Aetosaur

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Aetosaurs
Temporal range:
Ma
Skeletal mount of Desmatosuchus
Scientific classification Edit this classification
Domain: Eukaryota
Kingdom: Animalia
Phylum: Chordata
Class: Reptilia
Clade: Archosauria
Clade: Pseudosuchia
Clade: †Aetosauriformes
Order: †Aetosauria
Lydekker, 1887
Subgroups

Aetosaurs (

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 in which they are found.[1][2][3]

Nearly all aetosaurs (except for the genus

Desmatosuchia and Aetosaurinae, based primarily on broad differences in skull morphology.[4] 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.[3]

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.[3]

Description

Skull anatomy

Aetosauroides scagliai

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

carnivorous archosaurs.[7]

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

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.[9][5][10] 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.[8][4][7]

Postcranial anatomy

Desmatosuchus spurensis

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 "

transverse processes (rib attachments) developed into long massive pedestals, and the largest species even acquired hyposphene-hypantrum reinforcements between their vertebrae.[3][4]

Although aetosaurs were generally wide-bodied reptiles, there is some variation in the degree of this trend. The

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.[11] Desmatosuchus was likely one of the largest known aetosaurs, at 4–6 m (13–20 ft) in length and 280 kg (620 lb) in weight.[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.[3] 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.[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.[14] Others were larger, such as the basal-most aetosaur Aetosauroides and the early desmatosuchine Calyptosuchus.[3]

Armor

Skeletal diagram of Stagonolepis robertsoni, showing osteoderm terminology

Aetosaurs were very heavily armored, with rows of large, interlocking bony plates, known as

cancellous or spongy bone (also called diploë) and their outer portions are made up of compact bone.[3] In life, these plates were probably covered in a keratinous (horn) covering, like modern crocodilian scutes, which are another example of pseudosuchian osteoderms.[15] 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.[16][17][3]

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.[8][3][4][18]

Typothorax coccinarum

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.[19] This is taken to an extreme in desmatosuchines such as Longosuchus and Desmatosuchus, where the spike is enlarged into a sharply curved horn.[8]

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.[3][20]

History

Early European finds

Aetosaur material was first described by

lobe-finned fish Megalichthys due to their large size.[21]

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.[22]

The articulated skeletons of 22 Aetosaurus, discovered near Stuttgart, Germany and first described by Oskar Fraas in 1877. The skeletons are now in the State Museum of Natural History Stuttgart.

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.[23]

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.[24] Another species, E. haplocerus, was reassigned to Desmatosuchus in 1953.[25]

Stegomus
, described by Marsh in 1896

A third North American genus called Stegomus was named by Othniel Charles Marsh in 1896.[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. 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.[17] Marsh considered aetosaurs to be closely related to dinosaurs based on their elongated metatarsals (foot bones).[24]

As a distinct group, Aetosauria was named in 1889 by English

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.[24]

20th century updates

Aetosaur material continued to be described into the early 20th century, with notable paleontologists such as

E.C. Case in 1922) and "Typothorax" meadei (now Longosuchus, described by H.J. Sawin in 1947).[28] 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.[29]

Distribution

body fossils
have been found in North America, South America, Europe, and parts of Asia and Africa.

Aetosaur fossils have been found on all continents except

Timesgadiouine Formation in Morocco.[34][35] 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.[14]

trackways.

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.[36][1] In Brazil, fossils have been found in the Santa Maria and Caturrita Formations in Rio Grande do Sul (Paleorrota).[14][36] Chilean aetosaurs are represented by one fragmentary genus, Chilenosuchus, from the Antofagasta Region.[37] 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.[38] Reports of aetosaurs from Madagascar[39][40] are based on probable crocodylomorph scutes.[41] Supposed aetosaurian footprints and skeletal material from South Africa are also unsubstantiated.[42]

ichnogenus Brachychirotherium are often associated with aetosaurs.[43][44] Brachychirotherium are known from Rio Grande do Sul in Paleorrota, Brazil[36] as well as Italy,[45] Germany,[46] the eastern United States,[47] 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.[31] 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.[48]

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.[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. 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.[49]

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.[8] 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.[50]

List of genera

Genus Status Age Location Unit Notes Images

Acaenasuchus

Non-aetosaurian middle Norian (Adamanian)

 USA (Arizona)

Chinle Formation (Blue Mesa Member, Sonsela Member)

A valid species of non-aetosaur aetosauriform, previously considered a valid aetosaur or juvenile specimens of Desmatosuchus spurensis.

