Aphanosauria ("hidden lizards") is group of reptiles distantly related to dinosaurs (including birds). They were at the base of a group known as Avemetatarsalia, one of two main branches of archosaurs. The other main branch, Pseudosuchia, includes modern crocodilians. Aphanosaurs possessed features from both groups, indicating that they are the oldest and most primitive known clade of avemetatarsalians, at least in terms of their position on the archosaur family tree. Other avemetatarsalians include the flying pterosaurs, small bipedal lagerpetids, herbivorous silesaurids, and the incredibly diverse dinosaurs, which survive to the present day in the form of birds. Aphanosauria is formally defined as the most inclusive clade containing Teleocrater rhadinus and Yarasuchus deccanensis but not Passer domesticus (House sparrow) or Crocodylus niloticus (Nile crocodile). This group was first recognized during the description of Teleocrater. Although only known by a few genera, Aphanosaurs had a widespread distribution across Pangaea in the Middle Triassic.They were fairly slow quadrupedal long-necked carnivores, a biology more similar to basal archosaurs than to advanced avemetatarsalians such as pterosaurs, lagerpetids, and early dinosaurs. In addition, they seemingly possess 'crocodile-normal' ankles (with a crurotarsal joint), showing that 'advanced mesotarsal' ankles (the form acquired by many dinosaurs, pterosaurs, lagerpetids, and advanced silesaurids) were not basal to the whole clade of Avemetatarsalia. Nevertheless, they possessed elevated growth rates compared to their contemporaries, indicating that they grew quickly, more like birds than modern reptiles. Despite superficially resembling lizards, the closest modern relatives of aphanosaurs are birds.
Temporal range: Middle Triassic, Anisian–Ladinian
|Illustration of Teleocrater rhadinus|
Nesbitt et al., 2017
Members of this group were lightly-built and moderately-sized reptiles. They do not show any adaptations for bipedalism, which became much more common in other avemetatarsalians. In addition, their leg proportions indicate that they were not capable of sustained running, meaning that they were also slow by avemetatarsalian standards.
Very little skull material is known for the group as a whole. The only skull bones which can be confidently referred to this group consist of a few pterygoid and postorbital fragments belonging to Yarasuchus as well as some fragmentary material considered to belong to Teleocrater. These bones include a maxilla (tooth-bearing bone of the middle of the snout), frontal (part of the skull roof above the eyes), and a quadrate (part of the cranium's jaw joint). Although these fragments make it difficult to reconstruct the skull of aphanosaurs, they do show several notable features. For example, the shape of the maxilla shows that aphanosaurs had an antorbital fenestra, a large hole on the snout just in front of the eyes. Coupled with an antorbital depression (a collapsed area of bone which surrounded the fenestra), these indicate that aphanosaurs belonged to the group Archosauria. A partially-erupted tooth was also preserved on the lower edge of the maxilla. This tooth was flattened from the sides, slightly curved backwards, and serrated along its front edge. These tooth features indicate that aphanosaurs were carnivorous, as many meat-eating reptiles (including theropod dinosaurs such as Velociraptor) had the same features. The front edge of the maxilla also has a small pit, similar to some silesaurids. The rear part of the frontal possessed a round, shallow pit known as a supratemporal fossa. In the past it was believed that only dinosaurs possessed supratemporal fossae, but its presence in aphanosaurs (and Asilisaurus, a silesaurid) shows that it was variable among many avemetatarsalians. As a whole, known aphanosaurian skull material possessed no unique features, meaning that the rest of the skeleton would have to be used to characterize the group.
Aphanosaurs have many distinguishing features of their cervicals (neck vertebrae). The cervicals are very long compared to those of other early avemetatarsalians. As with most other reptiles, the vertebrae are composed of a roughly cylindrical main body (centrum) and a plate-like neural spine jutting out of the top. In the anterior cervicals (vertebrae at the front of the neck), a pair of low ridges run down the underside of the centrum. These ridges are separated by a wide area with other shallower ridges, making the centrum roughly rectangular in cross-section. The neural spines of the cervicals are also unique in aphanosaurs. They are hatchet shaped, with front edges that taper to a point and drastically overhang the centrum, at least in the front and middle parts of the neck. The upper edge of the neural spine is thin and blade-like, but the area immediately below the edge acquires a rough texture and forms a low, rounded ridge. These features are all unique to aphanosaurs.
