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 9 m (30 ft) long, and had a short neck. It weighed around 1.9–4 t (2.1–4.4 short tons), comparable to a modern
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
crocodylomorphs
.
History of discovery
Excavation of a specimen in 2000
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.[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 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.[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 named
Skeletal diagram showing known elements and size comparison
Sereno and the American palaeontologist Jeffrey 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 the National 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 to Darth Vader and a vacuum cleaner, and compared its tooth shear with a conveyor belt and sharpened piano keys.[6][7][8]
Numerous Nigersaurus specimens collected by French and American expeditions remain to be described.
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.[1]
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 9 m (30 ft) and a femur 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 modern elephant.[3][10] It had a short neck for a sauropod, with thirteen cervical vertebrae. Nearly all rebbachisaurids 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 were Rebbachisaurus[3] and Maraapunisaurus.[11]
The skull of Nigersaurus was delicate, with the four side
prognathous, the snout tip not protruding relative to the remainder of the tooth series.[1]Nigersaurus was distinct in that its frontal bone (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).[4] 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.[3][4]
Nigersaurus did not exhibit the same modifications seen in the jaws of other dinosaurs with dental batteries, or mammals with elaborate chewing functions. The
keratinous (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.[13]
The remains of Nigersaurus were initially described by Taquet in 1976 as belonging to a
Nigersaurinae, which includes Nigersaurus and closely related genera, was named by the American palaeontologist John A. Whitlock in 2011.[14]
The closely related genus
Tethys Sea.[15] 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.[16] Pneumatisation of the rebbachisaurid skeleton evolved progressively, culminating in the nigersaurines.[16]
Front view of reconstructed skullModel head at the Australian Museum, Sydney
Below is a cladogram following the 2013 analysis by Fanti and colleagues, which confirmed the placement of Nigersaurus as a basal nigersaurine rebbachisaurid.[16]
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 (since that name would have priority).[9] 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.[17]
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 (as well as the fact that the position of Rebbachisaurus could change in future analyses), 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.
palaeobiogeographic hypotheses for this group less reliable).[19]
Palaeobiology
CT-scanned
Nigersaurus limb elements
Though it had large nostrils and a fleshy snout, Sereno and colleagues found that Nigersaurus had an underdeveloped olfactory region of its brain and thus did not have an advanced sense of smell. Its
palaeoartist Mark 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-necked Nigersaurus. They pointed out that the eyes of Nigersaurus 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.[20]
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.[21]
A 2023 study by the French palaeontologist Rémi Lefebvre and colleagues examined the microanatomical structure of the limb bones of Nigersaurus through
selective pressure caused by bearing weight, but that features such as the columnar limbs (as seen in elephants), pneumaticity, and fleshy foot pads and cartilage 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.[22]
Diet and feeding
Crown form, wear pattern, and microstructure of the teeth
Nigersaurus was suggested by Sereno and colleagues to be a ground-level, non-selective
facets on the labial (externally facing) side of the upper teeth, similar to Dicraeosaurus and Diplodocus, 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 the maxillary 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 of Nigersaurus would have restricted the browsing range compared to other diplodocoids.[3]
The
herbaceous plants such as low-growing ferns.[13] 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 to Nigersaurus, sauropods with lower tooth replacement rates and broader tooth crowns are thought to have been canopy browsers.[12]
Grass did not evolve until the late Cretaceous, making
flamingos. He also suggested it could have low-browsed from short conifers and other low-growing plants.[20]
Head posture
sauropodomorph head postures proposed by Sereno and colleagues (left), and "prototype
" skull based on scans (right, dental batteries in red, green bones are reconstructed)
On the basis of
occiput 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 palaeontologist
Mike P. Taylor and colleagues agreed that Nigersaurus 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.[23] 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 that Nigersaurus habitually held its head like other sauropods.[24]
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.[25]
Palaeoenvironment
Gadoufaoua
in Niger
Nigersaurus is known from the
sediments are coarse- to medium-grained, with almost no fine-grained horizons.[3]Nigersaurus lived in what is now Niger about 115 to 105 million years ago, during the Aptian and Albian ages of the mid-Cretaceous.[4] It likely lived in habitats dominated by inland floodplains (a riparian zone).[3]
After the
bivalves have been found. The aquatic fauna consists entirely of freshwater inhabitants.[3][26]
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