Aetosaur Skull Diagrams

Aetosaurs () are heavily armored reptiles belonging to the extinct order Aetosauria (; from Greek, (aetos, "eagle") and (, "lizard")). They were medium- to large-sized omnivorous or herbivorous pseudosuchians, part of the branch of archosaurs more closely related to crocodilians than to birds and other dinosaurs. All known aetosaurs are restricted to the Late Triassic, and in some strata from this time they are among the most abundant fossil vertebrates. They have small heads, upturned snouts, erect limbs, and a body ornamented with four rows of plate-like osteoderms (bony scutes). Aetosaur fossil remains are known from Europe, North and South America, parts of Africa, and India. Since their armoured plates are often preserved and are abundant in certain localities, aetosaurs serve as important Late Triassic tetrapod index fossils. Many aetosaurs had wide geographic ranges, but their stratigraphic ranges were relatively short. Therefore, the presence of particular aetosaurs can accurately date a site that they are found in.

Nearly all aetosaurs (except for the genus '' Aetosauroides'') belong to the family Stagonolepididae. Over 20 genera of aetosaurs have been described, and recently there has been controversy regarding the description of some of these genera. Two distinct subdivisions of aetosaurs are currently recognized, Desmatosuchia and Aetosaurinae, based primarily on broad differences in skull morphology. Osteoderms structure is generally one of the most useful traits for inferring aetosaur relations more precisely. Among other archosaurs, aetosaurs are most closely related to '' Revueltosaurus'', a small reptile originally known from teeth mistakenly referred to herbivorous dinosaurs.

Aetosaur remains were first discovered in the early 19th century, although the very first remains that were described were mistaken for fish scales. Aetosaurs were later recognized as crocodile relatives, at which point they were interpreted as semiaquatic scavengers closely related to phytosaurs. Subsequent work has established that aetosaurs were entirely terrestrial animals, and were likely herbivorous to some extent. Some forms have characteristics that may have been adaptations to digging for food. Supposed nesting structures have also been referred to aetosaurs, but this connection is considered ambiguous.

Description

Skull anatomy

The skull of aetosaurs is relatively small compared to the body, and is quite distinctive in shape. Teeth are absent from both the front of the premaxilla (the bone forming the tip of the snout) and the front of the dentary (the toothed bone of the lower jaw). The teeth which are present are usually small and bulbous, ranging from basic conical forms to leaf-like shapes with large serrations. These are probably indicative of an omnivorous or herbivorous diet, and similar adaptations are seen in other archosaurs with less reliance on meat in their diet. A few aetosaurs have teeth with a ziphodont shape, meaning that the teeth are recurved, serrated, and flattened from the side. This shape, which is predominant in '' Aetosauroides'' and a small specimen tentatively referred to ''Coahomasuchus'', is typical of carnivorous archosaurs.

In some aetosaurs (particular members of the group Desmatosuchia), the tip of the snout is expanded sideways into a flattened 'shovel' shape, akin to the snout of a pig. The external nares (nostril holes) are elongated, much larger than the antorbital fenestrae (a hole on the side of the skull). Many aetosaurs have a small knob on the premaxilla which projects into the nares from below. In all aetosaurs except ''Aetosauroides'', the rear edge of the naris receives a contribution from the concave front edge of the maxilla bone. At the rear upper part of the skull, a hole known as the supratemporal fenestra is positioned and exposed on the side, unlike most other archosaurs where it is mostly visible when viewing the skull from above. The braincase is fairly standard by pseudosuchian standards, though the opening for the ''abducens'' nerve passes through the parabasisphenoid bone (at the lower front part of the braincase), rather than the prootic bone (at the upper front part). This trait is otherwise only seen in ''Revueltosaurus'' and crocodylomorphs among archosaurs.Archosaurian Anatomy and Palaeontology: Essays in Memory of Alick D. Walker, DB Norman & DJ Gower (eds.) The mandible (lower jaw) is described as 'slipper'-shaped in many aetosaurs. This is due to a combination of features: the front of the dentary strongly tapers to a point, while the underside of the dentary sometimes flexes into a 'chin' (downwards projection) which may expose the splenial bone as well. The jaw joint is set at a low position, and the articular (the bone of the lower jaw which connects to the cranium) often has a tall projection right behind the jaw joint.

