Irritator

of Brazil, with the discovery sites of three spinosaurine fossil specimens in the Araripe and São Luís-Grajaú Basins marked. From top to bottom: Oxalaia, Irritator, and Angaturama The holotype of Irritator was excavated from a chalk concretion containing the rear of a large skull with lower jaws near the town of Santana do Cariri in northeastern Brazil. This fossil was acquired by fossil traffickers and taken to the State Museum of Natural History in Stuttgart, Germany. At the time it was assumed to be the skull of a giant basal pterosaur, or flying reptile, since the Chapada do Araripe region is famous for its copious pterosaur finds, and the German museum often bought such pieces. As it promised to be a unique discovery of singular importance, German and British pterosaur experts were contacted to study the exemplar. A paper describing it as a pterosaur had already been submitted for publication when the authors, German paleontologist Eberhard Frey and British paleontologist David Martill, were disabused of this notion by the peer reviewers, who suggested the fossil belonged to a theropod dinosaur. Depictions (similar to the one at the bottom) based on this reconstruction were later featured in many dinosaur books and encyclopedias. The skull was flattened sideways somewhat and, as is common with fossils, was partly crushed. The right side was well preserved, while the left was extensively damaged during collection. Some of the skull's hindmost upper surface had eroded, and the lower jaw lacked its front end, both owing to breakage during fossilization. Parts of the specimen were also cracked due to being part of a septarian concretion. The tip of the upper jaw was also missing. Since there were no signs of erosion, it had most probably broken off during or after the fossil's collection. Evident corrosion on certain bones indicates acid preparation had been attempted. A vertical fracture was present across the middle of the skull, which had apparently been sealed with car body filler. In hopes of making it look more complete and valuable, the fossil traders had severely obscured the skull beneath plaster; The buyers were unaware of the modifications to the specimen until it was sent to universities in the United Kingdom for CT scan imaging. This revealed the collectors had tried to reconstruct the skull by grafting parts of the maxilla (main upper jaw bone) onto the front of the rostrum (snout). When Martill and colleagues first described Irritator challengeri, the holotype was still extensively encased in calcareous rock matrix. Under the supervision of American paleontologist Hans-Dieter Sues, technician Diane M. Scott from the University of Toronto Mississauga assumed the task of fully extracting the skull bones from the rock, allowing for a detailed redescription in 2002. Published by Sues, Frey, Martill, and Scott, this inspection of the now fully prepared specimen negated many of Martill and colleagues' original observations, which were based on misinterpretations of the damaged and largely concealed skull. The estimated length of the complete skull was shorter than previously proposed. What was originally thought to be a prominent head crest proved to be an unattached, indeterminate bone fragment. As in the previous study, Sues and colleagues regarded the African and possibly South American genus Spinosaurus as the most similar taxon to Irritator, because they shared many dental features, including mostly straight conical tooth crowns, thin enamel, well-defined edges with no , and lengthwise fluting. Since little was known of Spinosauruss skull at the time, these similarities were enough for the authors to suggest a possible junior synonymy of Irritator with Spinosaurus. Sues and colleagues noted that more overlapping skull material was needed for further diagnosis. Although the site of discovery is uncertain due to it having been trafficked out of Brazil and the lack of provenance documentation presented in the species description, the specimen most probably stems from the Romualdo Formation (previously designated the Romualdo Member of the then Santana Formation). tooth, as proposed by Sales and Schultz in 2017 In 1997, British paleontologists Alan J. Charig and Angela C. Milner considered Angaturama a likely junior synonym of Irritator, noting that both genera had retracted nostrils, long jaws, and characteristic spinosaurid dentition. Paul Sereno and colleagues in 1998 agreed with this possibility, and additionally observed that the holotype of Angaturama seems to complete that of Irritator (meaning that they could belong to the same specimen). Cristiano Dal Sasso and colleagues in 2005, Darren Naish in 2013, In their redescription of Irritator, Sues and colleagues pointed out that both holotypes are equally as narrow, and share transversely round teeth with defined yet unserrated edges. They also noted that a sagittal crest on Angaturamas premaxillae may correspond with that of Irritators nasal bones. A review of both fossils by the Brazilian paleontologists Marcos A. F. Sales and Cesar L. Schultz in 2017 noted that the specimens also differ in other aspects of their preservation: the Irritator specimen is brighter in color and is affected by a vertical crack, while the Angaturama specimen bears many cavities; the damage to the teeth of the Irritator challengeri holotype is also much less severe. Sales and Schultz also identified a possible point of overlap, the third left maxillary