| Cristatusaurus | |
|---|---|
 | |
| Diagram illustrating possible size and skeletal reconstruction combining severalfossil specimens | |
Scientific classification | |
| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Clade: | Dinosauria |
| Clade: | Saurischia |
| Clade: | Theropoda |
| Family: | †Spinosauridae |
| Subfamily: | †Baryonychinae |
| Genus: | †Cristatusaurus Taquet and Russell, 1998 |
| Species: | †C. lapparenti |
| Binomial name | |
| †Cristatusaurus lapparenti | |
| Synonyms | |
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Cristatusaurus is agenus oftheropoddinosaur that lived during theEarly CretaceousPeriod of what is nowNiger, 112 million years ago. It was abaryonychine member of theSpinosauridae, a group of largebipedalcarnivores with well-built forelimbs and elongated, crocodile-like skulls. Thetype speciesCristatusaurus lapparenti was named in1998 by scientistsPhilippe Taquet andDale Russell, on the basis of jaw bones and somevertebrae. Two clawfossils were also later assigned toCristatusaurus. The animal'sgeneric name, which means "crested reptile", alludes to asagittal crest on top of its snout; while thespecific name is in honor of the FrenchpaleontologistAlbert-Félix de Lapparent.Cristatusaurus is known from theAlbian toAptianElrhaz Formation, where it would have coexisted withsauropod andiguanodontian dinosaurs, other theropods, and variouscrocodylomorphs.
Originally proposed to be an indeterminate species ofBaryonyx, the identity ofCristatusaurus has been subject to debate, in part due to the fragmentary nature of its fossils. Some argue that it is probably the same dinosaur asSuchomimus, which has also been found in Niger, in the same sediment layers. In that case the genusCristatusaurus would havepriority, since it was named two months earlier. Others have concluded, however, thatCristatusaurus is anomen dubium, considering it indistinguishable from bothSuchomimus andBaryonyx. Some distinctions between the fossils ofCristatusaurus andSuchomimus have been pointed out, but it is uncertain whether these differences separate the two genera or if they are due toontogeny (changes in an organism during growth). A recent study differentiatedCristatusaurus fromSuchomimus and assigned it as a valid spinosaurid genus, placing the theropod just outside Baryonychinae.
The firstfossils ofCristatusaurus were found in 1973 by FrenchpaleontologistPhilippe Taquet atGadoufaoua, a locality within theElrhaz Formation inNiger. Theholotype specimen, cataloged under the numberMNHN GDF 366, consists of twopremaxillae (frontmost snout bones), a partial rightmaxilla (main upper jaw bone), and adentary fragment from themandible. Severalparatypes have been assigned: MNHN GDF 365, a snout of twoarticulated premaxillae; as well as MNHN GDF 357, 358, 359 and 361, fourdorsal vertebrae.[1][2] Two thumb claws from separate specimens were also subsequently attributed toCristatusaurus.[3] In 1984, the premaxilla specimens MNHN GDF 365 and 366 were first described in detail by Taquet, where he referred them to an unnamed newtheropod within thefamilySpinosauridae, because of shared characteristics with the holotype dentary ofSpinosaurus aegyptiacus.[1] At the time Taquet believed these specimens belonged to the creature's lower jaw, since no theropod was known then with over five teeth in the premaxilla, whileCristatusaurus had seven. This was later proven incorrect in 1996 by Brazilian paleontologistsAlexander Kellner and Diogenes Campos, in light of the discoveries of other spinosaurids preserving upper jaw tips with over five teeth.[1][4]
In a1986 publication, British paleontologistsAlan Charig andAngela Milner considered Taquet's jaw elements nearly indistinguishable from those of the spinosauridBaryonyx walkeri; which they were describing on the basis of a partial skeleton from theBarremian of theWeald Clay Formation,England.[5] A 1997 followup to this preliminary paper referred MNHN GDF 365 and 366 to an indeterminateBaryonyx species, regardless of their younger geological age.[6] In1998, Taquet and AmericangeologistDale Russell used the bones to erect the newgenusCristatusaurus, with thetype species beingCristatusaurus lapparenti. Itsgeneric name is derived from theLatincrista (for "crest"), and refers to asagittal crest on the snout.[2] Thespecific name honors the late French paleontologistAlbert-Félix de Lapparent, due to his contributions to dinosaur-related discoveries in the Sahara. In the same paper, several skull and vertebral fossils from theTademaït ofAlgeria were attributed to a new species ofSpinosaurus calledS. maroccanus, which was described and compared toCristatusaurus.[2]Spinosaurus maroccanus is now considered by most paleontologists either anomen dubium (name of uncertain application)[7][8][9] or onesynonymous withS. aegyptiacus.[10] Two months after Taquet and Russel published their paper, another spinosaurid genus and species was named from the Erlhaz Formation,Suchomimus tenerensis. Its describers, the American paleontologistPaul Sereno and colleagues, agreed with Charig and Milner in that there was no distinction between the skull fossils ofBaryonyx andCristatusaurus; concluding that the latter was anomen dubium.[10] In a 2003 analysis, German paleontologistOliver Rauhut concurred with this.[9]
