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Irritator challengeri
Topic Started: Jan 7 2012, 11:28 PM (5,559 Views)
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Irritator challengeri

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Fossil range: Early Cretaceous, 110 Ma

Scientific classification
Kingdom: Animalia
Phylum: Chordata
Class: Reptilia
Superorder: Dinosauria
Order: Saurischia
Suborder: Theropoda
Family: Spinosauridae
Subfamily: Spinosaurinae
Genus: Irritator
Species: Irritator challengeri

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). 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.

Martill et al. (1996) wrote that the generic name Irritator came "from irritation, the feeling the authors felt (understated here) when discovering that the snout had been artificially elongated."

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

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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. For example, "the posterior portion of the saggital crest... fabricated by fossil dealers." 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.

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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 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. It was named after Angaturama, a protective spirit in the aboriginal Tupi Indian culture of Brazil, and paleontologist Murilo R. de Lima, who informed Kellner of the specimen in 1991. Later research uncovered 60% of the complete skeleton, allowing a replica to be made and mounted for exhibit at the Federal University-owned Rio de Janeiro National Museum. Angaturama was diagnosed by the very strong lateral compression of the snout, and a thin sagittal crest (shape unknown) on top of the premaxillae. Fish may have formed a large part of its diet. However, a pterosaur fossil was found with an Angaturama tooth embedded in it, pointing to possible predation. Today many sources consider Angaturama a synonym of Irritator. Some scientists have even speculated that the two partial fossil skulls come from the same individual. Kellner and Campos (2000) as well as Machado and Kellner (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.

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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.

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The exact discovery site of the Irritator fossil skull is unknown; the fossil was collected by illegal 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.

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Cau reconstructing Irritator's skull.
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Irritator restoration by Fred Wierum: Posted Image
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Journal Reference:
Sales MAF, Schultz CL (2017) Spinosaur taxonomy and evolution of craniodental features: Evidence from Brazil. PLoS ONE12(11): e0187070. https://doi.org/10.1371/journal.pone.0187070

Fossil sites from Brazil have yielded specimens of spinosaurid theropods, among which the most informative include the cranial remains of Irritator, Angaturama, and Oxalaia. In this work some of their craniodental features are reinterpreted, providing new data for taxonomic and evolutionary issues concerning this particular clade of dinosaurs. The mesial-most tooth of the left maxilla of the holotype of Irritator is regarded as representing the third tooth position, which is also preserved in the holotype of Angaturama. Thus, both specimens cannot belong to the same individual, contrary to previous assumptions, although they could have been the same taxon. In addition, the position of the external nares of Irritator is more comparable to those of Baryonyx and Suchomimus instead of other spinosaurine spinosaurids. In fact, with regards to some craniodental features, Brazilian taxa represent intermediate conditions between Baryonychinae and Spinosaurinae. Such a scenario is corroborated by our cladistic results, which also leave open the possibility of the former subfamily being non-monophyletic. Furthermore, the differences between spinosaurids regarding the position and size of the external nares might be related to distinct feeding habits and degrees of reliance on olfaction. Other issues concerning the evolution and taxonomy of Spinosauridae require descriptions of additional material for their clarification.

Holotype and only known specimen.
SMNS 58022, an almost complete and articulated skull associated to the mandibular rami, lacking the anterior tip of the rostrum and the dentigerous portions of both mandibulae (Figs 2 and 3).

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Fig 2. Specimen SMNS 58022, holotype of Irritator challengeri.
A, Left lateral view. B, Right lateral view. The abbreviation for the third tooth of the left maxilla follows Hendrickx et al. . Additional abbreviations: ao.f, antorbital fenestra; c.c, crista cranii; e.n, external naris; m, maxilla; m.r, mandibular ramus; n, nasal; n.c, nasal sagittal crest; n.p, nasal process; o, orbit; pm, premaxilla; s, stapes. https://doi.org/10.1371/journal.pone.0187070.g002

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Fig 3. Dentition of Irritator challengeri.
A, Detail of the left maxillary tooth row. B, Detail of the first tooth crown preserved in situ of the left maxilla. C, Detail of the artificially detached fragment of the right maxilla bearing the right maxillary tooth row. D, Detail of the sixth tooth crown preserved in situ of the right maxilla in mesial view. Abbreviations for teeth follow Hendrickx et al. . Additional abbreviations: e.n, external naris; m, maxilla; m.r, mandibular ramus; m.u, marginal undulations; n, nasal; pm, premaxilla; r.m, rock matrix. https://doi.org/10.1371/journal.pone.0187070.g003

