Irritator

Irritator is a genus of spinosaurid dinosaur that lived during 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 80 cm skull discovered in eastern Brazil, was badly obscured by plaster which was added by the commercial fossil-collecting fossil-poachers who illegally sold it (the trade of fossils is prohibited by law in Brazil), in hopes of making the fossil look more complete and valuable. 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.

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 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.