Acompsosaurus

Nomen dubium Late Triassic (Adamanian)

 USA (New Mexico)

Chinle Group (Bluewater Creek Formation)

A dubious aetosaur based on a pelvis which is now lost. Possibly a junior synonym of Stagonolepis or Calyptosuchus.

Adamanasuchus

Valid early - middle Norian (Adamanian)

 USA (Arizona)

Chinle Formation (Blue Mesa Member)

A large desmatosuchian, closely related to Calyptosuchus and Scutarx.

Aetobarbakinoides

Valid late Carnian - early Norian

 Brazil

Santa Maria Formation

An aetosaur with aetosaurine-like osteoderms and desmatosuchine-like vertebrae.

Aetosauroides

Valid Carnian - Norian

 Argentina,
 Brazil

Ischigualasto Formation,
Santa Maria Formation

Currently considered the most basal aetosaur, as well as the most well-studied member of the group from Gondwana. It may have had a more carnivorous diet than most other aetosaurs.

Aetosaurus

Valid

middle Norian - early Rhaetian? (Revueltian)

 Germany,
 Italy,
 Greenland

Lower Stubensandtein
,
Calcare di Zorzino Formation
,
Fleming Fjord Formation

A small (1.5 m, 5 ft long) aetosaurine aetosaur with a wide geographic distribution in Europe.

Apachesuchus

Valid late Norian - Rhaetian (Apachean)

 USA

Redonda Formation

A large, late-surviving probable typothoracine known solely from smooth osteoderms.

Argentinosuchus

 Nomen dubium Carnian - Norian

 Argentina

Ischigualasto Formation

An indeterminate aetosaur similar to Aetosauroides and Stagonolepis

Calyptosuchus

Valid early - middle Norian (Adamanian)

 USA (Arizona, Texas)

Chinle Formation (Blue Mesa Member), Tecovas Formation

A desmatosuchian similar to Stagonolepis, which it was formerly considered to be a species of.

Chilenosuchus

Valid Late Triassic

 Chile

El Bordo Formation

A poorly-known aetosaur similar to Typothorax. It was found in strata that were originally thought to be Carboniferous or Permian in age, rather than Triassic.

Coahomasuchus

Valid Carnian? (Otischalkian)

 USA (Texas, North Carolina)

Colorado City Formation,
Pekin Formation

A small but wide-bodied aetosaur closely related to typothoracines.

Desmatosuchus

Valid early - middle Norian (Adamanian - earliest Revueltian?)

 USA (Texas, Arizona, New Mexico)

Cooper Canyon Formation,

Tecovas Formation,

Santa Rosa Formation,

Chinle Formation (Blue Mesa Member, Sonsela Member)

Namesake of the desmatosuchines, one of the largest, most common, and most impressively-armored aetosaurs.

Garzapelta

Valid Norian? (Adamanian - Revueltian)

 USA (Texas)

Cooper Canyon Formation

A possible paratypothoracine that possessed lateral osteoderms taht converged with those of desmatosuchines

Gorgetosuchus

Valid Carnian? (Otischalkian)

 USA (North Carolina)

Pekin Formation

A possible desmatosuchin characterized by cervical osteoderms that almost encircle the neck

Heliocanthus

 Junior synonym Junior synonym of Rioarribasuchus
Kocurypelta Valid Late Triassic  Poland Grabowa Formation A paratypothoracin diagnosed from skull fragments
Kryphioparma Valid early - middle Norian (Adamanian)  USA (Arizona) Chinle Formation (Blue Mesa Member) A typhothoracine based on fossils previously referred to Tecovasuchus

Longosuchus

Valid Carnian? (Otischalkian)

 USA (Texas)

Colorado City Formation

 A well-described desmatosuchin, the subject of major studies on aetosaur anatomy in the mid-20th century.

Lucasuchus

Valid Carnian? (Otischalkian)

 USA (Texas, North Carolina)

Colorado City Formation,
Pekin Formation

A desmatosuchin often conflated with Longosuchus, which it coexisted with.

