Temporal range: Albian
|An artist's illustration of Irritator challengeri|
David Martill et al., 1996
| †Irritator challengeri|
David Martill et al., 1996
Irritator (meaning "Irritating") was a genus of Spinosaurids, living during the early Cretaceous Brazil. It is estimated to be around 8 meters (26 feet) in length and around 1 metric ton. Like most Spinosaurs, Irritator had a small back sail and was most likely a piscivorous beast.
The main fossil piece is that of the skull, which bares a resemblance to that of Suchomimus tenerensis and Spinosaurus aegyptiacus. The genus may be a synonym to the genus Angaturama. The so far only species is Irritator Challengeri.
The first fossil, a skull, was found in the possession of fossil poachers, who had artificially added more teeth and plastered it to get a better price. The scientists who bought the fossil were unaware of this, and thus it took the poor paleontologists hours and hours to bring the skull to it's former glory, which is how the genus got its name. They also found the fossil had been artificially elongated, adding to the irritation.
The current fossil extraction point is not confirmed. After questioning the fossil dealers, they pointed to a village near Santana Do Cariri. The skull is considered to be the most complete of any Spinosaur skull. The skull is approximately 84 centimeters (33 inches) in length. It possesses a Sagittal crest on its forehead. The teeth were most likely went through a continual tooth change, and were estimated to be up to 40 millimeters in length.
In 2004, parts of a spinal column were discovered in the Santana Formation, the same origin as the skull. They rose high to form a small sail, and because of this, were assigned to Irritator.
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 University of São Paulo.
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 differed from teeth of other theropods, which seemed geared towards tearing or cutting off seized body parts. Particularly with Irritator and Suchomimus tenerensis 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 they 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. One more thing for awhile the only part of skeleton that was found was a skull.
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