The Triassic Period was one of the key episodes in the evolutionary history of life. It marks the transition from the Palaeozoic to the Mesozoic Era, preceded by the Permian/Triassic mass extinction – the greatest extinction event of all time. A conclusive explanation for it has still to be found, but it is presumed that massive eruptions of flood basalts in Siberia were a key factor. The resultant release of green house gases, such as carbon dioxide and sulphur dioxide, into the atmosphere may have led to a catastrophic rise of the global temperature and oxygen depletion. While other causes may have contributed to this extinction, its effects are clearly visible in the fossil record. Over 90% of marine species and about two thirds of terrestrial species died out. Among the tetrapods, amphibians and therapsids (mammal-like reptiles) suffered the most and the terrestrial ecosystems were heavily depleted.
Yet, on the other hand, this mass extinction cleared the way for several new groups of animals. The ecological niches on land, which had been vacated since the Permian/Triassic transition, were quickly occupied by two new groups – the rhynchosaurs and the archosaurs. The latter proved to be most successful, not least for the rapid evolution and diversification of the dinosaurs in the Late Triassic, which eventually gave rise to the birds. However, it was other groups among the archosaurs, which first took advantage of the newly vacated habitats in the Early and Middle Triassic. Many of these groups were restricted to the Triassic and are generally rather unfamiliar in contrast to the widely popular dinosaurs. The aetosaurs belong to one of these groups and they were among not only the first to appear, but also the first herbivorous archosaurs.
Aetosaurs or stagonolepidids (after the genus Stagonolepis) were remarkable and peculiar in many ways. Anatomically, they were highly specialised and their skeleton shows many adaptations to a primarily herbivorous lifestyle. They had small and pointed heads. Their snout was elongated and, depending on the taxon, more or less upturned. The premaxilla – the bone, which forms the foremost part of the snout – was either completely or partially toothless and its tip may have been covered by a cornified beak. Therefore, the skull of aetosaurs roughly resembles those of large birds of prey and this resemblance was probably responsible for the derivation of their name, which literally means “eagle lizard”. The rest of the upper jaw was equipped with small, leaf-shaped teeth with smooth edges. The lower jaw was similarly designed. Only the rearward part of the dentary held teeth, while its tip tapered to a toothless point. The whole skull was stoutly built and the temporal fenestrae – two distinct openings on the upper and lateral side of the skull of archosaurs – were considerably reduced in size.
The postcranial skeleton of the aetosaurs was broad and stocky. Their legs were short and stout, and connected to massive girdle bones. A specialised configuration of the joints between the pelvis and the hindlimbs allowed them to hold their legs straight under their body, allowing for an erect gait, which distinguished the aetosaurs from their sprawling ancestors. Their legs were of approximately equal length, with the front limbs being slightly shorter. The whole composition of their locomotor apparatus made the aetosaurs obligatory quadrupedal, but also allowed for an efficient locomotion. The hands and feet were equipped with short, but sharp claws. The spinal column was composed of massive vertebrae. In addition, the neural spines of the presacral vertebrae had a flattened and horizontally expanded tip, forming so-called spine tables. These formed a firm attachment point for osteoderms – bony scales, which were set into the dermal layer of the skin. Comparable osteoderms can be found in extant crocodiles and other Triassic archosaurs. The osteoderms of aetosaurs were rectangular and considerably wider than long, and their surface was characterised by an intricate ornamentation of pits and ridges. They were arranged in four rows along the dorsal and sacral region, with the single elements overlapping each other along their margin. Additionally, the underside of the animal, the tail, the sides and probably the limbs were covered by smaller osteoderms, providing protective armour for the whole body. Some specimens, like the North American Desmatosuchus, also possessed a series of spikes along each side of the neck, the shoulder region and the flanks, the largest of which were up to 45cm long.