| Nigersaurus | |
|---|---|
 | |
| Reconstructed skeleton in Japan | |
Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Reptilia |
| Clade: | Dinosauria |
| Clade: | Saurischia |
| Clade: | †Sauropodomorpha |
| Clade: | †Sauropoda |
| Superfamily: | †Diplodocoidea |
| Family: | †Rebbachisauridae |
| Genus: | †Nigersaurus Serenoet al.,1999 |
| Species: | †N. taqueti |
| Binomial name | |
| †Nigersaurus taqueti Serenoet al., 1999 | |
Nigersaurus (/niːˈʒɛərsɔːrəs,ˈnaɪdʒərsɔːrəs/) is agenus ofrebbachisauridsauropod dinosaur that lived during the middleCretaceous period, about 115 to 105 million years ago. It was discovered in theElrhaz Formation in an area calledGadoufaoua, inNiger. Fossils of this dinosaur were first described in 1976, but it was only namedNigersaurus taqueti in 1999 after further and more complete remains were found and described. The genus name means "Niger reptile", and the specific name honours the palaeontologistPhilippe Taquet, who discovered the first remains.
Small for a sauropod,Nigersaurus was about 9–14.1 m (30–46 ft) long, and had a short neck. It weighed around 1.9–4 t (2.1–4.4 short tons), comparable to a modernelephant. Its skull was very specialised for feeding, with largefenestrae and thin bones. It had a wide muzzle filled with more than 500 teeth, which were replaced at a rapid rate: around every 14 days. The jaws may have borne akeratinous sheath. Unlike othertetrapods, the tooth-bearing bones of its jaws were rotated transversely relative to the rest of the skull, so that all of its teeth were located far to the front. Its skeleton was highlypneumatised (filled with air spaces connected toair sacs), but the limbs were robustly built.
Nigersaurus and its closest relatives are grouped within the subfamilyRebbachisaurinae (formerly thought to be grouped in theeponymous Nigersaurinae) of the family Rebbachisauridae, which is part of the sauropod superfamilyDiplodocoidea.Nigersaurus was probably a browser, and fed with its head close to the ground. The region of its brain that detected smell was underdeveloped, although its brain size was comparable to that of other dinosaurs. There has been debate on whether its head was habitually held downwards, or horizontally like other sauropods. It lived in ariparian habitat, and its diet probably consisted of soft plants, such asferns,horsetails, andangiosperms. It is one of the most common fossil vertebrates found in the area, and shared its habitat with other dinosaurianmegaherbivores, as well as largetheropods andcrocodylomorphs.
Remains thought to belong toNigersaurus were first discovered during a 1965–1972 expedition to theRepublic of Niger led by French paleontologistPhilippe Taquet, and first mentioned in a paper published in 1976.[1][2] Although a common genus, the dinosaur had been poorly known until more material of other individuals was discovered during expeditions led by American palaeontologistPaul Sereno in 1997 and 2000. The limited understanding of the genus was the result of poor preservation of its remains, which arises from the delicate and highlypneumatic construction (filled with air spaces connected toair sacs) of the skull and skeleton, in turn causing disarticulation of the fossils. Some of the skull fossils were so thin that a strong light beam was visible through them. Therefore, no intact skulls or articulated skeletons have been found, and these specimens represent the most complete known rebbachisaurid remains.[3][1]
Nigersaurus was named and described in more detail by Sereno and colleagues only in 1999, based on remains of newly found individuals. The same article also namedJobaria, anothersauropod from Niger. The genus nameNigersaurus ("Niger reptile") is a reference to the country where it was discovered, and thespecific nametaqueti honours Taquet, who was the first to organise large-scale palaeontological expeditions to Niger.[4] Theholotype specimen (MNN GAD512) consists of a partial skull and neck. Limb material and ascapula (shoulder blade) found nearby were also referred to the same specimen. These fossils are housed at theNational Museum of Niger.[1]