tooth, and observed that the skull of Angaturama could have been larger than that of Irritator based on the proportions of the closely related genus Baryonyx. Therefore, Sales and Schultz concluded that the two specimens do not belong to the same individual, and that synonymy at the genus level requires verification based on more extensively overlapping remains. If Angaturama and Irritator are regarded as a member of the same genus, the latter would be the valid scientific name under rules of priority, since it was named almost a month earlier. Postcranial material and additional finds and sacral vertebrae (specimen MN 4819-V), National Museum of Rio de Janeiro Besides the skull, the snout fragment, and some isolated teeth, the Romualdo Formation has also yielded remains that may belong to spinosaurids, many of which are hitherto undescribed, and all of them pertaining to the Spinosaurinae subfamily. In 2004, parts of a spinal column (MN 4743-V) were unearthed at the formation. Brazilian paleontologist Jonathas Bittencourt and Kellner assigned these, due to their structure, to the Spinosauridae. It is uncertain whether this specimen can be referred to Irritator or Angaturama, given that both are based only on skull material. In 2007, Machado and Kellner tentatively referred a rib fragment (MN 7021-V) to the Spinosauridae. The skeleton was fully described in 2010 in an as-of-yet unpublished master's thesis by Machado. In 2018, Tito Aureliano and his team described LPP-PV-0042, part of a left tibia from a particularly large individual. The skeleton depicted the animal carrying an anhanguerid pterosaur in its jaws. In press releases of the exhibit's opening, Kellner informally implied MN 4819-V as belonging to Angaturama. In September 2018, a fire broke out at the palace housing the Museu Nacional, largely destroying the fossil collections and possibly the exhibited Angaturama skeleton and fossil elements. The holotype of Oxalaia quilombensis, which was stored in the same building, was thought to have been destroyed. Luckily, the Oxalaia holotype, while heavily damaged, was recovered. ==Description==

Even by maximal size estimates, Irritator was smaller than other known spinosaurids. Gregory S. Paul calculated its length at and weight at . Thomas R. Holtz Jr. published a higher estimate of , with a weight between . Estimates by Dougal Dixon were lower at long and high. Many elements from this specimen were incorporated into the skeletal mount in the National Museum of Rio de Janeiro, which had a length of and a height of . The opening behind the orbit (eye socket), the lateral temporal fenestra, was very large, while the antorbital fenestra, in front of the eyes, was long and elliptical. The orbit itself was deep and wider at the top (where the eyeball was placed) than the bottom. The bone separated the orbit from the antorbital fenestra, forming the upper and lower rear margins of the latter with two processes that enclosed a 40-degree angle; similar to Baryonyx, where it enclosed 35 degrees. Unlike in Baryonyx, Irritators lacrimal did not form a bony horn core. The prefrontal bones were large and sturdy, while the thinner bones, situated behind them, were smooth and concave on top; both of these bones formed the upper rim of the orbit. The preserved part of Irritators crest is deepest above the antorbital fenestra and lacks the vertical ridges seen in the crest of Spinosaurus. Also like its relatives, Irritator had two additional openings on the skull roof (called the postnasal fenestrae) as well as long and only partially diverging (bony extensions connecting the braincase with the palate). The back of the lower jaw was deep, its rear upper surface consisting mainly of the large bone, which with the shallower bone below it. The , a sideways-facing opening in the lower jaw, was oval and comparatively large. The (tooth-bearing bone of the mandible) is unknown in Irritator, save for a possible remnant at the front of the surangular. Irritator challengeris holotype is unique in that it is one of the few non-avian dinosaur fossils found with a preserved stapes. MN 4819-V comprises a largely intact pelvis, some dorsal (back) and (tail) vertebrae, five (hip) vertebrae, a partial right and (shin and calf bones), most of the right (thigh bone), and part of an (forearm bone). Like in all spinosaurids, the claw of the first finger (the "thumb") was enlarged. The pelvic bones are well preserved, with the right side better articulated than the left. The fused sacral vertebrae are still attached to the pelvis, which lacks the distal ends of both of its and (lower and rearmost hip bones). The (main hip bone) is long. The (front expansion) of the ilium was curved on the bottom and was somewhat shorter and deeper than the (rear expansion). The preacetabular ala were somewhat enlarged at the front, in contrast to the more slender condition of the postacetabular ala. The (groove at the bottom of the postacetabular ala) was concave, as was the rear margin of the ischium. The pubis bore a relatively large and almost closed , an indentation in the lower margin of the rear part of the bone that allowed for the passage of the obturator nerve. The upwards projecting neural spines of the sacrum were elongated, as is typical in spinosaurs. In life, these would have been covered in skin, forming a "sail" down the animal's back. MN 4819-V is distinguished from Suchomimus due to its longer and shallower ilium with a less curved upper margin, and from Baryonyx by having a more developed , a blade-like structure on the bottom of the ischium. ==Classification==

