''Azendohsaurus'' is an

extinct Extinction is the termination of an organism by the death of its Endling, last member. A taxon may become Functional extinction, functionally extinct before the death of its last member if it loses the capacity to Reproduction, reproduce and ...

genus Genus (; : genera ) is a taxonomic rank above species and below family (taxonomy), family as used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In bino ...

herbivorous A herbivore is an animal anatomically and physiologically evolved to feed on plants, especially upon vascular tissues such as foliage, fruits or seeds, as the main component of its diet. These more broadly also encompass animals that eat n ...

archosauromorph Archosauromorpha (Greek for "ruling lizard forms") is a clade of diapsid reptiles containing all reptiles more closely related to archosaurs (such as crocodilians and dinosaurs, including birds) than to lepidosaurs (such as tuataras, lizards, a ...

reptile Reptiles, as commonly defined, are a group of tetrapods with an ectothermic metabolism and Amniotic egg, amniotic development. Living traditional reptiles comprise four Order (biology), orders: Testudines, Crocodilia, Squamata, and Rhynchocepha ...

from roughly the late

Middle Middle or The Middle may refer to: * Centre (geometry), the point equally distant from the outer limits. Places * Middle (sheading), a subdivision of the Isle of Man * Middle Bay (disambiguation) * Middle Brook (disambiguation) * Middle Creek ...

to early

Late Late or LATE may refer to: Everyday usage * Tardy, or late, not being on time * Late (or the late) may refer to a person who is dead Music * ''Late'' (The 77s album), 2000 * Late (Alvin Batiste album), 1993 * Late!, a pseudonym used by Dave Groh ...

Triassic The Triassic ( ; sometimes symbolized 🝈) is a geologic period and system which spans 50.5 million years from the end of the Permian Period 251.902 million years ago ( Mya), to the beginning of the Jurassic Period 201.4 Mya. The Triassic is t ...

Period of

Morocco Morocco, officially the Kingdom of Morocco, is a country in the Maghreb region of North Africa. It has coastlines on the Mediterranean Sea to the north and the Atlantic Ocean to the west, and has land borders with Algeria to Algeria–Morocc ...

and

Madagascar Madagascar, officially the Republic of Madagascar, is an island country that includes the island of Madagascar and numerous smaller peripheral islands. Lying off the southeastern coast of Africa, it is the world's List of islands by area, f ...

. The

type species In International_Code_of_Zoological_Nomenclature, zoological nomenclature, a type species (''species typica'') is the species name with which the name of a genus or subgenus is considered to be permanently taxonomically associated, i.e., the spe ...

, ''Azendohsaurus laaroussii'', was described and named by Jean-Michel Dutuit in

1972 Within the context of Coordinated Universal Time (UTC) it was the longest year ever, as two leap seconds were added during this 366-day year, an event which has not since been repeated. (If its start and end are defined using Solar time, ...

based on partial jaw fragments and some teeth from Morocco. A second species from Madagascar, ''A. madagaskarensis'', was first described in

2010 The year saw a multitude of natural and environmental disasters such as the 2010 Haiti earthquake, the Deepwater Horizon oil spill, and the 2010 Chile earthquake. The 2009 swine flu pandemic, swine flu pandemic which began the previous year ...

by John J. Flynn and colleagues from a multitude of specimens representing almost the entire skeleton. The generic name "Azendoh lizard" is for the village of Azendoh, a local village near where it was first discovered in the

Atlas Mountains The Atlas Mountains are a mountain range in the Maghreb in North Africa. They separate the Sahara Desert from the Mediterranean Sea and the Atlantic Ocean; the name "Atlantic" is derived from the mountain range, which stretches around through M ...

. It was a bulky quadruped that unlike other early archosauromorphs had a relatively short tail and robust limbs that were held in an odd mix of sprawled hind limbs and raised forelimbs. It had a long neck and a proportionately small head with remarkably sauropod-like jaws and teeth. ''Azendohsaurus'' used to be classified as a herbivorous

dinosaur Dinosaurs are a diverse group of reptiles of the clade Dinosauria. They first appeared during the Triassic Geological period, period, between 243 and 233.23 million years ago (mya), although the exact origin and timing of the #Evolutio ...

, at first as an

ornithischian Ornithischia () is an extinct clade of mainly herbivorous dinosaurs characterized by a pelvic structure superficially similar to that of birds. The name ''Ornithischia'', or "bird-hipped", reflects this similarity and is derived from the Greek st ...

but more often as a "

prosauropod Sauropodomorpha ( ; from Greek, meaning "lizard-footed forms") is an extinct clade of long-necked, herbivorous, saurischian dinosaurs that includes the Sauropoda, sauropods and their ancestral relatives. Sauropods generally grew to very large siz ...

sauropodomorph Sauropodomorpha ( ; from Greek, meaning "lizard-footed forms") is an extinct clade of long-necked, herbivorous, saurischian dinosaurs that includes the sauropods and their ancestral relatives. Sauropods generally grew to very large sizes, had lo ...

. This was based only on its jaws and teeth, which share derived features typically found in herbivorous dinosaurs. The complete skeletal material from Madagascar, however, revealed more basal characteristics

ancestral An ancestor, also known as a forefather, fore-elder, or a forebear, is a parent or ( recursively) the parent of an antecedent (i.e., a grandparent, great-grandparent, great-great-grandparent and so forth). ''Ancestor'' is "any person from ...

to Archosauromorpha and that ''Azendohsaurus'' was not a dinosaur at all. Instead, ''Azendohsaurus'' was actually a more primitive archosauromorph that had

convergently evolved Convergent evolution is the independent evolution of similar features in species of different periods or epochs in time. Convergent evolution creates analogous structures that have similar form or function but were not present in the last comm ...

many features of the jaws and skeleton shared with the later giant

sauropod Sauropoda (), whose members are known as sauropods (; from '' sauro-'' + '' -pod'', 'lizard-footed'), is a clade of saurischian ('lizard-hipped') dinosaurs. Sauropods had very long necks, long tails, small heads (relative to the rest of their b ...

dinosaurs. It was found to be a member of a newly recognised group of specialised, mostly herbivorous archosauromorphs that was named the

Allokotosauria Allokotosauria is a clade of early archosauromorph reptiles from the Middle to Late Triassic known from Asia, Africa, North America and Europe. Allokotosauria was first described and named when a new monophyletic grouping of specialized herbivor ...

. It is also the namesake and typifier of its own

family Family (from ) is a Social group, group of people related either by consanguinity (by recognized birth) or Affinity (law), affinity (by marriage or other relationship). It forms the basis for social order. Ideally, families offer predictabili ...

of allokotosaurs, the

Azendohsauridae Azendohsauridae is a family of allokotosaurian archosauromorphs that lived during the Middle to Late Triassic period, around 242-216 million years ago. The family was originally named solely for the eponymous ''Azendohsaurus'', marking out its di ...

; initially the only member, the family now includes other similar allokotosaurs, such as the larger, horned azendohsaurid ''

Shringasaurus ''Shringasaurus'' (meaning "horned lizard", from Sanskrit शृङ्ग (''śṛṅga),'' "horn", and Ancient Greek (''sauros),'' "lizard") is an extinct genus of archosauromorph reptile from the Middle Triassic ( Anisian) of India. It is kn ...

'' from

India India, officially the Republic of India, is a country in South Asia. It is the List of countries and dependencies by area, seventh-largest country by area; the List of countries by population (United Nations), most populous country since ...

. Several other groups of archosauromorphs also adapted to herbivory in the

, sometimes with dinosaur-like teeth that also caused confusion in their classification. ''Azendohsaurus'' is notable, however, for also convergently evolving a similar body shape to sauropodomorphs in addition to its jaws and teeth. ''Azendohsaurus'' and sauropodomorphs likely independently evolved to fill a similar

ecological niche In ecology, a niche is the match of a species to a specific environmental condition. Three variants of ecological niche are described by It describes how an organism or population responds to the distribution of Resource (biology), resources an ...

as long-necked, relatively high browsing herbivores in their environments. However, ''Azendohsaurus'' predates the large Late Triassic sauropodomorphs it resembles by several million years, and did not evolve similar body plans under the same environmental conditions. It may then have been one of the first herbivores to fill the high-browsing role that only large sauropodomorphs were thought to occupy during the Triassic, expanding the known ecological diversity of herbivorous archosauromorphs outside of dinosaurs in the Triassic Period. ''Azendohsaurus'' is also significant as it may be one of the earliest

endothermic An endothermic process is a chemical or physical process that absorbs heat from its surroundings. In terms of thermodynamics, it is a thermodynamic process with an increase in the enthalpy (or internal energy ) of the system.Oxtoby, D. W; Gillis, ...

archosauromorphs known, and suggests that a warm-blooded metabolism was ancestral to the later archosaurs, including the dinosaurs.

Description

''Azendohsaurus'' was a stocky mid-sized reptile estimated to be roughly long. It had a small, box-shaped head with a short snout on a long neck that was raised above the shoulders. The body was broad, with a barrel-shaped chest and shoulders much taller than the hips, together with an unusually short tail. Its posture was semi-sprawled, with sprawling hind limbs and slightly elevated forelimbs. The limbs themselves are relatively short and particularly robust, with digits that are shorter and stouter compared to other early archosauromorphs, each with notably large, curved claws on all four feet. Superficially its appearance is comparable to that of

dinosaurs, along with various details of its skeleton, suggesting ''Azendohsaurus'' converged on similar traits for a relatively high-browsing, herbivorous lifestyle. ''A. laaroussii'' is poorly known compared to ''A. madagaskarensis'', and the two species are only known to differ in minor details of the jaw bones and teeth. Additional skeletal material of ''A. laaroussii'' has been reported from the type locality of the original skull fragments, but have yet to be formally described as of 2015.

