Herrerasaurus or Herrerasaurus: one of the most ancient theropods

Herrerasaurus, or Herrerasaurus (lat.: Herrerasaurus) is one of the most ancient and primitive genera of theropods. Fossils of this carnivorous dinosaur were found in deposits of the early Upper Triassic period (Carnian) in northwestern Argentina.

This genus was first scientifically described in 1963 by the Argentine paleontologist Osvaldo Reig using the only known species Herrerasaurus ischigualastensis as an example. At the same time, the generic name Herrerasaurus means “Gerrera lizard” and is given in honor of Victorino Herrera, a local shepherd who found the first fossils of Herrerasaurus in 1959 near the city of San Juan, and the specific name ischigualastensis comes from the name of the formation Ischigualasto (Spanish).: Ischigualasto). Thus, the full name of the species can be translated into Russian as “Herrera’s lizard from ExtraGalango”.

Herrerazaurs lived on Earth about 235-228 million years ago in the early Upper Triassic period (Carnius), that is, at a time when dinosaurs were not yet dominant land vertebrates.

The world of vertebrates in the formation of the eagerly and more young formation of the Los Colorados (Los Colorados) was mainly represented by many archosaurs from the Taxonomic Group of Krugoratsi (Crutarsi), as well as synapsides (Synapsida). In the formation of eagerly, only 6 % of fauna accounted for dinosaurs. Nevertheless, by the end of the Triassic period, they became dominant animals of land, however, this was accompanied by a reduction in the variety and numbers of other groups of archosaurs and synapsides.

It is assumed that Herrerasaurus was an active predator, as evidenced by its flexible jaws. He was larger and presumably more heavier than his smaller relative of the Stavricosaurus. Superobjects of the fauna of the formation of the Estigolast formation were presumably large ravizuhia (Rauisuchia). Herrerazavra and Ravizuhia hunted, possibly, on herbivorous rinhosaurus (Rhynchosauria), as well as synapsides.

Relating to the formation of the outgraduate deposit in which the fossil remains of Herrerazaurus were discovered are dated to the late Ladinsky tier to the early Carnia (Upper Triassic). In 1986, two more partial skeletons with skulls were described by the Argentine Paleontologist Fernando Novas as copies of the Frengielizaur (type of freenguellisaurus ischigualastensis), but are also currently classified as the remains of Herrerazavr.

Studies of early dinosaurs such as Herrerasaurus form an important basis for defining dinosaurs as a monophyletic group. So, the work of Paul Sereno (Paul Sereno), dedicated to the Herrerasaurus (1992.), suggests that many of the common newly acquired characters of early dinosaurs were not common derived characters (synapomorphies), but developed independently of each other (convergently) in different groups of dinosaurs.

The Ischigualasto Formation contains coprolites (petrified droppings) containing small bones but no plant remains that could have belonged to Herrerasaurs. The results of mineralogical and chemical analyzes of these coprolites, if the assumption of their ownership is correct, show that these dinosaurs were able to digest bones.

Comparison of the bony scleral rings of Herrerasaurus and modern birds and reptiles suggests that Herrerasaurus may have led a cathemeral lifestyle, that is, it became active at short intervals both during the day and at night.

One of the known skulls of Herrerasaurus showed signs of three healed stab wounds, which were inflicted at different angles and located on different parts of the skull, which indicates that they were received independently of each other. At the same time, the wounds were infected, as indicated by porous ridges around them, interpreted as traces of swelling.

These injuries may have been caused by a large saurosuchus rauisuchus, but fights with other herrerasaurs were more likely to have occurred, as indicated by the moderate size of the wounds and the fact that they did not lead to the death of the affected dinosaur.

Herrerasaurus was a relatively small carnivore with a light build. It is assumed that the length of his body was from 3 to 6 meters, and the height of the hips exceeded 1.1 meters. Herrerasaurus weighed presumably 250-350 kg.

The length of the skull of a very large specimen, which was previously attributed to a separate genus Frenguellisaurus (Frenguellisaurus), is 56 cm. In smaller specimens, it did not exceed 30 cm. The skull was long and narrow, like that of ancient archosaurs such as Euparkeria, but lacked the specialized features typical of other dinosaurs.

It had 5 cranial fenestrae on each side: a large antorbital fenestra between the orbit and the nasal cavity, a narrow promaxillary fenestra only 1 cm long, and a large infratemporal fenestra behind the orbit.

The lower jaw of Herrerasaurus had a flexible joint that made it easier to hold prey in the mouth. This specialized feature is not inherent in dinosaurs and evolved convergently (independently) in lizards. The jaws of the reptile were filled with large teeth with serrated edges. The neck was thin and flexible.