As in other reptiles, aphanosaurian vertebrae also have small structures which articulate with either other vertebrae or the ribs which connect to each vertebra. The structures which connect to vertebrae in front of them are called prezygapophyses, while those that connect to vertebrae behind them are called postzygapophyses. The structures which connect to the ribs also have different names. In most archosaurs, the heads of the ribs are two-pronged. As a result, there are two areas on the side of each vertebra for connecting to a rib: the diapophysis in the upper part of the centrum and the parapophysis in a lower position. However, some cervical ribs are very unusual in aphanosaurs due to possessing a three-pronged head, although this feature only occurs in ribs at the base of the neck. In conjunction with this feature, the vertebrae in that area have a facet for the third prong just above the parapophysis, which has sometimes been classified as a 'divided parapophysis'. The only other archosaurs with this feature were the poposauroids, which explains how Yarasuchus had been mistaken for a poposauroid in the past.
In addition to these features which are unique among avemetatarsalians, aphanosaurs also have a few more traits present in other groups. In vertebrae at the front and middle of the neck, the postzygapophyses have additional small prongs just above the articulating plates. These additional prongs are termed epipophyses, and are common in dinosaurs but likely independently evolved due to being absent in other groups of avemetatarsalians. The body vertebrae have a different type of secondary structure. A small structure (hyposphene) below the postzygapophyses fits into a lip (hypantrum) between the prezygapophyses of the following vertebra, forming additional articulations to assist the zygapophyses. These hyposphene-hypantrum articulations are present in saurischian dinosaurs as well as raisuchids, and are often considered to help make the spine more rigid.
Aphanosaurs have several characteristic features of the humerus (upper arm bone). This bone was robust, thin when seen from the side but wide when seen from the front. In anterior (front) view, its midshaft was pinched while the proximal (near) and distal (far) ends were wide, making the bone hourglass-shaped. The edge of the upper part of the humerus which faces away from the body has a rounded crest, known as a deltopectoral crest. This crest points forward and is fairly elongated, extending down about a third the length of the bone. Overall, the humerus of aphanosaurs closely resemble that of sauropod dinosaurs and Nyasasaurus, an indeterminate early dinosaur or dinosaur relative. The arm as a whole was robustly-built and somewhat shorter than the leg, but only the humerus possessed unique features. The hand is mostly unknown in members of this group, but it was presumably small and five-fingered as in most archosaurs (apart from specialized forms like pterosaurs or theropod dinosaurs).
The pelvis (hip) of aphanosaurs shares many similarities with those of early dinosaurs and silesaurids as well as the unrelated poposauroids. Most of these traits can be found in the ischium, a plank-shaped bone which makes up the lower rear branch of the hip. For example, each ischium (on either side of the hip) contacts each other at the hip's midline. This contact is very extensive, although they are not completely fused due to the contact not extending to the upper edge of each bone. In contrast, pterosaurs, lagerpetids, and Marasuchus (other avemetatarsalians) have their ischia only slightly contact at the middle portion of each bone. The tip of the ischium is also rounded and semi-triangular in cross-section, with the lateral (outer) face of each ischium thinning towards the lower edge of the bone while the medial (inner) face is flat and contacts the other ischium. Poposauroids and dinosaurs also have rounded ischia, but lack the semi-triangular shape, which is also known in Asilisaurus. The ischium also has a groove on the upper part of the shaft. Unlike dinosaurs, aphanosaurs have an acetabulum (hip socket) which is closed up by bone, although perhaps a small portion was open according to a notch near where the ischium contacts the ilium (upper blade of the hip).