Postcranial anatomy

In most respects apart from their skull and armor, the skeletal anatomy of aetosaurs was fairly standard among other large Triassic pseudosuchians. The hindlimbs developed a "pillar-erect" limb posture similar to that seen in " rauisuchians", a related grade of carnivorous Triassic pseudosuchians ancestral to crocodylomorphs. A pillar-erect limb posture is one where the femur articulates vertically with the acetabulum of the hip, which is angled downward, so that the leg is positioned beneath the body and acts as a weight-bearing pillar. Nevertheless, there was likely significant variation in the hip structure of aetosaurs, and the forelimbs may have had a semi-sprawling 'hybrid' stance. While the hindlimb posture is similar to rauisuchians, other traits are more plesiomorphic (typical to ancestral pseudosuchians), such as the stout pelvis and broad, five-toed feet. The forelimbs were smaller than the hind limbs, and the radius in particular was much shorter than the humerus. Nevertheless, their low and heavy body shape requires that all aetosaurs were quadrupeds. They had multiple adaptations to strengthen the body in response to their heavy armor: the ''iliofibularis'' muscle attached to a lower position on the fibula, the fourth trochanter of the femur was enlarged, the transverse processes (rib attachments) developed into long massive pedestals, and the largest species even acquired hyposphene-hypantrum reinforcements between their vertebrae.

Although aetosaurs were generally wide-bodied reptiles, there is some variation in the degree of this trend. The typothoracines, exemplified by ''Typothorax'' and ''Paratypothorax,'' had a very broad, disc-shaped carapace, edged by small spines or keels and transitioning to a narrow tail. The largest species of typothoracines may have been around 3 meters (9.8 feet) in length and 110 kg (243 lbs) in weight. The desmatosuchines ( Desmatosuchinae ''sensu stricto''), such as '' Desmatosuchus'' and '' Longosuchus'', had moderately narrower bodies and no belly armor. However, they also acquired spinier back armor, especially in the cervical (neck) region. ''Desmatosuchus'' was likely one of the largest known aetosaurs, at in length and in weight. Aetosaurs which do not fit into these two categories, such as '' Stagonolepis'' and '' Neoaetosauroides'', generally had narrow forms, slender limbs, and a restriction in the carapace above the hip. This body type is plesiomorphic (ancestral) to the other two shapes, with some narrow-bodied aetosaurs more closely related to typothoracines and others closer to desmatosuchines. Some plesiomorphic genera, like the widespread Norian genus '' Aetosaurus'' and the Carnian ''Coahomasuchus'', tended to be small, about a metre (3.2 ft) in length. Others were larger, such as the basal-most aetosaur ''Aetosauroides'' and the early desmatosuchine '' Calyptosuchus''.

Armor

Aetosaurs were very heavily armored, with rows of large, interlocking bony plates, known as osteoderms, protecting the back, sides, belly, and tail. These osteoderms generally have a quadrangular (four-sided) shape, and were most certainly used as a defense against predators. Most osteoderms are heavily pitted on their upper surfaces and smooth on their undersides. They have a heterogenous internal structure: the inner portion of each osteoderm is made of cancellous or spongy bone (also called diploë) and their outer portions are made up of compact bone. In life, these plates were probably covered in a keratinous (horn) covering, like modern crocodilian scutes, which are another example of pseudosuchian osteoderms. Osteoderms are useful in diagnosing aetosaur taxa, and aetosaur species can often be identified from individual scutes based on their shape, structure, or ornamentation pattern.

Aetosaurs have four rows of osteoderms running along their dorsal (back) side, forming a continuous plate often called the carapace. The inner two rows, which flank the midline of the spinal column, are known as paramedian osteoderms. These tend to be wider than long and strongly ornamented with radiating pits or grooves. Nearly all aetosaurs possess a small boss or raised surface, known as a dorsal eminence, on the upper surface of each plate. The dorsal eminence is often set posteriorly (backwards) or medially (inwards) on their respective paramedian osteoderm, though there are many exceptions within the group. The paramedian osteoderms almost always have raised or depressed anterior edges, where the plates are overlapped by the ones in front of them. If the anterior edge is raised, the area is called an anterior bar, while if it is depressed, the area is called an anterior lamina. Although two paramedian osteoderm rows are common in early archosauriforms, few reptiles approach aetosaurs in the complexity of their osteoderms. The osteoderms of doswelliids, erpetosuchids, and certain crocodylomorphs are occasionally confused or compared with those of aetosaurs. Both an anterior bar and dorsal eminence occur in '' Acaenasuchus'', a close relative of aetosaurs originally misidentified as a juvenile desmatosuchine.