Skull

The skull of ''A. madagaskarensis'' is almost completely known, and is robustly built with a short and boxy shape and a deep

snout A snout is the protruding portion of an animal's face, consisting of its nose, mouth, and jaw. In many animals, the structure is called a muzzle, Rostrum (anatomy), rostrum, beak or proboscis. The wet furless surface around the nostrils of the n ...

. The

premaxilla The premaxilla (or praemaxilla) is one of a pair of small cranial bones at the very tip of the upper jaw of many animals, usually, but not always, bearing teeth. In humans, they are fused with the maxilla. The "premaxilla" of therian mammals h ...

e are gently curved at the front of the upper jaw, forming a blunt, round snout tip, while the lower jaws have a deep, down-turned tip like those of sauropods. The bony nostrils are fused into a single (confluent) opening that faces forwards at the front of the snout, similar to those of

rhynchosaur Rhynchosaurs are a group of extinct herbivorous Triassic archosauromorph reptiles, belonging to the order Rhynchosauria. Members of the group are distinguished by their triangular skulls and elongated, beak like premaxillary bones. Rhynchosaurs ...

s. The skull has a number of traits convergent with sauropodomorphs, including the downward curving

dentary In jawed vertebrates, the mandible (from the Latin ''mandibula'', 'for chewing'), lower jaw, or jawbone is a bone that makes up the lowerand typically more mobilecomponent of the mouth (the upper jaw being known as the maxilla). The jawbone ...

, a robust dorsal process of the

maxilla In vertebrates, the maxilla (: maxillae ) is the upper fixed (not fixed in Neopterygii) bone of the jaw formed from the fusion of two maxillary bones. In humans, the upper jaw includes the hard palate in the front of the mouth. The two maxil ...

, and several features of the teeth. The process on the maxilla usually indicates the presence of an

antorbital fenestra An antorbital fenestra (plural: fenestrae) is an opening in the skull that is in front of the eye sockets. This skull character is largely associated with Archosauriformes, archosauriforms, first appearing during the Triassic Period. Among Extant ...

archosauriforms Archosauriformes (Greek for 'ruling lizards', and Latin for 'form') is a clade of diapsid reptiles encompassing archosaurs and some of their close relatives. It was defined by Jacques Gauthier (1994) as the clade stemming from the last common anc ...

, but in ''Azendohsaurus'' this space is occupied by the

lacrimal bone The lacrimal bones are two small and fragile bones of the facial skeleton; they are roughly the size of the little fingernail and situated at the front part of the medial wall of the orbit. They each have two surfaces and four borders. Several bon ...

in front of the eyes. This is a unique arrangement unknown in other Triassic archosauromorphs, except for the related ''Shringasaurus''. The

orbits In celestial mechanics, an orbit (also known as orbital revolution) is the curved trajectory of an physical body, object such as the trajectory of a planet around a star, or of a natural satellite around a planet, or of an satellite, artificia ...

are almost entirely occupied by the large

sclerotic ring The scleral ring or sclerotic ring is a hardened ring of plates, often derived from bone, that is found in the eyes of many animals in several groups of vertebrates. Some species of mammals, amphibians, and crocodilians lack scleral rings. The rin ...

s, suggesting large eyes. The lower temporal fenestra is open at the bottom, separating the

jugal The jugal is a skull bone found in most reptiles, amphibians and birds. In mammals, the jugal is often called the malar or zygomatic. It is connected to the quadratojugal and maxilla, as well as other bones, which may vary by species. Anatomy ...

and the

quadratojugal The quadratojugal is a skull bone present in many vertebrates, including some living reptiles and amphibians. Anatomy and function In animals with a quadratojugal bone, it is typically found connected to the jugal (cheek) bone from the front and ...

bones (a primitive trait for archosauromorphs). Also like other early archosauromorphs, ''Azendohsaurus'' has a small (3–5 mm across) parietal foramen ("third eye") on the roof of the skull. Azendohsaurus madagaskarensis pterygoid

The lower jaw is especially convergent with those of sauropodomorphs, with an

articular The articular bone is part of the lower jaw of most vertebrates, including most jawed fish, amphibians, birds and various kinds of reptiles, as well as ancestral mammals. Anatomy In most vertebrates, the articular bone is connected to two o ...

joint where the jaw hinges positioned below the level of the tooth row and downward curving dentaries, as well as the similarly shaped teeth. These features are variously found in other herbivorous Triassic archosauromorphs, but this combination is only known in ''Azendohsaurus'' and sauropodomorphs. The teeth are all roughly leaf-shaped (lanceolate) with expanded crowns and bulbous bases that are fused to the jaw bones (ankylothecodont). However, the upper and lower teeth are distinctly

heterodont In anatomy, a heterodont (from Greek, meaning 'different teeth') is an animal which possesses more than a single tooth morphology. Human dentition is heterodont and diphyodont as an example. In vertebrates, heterodont pertains to animals wher ...

, and can be readily distinguished from each other. The upper teeth are relatively short and broad at their base, with 4–6 denticles on each surface, similar to ornithischians; the lower teeth are almost twice as tall and have twice as many denticles, more closely resembling the teeth of sauropodomorphs. The four premaxillary teeth are the longest teeth in the upper jaw, and are more recurved back in shape than the rest. Azendohsaurus madagaskarensis maxilla medial

Azendohsaurus madagaskarensis maxilla medial

The

palate The palate () is the roof of the mouth in humans and other mammals. It separates the oral cavity from the nasal cavity. A similar structure is found in crocodilians, but in most other tetrapods, the oral and nasal cavities are not truly sep ...

is unusually covered in numerous fully developed palatal teeth, with up to four sets on the

pterygoid Pterygoid, from the Greek for 'winglike', may refer to: * Pterygoid bone, a bone of the palate of many vertebrates * Pterygoid processes of the sphenoid bone ** Lateral pterygoid plate ** Medial pterygoid plate * Lateral pterygoid muscle * Medial ...

and additional rows on the

palatine A palatine or palatinus (Latin; : ''palatini''; cf. derivative spellings below) is a high-level official attached to imperial or royal courts in Europe since Roman Empire, Roman times.

and

vomer The vomer (; ) is one of the unpaired facial bones of the skull. It is located in the midsagittal line, and articulates with the sphenoid, the ethmoid, the left and right palatine bones, and the left and right maxillary bones. The vomer forms ...

s. Mature ''Azendohsaurus madagaskarensis'' have at least 44 pairs of palatal teeth, in addition to the 4 teeth in each premaxilla and 11–13 in the maxilla each, along with a maximum of 17 teeth in the dentary. Palatal teeth are not uncommon in herbivorous reptiles, but in ''Azendohsaurus'' they are almost identical in shape to those along the jaw margins, but a bit stouter. Other archosauromorphs with palatal teeth have a much simpler palatal dentition of small, domed teeth. ''

Teraterpeton ''Teraterpeton'' (meaning "wonderful creeping thing" in Greek (language), Greek) is an extinct genus of trilophosaurid archosauromorphs. It is known from a partial skeleton from the Late Triassic Wolfville Formation of Nova Scotia, described in ...

'' is the only other archosauromorph with similarly well developed palatal teeth. The only described material of ''A. laaroussii'' are dentaries, maxillae, a premaxilla and several teeth. They broadly resemble ''A. madagaskarensis'' in general form but with a few distinguishing differences. The tooth count of ''A. laaroussii'' is higher, with 15–16 teeth in the maxilla compared to the 11–13 of ''A. madagaskarensis''. The teeth of ''A. laaroussii'' are also taller than those of ''A. madagaskarensis'' and have more closely packed denticles. Further distinguishing the two species is the presence of a prominent keel on the inside surface of the maxilla. This keel runs the whole length of the maxilla in ''A. laaroussii'', but is only found along the back half of it in ''A. madagaskarensis''. Any other possible differences between the two species cannot be determined without rest of the skull and skeleton.

Skeleton

All the known post-cranial information for the skeleton of ''Azendohsaurus'' comes from ''A. madagaskarensis''. Much of the

vertebral column The spinal column, also known as the vertebral column, spine or backbone, is the core part of the axial skeleton in vertebrates. The vertebral column is the defining and eponymous characteristic of the vertebrate. The spinal column is a segmente ...

is known in ''Azendohsaurus'', and although incomplete, it is estimated to have 24 presacral vertebrae (including the

atlas An atlas is a collection of maps; it is typically a bundle of world map, maps of Earth or of a continent or region of Earth. Advances in astronomy have also resulted in atlases of the celestial sphere or of other planets. Atlases have traditio ...

and

axis An axis (: axes) may refer to: Mathematics *A specific line (often a directed line) that plays an important role in some contexts. In particular: ** Coordinate axis of a coordinate system *** ''x''-axis, ''y''-axis, ''z''-axis, common names ...

). The

sacrum The sacrum (: sacra or sacrums), in human anatomy, is a triangular bone at the base of the spine that forms by the fusing of the sacral vertebrae (S1S5) between ages 18 and 30. The sacrum situates at the upper, back part of the pelvic cavity, ...

of the hips has only two vertebrae, and the full number of

caudal vertebrae Caudal vertebrae are the vertebrae of the tail in many vertebrates. In birds, the last few caudal vertebrae fuse into the pygostyle, and in apes, including humans, the caudal vertebrae are fused into the coccyx. In many reptiles, some of the caud ...

in the tail is unknown, but it is estimated to be only around 45–55 (low for an archosaur). The

cervical vertebrae In tetrapods, cervical vertebrae (: vertebra) are the vertebrae of the neck, immediately below the skull. Truncal vertebrae (divided into thoracic and lumbar vertebrae in mammals) lie caudal (toward the tail) of cervical vertebrae. In saurop ...

change shape down the neck, beginning as characteristically elongated with long and low

neural spines Each vertebra (: vertebrae) is an irregular bone with a complex structure composed of bone and some hyaline cartilage, that make up the vertebral column or spine, of vertebrates. The proportions of the vertebrae differ according to their spinal ...

