Amargasaurus: a genus of sauropod dinosaur

Amargasaurus (lat.: Amargasaurus) is a genus of sauropod dinosaurs from the Dicraeosauridae family that lived in the Lower Cretaceous period in what is now Argentina.

This genus is still known to science from a single, almost complete skeleton with skull fragments found in the La Amarga Formation (Spanish).: La Amarga) in the Argentine province of Neuquen (Spanish.: Neuquen). However, it is among the best known sauropods from the Lower Cretaceous.

The most prominent feature of this dinosaur are two rows of long spine-like processes of vertebrae (forked processes of the vertebrae) in the region of the cervical and thoracic spine, although their function is unknown.

As a member of the Dicreosaurid family, Amargasaurus was closely related to Dicreosaurus (lat.: Dicraeosaurus) and Brachytrachelopan (Brachytrachelopan), who lived on Earth in the Upper Jurassic. The only known species of the genus Amargasaurus is Amargasaurus cazaui.

Like all sauropods, it was a herbivorous dinosaur with a barrel-shaped body and a long neck and tail. However, the body size of Amargasaurus was relatively small for a sauropod the length of the only known specimen is estimated to be “only” 9-10 meters.

In 2004, a group of paleontologists led by the Uruguayan Gerardo Mazzetta (Italian.: Gerardo Mazzetta) estimated the body weight of this dinosaur at 2.6 tons. In dicreosaurids, to which Amargasaurus belonged, the neck was shorter than that of all other sauropods (in Amargasaurus itself, its length did not exceed 2.4 meters).

In 1991, Leonardo Salgado (Spanish.: Leonardo Salgado) and José Bonaparte (José Bonaparte) stated that, unlike some representatives of the Titanosaurs group (lat.: Titanosauria) this dinosaur had no signs of morphological adaptation to possible rearing and therefore, as paleontologists suggested, it must have moved on all fours.

As for the skull, only its posterior part has been preserved, mainly the brain skull and temporal regions, while the facial part and jaws are missing. However, it can be assumed that the skull of Amargasaurus corresponded to the typical structure of the skulls of representatives of the genera close to it, and the muzzle of the dinosaur was elongated, like that of a horse, and almost rectangular in horizontal projection, while its teeth had narrow, rod-shaped tops. Like its closest relatives, the nasal openings of Amargasaurus were displaced far back.

In the first reconstructions of the dinosaur skeleton, its skull was depicted only slightly tilted down. However, in 1999, Leonardo Salgado noted that the long spine-like processes of the cervical vertebrae of this sauropod anatomically ruled out the possibility of such an arrangement of the skull on the spine, and that, unlike relatives, its skull was in fact always tilted almost vertically down.

The skull of Amargasaurus can only be compared to a limited extent with the skulls of other related genera, since the species Dicraeosaurus hansemanni belonging to the genus Dicreosaurus is so far the only other representative of dicreosaurids with preserved bone material of the skull. However, both Amargasaurus cazaui and Dicraeosaurus hansemanni share common features of the skull that are absent in all other known sauropods: for example, the halves of their paired frontal bone were fused together. The processes of the basipterygoid, which connected the brain skull with the upper side of the oral cavity, were extremely elongated.

At the same time, this feature was more pronounced in Amargasaurus than in Dicreosaurus. In addition, the temporal (supratemporal) windows of the skull were greatly reduced and directed laterally, in contrast to other diapsid reptiles, in which they were always turned upwards. In contrast to Dicreosaurus and other sauropods, the basal tubercles (tuberous growths) on the underside of the brain skull of Amargasaurus were fused together.

The dinosaur spine consisted of 13 cervical, 9 dorsal and presumably 5 sacral vertebrae. Number of tail vertebrae unknown. The bodies of the cervical and anterior dorsal vertebrae were opisthocoelous, that is, they had an articular tubercle (bulge) in front and an articular fossa (concavity) behind. In contrast, the rest of the dorsal vertebrae were flat front and back.

Like other dicreosaurids, the dorsal vertebrae of Amargasaurus did not have lateral cavities (pleurocoels). A unique feature was the strongly developed transverse processes (diapophyses) of the anterior dorsal vertebrae, indicating that the dinosaur had powerful ribs.

The most unusual feature of this dinosaur was the extremely long spine-like processes of the cervical and dorsal vertebrae, as in other dicreosaurids, diverging in the form of a fork from the very arch of the vertebrae. However, the length and spiny shape of the processes markedly distinguishes the vertebrae of Amargasaurus from those of all other sauropods. These processes had a rounded cross section and were pointed upwards.