When describing thetaxon, Taquet and Russel basedCristatusaurus's separation fromBaryonyx on the former's "brevirostrine condition of premaxilla" (having a short snout).[2] The meaning of this diagnosis has been considered obscure by various subsequent authors, who describe the specimens as almost identical to those ofBaryonyx andSuchomimus.[11] In 2002,Eric Buffetaut and Mohamed Ouaja supportedCristatusaurus'sjunior synonymy withBaryonyx.[8] The same year,Hans-Dieter Sues and colleagues regarded bothCristatusaurus andSuchomimus as junior synonyms ofBaryonyx, stating that there is no fossil evidence indicating more than one spinosaur lived in the Elrhaz Formation.[12] More recent research has retainedSuchomimus andBaryonyx as distinct genera.[13][14][15] Others, such as Bertin Tor in 2010, and Carrano and colleagues in 2012, have referred toCristatusaurus as an indeterminatebaryonychine, because of how fragmentary its remains are.[11][16]
In 2016, Christophe Hendrickx,Octávio Mateus, and Buffetaut noted that Taquet and Russel might have interpretedCristatusaurus as having a shorter snout thanBaryonyx by mistaking the notch where the maxillae articulated with the premaxillae for the nostril openings. Since bothSuchomimus andBaryonyx have more completely preserved premaxillae, whileCristatusaurus only has the frontmost part of this bone known, Hendrickx and colleagues considered it possible thatCristatusaurus's snout was just as long as inBaryonyx. Therefore, they agreed with previous authors in the ambiguity of Taquet and Russel's diagnosis. Hendrickx and colleagues stated that sinceCristatusaurus andSuchomimus are nearly identical and both hail from the samestratigraphic unit, they are almost certainly synonyms. The researchers foundCristatusaurus andSuchomimus similar in that they both had premaxillary crests, similar size ratio of tooth sockets, and shallow depressions in front of their nostril openings. However, since these features are minor and may vary within species as well as depending on age and sex, Hendrickx and colleagues did not identify any definitiveautapomorphies (distinguishing features) ofCristatusaurus's holotype, and thus considered the taxon anomen dubium until itspostcranial remains are more closely examined.[17] Given that it was named first,Cristatusaurus lapparenti haspriority overSuchomimus tenerensis in the case that they become synonymized.[18]
In a 2017 study, Marcos Sales and Cesar Schultz compared the holotype ofCristatusaurus (MNHN GDF 366) to the referred snout ofSuchomimus (MNN GDF501). Both of them exhibit a narrow rim across the top of their premaxillae. However,Cristatusaurus's convex secondary palate is clearly visible in side view (situated under the premaxillary teeth), whereas inSuchomimus it is discernible only through cracks on the fossil snout. It was also pointed out that where known, the upward-sloping process ofCristatusaurus's maxilla is narrower than inSuchomimus. The researchers concluded that further study is needed to determine whether these differences are possibleautapomorphies (distinguishing features) between the taxa, or if they are the result ofontogenetic (developmental) changes, given that theCristatusaurus holotype represents a younger individual.[13] A 2021 study assignedCristatusaurus just outside of Baryonychinae and differentiated fromSuchomimus as a valid genus, supporting its independence as a genus.[19]
In 2012,American vertebrate paleontologistThomas R. Holtz Jr. tentatively estimatedCristatusaurus at around 10 m (33 ft) in length and weighing between 1 and 4 tonnes (1.1 and 4.4 short tons).[20][21] The holotype premaxillae are 115 mm (4.5 in) long and 55 mm (2.2 in) tall. The other known set of premaxillae (specimen MNHN GDF 365) are larger at 185 mm (7.3 in) long and 95 mm (3.7 in) tall.[4] The holotype's smaller size, smoother surface, and lack ofco-ossified (fused) sutures all indicate that it belongs to a juvenile individual; while MNHN GDF 365 probably represents an adult.[4]
The tip ofCristatusaurus's premaxilla was short and expanded, while the rear end was narrowed near thesuture with the maxilla; thisrosette-like snout shape was characteristic of spinosaurids. The front of the upper jaw was concave on the bottom, shaped to interlock with what would have been the convex and also enlarged tip of the mandible's dentary bone.[2][4][22] A thin sagittal crest ran lengthwise on top of the premaxillae, a condition present inBaryonyx andSuchomimus, and very prominent inAngaturama (a possible synonym ofIrritator).[13] Like all spinosaurids,Cristatusaurus'sexternal nares (bony nostrils) were positioned further back on the skull that in typical theropods.[4][13] Two bonyprocesses extended across the underside of the snout, in a convex structure that formed the animal'ssecondary palate. This condition is observed in all extantcrocodilians, but not in most theropod dinosaurs; however, it was a common trait among spinosaurids.[13][12]