, and getting progressively shorter in length towards the base of the neck, but with increasingly taller and narrower neural spines. This shortening is seen in the necks of other allokotosaurs like ''

Trilophosaurus ''Trilophosaurus'' (Greek language, Greek for "lizard with three ridges") is a lizard-like trilophosaurid allokotosaur known from the Late Triassic of North America. It was a herbivore up to 2.5 m long. Description ''Trilophosaurus'' had a sh ...

'', but is not found in other long-necked archosauromorphs (e.g. the middle cervicals are the longest in

tanystropheids Tanystropheidae is an extinct family of archosauromorph reptiles that lived throughout the Triassic Period, often considered to be " protorosaurs". They are characterized by their long, stiff necks formed from elongated cervical vertebrae with v ...

). The neck would have been held raised up above the body, indicated by the inclined angle of the zygapophyses that connect each

vertebra Each vertebra (: vertebrae) is an irregular bone with a complex structure composed of bone and some hyaline cartilage, that make up the vertebral column or spine, of vertebrates. The proportions of the vertebrae differ according to their spina ...

, as well as the front zygapophyses of each vertebra being higher than those at the back. The neck was also likely held in a gentle arc, based on an articulated set of cervicals in this position. Azendohsaurus madagaskarensis palatine

The

dorsal vertebrae In vertebrates, thoracic vertebrae compose the middle segment of the vertebral column, between the cervical vertebrae and the lumbar vertebrae. In humans, there are twelve thoracic vertebra (anatomy), vertebrae of intermediate size between the ce ...

of the back generally resemble the last cervicals, with tall, vertical neural spines. These vertebrae also decrease in length down the back, but less dramatically than in the neck. The last dorsal is unique, however, as it has a neural spine angled forward. Of the two sacrals, the first vertebra is larger and more robust, with a tall neural spines over its rear half. Both sacrals have large ribs completely fused to the vertebrae that articulate with the ilia (see below). The caudal vertebrae resemble the other vertebrae, but with backward inclined neural spines. The length of the caudals and the height of the neural spines gradually decreases down the tail, unlike some other archosauromorphs where the vertebrae elongate towards the tip. This implies that the tail was short and not tapering, but the very tip of the tail is unknown. They consistently have

chevron Chevron (often relating to V-shaped patterns) may refer to: Science and technology * Chevron (aerospace), sawtooth patterns on some jet engines * Chevron (anatomy), a bone * '' Eulithis testata'', a moth * Chevron (geology), a fold in rock la ...

facets from the 3rd or 4th vertebrae down to the last known caudals in the series. The

cervical rib Cervical ribs are the ribs of the neck in many tetrapods. In most mammals, including humans, cervical ribs are not normally present as separate structures. They can, however, occur as a pathology. In humans, pathological cervical ribs are usually no ...

s are long and thin, becoming more robust and tapered as they move down the neck. Some of the cervical ribs from bottom half of the neck have a slight facet on the inside surface at their tips that may have held the tip of the preceding rib, forming a rigid cervical rib series (also suggested for the long necked ''

Tanystropheus ''Tanystropheus'' (~ 'long' + 'hinged') is an extinct genus of archosauromorph reptile which lived during the Triassic period, Triassic Period in Europe, Asia, and North America. It is recognisable by its extremely elongated neck, longer than t ...

'') that would stiffen the neck. The

trunk Trunk may refer to: Biology * Trunk (anatomy), synonym for torso * Trunk (botany), a tree's central superstructure, and the stem of woody plants * Trunk of corpus callosum, in neuroanatomy * Elephant trunk, the proboscis of an elephant Comput ...

ribs are long and curve outwards, indicating ''Azendohsaurus'' had a broad and deep barrel-shaped chest. The length and curvature of the ribs decreases down the spine, and the last rib is short, fused completely to the final dorsal vertebra, and points directly outwards to the sides. Only a single set of

gastralia Gastralia (: gastralium) are dermal bones found in the ventral body wall of modern crocodilians and tuatara, and many prehistoric tetrapods. They are found between the sternum and pelvis, and do not articulate with the vertebrae. In these reptil ...

is known for ''Azendohsaurus'', and their very delicate build and rarity compared to other bones suggests that it did not have a well developed basket of gastralia under the belly like some other archosauromorphs (e.g. ''

Proterosuchus ''Proterosuchus'' is an extinct genus of archosauriform reptiles that lived during the Early Triassic. It contains three valid species: the type species ''P. fergusi'' and the referred species ''P. alexanderi'' and ''P. goweri''. All three spe ...

'').

Limbs and girdles

Azendohsaurus madagaskarensis premaxilla

The forelimbs and shoulders (pectoral girdle) of ''Azendohsaurus'' are well developed and robust. The

scapula The scapula (: scapulae or scapulas), also known as the shoulder blade, is the bone that connects the humerus (upper arm bone) with the clavicle (collar bone). Like their connected bones, the scapulae are paired, with each scapula on either side ...

(shoulder blade) is long, about twice as tall as it is wide, matching the length and curvature of the ribs to accommodate the deep chest. The blade is concave on each side with a slightly expanded tip that is pointed at the back. The

interclavicle An interclavicle is a bone which, in most tetrapods, is located between the clavicles. Therian mammals ( marsupials and placentals) are the only tetrapods which never have an interclavicle, although some members of other groups also lack one. In ...

is large and robust, and shares with ''Trilophosaurus'' and some rhynchosaurs a long "paddle-like" posterior process that is flattened and expanded towards the tip. It also has a unique forward-pointing process, a feature it shares only with ''

Protorosaurus ''Protorosaurus'' (from , 'earlier' and , 'lizard') is an extinct genus of reptile. Members of the genus lived during the late Permian period in what is now Germany and Great Britain. Once believed to have been an ancestor to lizards, ''Protoro ...

'' and some early diapsids (most other archosauromorphs have a notch instead). The

coracoid A coracoid is a paired bone which is part of the shoulder assembly in all vertebrates except therian mammals (marsupials and placentals). In therian mammals (including humans), a coracoid process is present as part of the scapula, but this is n ...

s are large and rounded, articulating with the scapula to form the glenoid (shoulder socket). The glenoid faces laterally, typical of sprawling reptiles, however, the scapular portion is directed slightly backwards, which could indicate the

humerus The humerus (; : humeri) is a long bone in the arm that runs from the shoulder to the elbow. It connects the scapula and the two bones of the lower arm, the radius (bone), radius and ulna, and consists of three sections. The humeral upper extrem ...

was held in a more raised posture. The humerus itself is large and broadly expanded at both ends, leaving a relatively narrow "waisted" mid-shaft, with a very well developed

deltopectoral crest The humerus (; : humeri) is a long bone in the arm that runs from the shoulder to the elbow. It connects the scapula and the two bones of the lower arm, the radius and ulna, and consists of three sections. The humeral upper extremity consists of ...

. The

radius In classical geometry, a radius (: radii or radiuses) of a circle or sphere is any of the line segments from its Centre (geometry), center to its perimeter, and in more modern usage, it is also their length. The radius of a regular polygon is th ...

is similarly stocky with slightly expanded ends, while the

ulna The ulna or ulnar bone (: ulnae or ulnas) is a long bone in the forearm stretching from the elbow to the wrist. It is on the same side of the forearm as the little finger, running parallel to the Radius (bone), radius, the forearm's other long ...

is greatly expanded at both ends, though to a lesser extent

distally Standard anatomical terms of location are used to describe unambiguously the anatomy of humans and other animals. The terms, typically derived from Latin or Greek roots, describe something in its standard anatomical position. This position provi ...

. The hips (pelvic girdle) are not as deep as the shoulders, with the three hip bones being roughly equal in size. The ilium is tall and curved along the top surface, with a short rounded process at the front and a longer tapering process behind it. The pubis points down and slightly forwards, and only has a slightly thickened expansion (boot) at the tip. The

ischium The ischium (; : is ...

is relatively short, shorter than the ilium, and roughly triangular in shape with straight edges and a rounded rear tip. The articulation surfaces between each ischia are unusually expanded compared to other archosauromorphs. All three contribute to forming a deep, rounded acetabulum (hip socket). Unlike the open socket of dinosaurs, the internal wall of the acetabulum in ''Azendohsaurus'' is solid bone. The large sacral ribs articulate with the ilium so that it is held almost vertically, although their slight downward angle would have deflected the hip socket to face not only out away from the body but also down by about 10° to 25° from vertical. The

femur The femur (; : femurs or femora ), or thigh bone is the only long bone, bone in the thigh — the region of the lower limb between the hip and the knee. In many quadrupeds, four-legged animals the femur is the upper bone of the hindleg. The Femo ...

is long and vaguely S-shaped, with a slightly expanded head that is not turned inwards, unlike those of dinosaurs, indicating it was not held upright. The femur is also twisted along its shaft so that the faces of the head and the knee are offset from each other by about ~75°. The

tibia The tibia (; : tibiae or tibias), also known as the shinbone or shankbone, is the larger, stronger, and anterior (frontal) of the two Leg bones, bones in the leg below the knee in vertebrates (the other being the fibula, behind and to the outsi ...

is roughly 75% the length of the femur, slightly bowed out, and is very robust compared those of other archosauromorphs except for the largest rhynchosaurs. The

fibula The fibula (: fibulae or fibulas) or calf bone is a leg bone on the lateral side of the tibia, to which it is connected above and below. It is the smaller of the two bones and, in proportion to its length, the most slender of all the long bones. ...

by contrast is slender and more prominently twisted along its length. The extremities of ''Azendohsaurus'' are well represented in the fossils, including both a complete hand (manus) and foot (pes) each in articulation. All of the carpals and

tarsal bones In the human body, the tarsus (: tarsi) is a cluster of seven articulating bones in each foot situated between the lower end of the tibia and the fibula of the lower leg and the metatarsus. It is made up of the midfoot (cuboid, medial, intermedi ...