Neoaetosauroides

Valid middle Norian (Revueltian)

 Argentina

Los Colorados Formation

A late-surviving basal desmatosuchian with apparent adaptations for a more carnivorous diet than most other aetosaurs.

Paratypothorax

Valid Norian

(Adamanian - Revueltian)

 Germany,
 USA (Arizona, New Mexico, Texas),
 Greenland

Lower Stubensandstein
,
Chinle Formation,

Dockum Group,
Fleming Fjord Formation

 The widespread and well-known namesake of the clade Paratypothoracini. Some authors have suggested that it represents an adult form of Aetosaurus.
Polesinesuchus Valid? late Carnian - early Norian  Brazil Santa Maria Formation A small aetosaur based on an immature specimen, which may be referrable to Aetosauroides.

Redondasuchus

Valid late Norian - Rhaetian (Apachean)

 USA (New Mexico)

Redonda Formation

A large typothoracinae often considered a close relative or species of Typothorax.

Rioarribasuchus

Valid middle - late Norian (Revueltian)

 USA (New Mexico, Arizona)

Chinle Formation (Petrified Forest Member, Sonsela Member)

A narrow-bodied paratypothoracin with elongated spines on paramedian osteoderms above the hip.

Scutarx

Valid middle Norian (Adamanian)

 USA (Arizona, Texas)

Chinle Formation (Sonsela Member),
Cooper Canyon Formation

A strongly-armored desmatosuchian closely related to Calyptosuchus.

Sierritasuchus

Valid early - middle Norian (Adamanian)

 USA (Texas)

Tecovas Formation

A desmatosuchin known from an immature specimen originally assigned to Desmatosuchus.

Stagonolepis

Valid Carnian

 Scotland,
 Poland

Lossiemouth Sandstone,
Krasiejów deposits

The first aetosaur to be described, previously used as a wastebasket taxon for other narrow-bodied basal aetosaurs.

Stegomus

 Junior synonym or Nomen dubium Norian (Revueltian)  USA (Connecticut, New Jersey, North Carolina) New Haven Arkose, Passaic Formation, Sanford Formation A small aetosaur only known from interior molds of a dorsal carapace. Often interpreted as a dubious genus or a junior synonym of Aetosaurus[32]

Stenomyti

Valid late Norian (Revueltian)

 USA (Colorado)

Chinle Formation (red siltstone member)

A small basal aetosaurine originally assigned to Aetosaurus, a close relative.

Tecovasuchus Valid early - middle Norian (Adamanian)  USA (Texas, New Mexico, Arizona?) Tecovas Formation,

Chinle Group (Bluewater Creek Formation, Blue Mesa Member?)

A typical wide-bodied paratypothoracin

Typothorax

Valid

middle - late Norian (latest Adamanian - Revueltian)

 USA (Arizona, Texas, New Mexico)

Chinle Formation (Sonsela Member, Petrified Forest Member),

Cooper Canyon Formation,

Bull Canyon Formation
,

An abundant wide-bodied typothoracine, known from multiple articulated skeletons.

Venkatasuchus[51]

Valid

middle Norian to Rhaetian

 India

Lower Dharmaram Formation

An wide-bodied typothoracine, known from a series of associated paramedian and lateral osteoderms as well as an isolated paramedian osteoderm.

Phylogeny

Cladogram from Parrish (1994)[52]


Rauisuchiformes 

Prestosuchidae

 Aetosauria 

Aetosaurus

Aetosauroides

Stagonolepis

Longosuchus

Desmatosuchus

Typothorax

Paratypothorax

Neoaetosauroides

Aetosaur

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.[52]

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

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.[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. 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.[53]

More recently, a 2007 analysis by paleontologist

stem-based taxon, claiming that Aetosauria included all crurotarsans that were more closely related to Desmatosuchus than to the immediate sister group of Aetosauria.[54] 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.[8]

A new genus of aetosaur,

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.[55]

Aetosauria

Aetosauroides

Stagonolepididae
Aetosaurinae
Stagonolepidinae

Calyptosuchus wellesi

Stagonolepis robertsoni

Desmatosuchinae

In 2016, William Parker conducted a new phylogenetic analysis of the Aetosauria, proposing an alternative hypothesis of aetosaur relationships. Below is the cladogram:[4]

 Aetosauria 

Aetosauroides scagliai

 
Stagonolepididae
 
 Aetosaurinae 

Stenomyti huangae

Aetosaurus ferratus

Coahomasuchus kahleorum

 Typothoracinae 

Apachesuchus heckerti

Typothorax coccinarum

Redondasuchus rineharti

 
Paratypothoracisini
 

Rioarribasuchus chamaensis

SMSN 19003

Tecovasuchus chatterjeei

Paratypothorax sp.