The gracile femur (thigh bone) of aphanosaurs possesses a characteristic set of features which can be used to diagnose the group. The proximal (near) surface of the bone, which connects to the hip socket, has a deep groove on it, rather than simply being a flat articulation surface. In addition, the bone's distal (far) articulation, which connects to the lower leg bones, is concave. The proximal part of the femur also has several bumps (tubers) on either the outer or inner edge of the bone. Many avemetatarsalians have two of these tubers on the inner edge, a small anteromedial tuber in front and a larger posteromedial tuber further back. However, aphanosaurians seem to have completely lost (or never even possessed) the anteromedial tuber. This is nearly unprecedented among archosaurs, but similar to the case in archosaur relatives such as Euparkeria.
A small ridge is present on the inner part of the bone, about a quarter the way down the shaft. This ridge, called a fourth trochanter, is an attachment point for the M. caudofemoralis, a tail muscle which helps to retract the hindlimbs. A scar on the anterolateral (front and outer) edge of the femur may have attached to the M. iliotrochantericus caudalis, a muscle which connects to the hip and helps to stabilize the thigh. This particular scar may be the same thing as the anterior (or lesser) trochanter, a specific structure present in dinosaurs and their close relative. A different scar is located somewhat further back on the bone and lower on the shaft. This scar may have attached to the M. iliofemoralis externus, a muscle which has a similar role to the M. iliotrochantericus caudalis. Likewise, its supposed equivalent in dinosaurs is a structure known as the trochanteric shelf. Aphanosaurs are unique among other avemetatarsalians in the fact that these two scars are separate from each other. In more advanced avemetatarsalians such as dinosaurs, the two structures and their corresponding muscles merge, a condition which is retained in modern birds.
The thin tibia and fibula (lower leg bones) of aphanosaurs do not possess unique traits to the same extent as the femur. However, they are also shorter than the femur. These proportions are rare among early avemetatarsalians, but more common among pseudosuchians and non-archosaur archosauriformes. A short lower leg is inversely correlated with running abilities, indicating that aphanosaurs were not as fast or agile as more advanced members of Avemetatarsalia.
Two different aphanosaurs (Yarasuchus and Teleocrater) each preserve a calcaneum, also known as a heel bone. Most avemetatarsalians have simple calcaneums which are firmly connected to a large bone known as an astragalus next to them. This type of heel, known as the 'advanced mesotarsal' condition, allows for more stability but less flexibility in the foot as it means the different bones of the ankle cannot flex against each other. Pseudosuchians (including modern crocodiles), as well as the crocodile-like phytosaurs have a different configuration, where the calcaneum is much larger and more complex, connecting to the astragalum with a joint that allows for movement between the two. This configuration is called a 'crocodile-normal' ankle, and reptiles which possess it are called crurotarsans. Some recent studies have suggested that phytosaurs are not actually archosaurs, but instead close relatives of the group. This indicates that 'crocodile-normal' ankles were the plesiomorphic (default) state in the first archosaurs, with 'advanced mesotarsal' ankles only later evolving within Avemetatarsalia, rather than at the base of the group.
The calcaneum of aphanosaurs supports this idea, as it more closely resembles that of 'crocodile-normal' ankles than 'advanced mesotarsal' ankles. The calcaneum lies on the outer side of the ankle, with its front or inner edge connecting to the astragalus, the upper surface connecting to the fibula, and the underside connecting to the fourth tarsal (a minor foot bone). In aphanosaurs, the socket for the astragalus is concave while the connection to the fibula manifests as a rounded dome. These are both characteristics of a 'crocodile-normal' ankle. In addition, the rear part of the calcaneum has a cylindrical structure known as a calcaneal tuber. Although this structure is smaller in aphanosaurs than in pseudosuchians, it is still much larger than in other avemetatarsalians, most of which don't even possess the structure. A few dinosauriformes also have small calcaneal tubers, although aphanosaurs have larger and rounder tubers than these taxa (Marasuchus and a few basal silesaurids). In cross-section, the calcaneal tubers of aphanosaurs are oval-shaped, taller than wide. Most foot material is fragmentary in this group, with only a few phalanges (toe bones) and metatarsals (primary elongated foot bones) known. Based on the length of the preserved metatarsals, the foot was likely rather elongated.