Revised diagnosis.
Spinosaurid that differs from Baryonyx, Suchomimus, and Cristatusaurus by the unserrated condition of its teeth and nearly half the number of maxillary alveoli (teeth). Differs from MSNM V4047 and MNHN SAM 124 by the relatively larger and more anteriorly placed external nares and the participation of the premaxillae in the anterior border of the external nares. Finally, an autapomorphy of Irritator is the presence of a sagittal crest formed by the conjoined nasals that ends in a knob-like projection over the frontals.

Exact geographical provenance unknown, but likely near Buxexé, 5 km south to Santana do Cariri Municipality, Ceará State, Araripe Basin, northeastern Brazil. Romualdo Formation; Albian, Lower Cretaceous.

Remarks and comparisons
Prior to its assignment to Spinosauridae, the unprepared skull of Irritator was thought to be a pterosaur. It was afterwards described as a theropod dinosaur, more specifically as a member of Bullatosauria. Much of this taxonomic confusion was due to the modifications and damages in the specimen, which were likely caused by its collectors or additional personnel that dealt with it before its study. Actually, the etymology of the generic name is an allusion to such problems. After a more careful preparation of SMNS 58022, Sues et al. were able to confirm the spinosaurid identity of Irritator, which was firstly proposed by Kellner.

Despite these damages, the holotype of Irritator is considered to be the most complete spinosaurid skull, at least in relation to the posterior portion (Fig 2). In fact, it is one of the few non-avian theropod specimens with one of its stapes preserved, in this case the right one (Fig 2B). Accordingly, the right side of the skull is best preserved. The amount of data recovered from Irritator must have been possible due to its fossilization within an early diagenetic carbonate concretion. This becomes even more reasonable when taking into account the scarcity of (articulated) spinosaurid postnarial bones. In this sense, Irritator reveals important aspects of the cranial morphology of Spinosauridae. The skull becomes progressively deeper towards its posterior end so that its dorsal outline projects posterodorsally (Fig 2), resembling the condition inferred for Baryonyx. This morphology is followed by other reconstructions of spinosaurid skulls but contrasts with that of Suchomimus.

Although Irritator was described and redescribed in detail, some of its craniodental features were not addressed further. For example, this taxon is remarkable because it is the spinosaurid skull with the highest number of in situ preserved teeth. However, due to the missing portion of its rostrum, including the anterior end of both maxillae, neither Martill et al. nor Sues et al. properly identified the preserved maxillary teeth in relation to their position along the tooth row.

The maxillary dentition of Irritator is composed of nearly straight conical crowns, which are labiolingually fluted but bear no serration on their carinae (Fig 3). These features are common in other spinosaurids and, particularly, the absence of serrations is characteristic of Spinosaurinae after Sereno et al. Some teeth also exhibit short marginal undulations close to the distal carina (Fig 3B). Regarding tooth size variation, in the left maxilla the first (mesial-most) crown preserved in situ is the second largest, being slightly smaller than the second crown, whereas the seven remaining teeth become gradually smaller (Figs 2A and 3A).

In spinosaurid specimens with the maxillary tooth row preserved, the largest crowns (and alveoli) are the third and the fourth. In addition, there is typically an increase in size from m1 to m4, which are followed by progressively smaller teeth. Thus, the spinosaurid maxillary dentition comprises a tooth size variation that begins with a tendency of increasing, reaching its maximum at m4, regarding the crown base length and crown height, and continues with an opposite trend. This is the case both for Baryonychinae, i.e., Baryonyx and Suchomimus, and Spinosaurinae, such as MNHN SAM 124 and MSNM V4047. This pattern is widely distributed among spinosaurids and no current evidence indicates that this condition is not also present in Irritator. The tooth size variation seen in the left maxilla of the Brazilian taxon matches the described pattern, so that the first two preserved teeth correspond to Lm3 and Lm4. This implies a total number of eleven left maxillary teeth (Figs 2A and 3A). Accordingly, Sues et al. inferred that the left maxilla would have comprised at least 11 teeth, although they neither justified their claim nor attributed a specific location within the tooth row for any of the preserved crowns. Furthermore, this number is nearly identical to that of the spinosaurine MSNM V4047, which displays 12 maxillary teeth (Fig 4C and 4D); this dissimilarity could be explained by the difference in size, interspecific variation, different ontogenetic stages, alveolar remodeling after the loss of teeth in vivo, other dental abnormalities, etc..

