Aetosauria

Aetosaurs were a group of extinct, heavily armored reptiles that lived exclusively during the Late Triassic. They belong to the pseudosuchian lineage of archosaurs, meaning they are 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. In certain rock layers from this time period, they are among the most common fossil vertebrates found. Their bodies featured small heads, upturned snouts, legs held upright beneath the body, and four rows of bony plates called osteoderms covering their backs. Fossils of aetosaurs have been uncovered across Europe, North and South America, parts of Africa, and India. Because their armored plates are frequently preserved and abundant at many sites, aetosaurs serve as important index fossils for dating Late Triassic terrestrial deposits. Many species had broad geographic ranges but existed for relatively short spans of time, so identifying a particular aetosaur can help pinpoint the age of a fossil site.

Nearly all aetosaurs belong to the family Stagonolepididae, with the genus *Aetosauroides* being the only known exception. Scientists have described over 20 genera, though some of these descriptions have sparked debate. Two main subgroups are recognized—Desmatosuchia and Aetosaurinae—based largely on differences in skull shape. The structure of the osteoderms is especially useful for working out relationships among aetosaurs. Among other archosaurs, their closest relatives appear to be *Revueltosaurus*, a small reptile originally known only from teeth that were mistakenly thought to belong to herbivorous dinosaurs.

Aetosaur remains were first discovered in the early 1800s, but the earliest described specimens were misidentified as fish scales. Later, they were recognized as relatives of crocodiles and interpreted as semiaquatic scavengers closely linked to phytosaurs. More recent research has shown that aetosaurs were fully terrestrial and likely herbivorous to some degree. Some species show features that may have helped them dig for food. Possible nesting structures have been 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 absent from the front of both the upper and lower jaws. The teeth that do exist are usually small and bulbous, ranging from simple cones to leaf-shaped forms with large serrations—traits typical of animals that eat plants or a mixed diet. A few aetosaurs, including *Aetosauroides* and a small specimen possibly belonging to *Coahomasuchus*, have ziphodont teeth (curved, serrated, and flattened), which are more common in meat-eating archosaurs. In some aetosaurs, particularly members of Desmatosuchia, the tip of the snout is widened sideways into a shovel-like shape, similar to a pig’s snout. The nostril openings are elongated and much larger than the antorbital fenestrae (holes on the side of the skull). Many aetosaurs have a small knob on the premaxilla that projects into the nostrils from below. In all aetosaurs except *Aetosauroides*, the rear edge of the nostril is formed by the concave front edge of the maxilla. At the back of the skull, the supratemporal fenestra is positioned on the side, unlike in most other archosaurs where it is mainly visible from above. The braincase is fairly typical for pseudosuchians, but the opening for the abducens nerve passes through the parabasisphenoid bone rather than the prootic bone—a feature 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 sometimes curves downward into a chin-like projection that may expose the splenial bone. The jaw joint sits low, and the articular bone often has a tall projection just behind the joint.

Aside from the skull and armor, aetosaur body skeletons were fairly standard for large Triassic pseudosuchians. Their hindlimbs had a “pillar-erect” posture, similar to that of rauisuchians (carnivorous pseudosuchians ancestral to crocodylomorphs), where the thigh bone fits vertically into a downward-angled hip socket, positioning the leg directly under the body as a weight-bearing pillar. However, hip structure likely varied among aetosaurs, and the forelimbs may have had a semi-sprawling stance. While the hindlimb posture resembles that of rauisuchians, other features—such as a stout pelvis and broad, five-toed feet—are more primitive. The forelimbs were smaller than the hindlimbs, with the radius being much shorter than the humerus.