Paratypothorax andressorum

 Desmatosuchia 
 Stagonolepidinae 

Polesinesuchus aurelioi

Stagonolepis robertsoni

 Desmatosuchinae 

"Stagonolepis" olenkae

Neoaetosauroides engaeus

Calyptosuchus wellesi

Adamanasuchus eisenhardtae

Scutarx deltatylus

 Desmatosuchini 

Gorgetosuchus pekinensis

Longosuchus meadei

Sierritasuchus macalpini

Lucasuchus hunti

Desmatosuchus smalli

Desmatosuchus spurensis

Origin and evolution

Life restoration of Revueltosaurus callenderi, a pseudosuchian that may be close to the ancestry of aetosaurs.

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

stem aetosaur" ("stem" meaning that it lies on the branch that includes aetosaurs, but is not itself an aetosaur).[56] However, other studies have considered Turfanosuchus part of a different family of pseudosuchians, the gracilisuchids.[57]

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.[56]

Paleobiology

Early interpretations

In 1904, American paleontologist

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.[28]

Restorations of Stagonolepis robertsoni, after skeletal by Hartman, 2016

Diet

Aetosaurs were herbivores, likely feeding on

olecranon process on the ulna which projects backward past the elbow, giving a large area for the insertion of the triceps muscle.[44] While many studies have suggested that aetosaurs had a fossorial or burrowing lifestyle,[52] 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.[44]

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.[52]

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.[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. 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.[60][61]

Nests

Life restoration of Desmatosuchus haplocerus. This is likely the animal that made the nests discovered there in 1996.

At least some aetosaurs built nests and protected their eggs.[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. The "nests" are compacted and appear very similar to the nests of the modern day crocodiles who guard their nests.[59] However, it seems that these "nests" are instead the result of sandstone weathering.[63]

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.[62]

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

lines of arrested growth on the undersides of paramedian osteoderms can be used to determine an individual's age.[3] 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.[64] 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.[3]

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

Apachean LVF.[2][69]

Biochrons for aetosaur genera have been developed for dating strata in the

Otischalkian LVF to the Apachean LVF and included genera such as Longosuchus, Tecovasuchus, and Typothorax.[66]

"Aetogate" naming controversy

Main articles: Rioarribasuchus and Redondasuchus

In 2007, paleontologists at the

Mike Taylor, Matt Wedel, and Darren Naish, who claimed that Lucas et al. knew that Parker would eventually redescribe the species and formally erect a new genus. According to Martz, Taylor, Wedel, and Naish, the authors rushed to publish their own name before Parker could publish his.[73]
At the time, Lucas, Hunt, and Spielmann were on the editorial board of the NMMNHS bulletin, while Hunt was also the museum director.

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.[74] In 2002, Jeff Martz came to the same conclusion in an unpublished thesis.[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.[73]

The allegations were rejected in an internal NMMNHS meeting, reported by Lucas in 2008.[76][77][78] They were also brought to the attention of an independent party, the Ethics Education Committee of the Society of Vertebrate Paleontology (SVP) in 2007, and a response was given in 2008. In regard to Redondasuchus, the SVP found no explicit evidence for plagiarism. 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. The SVP's response concluded with an update to its ethics policy and recommendations for how similar controversies could be better handled in the future.[79][80][81] The entire controversy came to be known as "Aetogate", in reference to the famous Watergate scandal of the 1970s.[73] It received wide attention from local Albuquerque newspapers and science blogs.[82] It was also the focus of a news article in a 2008 issue of the journal Nature.[83]

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General references

  • Vertebrate Paleontology
    , 2nd ed. Blackwell Science Ltd.

External links

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