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). In 2010, Gregory S. Paul gave lower estimations of 7.5 metres and one tonne. It was found in Brazil. Irritator was a theropod with an unusually shaped crest at the rear of its head, and most likely consumed 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 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.
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 well-known spinosaurid in the formation.
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.
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.
Source: NatGeo.com, Wiki.org, ifls.com