Nigersaurus
Nigersaurus is a genus of rebbachisaurid sauropod dinosaur that lived during the middle Cretaceous period, about 115 to 105 million years ago. It was discovered in the Elrhaz Formation in an area called Gadoufaoua, in Niger. Fossils of this dinosaur were first described in 1976, but it was only named Nigersaurus 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 palaeontologist Philippe Taquet, who discovered the first remains.
Small for a sauropod, Nigersaurus was about long, and had a short neck. It weighed around, comparable to a modern elephant. Its skull was very specialised for feeding, with large fenestrae 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 a keratinous sheath. Unlike other tetrapods, 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 highly pneumatised, but the limbs were robustly built.
Nigersaurus and its closest relatives are grouped within the subfamily Rebbachisaurinae of the family Rebbachisauridae, which is part of the sauropod superfamily Diplodocoidea. 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 a riparian habitat, and its diet probably consisted of soft plants, such as ferns, horsetails, and angiosperms. It is one of the most common fossil vertebrates found in the area, and shared its habitat with other dinosaurian megaherbivores, as well as large theropods and crocodylomorphs.
History of discovery
Remains thought to belong to Nigersaurus were first discovered during a 1965–1972 expedition to the Republic of Niger led by French paleontologist Philippe Taquet, and first mentioned in a paper published in 1976. Although a common genus, the dinosaur had been poorly known until more material of other individuals was discovered during expeditions led by American palaeontologist Paul 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 highly pneumatic construction 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.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 named Jobaria, another sauropod from Niger. The genus name Nigersaurus is a reference to the country where it was discovered, and the specific name taqueti honours Taquet, who was the first to organise large-scale palaeontological expeditions to Niger. The holotype specimen consists of a partial skull and neck. Limb material and a scapula found nearby were also referred to the same specimen. These fossils are housed at the National Museum of Niger.
Sereno and the American palaeontologist Jeffrey A. Wilson provided the first detailed description of the skull and feeding adaptations in 2005. 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 the National Geographic Society in Washington. 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 to Darth Vader and a vacuum cleaner, and compared its tooth shear with a conveyor belt and sharpened piano keys.
Numerous Nigersaurus specimens collected by French and American expeditions remain to be described. Teeth similar to those of Nigersaurus have been found on the Isle of Wight and in Brazil, but it is unknown whether they belonged to relatives of this taxon, or to titanosaurs, whose remains have been found in the vicinity. A lower jaw assigned to the titanosaur Antarctosaurus is likewise similar to that of Nigersaurus, but may have evolved convergently.
Description
Like all sauropods, Nigersaurus was a quadruped with a small head, thick hind legs, and a prominent tail. Among that clade, Nigersaurus was fairly small, with a body length of only and a femur reaching only ; it may have weighed around, comparable to a modern elephant. It had a short neck for a sauropod, with thirteen cervical vertebrae. Nearly all rebbachisaurids had relatively short necks and a length of or less. The only members of the family that reached the size of larger sauropods were Rebbachisaurus and Maraapunisaurus.Skull
The skull of Nigersaurus was delicate, with the four side fenestrae larger than in other sauropodomorphs. The total area of bone connecting the muzzle to the back of the skull was only. These connecting struts of bones were usually less than thick. Despite this, the skull was resistant to the sustained shearing of the teeth. Another unique trait it had among sauropodomorphs was a closed supratemporal fenestra. The nasal openings, the bony nostrils, were elongated. 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 all prognathous, the snout tip not protruding relative to the remainder of the tooth series. Nigersaurus was distinct in that its was elongate, and had a marked cerebral fossa. The maxillary tooth row was in its entirety transversely 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 as Nigersaurus.The slender teeth had slightly curved tooth 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. 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 comprised a total of more than 500 active and replacement teeth. Dental batteries erupted in unison, not each column individually. The enamel on the teeth of Nigersaurus was highly asymmetrical, ten times thicker on the outwards facing side than on the inner side. This condition is otherwise known only in advanced ornithischians.
Nigersaurus did not exhibit the same modifications seen in the jaws of other dinosaurs with dental batteries, or mammals with elaborate chewing functions. The lower jaw was L-shaped and divided into the subcylindrical transverse ramus, 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 a keratinous sheath. Nigersaurus is the only known tetrapod animal to have had jaws wider than the skull and teeth that extended laterally across the front. The snout was even broader than those of the "duck-billed" hadrosaurs.
Postcranial skeleton
Nigersaurus was distinct in that its vertebrae had paired pneumatic spaces at the base of the . The presacral vertebrae were heavily pneumatised to the point where the column consisted of a series of hollow "shells", each divided by a thin septum in the middle. It had little to no cancellous bone, making the centra thin bone plates filled with air spaces. The vertebral arches were so heavily pierced by extensions of the external air sacs that of their side walls little remained but thick intersecting laminae, the ridges between the pneumatic openings. The vertebrae of the tail, however, did have solid centra.The pelvic and pectoral girdle bones were very thin also, often only several millimetres thick. It had a prominent rugosity 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 of Nigersaurus were about two-thirds the length of the back legs, as in most diplodocoids.
Classification
The remains of Nigersaurus were initially described by Taquet in 1976 as belonging to a dicraeosaurid, but in 1999 Sereno and colleagues reclassified it as a rebbachisaurid diplodocoid. 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. Rebbachisauridae is the basalmost family within the superfamily Diplodocoidea, which also contains the long-necked diplodocids and the short-necked dicraeosaurids. The eponymous subfamily Nigersaurinae, which includes Nigersaurus and closely related genera, was named by the American palaeontologist John A. Whitlock in 2011.The closely related genus Demandasaurus 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 that carbonate platforms connected these landmasses across the Tethys Sea. This was supported in 2013 by the Italian palaeontologist Federico Fanti and colleagues in their description of the nigersaurine Tataouinea from Tunisia, which was more related to the European form than to Nigersaurus, despite being from Africa, then part of the supercontinent Gondwana. Pneumatisation of the rebbachisaurid skeleton evolved progressively, culminating in the nigersaurines.
Below is a cladogram following the 2013 analysis by Fanti and colleagues, which confirmed the placement of Nigersaurus as a basal nigersaurine rebbachisaurid.
A 2015 cladistic study by Wilsona and the French palaeontologist Ronan Allain found Rebbachisaurus itself to group with the nigersaurines, and the authors suggested that Nigersaurinae was therefore a junior synonym of Rebbachisaurinae. The same year, Fanti and colleagues supported the use of Rebbachisaurinae over Nigersaurinae, and found Nigersaurus to be the basalmost member of this "Euro-African" subclade.
In 2019, Mannion and colleagues pointed out that since Nigersaurus was found to be the sister taxon of all other nigersaurines in some studies, a Rebbachisaurinae clade may not necessarily include Nigersaurus itself, and supported the continued use of the name Nigersaurinae over Rebbachisaurinae for all rebbachisaurids more closely related to Nigersaurus than to Limaysaurus. They found that nigersaurines were restricted to North Africa and Europe, and that Limaysaurinae was strictly known from Argentina. The same year, the Brazilian palaeontologist Rafael Matos Lindoso and colleagues used the name Nigersaurinae following Mannion's recommendation, and found Itapeuasaurus from Brazil to group with the nigersaurines, thereby expanding this lineage more widely.