm, the radius and ulna, and consists of three sections. The humeral upper extremity consists of a round ...
(forearm bone) is solid in archosauromorphs, completely lacking a hole near the elbow known as the
entepicondylar foramen The entepicondylar foramen is an opening in the distal (far) end of the humerus (upper arm bone) present in some mammals. It is often present in primitive placentals, such as the enigmatic Madagascan '' Plesiorycteropus''. In most Neotominae and al ...
. This hole, present in most other tetrapods, is also absent in choristoderes yet not fully enclosed in some proterosuchids. In many advanced archosauromorphs, the capitullum and
trochlea Trochlea (Latin for pulley) is a term in anatomy. It refers to a grooved structure reminiscent of a pulley's wheel. Related to joints Most commonly, trochleae bear the articular surface of saddle and other joints: * Trochlea of humerus (part of t ...
(elbow joints) of the humerus are poorly developed. Early archosauromorphs retain well-developed elbow joints, but all archosauromorphs apart from ''Aenigmastropheus'' have a trochlea (
ulna The ulna (''pl''. ulnae or ulnas) is a long bone found in the forearm that stretches from the elbow to the smallest finger, and when in anatomical position, is found on the medial side of the forearm. That is, the ulna is on the same side of t ...
joint) which is shifted towards the outer surface of the humerus, rather than the midpoint of the elbow as in other reptiles. In conjunction with this shift, the
olecranon process The olecranon (, ), is a large, thick, curved bony eminence of the ulna, a long bone in the forearm that projects behind the elbow. It forms the most pointed portion of the elbow and is opposite to the cubital fossa or elbow pit. The olecranon ...
of the ulna is poorly developed in archosauromorphs apart from ''Aenigmastropheus'' and ''Protorosaurus''.


Hindlimbs

The ankle bones of archosauromorphs tend to acquire complex structures and interactions with each other, and this is particularly the case with the large proximal tarsal bones: the
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. S ...
. The calcaneum, for example, has a tube-like outer extension known as a calcaneal tuber in certain archosauromorphs. This tuber is particularly prominent in the ancient relatives of crocodylians, but it first appeared earlier at the last common ancestor of allokotosaurs, rhynchosaurs, and archosauriforms. The presence of a calcaneal tuber (sometimes known as a lateral tuber of the calcaneum) is a synapomorphy of the group Crocopoda, and is also responsible for its name.


Relationships

The cladogram shown below follows the most likely result found by an analysis of turtle relationships using both fossil and genetic evidence by M.S. Lee, in 2013. The following cladogram is based on a large analysis of
archosauriform Archosauriformes (Greek for 'ruling lizards', and Latin for 'form') is a clade of diapsid reptiles that developed from archosauromorph ancestors some time in the Latest Permian (roughly 252 million years ago). It was defined by Jacques Gauthier ...
s published by M.D. Ezcurra in 2016.


See also

*


References

{{Taxonbar, from=Q134676 Extant Permian first appearances Guadalupian first appearances Taxa named by Friedrich von Huene