are well ossified and distinct, and the complicated tarsus is made up of nine bones. The

metacarpals In human anatomy, the metacarpal bones or metacarpus, also known as the "palm bones", are the appendicular skeleton, appendicular bones that form the intermediate part of the hand between the phalanges (fingers) and the carpal bones (wrist, wris ...

in the hand are notable as they diverge in a smooth arc, with the length of the digits almost symmetrical around the long third digit as well as relatively non-diverged first and fifth digits. This contrasts with the hands of other reptiles where first and fifth digits are spread out from each other and the fourth digit is the longest. The

metatarsals The metatarsal bones or metatarsus (: metatarsi) are a group of five long bones in the midfoot, located between the tarsal bones (which form the heel and the ankle) and the phalanges (toes). Lacking individual names, the metatarsal bones are nu ...

and digits of the foot also diverge in a smooth arc, but unlike the hand they are not symmetrical, with a long fourth toe and a short, hooked fifth digit. All the digits of the hands and feet are unusually short for an archosauromorph, contrasting with the related ''Trilophosaurus''. The

claw A claw is a curved, pointed appendage found at the end of a toe or finger in most amniotes (mammals, reptiles, birds). Some invertebrates such as beetles and spiders have somewhat similar fine, hooked structures at the end of the leg or Arthro ...

s (or

ungual An ungual (from Latin ''unguis'', i.e. ''nail'') is a highly modified distal toe bone which ends in a hoof, claw, or nail. Elephants and ungulates have ungual phalanges, as did the sauropod Sauropoda (), whose members are known as sauropods (; ...

s) are all very large, narrow and sharply recurved, and are significantly larger than the preceding

finger bone A finger is a prominent digit on the forelimbs of most tetrapod vertebrate animals, especially those with prehensile extremities (i.e. hands) such as humans and other primates. Most tetrapods have five digits (pentadactyly), Chambers 1998 p. 60 ...

they were attached to. The digits and claws share features with those of

dromaeosaurid Dromaeosauridae () is a family (biology), family of feathered coelurosaurian Theropoda, theropod dinosaurs. They were generally small to medium-sized feathered carnivores that flourished in the Cretaceous period (geology), Period. The name Drom ...

and

troodontid Troodontidae is a clade of bird-like theropod dinosaurs from the Late Jurassic to Late Cretaceous. During most of the 20th century, troodontid fossils were few and incomplete and they have therefore been allied, at various times, with many dinos ...

maniraptorans Maniraptora is a clade of coelurosaurian dinosaurs which includes the birds and the non-avian dinosaurs that were more closely related to them than to '' Ornithomimus velox''. It contains the major subgroups Avialae, Dromaeosauridae, Troodontid ...

, as well as other reptiles such as the

turtle Turtles are reptiles of the order (biology), order Testudines, characterized by a special turtle shell, shell developed mainly from their ribs. Modern turtles are divided into two major groups, the Pleurodira (side necked turtles) and Crypt ...

Proganochelys ''Proganochelys'' is a genus of extinct, primitive stem-turtle. ''Proganochelys'' was named by Georg Baur in 1887 as the oldest turtle in existence at the time. The name ''Proganochelys'' comes from the Greek language, Greek word ''ganos'' meani ...

''. These shared traits are associated with well developed flexor tendons, and it is suggested to be an adaptation for withstanding forces involved in digging.

History of Discovery

''A. laaroussii''

Early discoveries

The first fossils of ''Azendohsaurus laaroussii'' were discovered in a northern part the

Timezgadiouine Formation The Timezgadiouine Formation, sometimes spelled as the Timesgadiouine Formation, is a Triassic geological formation in the Argana Basin of Morocco. It is a succession of red bed sediments spanning from the Olenekian to at least the Carnian, enco ...

, which is found within the Argana Basin of the

High Atlas The High Atlas, also called the Grand Atlas, is a mountain range in central Morocco, North Africa, the highest part of the Atlas Mountains. The High Atlas rises in the west at the Atlantic Ocean and stretches in an eastern direction to the Moro ...

. The fossil beds consist of

sandstone Sandstone is a Clastic rock#Sedimentary clastic rocks, clastic sedimentary rock composed mainly of grain size, sand-sized (0.0625 to 2 mm) silicate mineral, silicate grains, Cementation (geology), cemented together by another mineral. Sand ...

s and red clay

mudstone Mudstone, a type of mudrock, is a fine-grained sedimentary rock whose original constituents were clays or muds. Mudstone is distinguished from ''shale'' by its lack of fissility.Blatt, H., and R.J. Tracy, 1996, ''Petrology.'' New York, New York, ...

s, and were excavated by Jean-Michel Dutuit between 1962 and 1969. The fossils of ''A. laaroussii'' are known from only a single layer within the formation, in an

outcrop An outcrop or rocky outcrop is a visible exposure of bedrock or ancient superficial deposits on the surface of the Earth and other terrestrial planets. Features Outcrops do not cover the majority of the Earth's land surface because in most p ...

numbered XVI by Dutuit at the base of the T5 (or Irohalene) member. The T5 member has traditionally been roughly dated to the early Late Triassic in age using vertebrate

biostratigraphy Biostratigraphy is the branch of stratigraphy which focuses on correlating and assigning relative ages of rock strata by using the fossil assemblages contained within them.Hine, Robert. "Biostratigraphy." ''Oxford Reference: Dictionary of Biology ...

based on the presence of the

phytosaur Phytosaurs (Φυτόσαυροι in Greek, meaning 'plant lizard') are an extinct group of large, mostly semiaquatic Late Triassic archosauriform or basal archosaurian reptiles. Phytosaurs belong to the order Phytosauria and are sometimes ref ...

''"Paleorhinus" magnoculus'', as part of the Carnian dated '"''Palaeorhinus''" biochron', although this method of correlating and dating global Triassic sequences may be inaccurate and the date for the T5 member remains uncertain. The first fossils consisted of only a partial tooth-bearing dentary fragment and some associated teeth. This material was discovered by J. M. Dutuit in 1965 and described in 1972, who believed it to belong to a herbivorous ornithischian dinosaur, as well as one of the oldest dinosaurs yet discovered. He named the genus "Azendoh lizard", after the nearby Azendoh village located only 1.5 km to the west of where the fossils were discovered. The specific name, ''A. laaroussii'', is in honour of Laaroussi, the name of a technician from the Moroccan geological mapping service who first discovered the site where ''Azendohsaurus'' was found. Dutuit's description of ''Azendohsaurus'' as an ornithischian was soon challenged by

palaeontologist Paleontology, also spelled as palaeontology or palæontology, is the scientific study of the life of the past, mainly but not exclusively through the study of fossils. Paleontologists use fossils as a means to classify organisms, measure geolo ...

Richard Thulborn Richard Anthony (Tony) Thulborn is a British paleontologist. He is recognized as an expert in dinosaur tracks, and as one of the most productive paleontologists of his time. In 1982, Thulborn debunked the purported plesiosaur embryos discovered ...

two years later in 1974, who was the first to suggest that ''Azendohsaurus'' was a "prosauropod" instead. The same conclusion was made by José Boneparte after examining the material himself in 1976. This re-identification was favoured by researchers in subsequent publications, and it was variously referred to the "prosauropod" families

Anchisauridae ''Anchisaurus'' is a genus of basal sauropodomorph dinosaur. It lived during the Early Jurassic Period, and its fossils have been found in the red sandstone of the Upper Portland Formation, Northeastern United States, which was deposited from ...

and

Thecodontosauridae Thecodontosauridae is a family of basal sauropodomorph dinosaurs that are part of the Bagualosauria, known from fossil remains found exclusively in the Magnesian Conglomerate of Bristol, England, In 1985, palaeontologist

Peter Galton Peter Malcolm Galton (born 14 March 1942 in London) is a British vertebrate paleontologist who has to date written or co-written about 190 papers in scientific journals or chapters in paleontology textbooks, especially on ornithischian and prosau ...

suggested that Dutuit's original "Azandohsaurus " material included the jaw of a "prosauropod" and the tooth of a fabrosaurid ornithischian (a now defunct grouping of early ornithischians), based on the differences in the shape of the teeth. This suggestion was refuted by François-Xavier Gauffre in 1993 when he re-described the material, as well describing additional jaw bones and teeth including two maxillae. He correctly concluded that the material belonged to a single

taxon In biology, a taxon (back-formation from ''taxonomy''; : taxa) is a group of one or more populations of an organism or organisms seen by taxonomists to form a unit. Although neither is required, a taxon is usually known by a particular name and ...

, but assigned the genus to "Prosauropoda" ''

incertae sedis or is a term used for a taxonomy (biology), taxonomic group where its broader relationships are unknown or undefined. Alternatively, such groups are frequently referred to as "enigmatic taxa". In the system of open nomenclature, uncertainty ...

'' based again on the characteristics of the jaws and teeth. However, he could not determine its position within "Prosauropoda" due to the ambiguous distribution of these traits in early herbivorous dinosaurs, as well as a lack of any comparable Triassic reptiles, so he referred it to ''incertae sedis''. His assessment was accepted by many other researchers in the years following up until the description of the new material from the Madagascan species.