Sereno and the American palaeontologistJeffrey A. Wilson provided the first detailed description of the skull and feeding adaptations in 2005.[1] In 2007, a more detailed description of the skeleton was published by Sereno ad colleagues, based on a specimen discovered ten years earlier. The fossils, along with a reconstructed skeleton mount and a plastic model of the head and neck, were subsequently presented at theNational Geographic Society in Washington.[5]Nigersaurus was dubbed a "Mesozoic cow" in the press, and Sereno stressed that it was the most unusual dinosaur he had ever seen. He likened its physical appearance toDarth Vader and avacuum cleaner, and compared its tooth shear with aconveyor belt and sharpened piano keys.[6][7][8]
NumerousNigersaurus specimens collected by French and American expeditions remain to be described.[9] Teeth similar to those ofNigersaurus have been found on theIsle of Wight and inBrazil, but it is unknown whether they belonged to relatives of thistaxon, or totitanosaurs, whose remains have been found in the vicinity. A lower jaw assigned to the titanosaurAntarctosaurus is likewise similar to that ofNigersaurus, but may haveevolved convergently.[1]
Like all sauropods,Nigersaurus was aquadruped with a small head, thick hind legs, and a prominent tail. Among thatclade,Nigersaurus was fairly small, with a body length of only 9–14.1 m (30–46 ft) and afemur reaching only 1 m (3 ft 3 in); it may have weighed around 1.9–4 t (2.1–4.4 short tons), comparable to a modernelephant.[3][10][11] It had a short neck for a sauropod, with thirteencervical vertebrae. Nearly allrebbachisaurids had relatively short necks and a length of 10 m (33 ft) or less. The only members of the family that reached the size of larger sauropods wereRebbachisaurus[3] andMaraapunisaurus.[12]
.jpg)
The skull ofNigersaurus was delicate, with the four sidefenestrae (openings in the skull) larger than in othersauropodomorphs. The total area of bone connecting the muzzle to the back of the skull was only 1.0 cm2 (0.16 sq in). These connecting struts of bones were usually less than 2 mm (0.08 in) thick. Despite this, the skull was resistant to the sustained shearing of the teeth. Another unique trait it had among sauropodomorphs was a closedsupratemporal fenestra. The nasal openings, the bony nostrils, were elongated.[3] Though the nasal bones are not completely known, it appears the front margin of the bony nostril was closer to the snout than in other diplodocoids. The snout was also proportionately shorter, and the tooth row was not at allprognathous, the snout tip not protruding relative to the remainder of the tooth series.[1]Nigersaurus was distinct in that itsfrontal bone (which formed much of the skull-roof) was elongate (much narrower than long), and had a marked cerebralfossa (a depression on the surface of this bone inside the head).[4] The maxillary tooth row was in its entiretytransversely rotated, its normal rear 90° everted towards the front. This was matched by an identical rotation of the dentary of the lower jaw. This transverse orientation of the upper and lower tooth rows was unique to the dinosaur. Due to this configuration, no other tetrapod had all of its teeth located as far to the front asNigersaurus.[3][4]
The slender teeth had slightly curvedtooth crowns, which were oval in cross-section. The crowns were distinct in having prominent ridges on the margins of their midline and sides. The teeth in the lower jaw may have been 20–30% smaller than those in the upper jaw, but few are known, and they are of uncertain maturity. Apart from this, the teeth were identical.[4] Under each active tooth there was a column of nine replacement teeth within the jaw. With 68 columns in the upper jaws and 60 columns in the lower jaws, these so-called dental batteries (also present inhadrosaurs andceratopsians) comprised a total of more than 500 active and replacement teeth.[1] Dental batteries erupted in unison, not each column individually.[4] Theenamel on the teeth ofNigersaurus was highly asymmetrical, ten times thicker on the outwards facing side than on the inner side.[13] This condition is otherwise known only in advancedornithischians.[4]
Nigersaurus did not exhibit the same modifications seen in the jaws of other dinosaurs with dental batteries, or mammals with elaborate chewing functions. Thelower jaw was L-shaped and divided into thesubcylindrical transverseramus, which contained the teeth, and the back ramus, which was more lightweight and was the location for most of the muscle attachments. It was distinct in that the tooth row expanded to the sides from the plane of the main ramus of the lower jaw. The jaws also contained several fenestrae, including five that are not present in other sauropods. The front ends of the jaws had grooves that indicate the presence of akeratinous (horny) sheath.[3][4]Nigersaurus is the only known tetrapod animal to have had jaws wider than the skull and teeth that extended laterally across the front.[6] The snout was even broader than those of the "duck-billed" hadrosaurs.[14]
