Aetosaurs - Description

Description

The head is small relative to the large body, and quite distinctive in shape, being flat and blunt at the front, like the snout of a pig. Some teeth are chisel-shaped, small and leaf-like, indicating a probable herbivorous diet, although peg-like teeth and a keratinous snout have been considered possible adaptations in some species for feeding on colonial insects. A study of the braincase of Stagonolepis robertsoni has shown that there are similarities between it and those of crocodylomorphs, which may indicate a close relationship.

Aetosaurs had a "pillar-erect" erect limb posture similar to that seen in Rauisuchia, a related group of Triassic archosaurs. 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 pillar bearing weight. While the limb posture is similar to rauisuchians, the feet resemble those of phytosaurs (crocodile-like semiaquatic crurotarsans) in the retention of primitive characteristics. Although the forelimbs are much smaller than the hind limbs, all aetosaurs were quadrupeds.

Aetosaurs were very heavily armored (most certainly as a defense against predators), with large quadrangular, interlocking bony plates, or osteoderms, protecting the back and sides, belly, and tail. In life, these plates were probably covered in horn. Dorsal osteoderms, which are found on the backs of aetosaurs, are often ornamented with radial grooves. Dorsal paramedians, those found along the midline of the animal, are often wide and quadrangular with a small boss called a dorsal eminence on the dorsal surface of each plate. In aetosaurs, paramedian plates often have raised or depressed anterior edges where the plates articulate with 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. In lateral plates, which are positioned on either side of the paramedian plates, the dorsal eminence is often enlarged into a prominent spike. This spike is especially noticeable in desmatosuchines such as Longosuchus and Desmatosuchus. Osteoderms are useful in diagnosing aetosaur taxa, and aetosaur species can often be identified from individual scutes based on their ornamentation pattern.

Primitive genera, like the widespread Norian genus Aetosaurus and the Carnian Coahomasuchus, tended to be small, about a metre (3.2 ft) in length. However, more advanced forms were larger - about 3 metres (9.8 ft) in length - with some taxa, such as Typothorax and Paratypothorax, possessing broad turtle-like bodies, and others, like Desmatosuchus, a narrow-bodied genus at least 4 metres (13 ft) long, equipped with large spines over the shoulders, which added to the animal's defensive armament.