Azendohsaurus Madagaskarensis

''Azendohsaurus'' is an extinct genus of herbivorous archosauromorph reptile from roughly the late Middle to early Late Triassic Period of Morocco and Madagascar. The type species, ''Azendohsaurus laaroussii'', was described and named by Jean-Michel Dutuit in 1972 based on partial jaw fragments and some teeth from Morocco. A second species from Madagascar, ''A. madagaskarensis'', was first described in 2010 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. 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, at first as an ornithischian but more often as a "prosauropod" sauropodomorph. 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 to Archosauromorpha and that ''Azendohsaurus'' was not a dinosaur at all. Instead, ''Azendohsaurus'' was actually a more primitive archosauromorph that had convergently evolved many features of the jaws and skeleton shared with the later giant sauropod dinosaurs. It was found to be a member of a newly recognised group of specialised, mostly herbivorous archosauromorphs that was named the Allokotosauria. It is also the namesake and typifier of its own family of allokotosaurs, the Azendohsauridae; initially the only member, the family now includes other similar allokotosaurs, such as the larger, horned azendohsaurid ''Shringasaurus'' from India.

Several other groups of archosauromorphs also adapted to herbivory in the Triassic, 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 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 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 sauropodomorph 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. The premaxillae 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 rhynchosaurs.

The skull has a number of traits convergent with sauropodomorphs, including the downward curving dentary, a robust dorsal process of the maxilla, and several features of the teeth. The process on the maxilla usually indicates the presence of an antorbital fenestra in archosauriforms, but in ''Azendohsaurus'' this space is occupied by the lacrimal bone in front of the eyes. This is a unique arrangement unknown in other Triassic archosauromorphs, except for the related ''Shringasaurus''. The orbits are almost entirely occupied by the large sclerotic rings, suggesting large eyes. The lower temporal fenestra is open at the bottom, separating the jugal and the quadratojugal 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.

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

The palate is unusually covered in numerous fully developed palatal teeth, with up to four sets on the pterygoid and additional rows on the palatine and vomers. 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'' 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 is known in ''Azendohsaurus'', and although incomplete, it is estimated to have 24 presacral vertebrae (including the atlas and axis). The sacrum of the hips has only two vertebrae, and the full number of caudal vertebrae in the tail is unknown, but it is estimated to be only around 45–55 (low for an archosaur).

The cervical vertebrae change shape down the neck, beginning as characteristically elongated with long and low neural spines, 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'', but is not found in other long-necked archosauromorphs (e.g. the middle cervicals are the longest in tanystropheids). The neck would have been held raised up above the body, indicated by the inclined angle of the zygapophyses that connect each vertebra, 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.

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

The cervical ribs 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'') that would stiffen the neck. The trunk 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 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'').

Limbs and girdles

The forelimbs and shoulders (pectoral girdle) of ''Azendohsaurus'' are well developed and robust. The scapula (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 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'' and some early diapsids (most other archosauromorphs have a notch instead).

The coracoids 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 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 radius is similarly stocky with slightly expanded ends, while the ulna is greatly expanded at both ends, though to a lesser extent distally.

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 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 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 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 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 are well ossified and distinct, and the complicated tarsus is made up of nine bones. The metacarpals 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 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 claws (or unguals) are all very large, narrow and sharply recurved, and are significantly larger than the preceding finger bone they were attached to. The digits and claws share features with those of dromaeosaurid and troodontid maniraptorans, as well as other reptiles such as the turtle ''Proganochelys''. 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 in Morocco, which is found within the Argana Basin of the High Atlas. The fossil beds consist of sandstones and red clay mudstones, 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 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 based on the presence of the phytosaur ''"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 Richard Thulborn 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 and Thecodontosauridae

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