Aphanosauria is a recently named group, so it has a fairly short taxonomic history. Before it was named, its constituent genera were shuffled around Archosauria and its somewhat larger parent group, Archosauriformes. For example, Yarasuchus was first considered a prestosuchid and later a poposauroid by different analyses, with Martin Ezcurra (2016) placing both it and Dongusuchus as Euparkeria-grade archosaur relatives in his analysis. At the time of these analyses, Teleocrater (the most completely known aphanosaur) was not yet described.
In 2017, Aphanosauria was named and defined by Nesbitt et al. during the formal description of Teleocrater. The description was accompanied by two separate phylogenetic analyses, one derived from Nesbitt (2011)'s broad study on archosaurs and the other from Ezcurra (2016). Both analyses, reapplied with new information, gave a similar result for the position of aphanosaurs. They each placed the group at the base of Avemetatarsalia, outside of Ornithodira (the group containing pterosaurs, dinosaurs, and most other avemetatarsalians). A simplified strict consensus tree (a family tree with the fewest steps in evolution) using the Nesbitt (2011) analysis is given below:
Avemetatarsalia (meaning "bird metatarsals") is a clade name established by British palaeontologist Michael Benton in 1999 for all crown group archosaurs that are closer to birds than to crocodilians. An alternate name is Pan-Aves, or "all birds", in reference to its definition containing all animals, living or extinct, which are more closely related to birds than to crocodilians. Almost all avemetatarsalians are members of a similarly defined subgroup, Ornithodira. Ornithodira is defined as the last common ancestor of dinosaurs and pterosaurs, and all of its descendants.Members of this group include the Dinosauromorpha, Pterosauromorpha, the genus Scleromochlus, and Aphanosauria. Dinosauromorpha contains more basal forms, including Lagerpeton and Marasuchus, as well as more derived forms, including dinosaurs. Birds belong to the dinosaurs as members of the theropods. Pterosauromorpha contains Pterosauria, which were the first vertebrates capable of true flight. Aphanosauria is a Triassic group of gracile carnivorous quadrupeds which was recognized in 2017.Averostra
Averostra, or "bird snouts", is a clade that includes most theropod dinosaurs that have a promaxillary fenestra (fenestra promaxillaris), an extra opening in the front outer side of the maxilla, the bone that makes up the upper jaw. Two groups of averostrans, the Ceratosauria and the Orionides, survived into the Cretaceous period. When the Cretaceous–Paleogene extinction event occurred, ceratosaurians and two groups of orionideans within the clade Coelurosauria, the Tyrannosauroidea and Maniraptoriformes, were still extant. Only one subgroup of maniraptoriformes, Aves, survived the extinction event and persisted to the present day.Avetheropoda
Avetheropoda, or "bird theropods", is a clade that includes carnosaurians and coelurosaurs to the exclusion of other dinosaurs.Cerapoda
Cerapoda ("ceratopsians and ornithopods") is a clade of the dinosaur order Ornithischia.Dinosauriformes
Dinosauriformes is a clade of archosaurian reptiles that include the dinosaurs and their most immediate relatives. All dinosauriformes are distinguished by several features, such as shortened forelimbs and a partially to fully perforated acetabulum, the hole in the hip socket traditionally used to define dinosaurs. The oldest known member is Asilisaurus, dating to about 245 million years ago in the Anisian age of the middle Triassic period.Dongusuchus