The outer two rows of osteoderms, which lie beside the paramedians, are known as lateral osteoderms. They parallel the paramedians over nearly the entire backside, though the first two paramedians behind the head (known as nuchal osteoderms) are solitary. Lateral plates generally are separated into two surfaces, or flanges, flexed between their dorsal eminence. The upper, or dorsal flange lies in the same plane as the paramedian osteoderms. The lower/outer, or lateral flange wraps down onto the side of the body. The dorsal eminence between these flanges often has the form of a low blade, knob, or spike. In the cervical lateral osteoderms, which are positioned on the neck, the dorsal eminence tends to manifest as a prominent spike.Bill Parker (August 12, 2009).
Aetosaurs 101: Osteoderm Nomenclature
" ''Chinleana''. Accessed January 8, 2010. This is taken to an extreme in desmatosuchines such as '' Longosuchus'' and '' Desmatosuchus'', where the spike is enlarged into a sharply curved horn.

In most aetosaurs (a major exception being desmatosuchines), the underside of the animal is also protected by osteoderms. These ventral (belly) osteoderms are generally smaller and flatter than the dorsal series, and are arranged into a larger number of rows (usually 5 - 14 rows), at least in the torso. Ventral osteoderms rows usually curve outwards and separate under the hip, leaving a wide gap for the cloacal opening. Large, hooked spines occur around this opening in ''Typothorax'', one of the few exceptions to a general rule of smooth ventral osteoderms. Ventral rows break up into a shagreen of small plates on the neck, and a small number of wide rows under the tail. A dense assortment of small, non-overlapping plates, known as appendicular osteoderms, covered the front and hindlimbs.

History

Early European finds

Aetosaur material was first described by Swiss paleontologist Louis Agassiz in 1844. He named the genus '' Stagonolepis'' from the Lossiemouth Sandstone in Elgin, Scotland, but considered the fossil to be a Devonian fish rather than a Triassic reptile. This may be because he considered the strata to be part of the Old Red Sandstone, and thus Paleozoic in age. Agassiz mistook the osteoderms for large rhomboidal scales, which he thought were arranged in a similar pattern to those of gars. He also thought that these supposed scales were very similar to those of the lobe-finned fish '' Megalichthys'' due to their large size.

English biologist Thomas Henry Huxley reconsidered the fish scales described by Agassiz and considered them to belong to a crocodilian. He first proposed this to the Geological Society of London in 1858, and went into more detail in an 1875 paper in the society's quarterly journal. By this time, new material had been uncovered from Elgin that indicated that ''Stagonolepis'' was not a fish, but a reptile. However, ''Stagonolepis'' was still known primarily by scutes and imprints of scutes, many of which were not well preserved.

More complete aetosaur remains were found from the Lower Stubensandtein of Germany in the 1870s. Among them were complete articulated skeletons of 22 aetosaurs. These specimens were found in a large sandstone outcrop near Stuttgart and were preserved together in an area less than 2 square metres in size. The animals were probably buried under lake sediment soon after they died, with the flow of water repositioning their bodies on the lake bed and putting them in close proximity to one another. In 1877, German paleontologist Oscar Fraas assigned these specimens to the newly erected genus '' Aetosaurus''. Fraas named the genus after the skull's resemblance to the head of an eagle, with a narrow, elongate skull and a pointed snout.

First American aetosaurs

The first aetosaurs from North America were also being described in the 1870s. American paleontologist Edward Drinker Cope named ''Typothorax'' in 1875 and ''Episcoposaurus'' in 1877, both of which were from New Mexico. However, Cope considered these genera to be phytosaurs, crocodile-like aquatic archosaurs that were also common in the Late Triassic. While new material referable to ''Typothorax'' has been found in recent years throughout the American southwest, ''Episcoposaurus'' is no longer considered valid. Cope, along with later paleontologists such as Friedrich von Huene, recognized that remains of the type species ''E. horridus'' actually belonged to ''Typothorax''. At the time, Cope considered Aetosauria to belong to Rhynchocephalia, an order of reptiles that includes the living tuatara. He also thought that the tightly fitting osteoderms, which were thought to be fused to the ribs, indicated that aetosaurs were transitional between rhynchocephalians and turtles. Another species, ''E. haplocerus'', was reassigned to ''Desmatosuchus'' in 1953.

