Irritator: a genus of theropod dinosaur

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Irritator (lat.: Irritator) is a genus of theropod dinosaurs, represented by only one described species (type species). This genus belongs to the spinosaurids (Spinosauriden), a group of bipedal dinosaurs with a strongly elongated mouth and straight teeth. The animal, whose length probably reached 8 meters, lived in the territory of modern South America about 113-100 million years ago in the lower Cretaceous period (Albian stage).

Description

Of the bones belonging to the irritator, only a fossil skull about 80 cm long has been found so far, found in the Romualdo Bed, belonging to the suite of beds of the Brazilian Santana Formation. Since this skull is very similar to those of Suchomimus and Spinosaurus, it is assumed that the irritator, like these other two genera, had elongated spinous processes on the vertebrae.

According to the current generally accepted opinion, irritator is considered not a genus, but a species, and is identified with another species, Angaturama limai, which lived in the same period and in the same territory and is known only from a fragment of the front of the skull.

The first scientific description of the irritator was made in 1966 by paleontologists Martill, Cruickshank, Frey, Small and Clarke. This description was based on a skull, a stitch found in the formation of Santan in the southern part of the Brazilian state of Ceara Basin Araripe (Araripe). The skull lay in the layer of romuald belonging to the lower chalk period.

It was found by private collectors. They tried to supplement the missing fragments of the fossil skull with a gypsumlike mass for modeling, which later complicated its reconstruction with drugs. In addition, with the help of computed tomography, it was possible to determine that the skull was artificially elongated by attaching parts of the upper jaw to the predominant bone. Currently, this skull is stored in the collection of the State Museum of Natural Sciences in German Stuttgart under the number of SMNS 58022.

It has been preserved almost completely and is considered the most complete of all the skulls of the dinosaurs of the Spinosaurids family. It is distinguished primarily by its unusually long, arched and strongly laterally compressed mouth. The total length of the skull was supposedly 84 cm. It has a distinct sagittal crest running from the forehead through the nasal shields to the premaxilla,.e. along the entire skull; and although such a crest is found in some other dinosaurs, its presence has been confirmed in only a few theropods. The nostrils are displaced back almost to the orbits, and due to the fusion of the premaxilla with the secondary hard palate, the internal nasal openings, choanae, are also displaced back towards the pharynx.

Unique in fit, the highly elongated and straight teeth with conical apices featured mesolingual replacement, in which new teeth grew in between the existing teeth in use; based on its close relationship with the irritator, the same type of tooth change is currently attributed to the spinosaurus. At the same time, the length of the teeth of the holotype of the irritator is approximately from 6 to 40 millimeters.

The almost completely reduced upper part of both temporal (supratemporal) windows, and other features of the anatomy of the skull, allow a clear distinction between the irritator and other known genera of dinosaurs. A number of characters, such as, for example, a small number of teeth in the upper jaw and their shape, distinguish this genus from Spinosaurus. However, the presence of many other characters in the irritator, which were supposedly characteristic of higher taxa, cannot be confirmed with certainty, since specimens of these taxa often lack a skull.

In 2004, parts of the spine were found in the same Santana formation, including 3 sacral and 6 caudal vertebrae, which, judging by the structure of the vertebral bodies, could presumably belong to spinosaurids. At the same time, with a very high degree of probability, they also belong to the irritator, since it is the only so far known representative of spinosaurids in this formation.

The species Angaturama limai, which lived at the same time as Irritator challengeri, was also first described in 1996 (in February) by Alexander V.BUT. Kellner (Alexander W. A. Kellner) and diogenes. Camposom (Diogenes a. Campos) based on just one bone fragment from the same Santan formation and today it is identified by a majority of paleontologists with irritator.

Individual scientists even suggest that both of the aforementioned fragment of the remains are parts of the skull of the same Irritator individual. True, Kelner and Kampos (in 2000), as well as Machado (Machado) and Celner (in 2005) made a counterproposal that it was still about 2 different forms, and the Angaturama was compared to the irritator significantly higher and more flattened skull from the sides.

A fragment of the remains of the Angurama consists only of the front of the skull, which differs in that it is very narrow and has a predominant sagittal crest. On the premaxilla, one broken tooth with a partially preserved apex, corresponding to the irritator’s tooth, has been preserved; judging by the number of tooth sockets, there were 7 teeth in total on the premaxilla, with the third of them being the largest.

