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Classification


Martill and his team originally believed Irritator to be an unusual new genus of maniraptoran dinosaur in the clade Bullatosauria (now Maniraptoriformes ), as a close relative to the feathered ornithomimosaurs and troodontids. Given that its long snout, dental morphology, and assumed fin-shaped crest were features unknown in "other" maniraptorans, the researchers erected the new family Irritatoridae within the clade, based on these traits. They recognized Irritator 's affinities with Spinosaurus, in that they both had similarly shaped and unserrated teeth; noting, however, that the latter's holotype mandible would not conform with their reconstruction of Irritator 's upper jaw, and that other non-avian dinosaurs like Compsognathus and Ornitholestes also bore no serrations on some or all of their teeth. In 1996, Kellner found no evidence supporting Martill's referral of Irritator to the maniraptora, and instead resolved the genus as a spinosaurid; a conclusion agreed upon by later studies. Irritator was then assigned to the family Baryonychidae along with Angaturama, Baryonyx, Suchomimus, and Spinosaurus by Oliver Rauhut in 2003. Thomas Holtz and colleagues in 2004 considered the Baryonychidae synonymous with Spinosauridae, and placed these genera within that family. Most later revisions have upheld these classifications.

In 1998, Paul Sereno defined two subfamilies within the Spinosauridae on the basis of craniodental characteristics. They were Spinosaurinae, in which he placed Spinosaurus and Irritator; and Baryonychinae, to which he assigned Baryonyx, Suchomimus, and Cristatusaurus. Irritator was separated from baryonychines by its unserrated, straighter, and more widely spaced teeth.  In 2005, a reconstruction of Irritator 's skull by Dal Sasso and colleagues assumed the nostrils as being located above the middle of the maxillary tooth row; more posteriorly than in baryonychines, but less so than in Spinosaurus. However, Sales and Schultz's later identification of the third left maxillary tooth's position, placed Irritator 's nostrils in a comparable position to those of Baryonyx and Suchomimus. The more forward nostril placement was typically considered characteristic of baryonychines. Nevertheless, Irritator also bore unserrated teeth, a trait associated with spinosaurines. Sales and Schultz noted that the South American spinosaurids Irritator, Angaturama, and Oxalaia might represent intermediate forms between Baryonychinae and Spinosaurinae, and that with further research and evidence, the former could eventually be rendered a paraphyletic group. Irritator is further distinguished from Baryonyx, Suchomimus, and Cristatusaurus by having almost half as many teeth in the maxilla, and from Spinosaurus due to its comparatively larger and more forwardly positioned nostril openings; which unlike in Spinosaurus, are also formed by the premaxilla. The narrow saggital crest, which ends in a knob-like process above the frontals, is another autapomorphy (distinguishing feature) of Irritator from other spinosaurs.

Although the snout of Angaturama is generally narrower than in baryonychines, part of the extreme sideways flattening of the holotype may be due to taphonomy (changes during decay and fossilization); given that the specimen appears partly crushed and broken, with some of the preserved teeth having been sectioned off along their length. Therefore, Angaturama 's only valid autapomorphy is its sagittal crest; which extends further forwards on the rostrum and is more exaggerated than in other known spinosaurid skulls.

The following cladogram shows a 2009 analysis of the Megalosauroidea.

The next cladogram follows the results of the cladistic study by Sales and Schultz in 2017.