A third North American genus called ''Stegomus'' was named by Othniel Charles Marsh in 1896. Marsh had a long-time rivalry with Cope that was made famous in the Bone Wars of the late 19th century, in which the two tried to out-compete one another in the field and in scientific literature. Unlike Cope's aetosaurs, ''Stegomus'' was found from the eastern United States in Connecticut. Marsh also recognized ''Stegomus'' as an aetosaur rather than a phytosaur in his initial description of the genus. Like Cope, many paleontologists tended to consider aetosaur scutes to belong to phytosaurs during this time period. Marsh considered aetosaurs to be closely related to dinosaurs based on their elongated metatarsals (foot bones).

As a distinct group, Aetosauria was named in 1889 by English naturalist Richard Lydekker and zoologist Henry Alleyne Nicholson. They considered Aetosauria to be one of three suborders of the order Crocodilia, the other two being Parasuchia (a group including phytosaurs and other Triassic forms) and Eusuchia (a group including all post-Triassic crocodylomorphs). Nicholson and Lydekker placed a single family within the suborder, Aëtosauridæ. They considered aetosaurs to be similar to living crocodilians despite their longer metatarsals.

20th century updates

Aetosaur material continued to be described into the early 20th century, with notable paleontologists such as Barnum Brown and Charles Camp collecting specimens. Better remains of American aetosaurs helped to establish anatomical info for some common species, such as ''Desmatosuchus spurensis'' (described in detail by E.C. Case in 1922) and ''"Typothorax" meadei'' (now '' Longosuchus'', described by H.J. Sawin in 1947). However, aetosaur remains were still being confused with those of phytosaurs, and they were still considered to be predominantly carnivorous and semiaquatic "parasuchians" . A.D. Walker's 1961 redescription of ''Stagonolepis robertsoni'' helped to divert these notions and established a more modern view of aetosaurian anatomy and ecology.

Distribution

Aetosaur fossils have been found on all continents except Australia and Antarctica, giving them a nearly worldwide distribution during the Late Triassic. Aetosaur remains are most abundant in northern ( Laurasian) continents, and fossils are particularly common in the Chinle Formation and Dockum Group of the southwestern United States. Most fossils have been found in Arizona, New Mexico, and Texas. Some remains have also been found in Wyoming, Colorado, and Utah, particularly in Canyonlands National Park and Zion National Park. Aetosaurs are also known from the Newark Supergroup along the East Coast of the United States, in states such as Connecticut and North Carolina. ''Aetosaurus'' and ''Paratypothorax'' have been recovered from the Fleming Fjord Formation of Jameson Land, on the east coast of Greenland. These two genera were originally named from the Lower Stubensandstein of Germany. Scotland, Poland, and Italy have also produced aetosaur fossils. A few aetosaurs are known from northern Africa, with scutes having been found from the late Carnian Timesgadiouine Formation in Morocco. During the Late Triassic, Morocco would have been in close proximity with the Newark Supergroup of North America in the supercontinent of Pangaea. It is also possible that ''Desmatosuchus'' was present in Africa, as fossils from the Zarzaitine Series in Algeria have been referred to the genus.

Aetosaurs are also found in areas corresponding to Gondwana (southern Pangaea), though they are considerably less common or diverse than in Northern continents. South American aetosaurs are known from Argentina, Brazil, and Chile. In Argentina, '' Aetosauroides'' and '' Neoaetosauroides'' hail from the Carnian Ischigualasto Formation and Norian Los Colorados Formation, respectively. In Brazil, fossils have been found in the Santa Maria and Caturrita Formations in Rio Grande do Sul (Paleorrota). Chilean aetosaurs are represented by one fragmentary genus, ''Chilenosuchus'', from the Antofagasta Region. Aetosaurs have also been found in India, which, along with South America, was part of Gondwana during the Late Triassic. Early accounts of Indian aetosaurs were based on material from the Maleri Formation in south-central India, although these remains were too inadequate to assign specimens to any particular genus. Based on published descriptions, the Indian aetosaur fossils most closely resemble ''Longosuchus'' and ''Paratypothorax,'' which are considerably more specialized than most described Gondwanan aetosaurs. Reports of aetosaurs from Madagascar are based on probable crocodylomorph scutes. Supposed aetosaurian footprints and skeletal material from South Africa are also unsubstantiated.

