Aetosauria
Aetosaurs are heavily armored reptiles belonging to the extinct order Aetosauria and wikt:σαυρος ). They were medium- to large-sized omnivorous or herbivorous pseudosuchians, part of the branch of archosaurs more closely related to crocodilians than to birds and non-avian dinosaurs. All known aetosaurs are restricted to the Late Triassic, and in some strata from this time they are among the most abundant fossil vertebrates. They have small heads, upturned snouts, erect limbs, and a body ornamented with four rows of plate-like osteoderms. Aetosaur fossil remains are known from Europe, North and South America, parts of Africa, and India. Since their armoured plates are often preserved and are abundant in certain localities, aetosaurs serve as important Late Triassic tetrapod index fossils. Many aetosaurs had wide geographic ranges, but their stratigraphic ranges were relatively short. Therefore, the presence of particular aetosaurs can accurately date a site in which they are found.
Nearly all aetosaurs belong to the family Stagonolepididae. Over 20 genera of aetosaurs have been described, and recently there has been controversy regarding the description of some of these genera. Two distinct subdivisions of aetosaurs are currently recognized, Desmatosuchia and Aetosaurinae, based primarily on broad differences in skull morphology. Osteoderms structure is generally one of the most useful traits for inferring aetosaur relations more precisely. Among other archosaurs, aetosaurs are most closely related to Revueltosaurus, a small reptile originally known from teeth mistakenly referred to herbivorous dinosaurs.
Aetosaur remains were first discovered in the early 19th century, although the first remains that were described were mistaken for fish scales. Aetosaurs were later recognized as crocodile relatives, at which point they were interpreted as semiaquatic scavengers closely related to phytosaurs. Subsequent work has established that aetosaurs were entirely terrestrial animals, and were likely herbivorous to some extent. Some forms have characteristics that may have been adaptations to digging for food. Supposed nesting structures have also been referred to aetosaurs, but this connection is considered ambiguous.
Description
Skull anatomy
The skull of aetosaurs is relatively small compared to the body, and is quite distinctive in shape. Teeth are absent from both the front of the premaxilla and the front of the dentary. The teeth which are present are usually small and bulbous, ranging from basic conical forms to leaf-like shapes with large serrations. These are probably indicative of an omnivorous or herbivorous diet, and similar adaptations are seen in other archosaurs with less reliance on meat in their diet. A few aetosaurs have teeth with a ziphodont shape, meaning that the teeth are recurved, serrated, and flattened from the side. This shape, which is predominant in Aetosauroides and a small specimen tentatively referred to Coahomasuchus, is typical of carnivorous archosaurs.In some aetosaurs, the tip of the snout is expanded sideways into a flattened 'shovel' shape, akin to the snout of a pig. The external nares are elongated, much larger than the antorbital fenestrae. Many aetosaurs have a small knob on the premaxilla which projects into the nares from below. In all aetosaurs except Aetosauroides, the rear edge of the naris receives a contribution from the concave front edge of the maxilla bone. At the rear upper part of the skull, a hole known as the supratemporal fenestra is positioned and exposed on the side, unlike most other archosaurs where it is mostly visible when viewing the skull from above. The braincase is fairly standard by pseudosuchian standards, though the opening for the abducens nerve passes through the parabasisphenoid bone, rather than the prootic bone. This trait is otherwise only seen in Revueltosaurus and crocodylomorphs among archosaurs. The mandible is described as 'slipper'-shaped in many aetosaurs. This is due to a combination of features: the front of the dentary strongly tapers to a point, while the underside of the dentary sometimes flexes into a 'chin' which may expose the splenial bone as well. The jaw joint is set at a low position, and the articular often has a tall projection right behind the jaw joint.
