Temporal range: Early Cretaceous, 110 Ma
Martill et al., 1996
Irritator is a genus of spinosaurid theropod dinosaur that lived during the Early Cretaceous Period (Albian stage), around 110 million years ago, of what is now Brazil. It had an unusually shaped crest at the rear of its snout, and most likely consumed fish, fossil evidence is known of an individual that ate a pterosaur, possibly from hunting or scavenging it. Current estimates indicate a length of 8 meters (26 feet). In 2010, Gregory S. Paul gave lower estimations of 7.5 meters (24 ft) and 1 tonne (1.1 short tons)
It is only known from a partial skull from the Romualdo Member, a layer member of the Santana Formation. This skull strongly resembles the skulls of Suchomimus and Spinosaurus. The genus is often regarded today as identical (synonymous) with Angaturama, which lived in the same time and the same place as Irritator.
The holotype fossil of Angaturama consists only of the front part of the head; kept today under the number USP GP/2T-5 in the University of São Paulo. It is characterized particularly by its unusual length and curved lip region, which is strongly compressed laterally. In the premaxilla (snout), a broken-off tooth with partial tooth crown was recovered. The strongly extended and straight teeth with conical tooth crowns, which are 6 to 40 mm (2.3 to 15.7 in) long, are singly embedded. This indicates continuous tooth replacement where new teeth were pushed up between the old ones. Judging by the tooth sockets, altogether the premaxilla had seven teeth; the third tooth was the largest.
History of discovery
Near Santana do Cariri in eastern Brazil, commercial fossil-poachers dug up a chalk nodule containing a large skull. This nodule was acquired by fossil traders who illegally sold it — the trade of fossils is since 1973 prohibited by law in Brazil — to Rupert Wild of the State Museum of Natural History Stuttgart. At the time it was assumed to be the skull of a giant basal pterosaur. The region is famous for its pterosaur finds, and the German museum had often bought such pieces. As it promised to be a unique discovery of singular importance, German and British pterosaur experts were contacted to study the exemplar. A paper describing the specimen as a pterosaur had already been submitted for publication, when the authors were quickly disabused of the notion it had been a flying reptile by the peer review, in which it was pointed out that the fossil belonged to a theropod.
Irritator was first scientifically described and named in 1996 by David Martill, Arthur Richard Ivor Cruickshank, Eberhard Frey, Philip G. Small and Malcolm Clarke. Its only known fossil, a sixty centimetres long skull part, was badly obscured by plaster which was added by the fossil traders in hopes of making the fossil look more complete and valuable. For example, "the posterior portion of the saggital [sic] crest... [was] fabricated by fossil dealers." The buyers were not aware of the modifications to the illegally collected specimen, and it required them a great deal of work to reconstruct the original features — hence the name. Martill et al. (1996) wrote that the generic name Irritator came "from irritation, the feeling the authors felt (understated here) when discovering that the snout had been artificially elongated." The type species is I. challengeri, which honors the character of Professor Challenger in Arthur Conan Doyle's The Lost World.
The Romualdo Member of the Santana Formation is generally assigned to the Albian and thus the last section of the Early Cretaceous. The layers are aged to approximately 110 million years and to have come from a time when the continents of Africa and South America were still connected with one another in the northern part of Brazil. The material of I. challengeri, not counting that of A. limai, likely hails from the Romualdo Member of the Santana Formation in Brazil. The holotype is SMNS 58022, from the German name of the institution, Staatliches Museum für Naturkunde Stuttgart, and it consists of an incomplete skull and lower jaws, lacking the anterior (front) portion. The exact discovery site of the Irritator fossil skull is unknown; the fossil was collected by illegal fossil dealers. The skull and the matrix have been assigned to a rock of the Romualdo Member due to lithological characteristic. This classification was confirmed by microfossils of the ostracod Pattersoncypris. Questioning of local fossil dealers resulted in the identification of the discovery site near the village of Buxexé close Santana Do Cariri at the flank of the Chapada do Araripe, at a height of approximately 650 meters. Since rock from the Romualdo Member is indeed exposed there, this discovery site is regarded as very probable for the fossil.
In the year 2004 parts of a spinal column were discovered in the Santana Formation. These have been assigned, due to their structure, to the Spinosauridae. With a very high probability these fossils belong to Irritator, since this is the only well-known spinosaurid in the formation so far.