Footprints belonging to the ichnogenus ''Brachychirotherium'' are often associated with aetosaurs. ''Brachychirotherium'' are known from Rio Grande do Sul in Paleorrota, Brazil as well as Italy, Germany, the eastern United States, and many other aetosaur-bearing locales. They are also common in the southwestern United States, having been found in Canyonlands National Park and Dinosaur National Monument. Many of these tracks have a narrow gauge (meaning the left and right prints are placed closely together) and nearly overstep each other. A 2011 functional analysis of the skeleton of ''Typothorax coccinarum'' indicated that it had the range of movement necessary to produce the tracks.

Classification

Taxonomy

Aetosaurs belong to Pseudosuchia, a clade of archosaurs that includes living crocodilians and is characterized by the distinctive structure of the ankle bones. Aetosaurs were traditionally referred to a (now obsolete) group called the thecodonts, which included all "primitive" crocodilian relatives that lived in the Triassic. With the rise of phylogenetics, aetosaurs were later placed in a group called Suchia, which included many Triassic crurotarsans as well as later crurotarsans, including crocodilians.

Originally, all aetosaurs were considered members of the family Stagonolepididae. Early phylogenetic analyses split aetosaurs into two subfamilies, Aetosaurinae and Desmatosuchinae. Aetosaurines are characterized by projections called eminences on the dorsal paramedian osteoderms that are close to the midline of the back. Desmatosuchines have a few more distinguishing characteristics, including grooves on the dorsal paramedians that help them lock to the lateral plates in a tight articulation. Many desmatosuchines have long spikes projecting from the lateral plates. These spikes are especially prominent in ''Desmatosuchus''. Aetosaurines, on the other hand, tend to have less spikes. Many aetosaurines, such as ''Aetosaurus'' and ''Neoaetosauroides'', have smooth carapaces and lack spikes altogether. More recent studies (see below) have favored a third group, Typothoracinae, which like Desmatosuchinae has long spikes, but differs in having more sharply angled joints between osteoderms. Moreover, the genus '' Aetosauroides'' is now often classified outside Stagonolepididae as a non-stagonolepidid aetosaur, making the names Aetosauria and Stagonolepididae no longer synonymous.

List of genera

Phylogeny

Aetosaur phylogeny was first investigated in 1994 by paleontologist J. Michael Parrish. ''Aetosauroides'', ''Aetosaurus'', ''Desmatosuchus'', ''Longosuchus'', ''Neoaetosauroides'', ''Stagonolepis'', and ''Typothorax'' were included in the phylogenetic analysis. Aetosaurs were found to form a clade with rauisuchians, which Parrish termed Rauisuchiformes. Rauisuchiformes also included the superorder Crocodylomorpha, to which living crocodilians belong. Parrish found Aetosauria to be a monophyletic group and thus a true clade consisting of a common aetosaur ancestor and all of its descendants. To phylogenetically define Aetosauria, Parrish identified five synapomorphies, or shared characteristics. The first synapomorphy concerned the jaw, with the premaxilla at its tip being edentulous (toothless), upturned, and wide to form a "shovel". Moreover, the dentary bone in the lower jaw is also toothless, upturned, and broad. The reduced size and simple conical shape of the teeth was considered another synapomorphy. Two more synapomorphies of aetosaurs are shared with crocodylomorphs, but were not considered to be an indication of a close phylogenetic relationship; the body is covered in dorsal and ventral armor to form a complete carapace, and the paramedian osteoderms are much wider than they are long, with distinctive pitting. A final synapomorphy was found in the structure of the limb bones. In all aetosaurs, the limbs are very robust, with large muscle attachments such as the deltopectoral crest of the humerus, the fourth trochanter of the femur, the intracondylar ridge of the tibia, and the iliofibularis trochanter of the fibula.

In Parrish's phylogenetic analysis, ''Aetosaurus'' was found to be the most basal member of the clade, the earliest to diverge after the most recent common ancestor. After ''Aetosaurus'', there is a polytomy of three smaller clades in which it is unknown which clade diverged first from the group. Within this polytomy there was ''Neoaetosauroides'', a clade containing ''Aetosauroides'' and ''Stagonolepis'', and another polytomy that included ''Longosuchus'', ''Desmatosuchus'', and a clade containing ''Paratypothorax'' and ''Typothorax''.

