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. 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.
Hip and hindlimbs Pelvic girdle 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).
Leg 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.
Ankle 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. ==Classification==