The described fossil is currently stored under USP number GP/2T-5 at the University of São Paulo. It has been described as the first known fragment of a dinosaur skull from Brazil, but Irritator challengeri was described while the angaturama remains were still under investigation, and thus is in fact the first described dinosaur from the Santana Formation.

If we consider angaturama and irritator as different species of the same genus, then the correct scientific generic name should be the name “irritator” assigned to the species Irritator challengeri, which was described first (priority rule).

The exact location of the discovery of the remains of the irritator is unknown, since the skull became the property of science only after it was in the hands of traders and fossil collectors. The mass of rock, the matrix in which the skull was enclosed, could be presumably attributed by lithological features to the rock of the Romualdo formation in the suite of the Santana formation beds; and this classification was confirmed by the presence of microfossils of ostracods (a class of crustaceans) of the genus Pattersoncypris, as well as fish scales, which can be attributed to the genus Cladocylus of the Ichthyodectidae family.

A survey of local fossil dealers conducted by the authors of the first description testified that the skull was found near the small rural settlement of Buxexé, near the town of Santana do Cariri, on the edge of the Chapada do Araripe plateau. do Araripe), at an altitude of about 650 m above sea level. Since it does indeed contain rock from the Romualdo Formation, this find site is considered very likely.

The Romualdo Bed in the Santana Formation is usually assigned to the Albian Stage, the last segment of the Lower Cretaceous, based on the fish fauna present in it, although the presence of ostracod microfossils, often used in biostratigraphy, allows it to be assigned to the Aptian Stage. Accordingly, the age of the layers is about 110 million years, when the continents of Africa and South America were still connected to each other in the northern part of modern Brazil.

The Santana Formation horizon in which both fossils were found was very likely formed by sedimentation on the bottom of a shallow lake filled with fresh or brackish water. As for the degree of salinity of the water, the remains of dinosaurs found so far give a contradictory picture for example, the presence of some freshwater fish indicates fresh or low-salinity water, but at the same time, fish species that are considered typically marine were found there.

The remains of insects found in the same place testify, rather, in favor of fresh water, but the find of the (sea) turtle Santanachelys gaffneyi adapted to salt water, on the contrary, suggests that the water in this part of the formation was salty. Therefore, the most recognized theory is that there was a brackish lagoon in this place, connected to the sea. The climate was warm, tropical, and almost completely corresponded to the climate of modern Brazil.

Food

Irritator challengeri, like many of the pterosaurs found in the Santana Formation, probably fed mainly on fish that they hunted in the lake that existed at that time. At the same time, the irritator, like modern crocodiles, was probably a universal predator that fed not only on fish, but also on any other animals that it could catch.

A fossil of a pterosaur cervical spine with a tooth embedded in it, which is attributed to the irritator, is considered solid evidence that pterosaurs were included in the irritator’s diet. At the same time, it is not known whether he hunted them or devoured only fallen animals.

The fact that this dinosaur ate mainly fish is indicated by the elongated and very narrow mouth of all spinosaurids with sharp teeth of relatively the same shape.

The elongated conical teeth, which do not have sawtooth edges, are primarily suitable for capturing and holding prey, and this differs from the teeth of predators, which are designed to tear or cut parts of the prey after it has been grasped. The nostrils displaced far back, as well as the secondary hard palate, allowed the irritator to breathe even when the main part of the mouth was under water or held prey. In addition, especially the sagittal crest of the irritator indicates the presence of a pronounced cervical musculature in the irritator, which is necessary in order to overcome the water resistance with the muzzle and quickly turn the head back.

However, in 2002, Suez and his colleagues found that the assumption that spinosaurids specialized exclusively in catching fish is unfounded. They argue that such a skull morphology, on the contrary, testifies to the universal nature of the diet, which includes catching small animal prey and fish, and at the same time they cite the fact that parts of a young iguanodon, which is just a land herbivore.

In 2004, a group of paleontologist Naish (Naish) confirmed this assumption about the diet of the irritator and stated that it was a universal coastal predator and hunted both aquatic and land prey, and, moreover, probably also fed on carrion.

In 1996, the authors of the first description of the species Irritator challengeri assigned it to the Maniraptora clade within the Tetanurae group. Within this clade, they classified it as a separate family of irritorids (Irritatoridae), which they assigned to the superfamily of Bullatosaurus (Bullatosauria). They assigned Angaturama as a new genus to the spinosaurids, and in the same year Kellner assigned the irritator to the same family. This classification was adopted by Suez’s scientific group in 2002 for fossils, which by that time were already recognized as identical, so that at present such a classification is considered generally accepted.