A later study by paleontologists Andrew B. Heckert and Spencer G. Lucas in 1999 expanded the number of synapomorphies that diagnose Aetosauria to 18. New synapomorphies included temporal fenestrae, or holes, that opened on the side of the skull rather than the top, lateral osteoderms articulating with the paramedians, and osteoderms covering the limbs. ''Aetosaurus'' was still found to be the most basal member, but the phylogeny of more derived aetosaurs differed in that ''Typothorax'' and ''Paratypothorax'' were split into two different clades with their sister taxa being ''Desmatosuchus'' and ''Longosuchus'', respectively. More importantly, a new aetosaur called ''Coahomasuchus'' was included in the analysis. ''Coahomasuchus'' was found to be a basal aetosaur closely related to ''Stagonolepis'', and also appeared early in the fossil record of aetosaurs. Previously, basal members were only known from later times, occurring after more advanced aetosaurs.

In 2003, paleontologists Simon R. Harris, David J. Gower, and Mark Wilkinson examined previous phylogenetic studies of aetosaurs and criticized the way in which they used certain characters to produce cladograms. They concluded that only three hypotheses of aetosaur relationships from previous studies were still true: that ''Aetosaurus'' is the most basal aetosaur, that ''Aetosauroides'' is the sister taxon of ''Stagonolepis robertsoni'', and that ''Longosuchus'' and ''Desmatosuchus'' are more closely related to each other than either is to ''Neoaetosauroides''. They also went on to correct the trees from all previous analyses.

More recently, a 2007 analysis by paleontologist William G. Parker resulted in a larger tree of aetosaur phylogenetics with the inclusion of ''Heliocanthus''. Based on the tree, Parker defined the clades Typothoracisinae and Paratypothoracisini, both within Aetosaurinae. Parker also gave a revised phylogenetic definition of Aetosauria, mentioning that the previous definition, made by Heckert and Lucas in 2000, was somewhat ambiguous. Heckert & Lucas (2000) defined Aetosauria as a stem-based taxon, claiming that Aetosauria included all crurotarsans that were more closely related to ''Desmatosuchus'' than to the immediate sister group of Aetosauria. Because the immediate sister group of Aetosauria was uncertain, Parker offered a new definition with several non-aetosaur crurotarsan genera rather than one sister group. According to Parker, Aetosauria included all taxa more closely related to ''Aetosaurus'' and ''Desmatosuchus'' than to '' Leptosuchus'', '' Postosuchus'', '' Prestosuchus'', '' Poposaurus'', ''Sphenosuchus'', '' Alligator'', '' Gracilisuchus'', and '' Revueltosaurus''.

A new genus of aetosaur, '' Aetobarbakinoides'', was named in 2012. The phylogenetic analysis in that study found Aetosaurinae to be a paraphyletic grouping. As a paraphyletic group, aetosaurines would share a most recent common ancestor that is also the ancestor of other non-aetosaurine aetosaurs, and thus could not form their own clade. Parker's 2007 analysis accepted this definition. In 2002, Heckert and Lucas defined Aetosaurinae as "a stem-based taxon containing all taxa more closely related to ''Aetosaurus'' than to the last common ancestor of ''Aetosaurus'' and ''Desmatosuchus''". The 2012 study placed ''Aetosaurus'' at the base of the stagonolepidid clade, with traditional aetosaurine taxa placed in successively more derived positions. In the analysis, these taxa are actually more closely related to ''Desmatosuchus'' than to ''Aetosaurus''. Thus, under Heckert and Lucas's definition Aetosaurinae might be restricted to only ''Aetosaurus'' itself.

Another finding of this study was that ''Aetosauroides'' lies outside Stagonolepididae. If this phylogeny is correct, Stagonolepididae and Aetosauria would not be equivalent groupings, and ''Aetosauroides'' would be the first non-stagonolepidid aetosaur. The following cladogram simplified after an analysis presented by Devin K. Hoffman, Andrew B. Heckert, and Lindsay E. Zanno.