Later finds to present

New material from the type locality of ''A. laaroussii'', including parts of the post-cranial skeleton, was reported on in 2002 at the annual conference of the

Society of Vertebrate Paleontology The Society of Vertebrate Paleontology (SVP) is a professional organization that was founded in the United States in 1940 to advance the science of vertebrate paleontology around the world. Mission and activities SVP has about 2,300 members inte ...

by Nour-Eddine Jalil and Fabien Knoll. The additional material included presacral vertebrae, limb bones and limb girdles. The material was disarticulated, and was only attributable to ''A. laaroussii'' due to its association with recognised fragments of the skull and jaws. The post-cranial material was recognised as non-dinosaurian, but still believed to be an

ornithodiran Avemetatarsalia (meaning "bird metatarsals") is a clade of diapsid reptiles containing all archosaurs more closely related to birds than to crocodilians. The two most successful groups of avemetatarsalians were the dinosaurs and pterosaurs. Dinos ...

archosaur Archosauria () or archosaurs () is a clade of diapsid sauropsid tetrapods, with birds and crocodilians being the only extant taxon, extant representatives. Although broadly classified as reptiles, which traditionally exclude birds, the cladistics ...

related to dinosaurs. If correctly associated with the jaws and teeth, this indicated ''Azendohsaurus'' was not closely related to any herbivorous dinosaurs, despite their similarities. Similarly, teeth from other purported Triassic ornithischians were later found to belong to previously unrecognised herbivorous reptiles, such as the

pseudosuchia Pseudosuchia, from Ancient Greek ψεύδος (''pseúdos)'', meaning "false", and σούχος (''soúkhos''), meaning "crocodile" is one of two major divisions of Archosauria, including living crocodilians and all archosaurs more closely relat ...

n ''

Revueltosaurus ''Revueltosaurus'' ("Revuelto lizard") is an extinct genus of suchian pseudosuchian from Late Triassic (late Carnian to middle Norian stage) deposits of New Mexico, Arizona and North Carolina, United States. Many specimens, mostly teeth, have bee ...

'', highlighting the possibility for mistaken identities in other purported herbivorous Triassic dinosaurs, including ''Azendohsaurus''. The new post cranial material from ''A. laaroussii'' was described as part of a

Ph.D A Doctor of Philosophy (PhD, DPhil; or ) is a terminal degree that usually denotes the highest level of academic achievement in a given discipline and is awarded following a course of graduate study and original research. The name of the deg ...

thesis A thesis (: theses), or dissertation (abbreviated diss.), is a document submitted in support of candidature for an academic degree or professional qualification presenting the author's research and findings.International Standard ISO 7144: D ...

by Khaldoune in 2014, but as of 2019 this thesis has not yet been published and the material remains officially undescribed in published literature. However, it has now more confidently been referred to ''A. laaroussii'' after the description of the Madagascan material, and was found to share at least two diagnostic post cranial traits with the Madagascan species. All the material from ''A. laaroussii'', including the holotype and unpublished post crania, is housed at the

Muséum National d'Histoire Naturelle The French National Museum of Natural History ( ; abbr. MNHN) is the national natural history museum of France and a of higher education part of Sorbonne University. The main museum, with four galleries, is located in Paris, France, within the Ja ...

Paris Paris () is the Capital city, capital and List of communes in France with over 20,000 inhabitants, largest city of France. With an estimated population of 2,048,472 residents in January 2025 in an area of more than , Paris is the List of ci ...

France France, officially the French Republic, is a country located primarily in Western Europe. Overseas France, Its overseas regions and territories include French Guiana in South America, Saint Pierre and Miquelon in the Atlantic Ocean#North Atlan ...

''A. madagaskarensis''

Initial discoveries

In 1997 through to 1999, fossils of a new archosauromorph were discovered by an international expedition in southwestern

and were recovered over the following decade. The fossils were found in a single

bone bed A bone bed is any Geology, geological stratum or deposition (geology), deposit that contains bones of whatever kind. Inevitably, such deposits are Sedimentary rock, sedimentary in nature. Not a formal term, it tends to be used more to describe esp ...

, referred to as M-28, that was only tens of centimetres thick across a 100 metre stretch of outcrop exposed as an uplifted

river terrace Fluvial terraces are elongated terraces that flank the sides of floodplains and fluvial valleys all over the world. They consist of a relatively level strip of land, called a "tread", separated from either an adjacent floodplain, other fluvial t ...

not far from the east bank of the Malio River, just outside of the

Isalo National Park Isalo National Park is a National Park in the Ihorombe Region of Madagascar, in the southwestern corner of the Province of Fianarantsoa. The closest town is Ranohira, and the closest cities are Toliara and Ihosy. It is a sandstone landscape that ...

, north-west of the town of

Ranohira Ranohira is a town and commune in Madagascar. It belongs to the district of Ihosy, which is a part of Ihorombe Region. The population of the commune was 16,041 in 2018. The nearest towns are Ilakaka at 26 km and Ihosy 93 km distance.At ...

and to the east of Sakahara. The locality was from the base of the Middle–Late Triassic

Makay Formation Isalo II, also known as the Makay Formation, is an informal Triassic geological unit in Madagascar. It is described as "thick beds of mottled red or green clays associated with soft cross-bedded sandstones, light in colour and much finer-grained ...

, also referred to as Isalo II, a part of the Isalo "Group" in the

Morondava Basin Morondava Basin is one of the major sedimentary basins of Madagascar. It encompasses the large Tsimiroro heavy oil field Field may refer to: Expanses of open ground * Field (agriculture), an area of land used for agricultural purposes * Airfi ...

. Previously, the formation had been believed to be Early to Middle Jurassic in age, although the earlier discovery of the rhynchosaur '' Isalorhynchus'' had revised that estimate to the Middle Triassic. The tetrapod fossils recovered in the 1997–99 expedition confirmed the Isalo II was Triassic in age, but instead suggested a younger Carnian age. The age of the formation has also been correlated to the ''Santacruzodon'' Assemblage Zone (AZ) from the

Santa Maria Formation The Santa Maria Formation is a sedimentary rock formation found in Rio Grande do Sul, Brazil. It is primarily Carnian in age (Late Triassic), and is notable for its fossils of cynodonts, " rauisuchian" pseudosuchians, and early dinosaurs and othe ...

in South America based on the shared genera of

traversodontid Traversodontidae is an extinct family of herbivorous cynodonts. Traversodonts were primarily Gondwanan, with many species known from Africa and South America. Recently, traversodonts have also been found from Europe and North America. Traversodon ...

cynodonts Cynodontia () is a clade of eutheriodont therapsids that first appeared in the Late Permian (approximately 260 mya), and extensively diversified after the Permian–Triassic extinction event. Mammals are cynodonts, as are their extinct ances ...

, with a similar late Ladinian or early Carnian age. The ''Santacruzodon'' AZ has been more reliably dated through

radioisotope A radionuclide (radioactive nuclide, radioisotope or radioactive isotope) is a nuclide that has excess numbers of either neutrons or protons, giving it excess nuclear energy, and making it unstable. This excess energy can be used in one of three ...

U—Pb dating, suggesting a maximum depositional age of 237 ±1.5 million years in the early Carnian. The bone bed contained material from almost "a dozen" individuals of varying ages and size, all from a single species. The material was very well preserved, generally preserving the three-dimensional shape of the bones with very little crushing or distortion in some of the specimens. Based on the state of preservation, some of the bones were believed to have been buried rapidly while others were exposed for longer on the surface, where they were weathered, cracked and possibly trampled on before burial. As with ''A. laaroussii'', the teeth and jaws were the first material to be recovered and described from the bone bed. These were initially mistaken to belong two different species based on the difference in tooth shape in the upper and lower jaws, but one of them was recognised as closely resembling ''A. laaroussii'', sharing a keel on the inside surface of the maxilla and expanded, leaf-shaped teeth, among other features''.'' Like ''A. laaroussii'', both of these supposed species were also misidentified as "prosauropods"; a species of ''Azendohsaurus'' or a related taxon and another, more typical "prosauropod". Further discoveries of associated material clarified that all the jaw material and the rest of the skeletons were from a single, new species of ''Azendohsaurus''.

Reinterpretation

Even preliminary examination of the rest of the skull and skeleton from the Madagascan species confirmed that ''Azendohsaurus'' was not a dinosaur, and was instead an aberrant herbivorous archosauromorph that was only distantly related to dinosaurs, let alone sauropodomorphs. A description of the cranial material was published first in May, 2010 by John J. Flynn and colleagues, who also officially named and diagnosed it as a new species of ''Azendohsaurus'', ''Azendohsaurus madagaskarensis'', named for its country of origin. This was also the first time ''Azendohsaurus'' was determined to be a non-dinosaur in the published literature. In December, 2015, the rest of the skeleton of ''A. madagaskarensis'' was formally described and published by Sterling J. Nesbitt and colleagues, providing the first detailed examination of the full anatomy of ''Azendohsaurus'' from the now almost completely known skeleton. In addition to comparing its anatomy, they were also able to analyse its evolutionary relationships to other Triassic reptiles in a

phylogenetic In biology, phylogenetics () is the study of the evolutionary history of life using observable characteristics of organisms (or genes), which is known as phylogenetic inference. It infers the relationship among organisms based on empirical dat ...

context for the first time. The preservation of the material was described as "generally excellent" by Nesbitt and colleagues, and the amount of overlapping material made it easier to determine the original morphology from distorted and broken bones. Much of the material was found disarticulated and sometimes isolated, but a number of specific parts of the body were found articulated in life position, including sections of the neck, back, hands and feet. Most of the material was similarly sized, with a range of about 25% between the smallest and largest specimens, although the significance of this is not understood and it could be related to

ontogeny Ontogeny (also ontogenesis) is the origination and development of an organism (both physical and psychological, e.g., moral development), usually from the time of fertilization of the ovum, egg to adult. The term can also be used to refer to t ...

, individual variation or

sexual dimorphism Sexual dimorphism is the condition where sexes of the same species exhibit different Morphology (biology), morphological characteristics, including characteristics not directly involved in reproduction. The condition occurs in most dioecy, di ...

. The well preserved nature of much of the material also allowed for reinterpretations of parts of the skeleton of other archosauromorphs, such as the hand of ''Trilophosaurus''. The hands of other archosauromorphs are often poorly known, and so their preservation in ''Azendohsaurus'' is considered important for understanding their evolution in early archosauromorphs. All the specimens of ''A. madagaskarensis'' are permanently housed in both the

University of Antananarivo University of Antananarivo () is the primary public university of Madagascar, located in the capital Antananarivo. History The university traces its founding to 16 December 1955 and the formation of the Institute for Advanced Studies in Antan ...

in Madagascar (including the holotype) and at the

Field Museum of Natural History The Field Museum of Natural History (FMNH), also known as The Field Museum, is a natural history museum in Chicago, Illinois, and is one of the largest such museums in the world. The museum is popular for the size and quality of its educationa ...