s (Irritator in purple, first from left) compared with a human Martill and his team originally classified Irritator as a maniraptoran dinosaur in the clade Bullatosauria (a group no longer considered monophyletic Given that its dental morphology, particularly long snout, and assumed fin-shaped crest were features unknown in "other" maniraptorans, the researchers erected the new family Irritatoridae within the clade. They recognized Irritators affinities with Spinosaurus, in that they both had similarly shaped and unserrated teeth, but noted that the latter's mandible would not conform with the front of Irritators upper jaw, and that other non-avian dinosaurs like Compsognathus and Ornitholestes also bore no serrations on some or all of their teeth. Irritator was then assigned to the Baryonychidae along with Angaturama, Baryonyx, Suchomimus, and Spinosaurus by Oliver W.M. Rauhut in 2003. Holtz and colleagues in 2004 considered the Baryonychidae synonymous with Spinosauridae, and moved these genera to the latter family. Most later revisions have upheld these classifications. As spinosaurids, Irritator and Angaturama are placed within the superfamily Megalosauroidea, with Spinosauridae being a possible sister taxon to the Megalosauridae. In 2005, Dal Sasso and colleagues assumed Irritators nostrils as being located above the middle of the maxillary tooth row; more posteriorly than in baryonychines, but less so than in Spinosaurus''. |style=font-size:90%; line-height:90%;|label1=Spinosauridae}} }} Topology B: Sales and Schultz (2017) }} ==Paleobiology==

Diet and feeding , which has similarities to the head of Irritator In 1996, Martill and colleagues theorized that Irritator challengeri, with its elongated snout and unserrated conical teeth, likely had at least a partly piscivorous (fish-eating) diet. Sales and Schultz in 2017 found that Irritator and baryonychines might have relied more on their sense of smell for hunting than Spinosaurus did, since they had larger, less retracted nostrils and more room in their skulls for the nasal cavity. Spinosaurus itself probably made heavier use of senses like vision or the mechanoreceptors on the tip of its snout, like those used by crocodilians to sense prey moving in the water. Although to a lesser degree than most known spinosaurs, this feature was also present in the Angaturama limai holotype. A tooth belonging to Irritator was discovered still inserted into the fossil neck vertebral column of an anhanguerid pterosaur, likely with a wingspan of . This indicates that Irritator ate pterosaurs as well, although it is not known if it actively hunted these animals or simply scavenged the remains. In 2018, Aureliano and colleagues presented a possible scenario for the food web of the Romualdo Formation. The researchers proposed that spinosaurines from the formation may have also preyed on terrestrial and aquatic crocodyliforms, juveniles of their own species, turtles, and small to medium-sized dinosaurs. This would have made spinosaurines apex predators within this particular ecosystem.) to occupy a distinct ecological niche, therefore avoiding competition with more terrestrial theropods. Spinosaurines appear to have been more adapted for such lifestyles than baryonychines. In 2018, Aureliano and colleagues conducted an analysis on the Romualdo Formation tibia fragment. CT scanning of the specimen in the University of São Carlos revealed the presence of osteosclerosis (high bone density). The presence of this condition on the Brazilian leg fragment showed that compact bones had already evolved in spinosaurines at least 10 million years before the appearance of Spinosaurus in Morocco. According to phylogenetic bracketing—a method used to infer unknown traits in organisms by comparison with their relatives—osteosclerosis therefore might have been the norm in the Spinosaurinae. Neuroanatomy and senses of the holotype showing endocast and endosseous labyrinth In 2020, Schade and colleagues analyzed the anatomy of the holotype skull braincase through CT scans, revealing numerous details about behavioral capabilities of Irritator. With the scans, they created a 3D model of the skull and braincase, discovering that Irritator had elongated olfactory tracts and a relatively large floccular recesses (area that pierces through the semicircular canals and connects the brain with the inner ear). The flocculus, itself, is an important element in the coordination and control of head and ocular movements during gaze stabilization (visual ability during head movement), by being involved in the coordination of the vestibulo-ocular (VOR) and vestibulo-collic (VCR) reflexes. The flocculus appears to be enlarged in taxa that rely on quick movements of the head body. In addition, the vestibular part of the endosseous labyrinth has a large anterior semicircular canal with a lateral oriented semicircular canal. Both floccular recesses and semicircular canal suggest that Irritator could coordinate fast head movements and had a downward inclined snout posture, enabling an unobstructed, stereoscopic forward vision, which is important for distance perception and therefore precise snatching movements of the snout. These inferences seem to be an agreement with a piscivore life-style. They also noted that the relatively well-developed cochlear duct may have enable an average hearing frequency of 1,950 Hz with a frequency band width of 3,196 Hz. However, they considered these ranges as rough estimates and established an overall frequency range of 350–3,550 Hz, making Irritator to be placed under bird hearing but above crocodiles. ==Paleoenvironment and paleobiogeography==

