Aetosauria

Aetosaurs were a group of extinct, heavily armored reptiles that lived exclusively during the Late Triassic. They belonged to the pseudosuchian branch of archosaurs, meaning they were more closely related to modern crocodilians than to birds or non-avian dinosaurs. These animals were medium to large in size and likely ate both plants and animals, or were primarily herbivorous. Their bodies were covered with four rows of plate-like bony scutes, and they had small heads, upturned snouts, and limbs held upright beneath the body. In some rock layers from the Late Triassic, aetosaurs are among the most common fossil vertebrates found.

Fossils of aetosaurs have been discovered across Europe, North and South America, parts of Africa, and India. Because their armored plates preserved well and are abundant in certain deposits, aetosaurs serve as important index fossils for the Late Triassic. Many species had broad geographic ranges but existed for relatively short periods of time, so identifying a particular aetosaur can help date the rock layer where it was found.

Nearly all aetosaurs belong to the family Stagonolepididae, with the exception of the genus *Aetosauroides*. Over 20 genera have been described, though there has been recent debate about the validity of some. Researchers currently recognize two main subgroups—Desmatosuchia and Aetosaurinae—based largely on differences in skull shape. The structure of the osteoderms is especially useful for working out relationships between species. Among other archosaurs, aetosaurs are most closely related to *Revueltosaurus*, a small reptile whose teeth were once mistaken for those of herbivorous dinosaurs.

The first aetosaur remains were found in the early 1800s, but the earliest described specimens were misidentified as fish scales. Later, scientists recognized them as relatives of crocodiles and thought they were semiaquatic scavengers similar to phytosaurs. More recent studies have shown that aetosaurs were entirely terrestrial and likely ate plants to some degree. Some species have features that may have helped them dig for food. There have also been reports of possible nesting structures linked to aetosaurs, but this connection remains uncertain.

The skull of an aetosaur is small relative to its body and has a distinctive shape. Teeth are missing from the front of both the upper and lower jaws. The teeth that are present are usually small and bulbous, ranging from simple cones to leaf-like shapes with large serrations—adaptations seen in other archosaurs that ate less meat. A few aetosaurs, including *Aetosauroides* and a small specimen possibly belonging to *Coahomasuchus*, have ziphodont teeth (recurved, serrated, and flattened), which are typical of meat-eating archosaurs.

In some aetosaurs, especially members of Desmatosuchia, the tip of the snout is widened into a flattened, shovel-like shape. The nostril openings are elongated and much larger than the antorbital fenestrae. Many aetosaurs have a small knob on the premaxilla that projects into the nostril from below. In all aetosaurs except *Aetosauroides*, the rear edge of the nostril is formed by the concave front edge of the maxilla. The supratemporal fenestra, a hole at the rear top of the skull, is positioned on the side rather than on top as in most other archosaurs. The braincase is fairly typical for pseudosuchians, but the opening for the abducens nerve passes through the parabasisphenoid bone instead of the prootic—a trait otherwise seen only in *Revueltosaurus* and crocodylomorphs. The lower jaw is often described as slipper-shaped: the front of the dentary tapers to a point, and the underside may curve downward into a chin-like projection that exposes the splenial bone. The jaw joint sits low, and the articular bone often has a tall projection just behind it.

Apart from the skull and armor, the rest of the aetosaur skeleton was fairly standard for large Triassic pseudosuchians. The hindlimbs had a pillar-erect posture, similar to that of rauisuchians, where the femur fit vertically into a downward-angled hip socket, placing the leg directly under the body. However, hip structure likely varied among aetosaurs, and the forelimbs may have had a semi-sprawling stance. Other features, such as a stout pelvis and broad, five-toed feet, were more primitive. The forelimbs were smaller than the hindlimbs, and the radius was much shorter than the humerus.