Nigersaurus was distinct in that itsdorsal (back) vertebrae had paired pneumatic spaces at the base of theneural spines (the spines that projected upwards from the vertebrae). The presacral vertebrae (vertebrae in front of thesacrum) were heavily pneumatised to the point where the column consisted of a series of hollow "shells", each divided by a thinseptum in the middle. It had little to nocancellous bone, making thecentra thin bone plates filled with air spaces. Thevertebral arches were so heavily pierced by extensions of the external air sacs that of their side walls little remained but 2 mm (0.08 in) thick intersectinglaminae, the ridges between the pneumatic openings. The vertebrae of the tail, however, did have solid centra.[3][4]
Thepelvic andpectoral girdle bones were very thin also, often only several millimetres thick. It had a prominent rugosity (a roughly wrinkled area) on the midline aspect of the scapular blade's base, a distinguishing feature. Like other sauropods, its limbs were robust, contrasting with the extremely lightweight construction of the rest of the skeleton. The limbs were not as specialised as the rest of the skeleton, and the front legs ofNigersaurus were about two-thirds the length of the back legs, as in mostdiplodocoids.[3][4]
The remains ofNigersaurus were initially described by Taquet in 1976 as belonging to adicraeosaurid, but in 1999 Sereno and colleagues reclassified it as a rebbachisaurid diplodocoid.[4] These researchers speculated that since short necks and small size was known among basal diplodocoids, it may indicate these were ancestral features of the group.[3] Rebbachisauridae is thebasalmost family within the superfamily Diplodocoidea, which also contains the long-neckeddiplodocids and the short-necked dicraeosaurids. Theeponymous subfamilyNigersaurinae, which includesNigersaurus and closely related genera, was named by the American palaeontologist John A. Whitlock in 2011.[15]
The closely related genusDemandasaurus from Spain was described by the Spanish palaeontologist Fidel Torcida Fernández-Baldor and colleagues in 2011, and along with other animal groups that span the Cretaceous of Africa and Europe, this indicates thatcarbonate platforms connected these landmasses across theTethys Sea.[16] This was supported in 2013 by the Italian palaeontologist Federico Fanti and colleagues in their description of the nigersaurineTataouinea from Tunisia, which was more related to the European form than toNigersaurus, despite being from Africa, then part of the supercontinentGondwana.[17] Pneumatisation of the rebbachisaurid skeleton evolved progressively, culminating in the nigersaurines.[17]
Below is acladogram following the 2013 analysis by Fanti and colleagues, which confirmed the placement ofNigersaurus as a basal nigersaurine rebbachisaurid.[17]
A 2015 cladistic study by Wilsona and the French palaeontologist Ronan Allain foundRebbachisaurus itself to group with the nigersaurines, and the authors suggested that Nigersaurinae was therefore ajunior synonym of Rebbachisaurinae (since that name would have priority).[9] The same year, Fanti and colleagues supported the use of Rebbachisaurinae over Nigersaurinae, and foundNigersaurus to be the basalmost member of this "Euro-African" subclade.[18]
In 2019, Mannion and colleagues pointed out that sinceNigersaurus was found to be the sister taxon of all other nigersaurines in some studies, a Rebbachisaurinae clade may not necessarily includeNigersaurus itself (as well as the fact that the position ofRebbachisaurus could change in future analyses), and supported the continued use of the name Nigersaurinae over Rebbachisaurinae for all rebbachisaurids more closely related toNigersaurus than toLimaysaurus. They found that nigersaurines were restricted to North Africa and Europe, and that Limaysaurinae was strictly known from Argentina.[19] The same year, the Brazilian palaeontologist Rafael Matos Lindoso and colleagues used the name Nigersaurinae following Mannion's recommendation, and foundItapeuasaurus from Brazil to group with the nigersaurines, thereby expanding this lineage more widely (makingpalaeobiogeographic hypotheses for this group less reliable).[20]
