Aetosauria



The aetosaurs (literally "Eagle Lizards") are an extinct clade of heavily armoured, medium- to large-sized Late Triassic herbivorous archosaurs. Two distinct subdivisions of aeotosaurs are currently recognized, the Desmatosuchinae and the Aetosaurinae, based primarily on differences in the morphology of the bony scutes of the two groups.

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. The chisel-shaped teeth are small and leaf-like, indicating a probable herbivorous diet, although peg-like teeth and a keratinous snout have been described in at least one species as possible adaptations for feeding on colonial insects. Study of the braincase indicates that aetosaurs are actually closely related to crocodylomorphs.

As with the rauisuchians, aetosaurs had a "pillar-erect" erect limb posture. The feet, however, resemble those of the phytosaurs in the retention of primitive characteristics. In other respects, they have a typically crurotarsan (rauisuchian or crocodylian) body and large powerful tail. Although the fore-limbs are much smaller than the hind limbs, all aetosaurs were quadrupeds.

These animals 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.

Primitive genera, like the widespread Norian genus Aetosaurus and the Carnian Coahomasuchus (Heckert & Lucas, 1999), tended to be small, about a meter in length. However more advanced forms were larger - about 3 meters in length - with taxa some, such as Typothorax and Paratypothorax, possessing broad turtle-like bodies, and others, like Desmatosuchus, a narrow-bodied genus up to 5 meters long, equipped with large spines over the shoulders, which added to the animal's defensive armament.

At least some aetosaurs built nests and protected their eggs. In 1996, geologist Stephen Hasiotis discovered 220 million-year-old, fossilized, bowl-like nests in Arizona's Petrified Forest, in part of the Chinle Formation. The oldest such nests that have been found belonged to phytosaurs and aetosaurs. The nests are compacted and appear very similar to the nests of the modern day crocodiles who guard their nests. The nests were holes dug in the sand in the bank of an ancient river.

Aetosaur fossil remains are known from Scotland, Germany, Greenland, the southwest and the eastern United States, Argentina, and Madagascar. Since their armored plates are often preserved, and as they often have a wide geographic distribution, but a relatively short stratigraphic range, aetosaurs can serve, like phytosaurs, as important Late Triassic tetrapod index fossils.