In 2016, William Parker conducted a new phylogenetic analysis of the Aetosauria, proposing an alternative hypothesis of aetosaur relationships. Below is the cladogram:

Origin and evolution

Although aetosaurs are known exclusively from the Late Triassic, their currently accepted position in archosaur phylogeny indicates that they originated from more basal pseudosuchian archosaurs in the Early or Middle Triassic. Given that aetosaurs are highly specialized with many anatomical features not seen in other pseudosuchians, the group's evolutionary origins are poorly understood. The recent discovery of complete specimens of the Late Triassic pseudosuchian ''Revueltosaurus callenderi'' indicate that it may have been close to the ancestry of aetosaurs. Several phylogenetic analyses place it as the sister taxon or closest relative of Aetosauria. Like aetosaurs, ''Revueltosaurus'' has two rows of paramedian osteoderms along its back and, in the cheek region of the skull, a maxilla that fits into a groove of the jugal bone. One phylogenetic analysis places '' Turfanosuchus dabanensis'', a Middle Triassic pseudosuchian, as the sister taxon of ''Revueltosaurus'' and Aetosauria, potentially making it the earliest known "stem aetosaur" ("stem" meaning that it lies on the branch that includes aetosaurs, but is not itself an aetosaur). However, other studies have considered ''Turfanosuchus'' part of a different family of pseudosuchians, the gracilisuchids.

In 2012 another "stem aetosaur" was described from the Middle Triassic Manda Beds of Tanzania. It differs from other Middle Triassic pseudosuchians in having a long skull, a small antorbital fenestra that fits into a large antorbital fossa in front of the eye socket, sharp and curved teeth, and osteoderms covering much of its body. Like aetosaurs and ''Revueltosaurus'', it has a maxilla that fits into the jugal. ''Revueltosaurus'', ''Turfanosuchus'', and the unnamed Tanzanian pseudosuchian are all good fits for the hypothesized ancestor of aetosaurs because they both have double rows of leaf-shaped osteoderms along their backs that could potentially have evolved into the tightly fitting paramedian osteoderms of aetosaurs.

Paleobiology

Early interpretations

In 1904, American paleontologist Henry Fairfield Osborn described aetosaurs as carnivorous aquatic animals of the order Parasuchia, mentioning that " arasuchiaconstitutes an independent order, probably freshwater, littoral, carnivorous, short snouted (Aëtosaurus) or long snouted (Phytosaurus, Mystriosuchus) forms, analogous in their habits to the modern Crocodilia". Early aetosaur remains were often found in clays beside skeletons of aquatic animals such as phytosaurs and terrestrial animals such as dinosaurs and trilophosaurs. This may have led some paleontologists to believe that the animals had died in swampy environments. Because there were a large number of skeletons of animals that would not normally have inhabited swamps in these clays, some paleontologists even suggested that aetosaurs scavenged off the carcasses of animals that became trapped in the swamps and died. Doubts were later raised over this lifestyle, since aetosaur teeth show little indication of carnivory and the weight of the armor suggests that aetosaurs had "a passive mode of life." However, aetosaurs were still regarded as partly aquatic into the mid-20th century.

Diet

Aetosaurs were herbivores, likely feeding on ferns and seed ferns that were common in the Triassic. The upturned shape of their snout suggests that aetosaurs may have dug up roots and tubers. Aetosaurs have several anatomical features that may have been adaptations to digging, including a short radius relative to the humerus (seen in many other digging tetrapods) and a large deltopectoral crest on the humerus that served as an attachment for muscles. Aetosaurs also have large hind feet, or pes, with large claws that were likely used for skratch-digging. One aetosaur, ''Typothorax'', has an entepicondyle on the humerus, which is the origin of forearm pronator and manual flexor muscles often used in digging. Moreover, it has a large olecranon process on the ulna which projects backward past the elbow, giving a large area for the insertion of the triceps muscle. While many studies have suggested that aetosaurs had a fossorial or burrowing lifestyle, aetosaurs have few of the characteristics that fossorial animals have as adaptations to digging. Therefore, it is likely that aetosaurs were able to dig to some extent, possibly rooting for food, but were unable to burrow.

While features of the limbs indicate that aetosaurs probably dug for food, features of the skull and teeth can indicate what kind of food they were eating. Aetosaurs have many derived features not seen in other crurotarsans, which indicate that they are adapted to a different diet. Unlike the sharp, recurved teeth of other triassic archosaurs, aetosaurs had simple, conical teeth. The tips of the jaws were edentulous, or toothless, and probably supported a beak. The teeth have very little wear, suggesting that aetosaurs did not consume stiff and tough plant material. It is more likely that they consumed non-abrasive vegetation such as soft leaves.