The hind limbs of the dinosaur were more than twice as long as the front ones. At the same time, the shoulders and forearms were short, and the hands were elongated. The thumb and first two fingers of the hand ended in a sharp sickle-shaped claw. The fourth and fifth fingers were reduced and did not have a claw.

Unlike most reptiles of that time, Herrerasaurus was completely “bipedal”, that is, it moved exclusively on two hind limbs, which were strongly built and had a short thigh and relatively long foot, which indicates that this predator ran quickly.

The foot had 5 fingers, and the body weight of the dinosaur was distributed only on the middle three of them (fingers II, III and IV). The extreme toes (toes I and V) were small; while on the first finger there was a small claw. To balance the body when running, the tail of the reptile was partially devoid of mobility due to the fact that the processes of some tail vertebrae were pulled over the bodies of others.

Herrerasaurus has a mosaic of features from various derived (later) dinosaur groups, as well as basal archosaurs. Although most of the characteristics of Herrerasaurus are shared with dinosaurs, its other features, in particular the structure of the pelvic bone and bones of the hind limbs, are primitive and not typical of dinosaurs.

His pelvis was similar to that of lizard dinosaurs, but featured a ossified acetabulum that was only partially open. The ilium rested only on 2 sacral vertebrae, which is a basal sign. At the same time, the pubic bone was turned backwards, which is already a derived feature common to Herrerasaurus, the Dromaeosauridae family, and birds. The end of the pubic bone was L-shaped, like an aveteropod (Avetheropoda). The vertebral bodies were hourglass shaped like those of an Allosaurus.

The genus Herrerasaurus is an eponymous (naming) member of the Herrerasaurid family (Herrerasauridae), which was distributed in the Upper Triassic period and includes several of the most ancient and basal genera of dinosaurs.

The relationship of Herrerasaurus remained unclear for a long time, since the remains of its specimen, initially known to scientists, were very fragmentary. While some researchers have considered this reptile a basal theropod, others have considered it a basal sauropodomorph or a basal lizard dinosaur.

So, Reig suggested that this dinosaur was an early representative of the infraorder Carnosaurs (Carnosauria). In contrast, the English paleontologist Rodney Steel in 1970 classified Herrerasaurus as a member of the Prosauropoda subgroup. Another Englishman, Peter Galton, declared it not to be further classified as a lizard dinosaur.

Other researchers considered Herrerasaurus and Staurikosaurus to be very primitive non-lizard dinosaurs. In addition, scientists such as, for example, Martín Ezcurra, suggested that this dinosaur did not belong to either theropods or sauropodomorphs, but was a member of the Eusaurischia clade, sister to the basal lizard dinosaurs (Saurischia), and was very close to them. Some even ruled out his belonging to dinosaurs.

At the same time, the question of what other genera of dinosaurs should be classified as herrerasaurids is debatable. This family may have included the Sanjuansaurus, also found in the Argentine Ischigualasto Formation (Spanish).: Ischigualasto), a stauricosaurus from the South Brazilian Santa Maria Formation, a Chindesaurus from the Petrified Forest of the Chinle Formation of Arizona, and a Caseosaurus from the Texas Dockum Formation.

Other basal theropods could also be related to herrerasaurids, including Alwalkeria from the Indian Maleri Formation and Teyuwasu, known only from very fragmentary remains from Brazil.

Finally, in 1988, a group of researchers led by Paul Sereno managed to discover an almost completely preserved skeleton of a reptile with a skull. On the basis of newly obtained bone material, scientists such as Thomas Richard Holtz and José Fernando Bonaparte classified Herrerasaurus as a basal lizard dinosaur, not related to sauropodomorphs and theropods.

Sereno, on the contrary, preferred to consider him (and all herrerasaurids) as basal theropods. Most recent research supports both of these hypotheses: for example, Oliver Rauhut (in 2003.), as well as Jonathas Souza Bittencourt and Alexander Wilhelm Kellner (in 2004.) were supporters of the theory that Herrerasaurus belonged to basal theropods, while Max Langer (in 2004.), as well as he and Michael Benton (in 2006.), Randall Irmis (Randall Irmis) and co-authors (in 2007.) supported the hypothesis of a basal lizard dinosaur, with many researchers believing they are dealing with the oldest known theropod.

If Herrerazavr really were aeropode, it would mean that the teropodes, starfodomorphs and poultry dinosaurs (Ornithischia) developed before the evolution of Herrezavrides, possibly even before Middle Cartia, and that these large groups of reptiles acquired different typical signs for dinosaurs. , such as the ankle joint or an open switch extended forward.

In favor of this hypothesis, large three-fingered traces are probably left by the teroenal dinosaurs: these traces were discovered in the Argentine formation of Los Rastros and belong to the Ladinsky tier (middle trias), that is, they are 3-5 million older than them fossil remains of Herrerazaur.

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