Dongusuchus (meaning Donguz River crocodile in Greek, for the area where the type specimen was found) is an extinct genus of archosauriform. Fossils have been found from the Donguz Formation outcropping on the banks of the Donguz River in the Orenburg Oblast of Russia. They are associated with a fossil assemblage called the Eryosuchus Fauna, named after the capitosaurid Eryosuchus, the most common organism found from the assemblage. The locality dates back to the Anisian and early Ladinian stages of the Middle Triassic.Sennikov (1988) and Gower and Sennikov (2000) suggested that Dongusuchus was a gracile rausuchian with a long, sigmoidally curved neck, unlike the more typical robust short-necked rauisuchians that appear later in the Triassic. More recently, Nesbitt (2009) argued that Dongusuchus most probably represents a non-archosaurian archosauriform. According to Nesbitt (2009), the poorly-defined crista tibiofibularis and the absence of a distinct anteromedial tuber of the proximal portion in Dongusuchus suggest that it is not a member of Archosauria. Although Gower and Sennikov (2000) suggested that the distinct sigmoidal shape of Dongusuchus femur is unique, a similar shape is present in the femora of some phytosaurs. A paratype tibia was also found to be more closely related to Euparkeria and phytosaurs, on the basis of its convex and rounded distal surface. Additionally, the proximal surface of the tibia lacks a trait present in nearly all pseudosuchians, a depression on its lateral condyle. Nesbitt assigned Dongusuchus to Archosauriformes on the basis of the following traits: its femur has a low fourth trochanter, and the distal condyles do not expand markedly beyond the shaft. These traits suggest that Dongusuchus was an archosauriform more derived than Erythrosuchus.Dongusuchus was also excluded from Archosauria by Niedźwiedzki et al. (2014) and a new large cladistic analysis of archosauromorphs by Ezcurra (2016) found Dongusuchus to be the sister taxon to the Indian Yarasuchus. Both Dongusuchus and Yarasuchus were recovered in a clade with Spondylosoma and Teleocrater by Nesbitt et al. (2017) at the base of Avemetatarsalia, making them more closely related to bird-line archosaurs.Haya griva
Haya is an extinct genus of basal neornithischian dinosaur known from Mongolia.Jeholosauridae
Jeholosaurids were herbivorous neornithischian dinosaurs from the Cretaceous Period (Aptian - Santonian, with a possible Campanian record) of Asia. The family was first proposed by Han et al. in 2012. The jeholosaurids were defined as those ornithischians more closely related to Jeholosaurus shangyuanensis than to Hypsilophodon foxii, Iguanodon bernissartensis, Protoceratops andrewsi, Pachycephalosaurus wyomingensis, or Thescelosaurus neglectus. The Jeholosauridae includes the type genus Jeholosaurus and Yueosaurus.Jingshanosaurus
Jingshanosaurus (meaning "Jingshan lizard") is a genus of sauropodomorph dinosaurs from the early Jurassic period.Melanorosauridae
The Melanorosauridae were a family of sauropodomorph dinosaurs which lived during the Late Triassic and Early Jurassic. The name Melanorosauridae was first coined by Friedrich von Huene in 1929. Huene assigned several families of dinosaurs to the infraorder "Prosauropoda": the Anchisauridae, the Plateosauridae, the Thecodontosauridae, and the Melanorosauridae. Since then, these families have undergone numerous revisions. Galton and Upchurch (2004) considered Camelotia, Lessemsaurus, and Melanorosaurus members of the family Melanorosauridae. A more recent study by Yates (2007) indicates that the melanorosaurids were instead early sauropods.Neotheropoda
Neotheropoda (meaning "new theropods") is a clade that includes coelophysoids and more advanced theropod dinosaurs, and the only group of theropods who survived the Triassic–Jurassic extinction event. Yet all of the neotheropods became extinct during the early Jurassic period except for Averostra.Orionides
Orionides is a clade of tetanuran theropod dinosaurs from the Middle Jurassic to the Present. The clade includes most theropod dinosaurs, including birds.Orodrominae
Orodrominae is a subfamily of parksosaurid dinosaurs from the Cretaceous of North America and Asia.Raeticodactylidae
Raeticodactylidae is a family of eudimorphodontoid eopterosaurian pterosaurs that lived in Switzerland during the Late Triassic. The family includes Caviramus, and the type genus Raeticodactylus, which are both known from the Kössen Formation, around 205 mya. Raeticodactylidae was first used in 2014 by Andres et al., as a group of all pterosaurs closer to Raeticodactylus than Eudimorphodon. The following phylogenetic analysis follows the topology of Andres et al. (2014).Riojasauridae