Alternative theories have been proposed for the diet of aetosaurs. In 1947, H J Sawin proposed that the aetosaur ''Longosuchus'' was a scavenger based on the close proximity of some specimens to a large number of skeletons that were likely carcasses. A 2009 study of the jaw biomechanics of the South American genus ''Neoaetosauroides'' suggested that the animal may have fed on larvae and insects without hard exoskeletons. This is because ''Neoaetosauroides'' lacks serrations or wear facets on the teeth and has a jaw leverage that is not designed for strong forces such as crushing and chopping. The study recognized that northern aetosaurs such as ''Desmatosuchus'' and ''Stagonolepis'' did have jaws that would have supported a strong musculature, and were likely better suited to eating plant material.

Nests

At least some aetosaurs built nests and protected their eggs.

In 1996, geologist Stephen Hasiotis discovered 220‑million-year-old, fossilized, bowl-like pits in Arizona's Petrified Forest, in part of the Chinle Formation, assumed to be aetosaur and phytosaur nests. The "nests" are compacted and appear very similar to the nests of the modern day crocodiles who guard their nests. However, it seems that these "nests" are instead the result of sandstone weathering.

A second possible aetosaur nest site is known from northeastern Italy. The nests are preserved as depressions in carbonate rock that are circular or horseshoe-shaped, with high ridges around the sides. They appear to be unusually complex for nests created by Triassic reptiles. Archosaur footprints were found nearby that resembled aetosaurs, although they were not present in the same layer. Because the tracks were found so close to the nests, it is likely that aetosaurs built them.

Development

The ages of individual aetosaurs can be determined by examining their osteoderms. Some isolated osteoderms have been claimed to belong to juvenile aetosaurs based on their size and shape but these hypotheses have often been questioned. For example, juvenile osteoderms of ''Calyptosuchus'' were later identified as those of the small-bodied pseudosuchian '' Revueltosaurus'' (which is not an aetosaur), and juvenile osteoderms of ''Desmatosuchus'' have been reinterpreted as those of aetosaur ''Acaenasuchus'', which had a relatively small body size at maturity. Studies of the bone structure of paramedian osteoderms indicate that new bone was deposited along the edges of each plate over the course of an aetosaur's lifetime. This means that lines of arrested growth on the undersides of paramedian osteoderms can be used to determine an individual's age. Comparing the ages of individual specimens with their total body lengths indicates that aetosaurs increased in length at relatively constant rates, but increased in body mass at different rates depending on whether they had wide bodies like ''Typothorax'' or narrow bodies like ''Aetobarbakinoides''. Aetosaurs also seem to have grown more slowly than modern crocodilians. Analysis of the limb bones of aetosaurs indicates that they grew quickly when young and more slowly when adults. This pattern of growth is seen in most other pseudosuchians.

Biochronology

Because species of aetosaurs typically have restricted fossil ranges and are abundant in the strata they are found in, they are useful in biochronology. Osteoderms are the most common remains associated with aetosaurs, so a single identifiable scute can accurately date the layer it is found in.

One aetosaur, ''Typothorax coccinarum'', has been used to define the Revueltian land vertebrate faunachron. A land vertebrate faunachron (LVF) is a time interval that is defined by the first appearance datum (FAD), or first occurrence, of a tetrapod index fossil and is commonly used to date Late Triassic and Early Jurassic terrestrial strata. Since the FAD of ''T. coccinarum'' is at the beginning of the Norian stage, the Revueltian LVF starts at the beginning of the Norian around 216 million years ago. The Revueltian ends with the next FAD, which happens to be that of the phytosaur '' Redondasaurus'' and the start of the Apachean LVF.

Biochrons for aetosaur genera have been developed for dating strata in the Chinle Group of the southwestern United States. Up to 13 genera of aetosaurs are known from the Chinle Group, with most occurring in multiple localities and over short time spans. In 1996, paleontologists Spencer G. Lucas and Andrew B. Heckert recognized five biochrons based on the presence of aetosaurs throughout the Chinle Group. The number of biochrons grew to 11 in a 2007 study by Heckert and Lucas along with Adrian P. Hunt