Aphanosauria
Aphanosauria is an extinct group of reptiles distantly related to dinosaurs. They are 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 or Crocodylus niloticus. 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, showing that 'advanced mesotarsal' ankles 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 other modern reptiles. Despite superficially resembling lizards, the closest modern relatives of aphanosaurs are birds.
Description
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.Skull
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, frontal, and a quadrate. 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, 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 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 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.Vertebrae
Aphanosaurs have many distinguishing features of their cervicals. 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 and a plate-like neural spine jutting out of the top. In the anterior cervicals, 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 below the postzygapophyses fits into a lip 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.
Forelimbs
Aphanosaurs have several characteristic features of the humerus. This bone was robust, thin when seen from the side but wide when seen from the front. In anterior view, its midshaft was pinched while the proximal and distal 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.Hip and hindlimbs
Pelvic girdle
The pelvis 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 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 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 face of each ischium thinning towards the lower edge of the bone while the medial 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 which is closed up by bone, although perhaps a small portion was open according to a notch near where the ischium contacts the ilium.Leg
The gracile femur of aphanosaurs possesses a characteristic set of features which can be used to diagnose the group. The proximal 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 articulation, which connects to the lower leg bones, is concave. The proximal part of the femur also has several bumps 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 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 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 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 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.