Chicago Chicago is the List of municipalities in Illinois, most populous city in the U.S. state of Illinois and in the Midwestern United States. With a population of 2,746,388, as of the 2020 United States census, 2020 census, it is the List of Unite ...

Illinois Illinois ( ) is a U.S. state, state in the Midwestern United States, Midwestern United States. It borders on Lake Michigan to its northeast, the Mississippi River to its west, and the Wabash River, Wabash and Ohio River, Ohio rivers to its ...

, including casts of some of the original specimens.

Classification

Initial attempts

''Azendohsaurus'' was first misidentified as an ornithischian dinosaur by Dutuit, based on shared characteristics of its teeth such as the leaf-like shape and the number of denticles. It was later believed to be a sauropodomorph instead by other researchers, assigned to the defunct infraorder "Prosauropoda" (then believed to be a distinct

monophyletic In biological cladistics for the classification of organisms, monophyly is the condition of a taxonomic grouping being a clade – that is, a grouping of organisms which meets these criteria: # the grouping contains its own most recent co ...

group related to sauropods, now known to be a

paraphyletic Paraphyly is a taxonomic term describing a grouping that consists of the grouping's last common ancestor and some but not all of its descendant lineages. The grouping is said to be paraphyletic ''with respect to'' the excluded subgroups. In co ...

grade) based on the morphology of its lower jaw, maxilla and teeth, such as the downward curving dentary and absence of a predentary bone, one of the characteristic traits of ornithischians. These misidentifications were caused by the convergence in jaw and tooth shape between it and the herbivorous dinosaurs while its true phylogenetic relationships could not be realised due to the absence of other bones of the skull and skeleton. The non-dinosaurian identity of ''Azendohsaurus'' was first hinted at after the discovery of additional skeletal material recovered from the type locality. This was based on the presence of traits such as a solid hip socket (acetabulum), and a proximal

fourth trochanter The fourth trochanter is a shared characteristic common to archosaurs. It is a protrusion on the posterior-medial side of the middle of the femur shaft that serves as a muscle attachment, mainly for the '' musculus caudofemoralis longus'', the m ...

on the femur that also lacked an inward-facing head, which are typical of dinosaur skeletons. While evidently not a dinosaur, it was tentatively interpreted as an ornithodiran archosaur, still closely related to dinosaurs. The discovery of more complete material from Madagascar prompted the first formal classification ''Azendohsaurus'' as non-dinosaurian by Flynn and colleagues in 2010 through a detailed description of its cranial anatomy and were able to clarify its relationships further. They instead recognised it to be a more basal archosauromorph, far removed dinosaurs and closely related to but outside of the clade of

Archosauriformes Archosauriformes (Ancient Greek, Greek for 'ruling lizards', and Latin for 'form') is a clade of diapsid reptiles encompassing Archosaur, archosaurs and some of their close relatives. It was defined by Jacques Gauthier (1994) as the clade stemmin ...

. As well as the derived, sauropodomorph-like features, the skull also had numerous primitive traits for archosauromorphs, including an open-bottomed lower temporal fenestra, extensive palatal teeth, a pineal foramen and no external

mandibular In jawed vertebrates, the mandible (from the Latin ''mandibula'', 'for chewing'), lower jaw, or jawbone is a bone that makes up the lowerand typically more mobilecomponent of the mouth (the upper jaw being known as the maxilla). The jawbone i ...

or antorbital fenestrae. However, its exact relationships still remained unknown beyond a position as an indeterminate non-archosauriform archosauromorph.

Recognition of Allokotosauria

''Azendohsaurus'' was included in a

phylogenetic analysis In biology, phylogenetics () is the study of the evolutionary history of life using observable characteristics of organisms (or genes), which is known as phylogenetic inference. It infers the relationship among organisms based on empirical data ...

of Triassic archosauromorphs for the first time in 2015 by Nesbitt and colleagues, utilising all of the new information from the skull and skeleton and a broad sample of various Triassic archosauromorph species, where it was recovered as closely related to other enigmatic herbivorous Triassic reptiles such as ''

'' and ''

''. This newly recognised grouping of archosauromorphs was named

, meaning "strange reptiles", for the unusual qualities of the reptiles that belonged to the group. ''Azendohsaurus'' was found to be the

sister taxon In phylogenetics, a sister group or sister taxon, also called an adelphotaxon, comprises the closest relative(s) of another given unit in an evolutionary tree. Definition The expression is most easily illustrated by a cladogram: Taxon A and ...

of the

Trilophosauridae Trilophosaurs are lizard-like Triassic allokotosaur reptiles related to the archosaurs. The best known genus is '' Trilophosaurus'', a herbivore up to long. It had a short, unusually heavily built skull, equipped with massive, broad flattened ...

, and was recognised as the sole member of its own family, the

, due its distinctiveness even amongst other allokotosaurs. A similar result was recovered by another large analysis of archosauromorph phylogeny in 2016 by

Martín D. Ezcurra Martin may refer to: Places Antarctica * Martin Peninsula, Marie Byrd Land * Port Martin, Adelie Land * Point Martin, South Orkney Islands Europe * Martin, Croatia, a village * Martin, Slovakia, a city * Martín del Río, Aragón, Spain * Martí ...

, who found a monophyletic Allokotosauria containing ''Azendohsaurus'' and ''Trilophosaurus''. Allokotosaurs are recognised as often having specialised jaws and teeth, as well as sharing a number of

synapomorphies In phylogenetics, an apomorphy (or derived trait) is a novel character or character state that has evolved from its ancestral form (or plesiomorphy). A synapomorphy is an apomorphy shared by two or more taxa and is therefore hypothesized to ...

that include several reversals to more

plesiomorphic In phylogenetics, a plesiomorphy ("near form") and symplesiomorphy are synonyms for an ancestral character shared by all members of a clade, which does not distinguish the clade from other clades. Plesiomorphy, symplesiomorphy, apomorphy, an ...

(ancestral) traits of archosauromorphs, as well as at least two derived traits. The clade is considered to be well supported in these analyses. However, although closely related, the craniodental characteristics of allokotosaurs vary dramatically, and among them ''Azendohsaurus'' was characterised by having laterally compressed, serrated teeth present throughout the length of the jaws (unlike the 'beaked' jaws of trilophosaurids). ''Azendohsaurus'' broadly shares with other azendohsaurids features such as confluent nares, leaf-shaped teeth and a long neck, but ''Azendohsaurus'' itself is distinguished by the distinctive groove on the inside surface of the maxilla and tooth crowns that are expanded above the base. Shringasaurus indicus life restoration

In 2017, another large allokotosaur was described from the Middle Triassic of

by Saradee Sengupta and colleagues, named ''

Shringasaurus indicus ''Shringasaurus'' (meaning "horned lizard", from Sanskrit शृङ्ग (''śṛṅga),'' "horn", and Ancient Greek (''sauros),'' "lizard") is an extinct genus of archosauromorph reptile from the Middle Triassic (Anisian) of India. It is known ...

.'' ''Shringasaurus'' was very similar to ''Azendohsaurus'', and they were found to be closely related, supporting the existence of Azendohsauridae as a distinct family from the trilophosaurids. The same analysis also recovered ''

Pamelaria ''Pamelaria'' is an extinct genus of allokotosaurian archosauromorph reptile known from a single species, ''Pamelaria dolichotrachela'', from the Middle Triassic of India. ''Pamelaria'' has sprawling legs, a long neck, and a pointed skull with no ...

'', another long necked archosauromorph from India, as a basal azendohsaurid. Similarities between ''Pamelaria'' and ''Azendohsaurus'' had been noted by Nesbitt and colleagues in 2015, including confluent nares, serrated teeth and low cervical spines, but their analysis favoured a position in Allokotosauria basal to azendohsaurids. The 2017 analysis also confirmed the close relationship between ''A. laaroussii'' and ''A. madagaskarensis'' within Azendohsauridae, strengthening their shared referral to the genus ''Azendohsaurus''. A 2018 analysis of Triassic archosauromorphs failed to recover Allokotosauria, but still recovered both species of ''Azendohsaurus'' within a clade of azendohsaurids. The cladogram below follows the results of Sengupta and colleagues in 2017: A 2022 phylogenetic analysis supported a monophyletic Allokotosauria and confirmed that ''Azendohsaurus'' was a sister taxon of ''Shringasaurus'', and that the clade consisting of the two genera was in turn most closely related to ''Malerisaurus'' and ''Pamelaria''.

Evolutionary significance

The number of convergent traits shared between ''Azendohsaurus'' and sauropodomorph dinosaurs is remarkably high, especially as all the shared features are interpreted as being homoplastic, meaning that they evolved completely independently of each other. Some of the shared adaptations of the skeleton between ''Azendohsaurus'' and sauropodomorphs were previously considered to be unique to sauropodomorphs. However, the

convergent evolution Convergent evolution is the independent evolution of similar features in species of different periods or epochs in time. Convergent evolution creates analogous structures that have similar form or function but were not present in the last comm ...

of these traits in ''Azendohsaurus'' as adaptations towards a herbivorous lifestyle show that they may be more broadly distributed amongst Triassic archosauromorphs, and do not necessarily indicate a close relationship to sauropodomorphs in fossil taxa. The pattern of convergences in ''Azendohsaurus'' is unusual, as they appear to have arisen in only the front half of the animal, while the sprawling back legs and short tail of ''Azendohsaurus'' are characteristically primitive of earlier archosauromorphs, very unlike the columnar hind limbs and long tail of sauropodomorphs. This further highlights the non-uniform distribution and acquisition of typically sauropodomorph traits in other herbivorous archosauromorphs. The age of ''Azendohsaurus'' is also significant, as it was roughly coeval with the earliest known sauropodomorphs from

Carnian The Carnian (less commonly, Karnian) is the lowermost stage (stratigraphy), stage of the Upper Triassic series (stratigraphy), Series (or earliest age (geology), age of the Late Triassic Epoch (reference date), Epoch). It lasted from 237 to 227.3 ...

