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 (filled with air spaces connected to air sacs), but the limbs were robustly built.

''Nigersaurus'' and its closest relatives are grouped within the subfamily Rebbachisaurinae (formerly thought to be grouped in the eponymous Nigersaurinae) 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 (filled with air spaces connected to air 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.

''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'' ("Niger reptile") 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 (MNN GAD512) consists of a partial skull and neck. Limb material and a scapula (shoulder blade) 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 (openings in the skull) 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 (which formed much of the skull-roof) was elongate (much narrower than long), and had a marked cerebral fossa (a depression on the surface of this bone inside the head). 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 (also present in hadrosaurs and ceratopsians) 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