Aetosaurs (/eɪˌɛtoʊˈsɔːr/) are heavily armored reptiles belonging to the extinct order Aetosauria (/eɪˌɛtoʊˈsɔːriə/; from Greek, ἀετός (aetos, "eagle") and σαυρος (sauros, "lizard")).

The hindlimbs developed a "pillar-erect" limb posture similar to that seen in "rauisuchians", a related grade of carnivorous Triassic pseudosuchians ancestral to crocodylomorphs.

[1] 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.

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 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.

[12][3][13] 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.

[4] 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.

[3] Aetosaurs were very heavily armored, with rows of large, interlocking bony plates, known as osteoderms, protecting the back, sides, belly, and tail.

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.

[26] 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.

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.

Early accounts of Indian aetosaurs were based on material from the Maleri Formation in south-central India, although most of these remains were too inadequate to assign specimens to any particular genus.

[dubious – discuss] Aetosaurs were traditionally referred to a (now obsolete) group called the thecodonts, which included all "primitive" crocodilian relatives that lived in the Triassic.

Prestosuchidae Aetosaurus Aetosauroides Stagonolepis Longosuchus Desmatosuchus Typothorax Paratypothorax Neoaetosauroides Aetosaur phylogeny was first investigated in 1994 by paleontologist J. Michael Parrish.

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.

[14] 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.

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.

[55] Aetosauroides Coahomasuchus Aetosaurus Gorgetosuchus Redondasuchus Typothorax Tecovasuchus Rioarribasuchus Paratypothorax 19003 Paratypothorax type Calyptosuchus wellesi Stagonolepis robertsoni Neoaetosauroides Polesinesuchus Aetobarbakinoides Longosuchus Sierritasuchus Acaenasuchus Desmatosuchus smalli Desmatosuchus spurensis In 2016, William Parker conducted a new phylogenetic analysis of the Aetosauria, proposing an alternative hypothesis of aetosaur relationships.

Below is the cladogram:[4] Aetosauroides scagliai Stenomyti huangae Aetosaurus ferratus Coahomasuchus kahleorum Apachesuchus heckerti Typothorax coccinarum Redondasuchus rineharti Rioarribasuchus chamaensis

Paratypothorax andressorum Polesinesuchus aurelioi Stagonolepis robertsoni "Stagonolepis" olenkae Neoaetosauroides engaeus Calyptosuchus wellesi Adamanasuchus eisenhardtae Scutarx deltatylus Gorgetosuchus pekinensis Longosuchus meadei Sierritasuchus macalpini Lucasuchus hunti Desmatosuchus smalli Desmatosuchus spurensis 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.

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.

[56] In 1904, American paleontologist Henry Fairfield Osborn described aetosaurs as carnivorous aquatic animals of the order Parasuchia, mentioning that "[Parasuchia] constitutes an independent order, probably freshwater, littoral, carnivorous, short snouted (Aëtosaurus) or long snouted (Phytosaurus, Mystriosuchus) forms, analogous in their habits to the modern Crocodilia".

[28] 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.

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.

[62] 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.

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.

[71] However, four years earlier paleontologist William Parker reassigned "D." chamaensis to the newly named genus Heliocanthus in an unpublished thesis, which was widely disseminated among aetosaur researchers.

A coinciding controversy occurred after Spielmann, Hunt, and Lucas published a 2006 paper mentioning that the holotype of Redondasuchus was not a left paramedian, but instead a right one.

[75] He, along with Taylor, Wedel, and Naish, claimed that this was another form of plagiarism, as Martz's 2002 thesis was cited by Spielmann et al. (2006), even if his conclusion on Redondasuchus was not mentioned.

In the case of Heliocanthus and Rioarribasuchus, the SVP did not try to resolve the issue, as Lucas et al. and Parker offered conflicting accounts regarding communication and intent.