Irritator

Taxobox
name = "Irritator"

---

image_width = 200px
fossil_range = Early Cretaceous
regnum = Animalia
phylum = Chordata
classis = Sauropsida
superordo = Dinosauria
ordo = Saurischia
subordo = Theropoda
familia = Spinosauridae
subfamilia = Spinosaurinae
genus = "Irritator"
genus_authority = Martill "et al.", 1996
subdivision_ranks = Species
subdivision =
* "I. challengeri" Martill "et al.", 1996 (type)
synonyms =
* "Angaturama"? Kellner & Campos, 1996

"Irritator" is a genus of spinosaurid dinosaur that lived in the early Cretaceous Period (Albian stage), around 110 million years ago. Current estimations indicate a length of 8 meters (26 feet) and a height of 3 meters (9 feet). It was found in Brazil. "Irritator" was a theropod with an unusually shaped crest at the rear of its head, and probably ate fish.

So far the only fossil that has been found was an 80 centimeter long fossil skull in the Romualdo Member, a layer member of the Brazilian 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".

Discovery

"Irritator" was first scientifically described in 1996 by paleontologists Martill, Cruikshank, Frey, Small and Clarke. Its only known fossil, an 80cm skull discovered in eastern Brazil, was badly obscured by plaster which was added by the commercial fossil-collecting amateurs who discovered it, in hopes of making the fossil look more complete and valuable. It required a great deal of work to reconstruct the original features — hence the name.

It is probably synonymous with "Angaturama limai", another spinosaurid from the same time and place, whose remains curiously seem to complete "Irritator"'s skull, meaning that they could belong to the same specimen.

The type species is "I. challengeri". It honors the character of Professor Challenger in Arthur Conan Doyle's "The Lost World.

Material

Material of "I. challengeri", not counting that of "A. limai", hails from the Romualdo Member of the Santana Formation in Brazil. The holotype is SMNS 58022, from the Stuttgart State Museum of the Natural Sciences, and it consists of an incomplete skull, lacking the anterior (front) portion.

The skull was recovered nearly complete and is considered the most complete head find of a Spinosaurid. It is characterized particularly by its unusual length and curved lip region, which is strongly compressed laterally. The overall length of the complete head is estimated at approximately 84 centimeters. It possesses a clear Sagittal crest; such a comb is found also with some other dinosaurs. The teeth exhibit a single embedding of the strongly extended and straight teeth with conical tooth crowns, which indicates a continual tooth change, as new teeth were pushed up between the old ones. The teeth exhibit lengths from 6 to approximately 40 millimeters.

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 very high probability these fossils belong to "Irritator", since this is the so far the only so far well-known Spinosaurid of the formation.

"Angaturama limai", from the same time and place as "Irritator challengeri", was described by A. Kellner and Diogenes A. Campos in February 1996 on the basis of a fossil from the Santana formation. Today most sources consider "Angaturama" a synonym of "Irritator". Some scientists have even speculated that the two partial fossil skulls come from the same individual. Waiters and Campos (2000) as well as Machado and Waiters (2005) hold however the opinion that the fossils come from two different genera and that "Angaturama" had a clearly higher and laterally more flattened head than "Irritator". If "Angaturama" and "Irritator" are actually regarded as a member of the same genus, "Irritator challengeri" would be the valid scientific name under the priority rules.

The fossil consists only of the front part of the head, which is characterized by the fact that it is very narrow and carries a premaxillary sagittal crest. In the premaxilla a broken-off tooth with partial tooth crown was recovered which corresponds to that of "Irritator". Altogether the premaxilla had seven teeth; the third tooth was the largest. The fossil is kept today under the number USP GP/2T-5 in the Universidade de São Paulo.

Systematics

clade| style=font-size:80%;line-height:80%
label1= Spinosauridae
1=clade
1= ?"Chilantaisaurus"
2= ?"Suchosaurus"
3=clade
label1= Baryonychinae
1=clade
1= "Baryonyx"
2= "Cristatusaurus"
3= "Suchomimus"
4=clade
label1= Spinosaurinae
1=clade
1= "Irritator"
2= "Angaturama"
3= ?"Siamosaurus"
4= "Spinosaurus" "I. challengeri" was a member of the Spinosauridae, more specifically the subfamily Spinosaurinae. It shares a close relationship with "Spinosaurus" and possibly "Siamosaurus", though this last genus is not well-known from fossil material.

"Irritator" was originally described as a Maniraptoran within the Tetanurae. It was 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.

Paleobiology

The exact discovery site of the "Irritator" fossil skull is unknown; the fossil was collected by fossil dealers. The skull and the matrix have been assigned to a rock of the Romualdo Member of the Santana formation due to lithological characteristics; this classification was confirmed by microfossils of the ostracod "Pattersoncypris". A questioning of the 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.

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 in which the continents of Africa and South America were still connected with one another in the northern part of Brazil.

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 seawaters, 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.

"Irritator" probably nourished itself on fish, like the pterosaurs found in large number in the Santana formation. "Irritator" 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 "Irritator" 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 particularly found in crocodiles such as the Sunda Gavial. The long conical teeth, which do not possess serrated edges, are suitable particularly to grabbing and holding of prey. They differ from teeth of carnivores, which must tear or cut off the 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. For example, "Baryonyx" fossils from Portugal were originally described as "Suchosaurus" and only in 2007 were the 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 landliving 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.

References

*.

External links

* [http://home.comcast.net/~eoraptor/Megalosauroidea.htm#Irritatorchallengeri "Irritator" at The Theropod Database]