Postcranial anatomy
In most respects apart from their skull and armor, the skeletal anatomy of aetosaurs was fairly standard among other large Triassic pseudosuchians. The hindlimbs developed a "pillar-erect" limb posture similar to that seen in "rauisuchians", a related grade of carnivorous Triassic pseudosuchians ancestral to crocodylomorphs. A pillar-erect limb posture is one where the femur articulates vertically with the acetabulum of the hip, which is angled downward, so that the leg is positioned beneath the body and acts as a weight-bearing pillar. Nevertheless, there was likely significant variation in the hip structure of aetosaurs, and the forelimbs may have had a semi-sprawling 'hybrid' stance. While the hindlimb posture is similar to rauisuchians, other traits are more plesiomorphic, such as the stout pelvis and broad, five-toed feet. The forelimbs were smaller than the hind limbs, and the radius in particular was much shorter than the humerus. Nevertheless, their low and heavy body shape requires that all aetosaurs were quadrupeds. They had multiple adaptations to strengthen the body in response to their heavy armor: the iliofibularis muscle attached to a lower position on the fibula, the fourth trochanter of the femur was enlarged, the transverse processes developed into long massive pedestals, and the largest species even acquired hyposphene-hypantrum reinforcements between their vertebrae.Although aetosaurs were generally wide-bodied reptiles, there is some variation in the degree of this trend. The typothoracines, exemplified by Typothorax and Paratypothorax, had a very broad, disc-shaped carapace, edged by small spines or keels and transitioning to a narrow tail. The largest species of typothoracines may have been around 3 meters in length and 110 kg in weight. The desmatosuchines, such as Desmatosuchus and Longosuchus, had moderately narrower bodies and no belly armor. However, they also acquired spinier back armor, especially in the cervical region. Desmatosuchus was likely one of the largest known aetosaurs, at in length and in weight. Aetosaurs which do not fit into these two categories, such as Stagonolepis and Neoaetosauroides, generally had narrow forms, slender limbs, and a restriction in the carapace above the hip. This body type is plesiomorphic to the other two shapes, with some narrow-bodied aetosaurs more closely related to typothoracines and others closer to desmatosuchines. Some plesiomorphic genera, like the widespread Norian genus Aetosaurus and the Carnian Coahomasuchus, tended to be small, about a metre in length. Others were larger, such as the basal-most aetosaur Aetosauroides and the early desmatosuchine Calyptosuchus.
Armor
Aetosaurs were very heavily armored, with rows of large, interlocking bony plates, known as osteoderms, protecting the back, sides, belly, and tail. These osteoderms generally have a quadrangular shape, and were most certainly used as a defense against predators. Most osteoderms are heavily pitted on their upper surfaces and smooth on their undersides. They have a heterogenous internal structure: the inner portion of each osteoderm is made of cancellous or spongy bone and their outer portions are made up of compact bone. In life, these plates were probably covered in a keratinous covering, like modern crocodilian scutes, which are another example of pseudosuchian osteoderms. Osteoderms are useful in diagnosing aetosaur taxa, and aetosaur species can often be identified from individual scutes based on their shape, structure, or ornamentation pattern.Aetosaurs have four rows of osteoderms running along their dorsal side, forming a continuous plate often called the carapace. The inner two rows, which flank the midline of the spinal column, are known as paramedian osteoderms. These tend to be wider than long and strongly ornamented with radiating pits or grooves. Nearly all aetosaurs possess a small boss or raised surface, known as a dorsal eminence, on the upper surface of each plate. The dorsal eminence is often set posteriorly or medially on their respective paramedian osteoderm, though there are many exceptions within the group. The paramedian osteoderms almost always have raised or depressed anterior edges, where the plates are overlapped by the ones in front of them. If the anterior edge is raised, the area is called an anterior bar, while if it is depressed, the area is called an anterior lamina. Although two paramedian osteoderm rows are common in early archosauriforms, few reptiles approach aetosaurs in the complexity of their osteoderms. The osteoderms of doswelliids, erpetosuchids, and certain crocodylomorphs are occasionally confused or compared with those of aetosaurs. Both an anterior bar and dorsal eminence occur in Acaenasuchus, a close relative of aetosaurs originally misidentified as a juvenile desmatosuchine.
The outer two rows of osteoderms, which lie beside the paramedians, are known as lateral osteoderms. They parallel the paramedians over nearly the entire backside, though the first two paramedians behind the head are solitary. Lateral plates generally are separated into two surfaces, or flanges, flexed between their dorsal eminence. The upper, or dorsal flange lies in the same plane as the paramedian osteoderms. The lower/outer, or lateral flange wraps down onto the side of the body. The dorsal eminence between these flanges often has the form of a low blade, knob, or spike. In the cervical lateral osteoderms, which are positioned on the neck, the dorsal eminence tends to manifest as a prominent spike. This is taken to an extreme in desmatosuchines such as Longosuchus and Desmatosuchus, where the spike is enlarged into a sharply curved horn.
In most aetosaurs, the underside of the animal is also protected by osteoderms. These ventral osteoderms are generally smaller and flatter than the dorsal series, and are arranged into a larger number of rows, at least in the torso. Ventral osteoderms rows usually curve outwards and separate under the hip, leaving a wide gap for the cloacal opening. Large, hooked spines occur around this opening in Typothorax, one of the few exceptions to a general rule of smooth ventral osteoderms. Ventral rows break up into a shagreen of small plates on the neck, and a small number of wide rows under the tail. A dense assortment of small, non-overlapping plates, known as appendicular osteoderms, covered the front and hindlimbs.