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.
As a whole, aetosaurs were comparatively small, with an average body length of approximately 2m. Aetosaurus ferratus had a head-to-tail length of just about 1 to 1.5m and represents one of the smallest specimens. On the other hand, the heavily armoured, Desmatosuchus, may have reached a length of up to 5m or more, ranging on the upper margin of the aetosaur size-range.
Aetosaur lives and lifestyles
Aetosaurs have traditionally always been considered as being herbivorous. The leaf-shaped teeth, with bulbous crowns and without denticles, the edentulous tip of the snout, a possible keratinous beak and a jaw (mandibular) joint that is positioned below the level of the tooth row, all indicate a diet consisting of plants. The bodies of the aetosaurs were broad and the pelvis had been modified, so that the pubis – the element of the pelvis normally projecting forward – was arched posteriorly, providing space for a capacious gut. The latter would have been necessary to process and digest large amounts of plant matter. Therefore, most aetosaurs were probably low browsers, feeding on shrubs or using their upturned snout and stout forelimbs to dig up plants and roots. Furthermore, the extensive body armour suggests that aetosaurs were peaceful herbivores, rather than carnivorous predators.
Nevertheless, recent analysis of the skulls, teeth structure and functional morphology of aetosaurs provides a different insight into their feeding behaviour. Closer examination showed that aetosaur teeth are generally lacking signs of wear facets – microscopic scratches on the enamel, which are generated either by the occlusion of the upper and lower teeth or by abrasive food. Therefore, aetosaurs must have fed on non-abrasive and soft components, though not necessarily only plants. Insects and larvae might also have been an integral part of their diet.
A recent investigation into the functional morphology of aetosaur skulls and jaws showed that there are significant differences in the function of the adductor and depressor jaw musculature. These muscles are responsible for the closing and opening of the jaws, and their attachment points in the skull and their morphology gives a measure of the force and speed of a bite moment. This investigation concluded that the jaw musculature of Desmatosuchus or Stagonolepis was designed for strong, powerful biting. This can generally be associated with herbivory, where force is necessary for slicing and chopping of large amounts of plant matter. The South American taxon, Neoaetosauroides, on the other hand, was shown to rely on fast, rather than powerful biting – an important requirement to catch moving prey.
Tooth number and morphology vary among the single taxa and it is quite possible that the various aetosaur species pursued different feeding habits from completely herbivorous to insectivorous or omnivorous. There has even been a report on a carnivorous aetosaur with recurved and pointed teeth, though this has yet to be verified.
Therefore, aetosaurs were most likely generalist feeders and their mode of life has been compared to that of recent armadillos (because of which, aetosaurs have sometimes been termed “armadillodiles”, in reference to their crocodilian relationships and the armadillo-like behaviour). Armadillos have an equally elongated and tapering snout, which holds only few peg-like teeth at the back. Likewise, the body of armadillos is covered by small epidermal scales, which form a sturdy armour. They use their powerful limbs to dig up roots and tubers, but also social insects like ants or termites from subsurface nests. Even small vertebrates and carrion can form a considerable part of their diet, although several species have specialised on particular food. The same concept probably applied to aetosaurs with a general omnivorous feeding behaviour, although the single species diversified according to the various ecological niches by becoming insectivorous or herbivorous specialists.
With their extensive armour, the aetosaurs were well protected against small or medium-sized predators up to the same size. Pathological osteoderms of Paratypothorax have been found, which show an osteological deformation. The single osteoderms were fused together displaying an extensive bone growth in response to an injury. The aetosaur must have survived this attack for the bone to continue growing afterwards. As no teeth or tooth impressions were found, the identity of the attacker remains a mystery. The development of spikes along their sides in taxa like Desmatosuchus or Paratypothorax may even have permitted them to defend themselves actively against larger predators. The latter mainly consisted of large crurotarsan archosaurs, like rauisuchians, ornithosuchids or phytosaurs. The first carnivorous dinosaurs were comparatively small at that time and might have preferred smaller and less armoured prey.
Aetosaurs in time and stratigraphy
Aetosaurs were a widespread group and a common faunal element in the Late Triassic. The first unequivocal finds are referred to the Carnian Stage, but recent discoveries from the German Lettenkeuper confirm their presence several million years earlier in the Middle Triassic (Ladinian). This shifts the origin of the whole group well back into the Middle or even the Early Triassic. During the Carnian and Norian, aetosaurs quickly diversified and became an integral component of the terrestrial ecosystems of the Late Triassic, with over a dozen documented genera. Towards the Rhaetian, their number declined and they died out at the end of the Triassic, alongside other crurotarsan archosaurs, including rauisuchians and phytosaurs.
The large number of different species, and their comparably short temporal distribution, has prompted some authors to suggest a biostratigraphic value for aetosaurs. As the osteoderms of the various genera and species seem to be highly characteristic in their ornamentation and morphology, they could be used as index fossils to determine the age of a specific rock unit. Especially for the North American Chinle and Dockum Formation of the Upper Triassic, where such osteoderms are abundant and more than ten different taxa have been described, several vertebrate biochrones have been proposed. Nevertheless, one has to keep in mind that some genera are based on single osteoderms only and that the variations in their ornamentation are often intraspecific and dependent on the body region or the age of the individual animal.
For the same reason, the phylogenetic relationships of aetosaurs are not completely understood yet. They form a distinct group within the crurotarsan or crocodilian branch of the archosaurs and, therefore, are closer related to crocodiles than to dinosaurs. Relationships within Aetosauria are strongly debated, mainly because of the fragmentary fossil remains and aetosaurian precursors are so far unknown. No transient forms have been found yet, so that their ancestry remains shrouded.
Geographically, the aetosaurs were restricted to Europe and the American continents, with isolated finds additionally from Morocco and India. Fossil remains consist mainly of isolated osteoderms and articulated parts of the armour, while cranial elements or even complete specimens are generally rare.
The first aetosaur remains were found in 1844. They consisted of single scales of Stagonolepis robertsoni from the famous Lossiemouth Sandstone Formation of Elgin, Scotland and, therefore, were not correctly identified at the time. In fact, the famous zoologist and palaeontologist, Louis Agassiz, described them as large scales of a ganoid fish, hence the name Stagonolepis, which literally means “drop-like scale”. Not until the discovery of more complete material from the same locality in the following years was their archosaurian nature affirmed. So far, more than 20 individual, more or less complete specimens of Stagonolepis robertsoni have been collected from Elgin.
In 1875, a truly exceptional find was made near Stuttgart, Germany, in the sediments of the Norian Stubensandstein. In a block of sandstone of less than 1.5m², the nearly complete and fully articulated skeletons of 24 specimens of Aetosaurus ferratus had been preserved. What event gave rise to this accumulation of animals of different ages and sizes remains a mystery. Whether they were buried in subterranean burrows or were trapped in quicksand is unknown. The surrounding sediments offer little explanation.
Since then, a multitude of new discoveries have been made, especially in North and South America, giving new insights into a diverse and fascinating group of extinct reptiles.