The Aetosaurs

a Desmatosuchus searching for food in a fire-scorched wasteland

The aetosaurs – ‘eagle lizards’ (or, as those who study them call them, 'armadillodiles') – were heavily-armored, medium- to large-sized Late Triassic herbivorous reptiles. They were easily distinguishable, having small heads, upturned snouts, erect limbs positioned beneath their bodies, and most notably they were covered by plate-like bony scutes. Over twenty different kinds of aetosaurs have been discovered, and their remains come from every continent but Australia and Antarctica, giving them a nearly worldwide distribution during the Late Triassic. 

Aetosaurus

All aetosaurs belong to the family Stagonolepididae, but all fall within two distinct subdivisions: the Aetosaurinae and the Desmatosuchinae. The differences between these two groups focus on the arrangement of their armor. The Aetosaurines have projections called eminences on their dorsal paramedian osteoderms close to the midline of their backs. The Desmatosuchines have grooves on their dorsal paramedians that locked the lateral plates in a tight articulation, and many desmatosuchines had long spikes that projected from their lateral plates. These spikes are especially pronounced in Desmatosuchus, the group’s namesake. Some aetosaurines had spikes, but they had less and these weren’t as prominent. Some aetosaurines, such as Aetosaurus, had smooth carapaces that lacked spikes. Phylogenetic studies indicate that the aetosaurs may have been closely related to crocodylomorphs. Aetosaur evolution is pretty murky, as they show up on the scene in the Late Triassic fully-formed and with no ‘transitional species’ from ancestral archosaurs. Scientists postulate that the aetosaurs emerged in the Early or Middle Triassic before diversifying and flourishing in the Late Triassic. Primitive genera, such as the successful Aetosaurus in the Norian stage and Coahomasuchus in the Carnian stage, tended to be small, reaching three feet in length; later advanced forms could reach up to ten feet in length. Some aetosaurs, such as Typothorax, had broad, turtle-like bodies and reached eight feet in length; the spiked and narrow-bodied Desmatosuchus reached at least thirteen feet in length. 

an Aetosaurus roots around in the soil, looking for food

Aetosaurs lived on land and ate the ferns and seed ferns that flourished in the Triassic. Aetosaur teeth were simple and conical, designed for vegetation rather than meat. The tips of their upturned jaws were toothless and probably supported a keratinous beak. Because aetosaurs teeth show little wear and tear, it’s assumed that they didn’t eat tough and fibrous plant material but the softer leaves of ferns. They may have dug up roots and tubers with their upturned snouts, and there’s further evidence that they were diggers: their muscles were designed for digging motions, and their oversized hind feet sported large claws that could’ve been used for scratch-digging. Though some studies have indicated aetosaurs may have been burrowers – at least in the case of the smaller, more primitive forms – aetosaurs lack several characteristics seen in other burrowers. A 2009 study of the aetosaur genus Neoaetosauroides suggests that these animals may have fed on bug larvae and insects that lacked hard exoskeletons; this genus of aetosaur had a jaw leverage that wouldn’t be suitable for strong forces such as crushing and chopping needed for an herbivorous diet. Thus some aetosaurs, though certainly not all, may have been insectivores. 

several aetosaurs find themselves wedged between a forest
fire and some hungry phytosaurs

Aetosaur heads were flat and blunt at the front, resembling a pig’s snout. They had a ‘pillar-erect’ posture with their legs positioned underneath their bodies. Their feet resembled those of the crocodile-like semiaquatic phytosaurs. Though aetosaurs hind-limbs were much larger than the forelimbs, they remained nevertheless quadrupeds. Even if they’d wanted to go about on two legs, such a feat would’ve been exhausting because of their heavy armor. This armor, undoubtedly designed as a defense against predators, consisted of large quadrangular interlocking bony plates (called osteoderms) that protected the back and sides, belly, and tail. Most osteoderms were heavily pitted on their upper surfaces and smooth on their undersides. The center of the osteoderms was made up of spongy bone, and the outer parts were formed by compact bone. These plates were probably covered in horn. Dorsal osteoderms – found on the back of the animal – were often ornamented with radial grooves. Dorsal paramedians – found along the animal’s midline – were usually wide and quadrangular with a small boss called a dorsal eminence on the surface of each plate. The paramedian plates on aetosaurs often had raised or depressed edges where the plates articulated with the ones in front of them, giving them an almost ‘scaly’ appearance. In the lateral plates – positioned on either side of the paramedian plates – the dorsal eminence was often enlarged into a pronounced spike. These spikes are vividly evident in Desmatosuchus and Longosuchus. 

plate armor didn't mean aetosaurs were invincible: here a Postosuchus defends its kill
against a rival