Though it had large nostrils and a fleshy snout, Sereno and colleagues found thatNigersaurus had an underdeveloped olfactory region of its brain and thus did not have an advanced sense of smell. Itsbrain-to-body-mass ratio was average for a reptile, and smaller than those of ornithischians and non-coelurosauriantheropods. Thecerebrum comprised about 30% of the brain volume, as in many other dinosaurs.[3] The AmericanpalaeoartistMark Hallett and paleontologist Mathew J. Wedel and stated in 2016 that while sauropods in general could use their long necks to detect predators from afar, this would not apply to the short-neckedNigersaurus. They pointed out that the eyes ofNigersaurus were placed further towards the top of the skull than in most other sauropods, above the muzzle, which would give it overlapping fields of view. Its visual field would have been at or close to 360 degrees, and hypersensitivity of movement would have been important to a vulnerable prey-animal.[21]
In 2017, the Argentinian palaeontologist Lucio M. Ibiricu and colleagues examined the postcranial skeletal pneumaticity in the skeletons of rebbachisaurids, and suggested that it was an adaptation for lowering the density of the skeleton, and that this could have decreased the muscle energy needed to move the body, as well as the heat generated in the process. Since several rebbachisaurids inhabited latitudes that would have been tropical to subtropical in the Middle Cretaceous, this pneumaticity may have helped the animals cope with the very high temperatures. According to Ibiricu and colleagues, this adaptation may be a reason why rebbachisaurids were the only group of diplodocoids that survived into the Late Cretaceous.[22]
A 2023 study by the French palaeontologist Rémi Lefebvre and colleagues examined the microanatomical structure of the limb bones ofNigersaurus throughCT-scans, the first such study of a sauropod. Virtual sections of the limb bones sampled from individuals of different sizes were explored to determine the range of variation. Heavy-weight bearing animals often have features associated with this condition, affecting the inner structure of the limb bones, but the variation between such bones is poorly known. While other heavy animals examined (such asrhinoceroses) had thick and variablebone cortices, the cortex ofNigersaurus was rather thin. These researchers suggested that the microanatomy of sauropods was not affected by drasticselective pressure caused by bearing weight, but that features such as the columnar limbs (as seen in elephants), pneumaticity, and fleshy foot pads andcartilage relaxed pressure on the bones. They also suggested that sauropods may therefore have been lighter in weight than expected for their size, supporting the lowest body mass estimates for these dinosaurs.[23]
Nigersaurus was suggested by Sereno and colleagues to be a ground-level, non-selectivebrowser. The width of the muzzle and lateral orientation of the tooth row show that the sauropod could gather much food and crop it close to the ground, within 1 m (3 ft 3 in) of the surface.[3][1] This is further supported byfacets on the labial (externally facing) side of the upper teeth, similar toDicraeosaurus andDiplodocus, which are evidence that food or substrate wore the animal's teeth as it fed.Nigersaurus also bears signs of low-angle tooth-to-tooth wear on the inside of themaxillary crowns, which suggests that jaw movement was limited to precise up-and-down motions. Worn teeth from the lower jaw have not yet been discovered, but they are expected to show opposing tooth-to-tooth wear. The ability to raise their heads well above the ground does not necessarily mean they browsed on items there, and the short neck ofNigersaurus would have restricted the browsing range compared to other diplodocoids.[3]
Theadductor muscle of the jaw appears to have attached to thequadrate instead of the supratemporal fenestra. Both this and the othermastication muscles were likely weak, andNigersaurus is estimated to have had one of the weakest bites of the sauropods.[3] In addition, according to Whitlock and colleagues in 2011, the small, nearly parallel nature of the tooth scratches and pits (caused by grit, which would not be obtained as often by high-browsers) indicate that it ate relatively soft,herbaceous plants such as low-growing ferns.[14] Because of the lateral orientation of the teeth, it probably would not have been able to chew.[1]Nigersaurus wore its tooth crowns down faster than other dinosaurian herbivores,[3] and its tooth replacement rate was the highest of any known dinosaur. Each tooth was replaced once every 14 days; the rate had previously been estimated lower. In contrast toNigersaurus, sauropods with lower tooth replacement rates and broader tooth crowns are thought to have beencanopy browsers.[13]