Synonymy with Angaturama
Angaturama limai, from the same time and place as Irritator challengeri, was described by Kellner and Campos in February 1996 on the basis of a fossil from the Santana Formation. The generic name is in reference to Angaturama, a protective spirit in the aboriginal Tupi Indian culture of Brazil, and the specific name alludes to paleontologist Murilo R. de Lima, who informed Kellner of the specimen in 1991. Later research uncovered 60% of the complete skeleton, allowing a replica to be made and mounted for exhibit at the Federal University-owned Museu Nacional do Rio de Janeiro (National Museum of Brazil). The remains of Angaturama were diagnosed by very strong lateral compression of the snout, and a thin sagittal crest (shape unknown) on top of the premaxilla.
Some sources consider Angaturama a synonym of Irritator, citing the fact that the remains of Angaturama allegedly seem to complete Irritator's skull (meaning that they could belong to the same specimen). However, substantial objection has been raised to this assertion. Kellner and Campos (2000) and Machado and Kellner (2005) both expressed the opinion that the fossils come from two different genera, and that the fossil of Angaturama was clearly more laterally flattened than Irritator. A review of both fossils by Sales and Schultz (2017) also noted that the specimens differ in other aspects of their preservation: the Irritator specimen is brighter in color; the Irritator specimen is affected by a vertical crack while the Angaturama specimen bears many cavities; and the damage to the teeth of the Irritator specimen is much less severe. They also identified a possible point of overlap, the third left maxillary tooth, and observed that the skull of Angaturama may have been larger than that of Irritator based on the proportions of Baryonyx. Nevertheless, although the two specimens evidently do not belong to the same individual, Sales and Schultz noted that synonymy at the genus level would need to be verified by more extensively overlapping remains. If Angaturama and Irritator are actually regarded as a member of the same genus, Irritator challengeri would be the valid scientific name under rules of priority.
I. challengeri is a member of the Spinosauridae family, more specifically the subfamily Spinosaurinae. It shares a close relationship with Spinosaurus and possibly Siamosaurus, though the latter genus is not well known from fossil material. Irritator was originally described as a Maniraptoran within the Tetanurae. It was then assigned to the family Baryonychidae, along with Angaturama, Baryonyx, Suchomimus and Spinosaurus by Oliver Rauhut in 2003. Holtz et al. (2004) considered the Baryonychidae synonymous with the family Spinosauridae, and placed these genera within that family. Most later revisions have upheld these classifications.
Irritator likely nourished itself on fish, like the pterosaurs found in large numbers at the Santana Formation. It was probably, like today's crocodiles, a food generalist, eating all other animals that it could catch besides fish. A tooth belonging to Irritator still inserted into a fossil neck vertebral column of a pterosaur, indicates that it ate pterosaurs as well, although it is not known if it actively hunted these animals, or simply scavenged the remains.
All spinosaurids had very narrow jaws with relatively homogeneous pointed teeth. This arrangement is found particularly in crocodiles such as the false gharial. The long conical teeth, which do not possess serrated edges, are suitable for the grabbing and holding of prey. They differed from teeth of other theropods, which seemed geared towards tearing or cutting off seized body parts. Particularly with Irritator and Suchomimus a convergence with crocodiles is regularly discussed in the literature. Individual fossils belonging to the Spinosauridae were regarded in the past as crocodile fossils, such as in the case of Baryonyx, whose fossils from Portugal were originally described as Suchosaurus; at the time thought to be a genus of crocodile. Only in 2007 were the fossils recognized as those of a spinosaurid.
The nostrils of Irritator were shifted far to the rear of the skull, and the secondary palate make respiration possible even if the majority of the jaw was under water or held prey. In particular, the sagittal crest of Irritator is an indication for a pronounced neck musculature, which would have been necessary in order to pull the jaw closed quickly against water resistance and withdraw the head fast. Sues et al. (2002) point out, however, that there would be no reason to assume that the Spinosauridae specialized completely in fishing. They stress rather that this head morphology indicates a generalistic feeding, particularly on small prey animals. In fact, portions of a young Iguanodon, a terrestrial herbivore, were found inside the fossil skeleton of one Baryonyx. Naish et al. (2004) support the theory that Irritator hunted both aquatic and terrestrial animals as a generalist within the coastal area and in addition probably also searched for carrion.
The horizon of the Santana Formation, in which both fossils were found, resulted with very high probability from sedimentation in a flat lake, which was filled with fresh or brackish water. The fossil finds made so far create an ambivalent picture. The fossil insects which have been recovered are an indication for fresh water; the find of the turtle Santanachelys, which was adapted to seawater, indicate a saltwater environment. One theory is that the site was a brackish lagoon, which was connected to the sea. The climate was tropical and corresponded to today's climate in Brazil to a large extent.
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