South America, such as the lightweight, bipedal ''

Saturnalia Saturnalia is an Roman festivals, ancient Roman festival and holiday in honour of the List of Roman deities, god Saturn (mythology), Saturn, held on 17 December in the Julian calendar and later expanded with festivities until 19 December. By t ...

''. However, ''Azendohsaurus'' resembles the later

Norian The Norian is a division of the Triassic geological period, Period. It has the rank of an age (geology), age (geochronology) or stage (stratigraphy), stage (chronostratigraphy). It lasted from ~227.3 to Mya (unit), million years ago. It was prec ...

sauropodomorphs more closely, both in general anatomy and its larger body size. This suggests that azendohsaurids had been the first reptiles to evolve as high browsing herbivores in Triassic ecosystems, prior to the evolution of the larger sauropodomorphs, which had previously been assumed to have been the first high browsing herbivores. It also indicates that the convergence between ''Azendohsaurus'' and sauropodomorphs did not occur under the same environmental circumstances, as ''Azendohsaurus'' was part of an initial wave of herbivory in archosauromorphs (along with rhynchosaurs, silesaurids and cynodonts) and large sauropodomorphs in a second wave (along with herbivorous pseudosuchians). ''Azendohsaurus'' also demonstrates that archosauromorphs were occupying roles as large herbivores in Triassic ecosystems earlier in their evolutionary history than had been assumed. These roles were previously thought to be dominated by large

synapsid Synapsida is a diverse group of tetrapod vertebrates that includes all mammals and their extinct relatives. It is one of the two major clades of the group Amniota, the other being the more diverse group Sauropsida (which includes all extant rept ...

s such as

dicynodont Dicynodontia is an extinct clade of anomodonts, an extinct type of non-mammalian therapsid. Dicynodonts were herbivores that typically bore a pair of tusks, hence their name, which means 'two dog tooth'. Members of the group possessed a horny, t ...

s prior to the radiation of archosaurs in the Late Triassic, but ''Azendohsaurus'' suggests that earlier archosauromorphs were also capable of competing with synapsid herbivores.

Palaeobiology

Feeding and diet

Azendohsaurus madagaskarensis maxilla and teeth

The leaf-shaped teeth of ''Azendohsaurus'' are clearly suited for a herbivorous lifestyle, and microwear—marks left on the tooth surface during feeding—on the teeth of ''A. madagaskarensis'' suggest they were used for browsing on vegetation that was not especially tough or woody, preferring softer (but firm) vegetation. The microwear patterns also show that it used a simple up-and-down motion of the jaw, and did not use complex jaw movement to chew its food like ornithischian dinosaurs or contemporary cynodonts. This microwear has not yet been observed on the teeth of ''A. laaroussii'', but it is unknown if this is a genuine feature relating to a difference in diet and feeding habits between the two species or if it is just a feature of preservation. The fully developed palatal teeth suggest that it was using them for feeding in a specialised manner. However, no functional studies have been performed on the palatal teeth so it is unknown exactly what they were used for, although their similar shape to the marginal teeth suggests they were used for processing similar food. A pterygoid from a younger individual of ''A. madagaskarensis'' has fewer rows of palatal teeth that are smaller in size than those of the larger, mature individuals, indicating that ''Azendohsaurus'' increased both the number and size of its palatal teeth as it grew into adulthood. Younger individuals also had fewer dentary teeth than adults, although the difference was much less extreme compared to the palatal teeth (16 compared to the 17 of mature specimens).

Body posture

The body posture inferred for ''Azendohsaurus'' is a mixture of sprawled and semi-sprawled. The hind limbs have been interpreted as being completely sprawled outwards from the body, with its femur held straight out and the lower leg bent 90° beneath it at the knee, like a

lizard Lizard is the common name used for all Squamata, squamate reptiles other than snakes (and to a lesser extent amphisbaenians), encompassing over 7,000 species, ranging across all continents except Antarctica, as well as most Island#Oceanic isla ...

. The forelimbs and shoulder girdle, however, suggest that the front of the body was held more upright than the hind quarters, with a partly downward directed shoulder socket and a humerus more suited for being held partially erect, and was similar in shape to those of sauropodomorphs. This unusual combination suggests that ''Azendohsaurus'' stood with its front end raised up off from the ground, which combined with its long, arched neck and small head, allowed it to browse relatively high off the ground, unlike contemporary low-browsing rhynchosaurs and cynodonts. Adapting to high-browsing could possibly explain the convergence between ''Azendohsaurus'' and sauropodomorphs, acquiring similar traits of the neck, forelimbs and spine to perform in similar niches. However, the more sprawling posture of ''Azendohsaurus'' probably inhibited high-browsing like that of the fully erect sauropodomorphs.

Palaeopathology

Despite the multitude of specimens present in the bone bed that was examined, only a single

pathology Pathology is the study of disease. The word ''pathology'' also refers to the study of disease in general, incorporating a wide range of biology research fields and medical practices. However, when used in the context of modern medical treatme ...

has been recorded in ''A. madagaskarensis''. Specimen UA 7-16-99-620, one of the three preserved interclavicles, had been malformed so that the long posterior process had been sharply bent to the right, compared to the normal straight posterior processes of the other two interclavicles.

Metabolism and growth

In 2019, thin slices were cut from the humerus, femur and tibia of specimens attributed to ''A. laaroussii'' for

histological Histology, also known as microscopic anatomy or microanatomy, is the branch of biology that studies the microscopic anatomy of biological tissue (biology), tissues. Histology is the microscopic counterpart to gross anatomy, which looks at large ...

examination of the microscopic bone structure to try and determine the rate of growth in ''Azendohsaurus''. The vascular density (the density of blood vessels in the bone tissue) in all three limb bones was found to be comparable to those of fast-growing

birds Birds are a group of warm-blooded vertebrates constituting the class (biology), class Aves (), characterised by feathers, toothless beaked jaws, the Oviparity, laying of Eggshell, hard-shelled eggs, a high Metabolism, metabolic rate, a fou ...

and

mammals A mammal () is a vertebrate animal of the class Mammalia (). Mammals are characterised by the presence of milk-producing mammary glands for feeding their young, a broad neocortex region of the brain, fur or hair, and three middle e ...

, and the types of bone tissue identified—particularly energy-consuming fibrolamellar bone tissue—were interpreted as indicating a high

resting metabolic rate Resting metabolic rate (RMR) refers to whole-body mammal (or other vertebrate) metabolism during a time period of strict and steady ''resting conditions'' that are defined by a combination of assumptions of physiological homeostasis and List of typ ...

that was in the range of living birds and mammals. It was inferred then that, like birds and mammals, ''Azendohsaurus'' would also likewise have been

, or "warm-blooded". High resting metabolic rates similar to those of ''Azendohsaurus'' had been identified in other more derived archosauromorphs (such as ''

Prolacerta ''Prolacerta'' is a genus of Archosauromorpha, archosauromorph from the lower Triassic of South Africa and Antarctica. The only known species is ''Prolacerta broomi''. ''Prolacerta'' was a small and slender reptile, with a rather long neck, low s ...

''), and analyses suggested that endothermy may then have been ancestrally present in archosauromorphs as far back as their common ancestor with allokotosaurs. This suggests that ''Azendohsaurus'' may then have been ancestrally endothermic. By contrast, the related allokotosaur ''Trilophosaurus'' was previously found to not have any fibrolamellar bone tissue in its limb bones and so was inferred to have grown slowly.

Palaeoecology

Although the two species of ''Azendohsaurus'' are known from disparate locations in North Africa and Madagascar, during the Middle to Late Triassic these regions were connected as part of the

supercontinent In geology, a supercontinent is the assembly of most or all of Earth's continent, continental blocks or cratons to form a single large landmass. However, some geologists use a different definition, "a grouping of formerly dispersed continents", ...

Pangaea Pangaea or Pangea ( ) was a supercontinent that existed during the late Paleozoic and early Mesozoic eras. It assembled from the earlier continental units of Gondwana, Euramerica and Siberia during the Carboniferous period approximately 335 mi ...

. Because of this, the two regions share broadly similar faunas, as well as sharing some with other regions of the globe at the time. For example, the cynodonts in Madagascar are similar to those also found in South America, and the Moroccan temnospondyls may be related to those found in eastern North America. The climate was hot and dry at this time, but with evidence suggesting higher levels of rainfall during the Carnian, interrupting the increasing aridity trend and creating wetter environments around the globe. Moghreberia

Timezgadiouine Formation, Morocco

Other reptiles from the base of the Irohalene (T5) member of the

contemporaneous with ''A. laaroussii'' include the

'' Arganarhinus'', the predatory

rauisuchid Rauisuchidae is a group of large (up to ) predatory Triassic archosaurs. Some disagreement exists over which genera should be included in the Rauisuchidae and which should be in the related Prestosuchidae and Poposauridae, and indeed whether thes ...

Arganasuchus ''Arganasuchus'' is an extinct genus of " rauisuchian" (loricatan) archosaur. It is known from a single species, ''Arganasuchus dutuiti''. Fossils of this genus have been found in Upper Triassic rocks of the Argana Basin, Morocco. Though its re ...

'', the herbivorous

silesaurid Silesauridae is an extinct family of Triassic dinosauriforms. It is most commonly considered to be a clade of non-dinosaur dinosauriforms, and the sister group of dinosaurs. Some studies have instead suggested that most or all silesaurids compr ...