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Fig 4. Schematic illustration of spinosaurid cranial remains.
A, Premaxillae and maxillae of Baryonyx walkeri. B, Partial rostrum (MNN GDF501) referred to Suchomimus tenerensis. C, Specimen MSNM V4047. D, Reconstructed skull of Irritator challengeri. All specimens are aligned based on the position of the fourth maxillary tooth, whereas the last maxillary tooth (or alveolus) in each is indicated by a black star. Abbreviations for teeth follow Hendrickx et al. . Additional abbreviations: n.c, nasal contact; ao.f, antorbital fenestra; e.n, external naris. A, B, C, and D are modified from Charig and Milner , Sereno et al., Dal Sasso et al. , and Sues et al. , respectively. https://doi.org/10.1371/journal.pone.0187070.g004

Regarding the artificially detached fragment of the right maxilla of Irritator, nine teeth are preserved in situ as well as the broken base and partial impression in the matrix of the tenth (distal-most) crown. The second and the fourth are replacement teeth (Fig 3B). In general, the first four crowns are more damaged than those from the left side. This precludes an accurate estimation of their dimensions, although the mesial-most crown seems to have been the largest one. If the latter was indeed the largest, the preserved portion of the right maxilla would also comprise teeth from m4 to m11.

The identification of the maxillary teeth of Irritator implies the reinterpretation of the location of its external nares. Dal Sasso et al. considered that these were restricted to the mid-part of the maxillary tooth row. However, their anterior-most outline must have been placed somewhere between m3 and m4 (Figs 2A, 3A and 4D). Therefore, the external nares of Irritator begin in a comparable position to those of Baryonyx and Suchomimus, in which the external nares start between m2 and m3 and between m3 and m4, respectively (Fig 4A and 4B). On the other hand, the external nares of MSNM V4047 start only at the level of m9 (Fig 4C).

Another difference between Irritator and African material attributed to Spinosaurus is the relationship between the bones that bind the external nares. In the former genus, the premaxillae form a minor part of the ventral outline of the nares, whereas their dorsal and ventral outlines are surrounded by the nasals and maxillae (Figs 2A, 3A and 4D). In MSNM V4047, the premaxillae are completely excluded from the external nares (Fig 4C). Both spinosaurids differ from Baryonyx and Suchomimus; in these, the premaxillae are major components of the external nares, but they differ from each other in relation to the contribution of the maxillae. In Baryonyx, the maxillae form the posterior half of the preserved ventral border of the left naris (Fig 4A). In Suchomimus, the maxillae are minor components of the ventral outline of the nares and are constricted by the premaxillae and descending rami (= maxillary processes) of the nasals (Fig 4B). Furthermore, Dal Sasso et al. considered the exclusion of premaxillae from the nares of MSNM V4047 as an autapomorphy of Spinosaurus. Among spinosaurids, MSNM V4047 is also unique in presenting premaxillae, maxillae, and nasals meeting at a single point, whereas Irritator, Baryonyx, and Suchomimus do not present such a triple joint (Fig 4).

Another important issue related to spinosaurid external nares is their size. The external nares of Irritator are both absolutely and comparatively (in relation to the antorbital fenestrae) smaller than those of the baryonychine Suchomimus and, possibly, Baryonyx (Fig 4A, 4B and 4D). On the other hand, they are larger than those of the spinosaurine MSNM V4047 in absolute length (Fig 4C and 4D), despite the Brazilian taxon being inferred to have had a shorter skull (c. 60 cm) than the African specimen (c. 175 cm).

Irritator is also unique in displaying a sagittal crest formed by the conjoined nasals that ends in a knob-like projection overhanging the frontals. In fact, where observable, the unfluted condition of the nasal crest of Irritator might also distinguish it from an isolated fragmentary pair of fused nasals from Morocco (UCPC-2) and those reported by Ibrahim et al. (and, consequently, the taxon to which they belonged). Finally, additional differences between Irritator and African material assigned to Spinosaurus might include features of the quadrates, which were well addressed by Hendrickx et al. and, as such, are not further considered here.

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