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.