The scientific group of Paul Sereno presented in 1998 in the first description of suchomimus (Suchomimus tenerensis) a phylogenetic genealogical tree of spinosaurids, in which this family was divided into 2 subfamilies: spinosaurines (Spinosaurinae) and baryonychines (Baryonychinae), and the irritator was defined as sister species of the genus Spinosaurus.

In 2006, this family tree was confirmed by paleontologist Dal Sasso and his colleagues. According to this view, Suchomimus and Baryonyx, the only species of which, Baryonyx walkeri found in England, was described in 1986, together form the subfamily of Baryonychines, while Irritator and Spinosaurus form the subfamily of Spinosaurines. In addition, Dahl Sasso and colleagues added to the baryonychins the cristatusaurus (Cristatusaurus lapparenti) described in 1998, which had not yet been described in the original classification of Paul Sereno’s scientific group.

As a sister family to spinosaurids, they call torvosaurids (Torvosauridae), which include the genera Torvosaurus (Torvosaurus) and eustreptospondyl (Eustreptospondylus), although these 2 genera are currently classified as megalosaurids (Megalosauridae).

Such a family tree is substantiated primarily on the basis of the signs of the morphology of the skulls, since this part of the skeleton is present at least partially in specimens of most dinosaur species. Merging into the superfamily Spinosauroids (Spinosauroidea) is justified primarily by the fact that its representatives had developed powerful forelimbs with a sickle-shaped claw on the big toes.

As a result of specialization in fishing, the spinosauroids had an elongation of the mouth region due to the simultaneous elongation of both the upper jaw (Maxillare) and the premaxilla. At the same time, the specific shape of the nasal cavity, as well as the formation of a secondary hard palate, are very significant unique features (apomorphies) that confirm the monophyly of spinosaurids.

In addition, scientists cite such features as nostrils displaced far back and conical teeth, a small postnasal cranial fenestra, and a specific shape of the brain skull, which, compared with the brain skulls of other theropods, is relatively short, but deep. In spinosaurines, there was a decrease in the number of teeth of the premaxilla and upper jaw with a simultaneous increase in the distances between the teeth, as well as the formation of almost straight teeth, which are present in both the irritator and the spinosaurus.

The opposition of Spinosaurus to the irritator is mainly based on the presence of strongly elongated spinous processes on the spine and the resulting dorsal crest, although it is not known how it looked like in the irritator.

Although even after joining the two known fossils, the remains of the irritator consist of only one skull, it is possible to perform a relatively realistic reconstruction of the entire animal. In this case, paleontologists often refer to species that are classified as the closest relatives of the irritator, and whose specimens exist or did exist (for example, the Spinosaurus skeleton stored in Munich was destroyed during World War II) very complete skeletons.

By comparing the features, it can be assumed that the irritator has many plesiomorphic features that already existed in the general original species, including features of the position and shape of the limbs, as well as the general structure and position of the body (“posture”).

Since, for example, all fossil theropods were bipedal and terrestrial, and the same applies to all other genera of spinosauroids, we can assume the presence of the same characters in the irritator after all, it is extremely unlikely that in him these strong common characters developed apomorphically in a sharply different form.

In the same way, it can be assumed with a very high degree of probability that the irritator has almost all the features common to the genera Baryonyx, Spinosaurus, and Suchomime, which are close to it, and this allows for a basic reconstruction of this dinosaur. On the contrary, it is impossible to assume with certainty that it has a dorsal crest, which is confirmed only in Spinosaurus; however, since suchomime also had highly elongated spinous processes on the vertebrae, it can be assumed that the irritator had at least this feature.

The scientific group of David Martill (David Martill) described in their report, published in 1996, both the genus Irritator and the species within this genus. The authors of this classification explain their generic name “irritator” (lat. and English.: Irritator) by being irritated (engl.: irritation) when an artificial mouth extension was discovered on a fossil skull. And the specific name “challengeri” was given by them in honor of the fantastic Professor Challenger, the hero of the novel by Arthur Conan Doyle (Arthur Conan Doyle) “The Lost World”. Thus, Irritator challenger became the second species, after the pterosaur Arthurdactylus conandoylei (also found in the Santana Formation), whose name is associated with Conan Doyle.

The generic name “Angaturama”, given by the paleontologists by the cellner and the camposos of the fossil described by them, in the translation from the language of the dull means “noble”. The species name Limai was given by them in honor of the Brazilian paleontologist Murilo Rodolfo de Lima, who discovered and conveyed this fossil to the authors of her first description.

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