Grass did not evolve until the late Cretaceous, makingferns,horsetails, andangiosperms (which had evolved by the middle Cretaceous) potential food forNigersaurus. Sereno and colleagues stated it was unlikely thatNigersaurus fed onconifers,cycads, oraquatic vegetation, due respectively to their height, hard and stiff structure, and lack of appropriate habitat.[3] Wedel suggested that the evenly spaced teeth ofNigersaurus could have functioned like a comb, by straining water plants or invertebrates, similar toflamingos. He also suggested it could have low-browsed from short conifers and other low-growing plants.[21]
On the basis ofmicrotomography scans of skull elements of the holotype specimen, Sereno and colleagues created a "prototype"Nigersaurus skull they could examine. They also made anendocast of the brain and scanned thesemicircular canals of itsinner ear, which they found to be oriented horizontally. In their 2007 study, they stated that the structure of theocciput at the back of the skull and cervical vertebrae would have limited the upward and downward movement of the neck and the rotation of the skull. Based on this biomechanical analysis, the team concluded that the head and muzzle were habitually oriented 67° downwards and close to ground level, as an adaptation for ground-level browsing. This is unlike the way other sauropods have been restored, with their heads held more horizontally.[3]
A 2009 study by the British palaeontologistMike P. Taylor and colleagues agreed thatNigersaurus was able to feed with the downturned head and neck posture proposed by the 2007 study, but contested that this was the habitual posture of the animal. The study noted that the "neutral" head and neck posture of modern animals does not necessarily correspond to their habitual head posture. It further argued that the orientation of the semicircular canals varies significantly within modern species, and is therefore not reliable for determining head posture.[24] This was supported by the Spanish palaeontologist Jesús Marugán-Lobón and colleagues in a 2013 study that suggested the methods used by Sereno's team were imprecise, and thatNigersaurus habitually held its head like other sauropods.[25]
In 2020, the French palaeontologist Julien Benoit and colleagues tested lateral semicircular canal correlation to head posture on modern mammals, and found that while there was significant correlation between the reconstructed and actual head postures, the plane of the semicircular canal was not held horizontally in the resting pose as inferred. The authors therefore cautioned against using semicircular canals as proxy to infer the precise orientation of skulls. They found that diet correlated strongly with semicircular canal orientation, but not with head posture, while head posture and semicircular canal orientation were strongly correlated with phylogeny.[26]
Nigersaurus is known from theElrhaz Formation of theTegama Group in an area of theTénéré Desert calledGadoufaoua, located in Niger. It is one of the most commonly found vertebrates in the formation. The Elrhaz Formation consists mainly offluvial sandstones with low relief, much of which is obscured by sand dunes.[4][27] Thesediments are coarse- to medium-grained, with almost no fine-grainedhorizons.[3]Nigersaurus lived in what is now Niger about 120 to 112 million years ago, during theAptian andAlbian ages of the mid-Cretaceous.[4] It likely lived in habitats dominated by inlandfloodplains (ariparian zone).[3]
After theiguanodontianLurdusaurus,Nigersaurus was the most numerousmegaherbivore. Other herbivores from the same formation includeOuranosaurus,Elrhazosaurus, and an unnamedtitanosaur. Together, these compose one of the few associations of megaherbivores with a balance of sauropods and largeornithopods. It also lived alongside thetheropodsKryptops,Suchomimus,Eocarcharia, andAfromimus.Crocodylomorphs likeSarcosuchus,Anatosuchus,Araripesuchus, andStolokrosuchus also lived there. In addition, remains of apterosaur,chelonians, fish, ahybodont shark, and freshwaterbivalves have been found. The aquatic fauna consists entirely of freshwater inhabitants.[3][27][28]