The longest were the spiky processes of the vertebrae of the middle region of the cervical spine, while the length of the process of the 8th cervical vertebra was 60 cm. It is noteworthy that the spinous processes of the cervical vertebrae were bent back at an angle of about 50 degrees, so that the ends of the processes of each anterior vertebra rose above the corresponding lower vertebra.

The processes of the last two dorsal vertebrae, as well as the sacral and anterior caudal vertebrae, were also strongly elongated, but differed significantly from the spiny processes of the previous vertebrae, since they were not bifurcated and expanded upwards, which gave them the shape of an oar.

The bones of the shoulder and pelvic belts, as well as the front and hind limbs of the Amargazaurus, were similar to the corresponding bones of the related digrezaur and had no unique features. The pelvis was relatively wide due to the increased distance between the ilium bones located on either side of the sacrum.

The bones of the hands and feet have not been preserved, however, presumably these parts of the fore and hind limbs of the dinosaur were five-rayed, like in all sauropods. Moreover, the fingers of the hand were greatly reduced, and the toes were normal.

Amargasaurus is a member of the Dicreosaurid family, one of three within the Diplodocoid superfamily (lat.: Diplodocoidea). Currently, dicozaurids are divided into 3 kinds, numbering a total of 4 types of, along with Amargasaurus Cazaui, this family includes such species as Dicraeosaurus Hansemanni and Dicraeosaurus SatTleri, who lived on the territory of modern Tanzania in the Verkhneyuyra period, as well as only recently described by Brachyopan Mesai, who also lived in the Verkhneyursky period, but already on the territory of the current Argentina.

Thus, Amargasaurus is the only named member of this family from the Lower Cretaceous, although one unnamed find from Brazil shows that its members still existed at the end of the Lower Cretaceous.

Relationships within the Dicreosaurid family are relatively undisputed. Thus, recent analyzes carried out in 2005, 2007 and 2011 led to the general result that Dicreosaurus and Brachytrachelopan were more closely related to each other than to Amargasaurus, which, apparently, formed a taxon, sister in relation to the clade that included Brachytrachelopan and Dicreosaurus.

A 1999 study led to a different result, which suggests that Dicreosaurus is a paraphyletic group and that Dicraeosaurus sattleri is in fact the second species of the genus Amargasaurus.

The only skeleton of the Amargazaurus was found in February 1984 by the paleontologist Guillermo Ruzhee.: Guillermo Rougier) during the expedition led by Jose Bonaparte. It was the eighth paleontological expedition, which took place in Patagonia as part of a project for the study of the world of vertebrates that inhabited the territory of modern South America in the Jurassic and Cretaceous periods, and launched back in 1976. In addition, a skeleton of Teropod Carnotaurus was also discovered during it (lat.: Carnotaurus).

The place of the finds of the remains of Amargazaur is located in the La-Amarga formation in the Picun-Leuf department (SPP.: Pic push leufú), which is part of the Argentine province of Cent.

The skeleton (a holotype, copy No. MacN-N 15) was preserved for the most part in the original anatomically related state: for example, the cervical and thoracic spine, consisting of a total of 22 vertebrae, were articulated with the skull and the sacrum, respectively. From the skull, only the temporal regions, as well as the cerebral skull, were preserved, while the sacrum completely survived, but partially collapsed even before the corpse of Amargasaurus was covered with sedimentary rocks from above. Of the tail vertebrae, only 3 anterior, 3 middle, 1 posterior, as well as fragments of the bodies of several more vertebrae have survived.

The shoulder girdle of the found skeleton is represented only by a scapula with a coracoid (crow bone), and the pelvis is represented only by the ilium. The bones of the limbs are also not completely preserved. So, the skeleton lacks a hand and, apart from two metatarsal bones, a foot.

Currently, the skeleton of Amargasaurus is kept in the collection of the Argentine Museum of Natural Sciences named after Bernardino Rivadavia (Spanish).: Museo Argentino de Ciencias Naturales Bernardino Rivadavia) in Buenos Aires.

The first, however, unofficial mention of a new dinosaur was published by the paleontologist Bonaparte in 1984 in the book “In the Footsteps of Dinosaurs” (Italian.: Sulle Orme dei Dinosauria). In it, Bonaparte gave the name “Amargasaurus groeberi” to the new species, although this specific name was changed in the first official description in 1991 to “Amargasaurus cazaui”.