Cristatusaurus's dentalalveoli (tooth sockets) were closely spaced, those in the maxilla and dentary were flattened somewhat sideways; while the ones in the premaxillae were large and mostly circular, with the frontmost alveoli being the largest.[2][4] Partialtooth crowns preserved in some alveoli show that the teeth were finelyserrated, with flutes (lengthwise ridges) on theirlingual (inward-facing side of teeth) surface.[4] Both premaxilla specimens had seven alveoli on each side, the same number as inSuchomimus,Angaturama,Oxalaia, and theSpinosaurus maroccanus specimen.[2][23]
One of the dorsal vertebrae (MNHN GDF 358) measured 13.5 cm (5.3 in) incentrum length, which is equal to the largest known vertebrae ofSpinosaurus maroccanus. The preserved base of one ofCristatusaurus's vertebralneural spines (MNHN GDF 359) was 15 mm (0.59 in) thick in comparison to the 25 mm (0.98 in) measurement seen in an equivalentSpinosaurus vertebra, indicating thatCristatusaurus's neural spines were probably not as tall as those ofSpinosaurus.[2] Of the two manualunguals (claws) referred toCristatusaurus, one was equivalent in size to those found forSuchomimus andBaryonyx, while the other was about 25 to 30 percent smaller.[3] As a spinosaur, it would have wielded these claws with three-fingered hands carried by robust arms.[22]
Spinosaurids were largebipedalcarnivores with well-built forelimbs and elongated, crocodile-like skulls. Thetaxonomic andphylogenetic affinities of the group are subject to active research and debate, given that in comparison to other theropod groups, many of the family's taxa (includingCristatusaurus) are based on relatively poor fossil material.[22] Traditionally the family has been divided into twosubfamilies: Spinosaurinae, which includes genera likeIrritator,Spinosaurus, andOxalaia; and Baryonychinae, which includesBaryonyx andSuchomimus. Although the genus and species placement ofCristatusaurus lapparenti is disputed, its fossils certainly belong to a member of the baryonychinae, because of its more forwardly placed external nostrils; relatively larger first premaxillary teeth; and more closely spaced tooth sockets than in spinosaurines; as well as the presence of fine serrations, in contrast to spinosaurines lacking them entirely.[22][12][23] However, authors like Sales and Schultz have questioned themonophyly of Baryonychinae (meaning it might be an unnatural group), stating that the South American spinosauridsAngaturama andIrritator represent intermediate forms between Baryonychinae and Spinosaurinae, based on their craniodental (skull and tooth) features. Theircladogram can be seen below.[13]
The Elrhaz Formation, part of theTegama Group, consists mainly offluvialsandstones with low relief, much of which is obscured by sand dunes.[24][25] Thesediments are coarse- to medium-grained, with almost no fine-grainedhorizons.[26]Cristatusaurus lived in what is now Niger, during the lateAptian to earlyAlbianstages of theEarly CretaceousPeriod, 112 million years ago.[27][28] The sediment layers of the formation have been interpreted as an inland habitat of extensive freshwaterfloodplains and fast-moving rivers, with a tropical climate that likely experienced seasonal dry periods.[27]
This environment was home to a variety of fauna including dinosaurs,pterosaurs,chelonians, fish,hybodont sharks, and freshwaterbivalves.[28][25] BesidesCristatusaurus lapparenti andSuchomimus tenerensis, theropods such as theabelisauridKryptops palaios, thecarcharodontosauridEocarcharia dinops and thenoasauridAfromimus tenerensis have been found. Herbivorous dinosaurs of the region includediguanodontians likeOuranosaurus nigeriensis,Elrhazosaurus nigeriensis,Lurdusaurus arenatus, and twosauropods:Nigersaurus taqueti, and an unnamedtitanosaur.Crocodylomorphs were abundant; represented by the giantpholidosaur speciesSarcosuchus imperator, as well as smallnotosuchians likeAnatosuchus minor,Araripesuchus wegeneri, andStolokrosuchus lapparenti.[25] The local flora probably consisted mainly offerns,horsetails, andangiosperms, based on the dietary adaptations of the sauropods that lived there.[27]
Asemiaquatic lifestyle has been proposed for many spinosaurids, on account of their unusual anatomical traits and bonehistology.Cristatusaurus's teeth would have likely been used for piercing and gripping prey items, rather than slicing flesh, as indicated by their subcircular cross section and reduced serrations. Its teeth, combined with thesinusoidal (wave-like) curvature of the jaws, would have performed as tan efficient trap for fish. The retracted nostrils would have allowed it to submerge its snout further underwater than most theropods, while still being able to breathe; and the bony secondary palate is theorized to have reinforced the skull against bending stresses when feeding. The use of the giant recurved manual unguals of spinosaurs is still under debate; suggested functions have ranged fromgaffing aquatic prey out of the water, to scavenging carcasses or digging.[4][12][13][29]
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