Irritator and Angaturama are known from the Romualdo Formation, whose rocks are dated to the Albian stage of the Early Cretaceous Period, about 110 million years ago. The formation is interpreted as a coastal lagoon with irregular freshwater influence that contended with cycles of transgressing and regressing sea levels. Barbosania, Maaradactylus, Tropeognathus, and Unwindia. The known dinosaur fauna besides Irritator was represented by other theropods like the tyrannosauroid Santanaraptor, the compsognathid Mirischia, an indeterminate unenlagiine dromaeosaurid, and a megaraptoran. as well as the turtles Brasilemys, Cearachelys, Araripemys, Euraxemys, and Santanachelys, are known from the deposits. There were also clam shrimps, sea urchins, ostracods, and molluscs. Various well-preserved fish fossils record the presence of: hybodont sharks, guitarfish, gars, amiids, ophiopsids, oshuniids, pycnodontids, aspidorhynchids, cladocyclids, bonefishes, chanids, mawsoniids and some uncertain forms. According to Naish and colleagues, the lack of herbivorous dinosaurs could mean that the local vegetation was scant and thus incapable of sustaining a large population of them. The abundant carnivorous theropods would have then likely turned to the lush aquatic life as a primary food source. They also hypothesized that following storm events, pterosaur and fish carcasses might have washed up on the shoreline, providing theropods with plenty of carrion. Paul Sereno and colleagues in 1998 suggested that with the opening of the Tethys Sea, spinosaurines would have evolved in the south (Africa, in Gondwana) and baryonychines in the north (Europe, in Laurasia). But the paleobiogeography of spinosaurids remains highly theoretical and uncertain, with discoveries in Asia and Australia indicating that it may have been complex. Taphonomy The taphonomy (changes between death and fossilization) of the Irritator challengeri holotype specimen has been discussed by some researchers. The skull was found lying on its side. Preceding fossilization, several bones from the back of the braincase, as well as the dentary, splenial, coronoid, and right angular bones from the lower jaw, were lost. Other bones, mostly from the skull rear, had become and displaced towards alternate regions of the head before burial. Naish and colleagues in 2004 asserted that the Romualdo Formation dinosaur fauna is represented by animals that died near shorelines or rivers before being carried out to sea, where their floating remains were eventually fossilized. In 2018, Aureliano and colleagues argued against this scenario, stating that the Irritator challengeri holotype's mandible was preserved in articulation with the rest of the skull, whereas it would have likely detached had the carcass been floating at sea. They also noted that the corpse would have quickly sunk due to the osteosclerosis of the skeleton. The researchers, therefore, concluded that fossils from the Santana Group represent organisms that were buried in their natural habitat, instead of having been deposited allochthonously (other than at their present position). == Controversies and Restitution Dispute ==

Following the publication of the reappraisal study in May 2023, the fact that the Brazilian holotype specimen was kept in a German collection attracted criticism. The authors had included an "Ethics Statement" which acknowledged recent debates surrounding the ethical and legal concerns regarding research on Brazilian fossils. According to the authors, however, the fossils had been exported from Brazil before legislation requiring permits for the export of specimens entered into force. Accordingly, they claimed the holotype specimen to be the property of the German Bundesland of Baden-Württemberg, with a clarification of its legal status lying neither within the authors' power nor within that of the local curator.. Brazilian paleontologists and legal specialists disputed this claim and called for the return of the fossil to Brazil. They also formulated this demand in an open letter to Petra Olschowksi, State Minister of Science and Art of Baden-Württemberg; two authors of the criticised study were among the signatories. Some argue that the Brazilian state is the legal owner of the fossil and that the State Museum of Natural History most likely did not acquire good title to it in good faith. A civil action of restitution under Article 985 of the German Civil Code is, however, precluded due to a statute of limitations of 30 years. In April 2025, the Brazilian think thank Rede Patrimônio em Pauta (Heritage in Focus Network) published an online petition demanding the return of the Irritator to its region of origin and has gathered over 34.600 signatures. In April 2026, theMinistry of Foreign Affairs of Brazil and the German Press Office shared the Joint Declaration signed during the 3rd German-Brazilian Intergovernmental Consultations, which stated that: "both governments welcome the willingness of the State of Baden-Württemberg and the State Museum of Natural History in Stuttgart to hand over the Irritator challengeri fossil to Brazil". . No timeframe has been announced for the return. == References ==