The first description, compiled in Spanish by paleontologists Leonardo Salgado and José Bonaparte, was published in the scientific journal Ameghiniana. A year later, another description appeared, dedicated specifically to the skull of a dinosaur.

The generic name “Amargasaurus”, derived from the Spanish name of the formation (La Amarga) and the Greek word “sauros” (“lizard”), indicates that the dinosaur skeleton was found in the La Amarga Formation. The specific epithet “cazaui” is given in honor of the scientist Louis B. Casau (Spanish).: Louis B. Cazau, an employee of the Argentinean oil company YPF Sociedad Anónima, who brought the paleontological significance of the La Amarga formation to the fore.

The formation of layers in which the skeleton of Amargasaurus was found belongs to the Neuquen River basin and dates from the period from the Barremian to the Lower Aptian. Fossils of this dinosaur, as well as fossils of most other vertebrates from the La Amarga Formation, were found in the lowest of its 3 layers (subformations), the Puesto Antigual layer (Spanish.: Puesto Antigual). This formation is about 29 meters thick and is composed of sandstones that resulted from the settling of particles from the water of a multi-channel river system under conditions of high-energy flows.

Fossil remains of Amargasaurus itself were recovered from a layer of sandy conglomerate. Other bones found in this formation are from herbivorous dinosaurs such as the basal diplodocoid Zapalasaurus (lat.: Zapalasaurus), titanosaurus amargatitan (Amargatitanis), in addition, the basal representatives of titanosauroforms (Titanosauriformes), as well as rebbachisaurids (Rebbachisauridea) and so far the only specimen of Stegosaurus (Stegosaurus) found in South America.

The remains of predatory dinosaurs of this formation are represented by the bones of a stunted abelisaurid ligabueino (lat.: Ligabueino), as well as teeth indicating the presence here of a large representative of the Tetanurae group (Tetanurae). It was also proved that in addition to dinosaurs, a relative of the crocodile amargazukh (lat.: Amargasuchus) its upper jaw was found along with the bones of Amargasaurus.

It is assumed that the spiny processes of the cervical and dorsal vertebrae of Amargasaurus were exposed to the outside. The rough surface of the upper part of the spine-like processes indicates that in living dinosaurs they may have been covered with a horny substance. At the same time, it is not known whether they were separated from each other, or connected to each other by a skin membrane.

In 1991, Leonardo Salgado and José Bonaparte suggested that the spines of Amargasaurus were a morphological adaptation designed to protect against carnivorous dinosaurs, as evidenced by the pointed shape of the spines. According to these researchers, spiny processes could also serve as a display function in front of their relatives, that is, serve to attract females or to scare away rivals.

In 1997, paleontologist Jack Bailey.: Jack Bailey) pointed out that the spine-like processes of Amargasaurus had similarities with the spinous processes of pelycosaurs (lat.: Pelycosauria), which formed a leathery dorsal crest (“sail”), which, in his opinion, indicates that Amargasaurus also had such a crest.

True, unlike pelycosaurs, the spine-like processes of Amargasaurus were forked and formed 2 parallel ridges. However, since the distance between the two rows of spines was only 3-7 cm, Bailey ruled out the possibility that the dinosaur had 2 separate “sails”. Instead, 2 rows of spikes, in his opinion, formed a frame, completely covered on the outside with a single layer of leather.

In addition, this researcher pointed out that, starting from the penultimate dorsal vertebrae, the spine-like processes differed significantly in shape from the processes of previous vertebrae. According to Jack Bailey, these oar-like processes are similar to those of modern humpbacked ungulate mammals, such as bison, which may indicate that in the back of the body the crest of Amargasaurus turned into a fleshy hump.

Salgado and Bonaparte suggested that the Amargazavr was able to move only at low speed, since its forearms and lower legs were proportionally short, like other animals that were not adapted to quick run. On the contrary, Gerardo Mazzetta and Richard Farinya (POP.: Richard Fariña) In 1999 they came to the conclusion that the Amargazaur was capable of quick running: these paleontologists argued their conclusion that, when running the bones of the hind limbs of the dinosaurus, they were especially strong under the influence of bending moments and, insufficient resistance of the bones of the limbs The bending moments is a limiting factor for the maximum speed of any animal.

Mazzette and Farinya explained that the bones of the rear limbs of Amargazaur were even more powerful than that of a modern white rhino (lat.: Ceratotherium simum), which is adapted for galloping.

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