Riojasauridae is a family of sauropod-like dinosaurs from the Upper Triassic. It is known primarily from the genera Riojasaurus and Eucnemesaurus. Sites containing Riojasauridae include the Lower Elliot Formation of Orange Free State, South Africa (where fossils of Eucnemesaurus have been found), and Ischigualasto, in La Rioja Province, Argentina ( where fossils of Riojasaurus have been recovered).Spondylosoma
Spondylosoma (meaning "vertebra body") is a genus of avemetatarsalian archosaur belonging to the clade Aphanosauria from the late Ladinian-age Middle Triassic Lower Santa Maria Formation in Paleorrota Geopark, Brazil.Teleocrater
Teleocrater (meaning "completed basin", in reference to its closed acetabulum) is a genus of avemetatarsalian archosaur from the Middle Triassic Manda Formation of Tanzania. The name was coined by English paleontologist Alan Charig in his 1956 doctoral dissertation, but was only formally published in 2017 by Sterling Nesbitt and colleagues. The genus contains the type and only species T. rhadinus. Uncertainty over the affinities of Teleocrater have persisted since Charig's initial publication; they were not resolved until Nesbitt et al. performed a phylogenetic analysis. They found that Teleocrater is most closely related to the similarly enigmatic Yarasuchus, Dongusuchus, and Spondylosoma in a group that was named the Aphanosauria. Aphanosauria was found to be the sister group of the Ornithodira, the group containing dinosaurs and pterosaurs.
A carnivorous quadruped measuring 7–10 feet (2.1–3.0 m) long, Teleocrater is notable for its unusually long neck vertebrae. The neural canals in its neck vertebrae gradually become taller towards the back of the neck, which may be a distinguishing trait. Unlike the Lagerpetidae or Ornithodira, the hindlimbs of Teleocrater are not adapted for running; the metatarsal bones are not particularly elongated. Also unlike lagerpetids and ornithodirans, Teleocrater inherited the more flexible ankle configuration present ancestrally among archosaurs, suggesting that the same configuration was also ancestral to Avemetatarsalia but was lost independently by several lineages. Histology of the long bones of Teleocrater indicates that it had moderately fast growth rates, closer to ornithodirans than crocodilians and other pseudosuchians.Xixiposaurus
Xixiposaurus is a genus of prosauropod dinosaur which existed in what is now Lower Lufeng Formation, China during the lower Jurassic period. It was first named by Sekiya Toru in 2010 and the type species is Xixiposaurus suni.Yarasuchus
Yarasuchus (meaning "red crocodile") is an extinct genus of avemetatarsalian archosaur that lived during the Anisian stage of the Middle Triassic of India. The genus was named and described in 2005 from a collection of disarticulated but fairly complete fossil material found from the Middle Triassic Yerrapalli Formation. The material is thought to be from two individuals, possibly three, with one being much more complete and articulated than the other. The type and only species is Y. deccanensis. Yarasuchus was a quadruped roughly 2–2.5 metres (6.6–8.2 ft) long, with an elongated neck and tall spines on its vertebrae. Unlike other quadrupedal Triassic reptiles, the limbs and shoulders of Yarasuchus were slender, and more like those of ornithodirans.
Yarasuchus has had a complicated taxonomic history, after originally being described as a "prestosuchid rauisuchian", it was later variously recovered as a poposauroid pseudosuchian and a non-archosaurian archosauriform of unstable position. In 2017 it was determined to be related to the similarly enigmatic Triassic reptiles Teleocrater, Dongusuchus and Spondylosoma. Together, they belong to a group called Aphanosauria and are placed at the base of Avemetatarsalia, sister to Ornithodira, making Yarasuchus one of the earliest diverging bird-line archosaurs known. The relative completeness of Yarasuchus and its evolutionary position helps to shed light on the origins of later, well known bird-line archosaurs such as the dinosaurs and pterosaurs.