Diodorus Diodorus Siculus or Diodorus of Sicily (; 1st century BC) was an ancient Greek historian from Sicily. He is known for writing the monumental universal history '' Bibliotheca historica'', in forty books, fifteen of which survive intact, b ...

'', a paratypothoracisine

aetosaur Aetosaurs () are heavily armored reptiles belonging to the extinct order (biology), order Aetosauria (; from Ancient Greek, Greek, (aetos, "eagle") and (, "lizard")). They were medium- to large-sized Omnivore, omnivorous or Herbivore, herbivor ...

, and

procolophonid Procolophonidae is an extinct family of small, lizard-like parareptiles known from the Late Permian to Late Triassic that were distributed across Pangaea, having been reported from Europe, North America, China, South Africa, South America, Antarc ...

parareptile Parareptilia ("near-reptiles") is an extinct group of Basal (phylogenetics), basal Sauropsida, sauropsids ("Reptile, reptiles"), traditionally considered the sister taxon to Eureptilia (the group that likely contains all living reptiles and birds ...

s, as well as the

stahleckeriid Stahleckeriidae is a family of dicynodont therapsids whose fossils are known from the Triassic of North America, South America, Asia and Africa. Classification Phylogeny Below is a cladogram A cladogram (from Greek language, Greek ''clados ...

dicynodont '' Moghreberia'', a synapsid.

Temnospondyl Temnospondyli (from Greek language, Greek τέμνειν, ''temnein'' 'to cut' and σπόνδυλος, ''spondylos'' 'vertebra') or temnospondyls is a diverse ancient order (biology), order of small to giant tetrapods—often considered Labyrinth ...

amphibian Amphibians are ectothermic, anamniote, anamniotic, tetrapod, four-limbed vertebrate animals that constitute the class (biology), class Amphibia. In its broadest sense, it is a paraphyletic group encompassing all Tetrapod, tetrapods, but excl ...

s are represented by several genera of metoposauroids, including the metoposaurids ''

Arganasaurus ''Arganasaurus'' is an extinct genus of temnospondyls belonging to the family Metoposauridae that lived in Morocco during the Late Triassic (Carnian). Taxonomy The type species of ''Arganasaurus'', ''A. lyazidi'', was originally described as ''M ...

'' and ''

Dutuitosaurus ''Dutuitosaurus'' is a genus of metoposaurids, a group of temnospondyls that lived during the Late Triassic period. ''Dutuitosaurus'' was discovered in the early 1960s in Morocco and is known from the lower t5 units of the Timezgadiouine Formati ...

,'' and the

latiscopid Latiscopidae is an extinct family of Triassic temnospondyls. It was first described in 1940 based on a fossil skull found in Texas. It includes small sized temnospondyls that are thought to have engaged in burrowing and seasonal aestivation ...

Almasaurus ''Almasaurus'' is an extinct genus of trematosaurian temnospondyl within the family Latiscopidae. It is known from several skulls and some postcranial material found from the Argana Formation in Morocco, which dates back to the Late Triassic. ...

''. Fish are also known from the T5 member, including various ray-finned

actinopterygians Actinopterygii (; ), members of which are known as ray-finned fish or actinopterygians, is a class of bony fish that comprise over 50% of living vertebrate species. They are so called because of their lightly built fins made of webbings of skin ...

such as the locally endemic '' Dipteronotus gibbosus'' and '' Mauritanichthys'', as well as other perleidiform and redfieldiiform fishes, alongside

lobe-finned Sarcopterygii (; )—sometimes considered synonymous with Crossopterygii ()—is a clade (traditionally a class or subclass) of vertebrate animals which includes a group of bony fish commonly referred to as lobe-finned fish. These vertebrates ar ...

actinistia Coelacanths ( ) are an ancient group of lobe-finned fish (Sarcopterygii) in the class Actinistia. As sarcopterygians, they are more closely related to lungfish and tetrapods (the terrestrial vertebrates including living amphibians, reptiles, bi ...

ns and

lungfish Lungfish are freshwater vertebrates belonging to the class Dipnoi. Lungfish are best known for retaining ancestral characteristics within the Osteichthyes, including the ability to breathe air, and ancestral structures within Sarcopterygii, inc ...

such as ''

Asiatoceratodus ''Asiatoceratodus'' is an extinct genus of lungfish which lived during the Middle Triassic, Middle-Late Triassic, Jurassic and Cretaceous periods in what is now Asia (Kyrgyzstan), Africa (Ethiopia, Niger, Algeria, Morocco) and South America (Urug ...

''. The T5 member is composed of cyclical layers of bioturbated

and

deposited in a broad, semi-humid basin. It has been interpreted as a system of brackish permanent lakes and sandbars, or alternatively sandy meandering rivers on a muddy floodplain. The fluvial or lacustrine sediments of the T5 member contrast with the playa sediments of the preceding T4 member, suggesting that it was deposited during an interval of increased rainfall. Numerous tracks and trackways from various animals are preserved, typically those of animals known from fossil remains such as phytosaurs, pseudosuchians,

dinosauromorphs Dinosauromorpha is a clade of avemetatarsalians (archosaurs closer to birds than to crocodilians) that includes the Dinosauria (dinosaurs) and some of their close relatives. It was originally defined to include dinosauriforms and lagerpetids, ...

and basal archosauromorphs. The tracks also appear to indicate the presence of large to very-large dinosauromorphs or

paracrocodylomorphs Paracrocodylomorpha is a clade of pseudosuchian archosaurs. The clade includes the diverse and unusual group Poposauroidea as well as the generally carnivorous and quadrupedal members of Loricata, including modern crocodylians. Paracrocodylomor ...

that are currently not yet known from skeletal remains. Additional traces mark the presence of burrowing invertebrates,

bivalves Bivalvia () or bivalves, in previous centuries referred to as the Lamellibranchiata and Pelecypoda, is a class of aquatic molluscs (marine and freshwater) that have laterally compressed soft bodies enclosed by a calcified exoskeleton consis ...

, and

clam shrimp Clam shrimp are a group of bivalved branchiopod crustaceans that resemble the unrelated bivalved molluscs. They are extant and also known from the fossil record, from at least the Devonian period and perhaps before. They were originally classif ...

Makay Formation, Madagascar

In Madagascar, ''Azendohsaurus'' co-existed with the hyperodapedontine rhynchosaur ''Isalorhynchus'', the herbivorous traversodontid cynodonts ''

Dadadon ''Dadadon'' is an extinct genus of traversodontid cynodonts which existed in Madagascar during the late Middle Triassic In the geologic timescale, the Middle Triassic is the second of three epoch (geology), epochs of the Triassic period (geol ...

'' and '' Menadon'', and the predatory

chiniquodontid Chiniquodontidae is an extinct family of basal probainognathian cynodonts that lived in what is now Africa and South America during the Middle and Late Triassic. It is currently thought to include four valid genera: ''Aleodon'', ''Chiniquodon'', ...

cynodont '' Chiniquodon kalanoro'', as well as an undescribed

kannemeyeriiform Kannemeyeriiformes is a group of large-bodied Triassic dicynodonts. As a clade, Kannemeyeriiformes has been defined to include the species ''Kannemeyeria simocephalus'' and all dicynodonts more closely related to it than to the species ''Lystrosa ...

dicynodont, a sphenodontian reptile, a

, the diminutive

lagerpetid Lagerpetidae (; originally Lagerpetonidae) is a family of basal avemetatarsalians (early-diverging members of the reptile lineage leading to birds and other dinosaurs). Though traditionally considered the earliest-diverging dinosauromorphs (ar ...

Kongonaphon ''Kongonaphon'' is an extinct genus of lagerpetid avemetatarsalians from the Middle to Late Triassic of Madagascar. It contains a single species, ''Kongonaphon kely'', which is known from a fragmentary partial skeleton. This fossil hails from ...

'', various other undescribed dinosauromorphs, and an "enigmatic archosaur" of uncertain classification. The faunal composition of the Isalo II is believed to represent a Middle Triassic

Ladinian The Ladinian is a stage and age in the Middle Triassic series or epoch. It spans the time between Ma and ~237 Ma (million years ago). The Ladinian was preceded by the Anisian and succeeded by the Carnian (part of the Upper or Late Triassic ...

aged assemblage, existing prior to the appearance of dinosaurs and associated Late Triassic faunas, particularly

s and

s that are absent from the formation, and also inferred from the dominance of traversodonts in the fauna. However, this age assessment remains uncertain, and the formation is possibly from the younger early Late Triassic during the Carnian, as has been proposed for the T5 member of the Argana Formation. Fossils of ''A. madagaskarensis'' have been exclusively recovered from a deposit of fine grained red mudstone, while other fossil bearing localities in the formation consist of medium grained channel sands, possibly reflecting a habitat preference in the ecosystem distinct from other animals or unique behavioural trait. The absence of any other species in the bone bed may also support this. However, this speculation cannot be confirmed, and it could instead be attributed to

preservation bias Taphonomy is the study of how organisms decay and become fossilized or preserved in the paleontological record. The term ''taphonomy'' (from Greek , 'burial' and , 'law') was introduced to paleontology in 1940 by Soviet scientist Ivan Efremov ...

. Possible niche partitioning in diet, though, is supported by differences in the tooth microwear of ''A. madagaskarensis'' and the contemporary traversodont ''Dadadon''. ''Dadadon'' was inferred to be capable of feeding on tough, hardy vegetation by using complex chewing, in contrast to the simpler dentition and processing of ''Azendohsaurus'', which was better suited for eating leaves. This may also be supported by its more elevated body posture and long neck.

References

{{Portal bar, Paleontology, Reptiles, Morocco, Madagascar Azendohsauridae Prehistoric reptile genera Middle Triassic reptiles of Africa Late Triassic reptiles of Africa Fossils of Morocco Fossils of Madagascar Triassic Madagascar Fossil taxa described in 1972