Ouranosaurus is a genus of herbivorous basal hadrosauriform dinosaur that lived during the Aptian stage of the Early Cretaceous of modern-day Niger and Cameroon. Ouranosaurus measured about long and weighed . Two rather complete fossils were found in the Elrhaz Formation, Gadoufaoua deposits, Agadez, Niger, in 1965 and 1970, with a third indeterminate specimen known from the Koum Formation of Cameroon. The animal was named in 1976 by French paleontologist Philippe Taquet, the type species being Ouranosaurus nigeriensis. The generic name is a combination of ourane, a word with multiple meanings, and sauros, the Greek word for lizard. The specific epithet nigeriensis alludes to Niger, its country of discovery (in Latin, the adjectival suffix -iensis means "originating from"). As such, Ouranosaurus nigeriensis could be interpreted as "brave lizard originating from Niger". On the first expedition, lasting from January–February 1965, eight iguanodontian specimens were discovered at the "niveau des Innocents" site, east of the Emechedoui wells. An additional two skeletons were discovered southeast of Elrhaz in the "Camp des deux Arbres" locality, which were given the field numbers GDF 300 and GDI 381. Both were collected in the February–April 1966 expedition, the former including a nearly complete but scattered skeleton and the latter a skeleton that was two-thirds preserved. Additional description for bones unpreserved in the holotype was based on Taquet's MNHN GDF 381, which was not mentioned as having been sent to Venice and renumbered as MSNVE 3714, although this was confirmed by Italian palaeontologist Filippo Bertozzo and colleagues in 2017. The holotype itself was returned to Niger after being described and having its bones cast and mounted, and is now on display at the Musée National Boubou Hama in Niamey. The generic name Ouranosaurus carries a double meaning, it is both taken from Arabic meaning "valour", "bravery" or "recklessness" and also from the local Tuareg language of Niger where it is the name they call the desert monitor. The specific name refers to Niger, the country of discovery. Taquet had used the name "Ouranosaurus nigeriensis" previously, first in a public presentation of the skeleton MNHN GDF 300 in July 1972, then later in September 1972 in a news article and again in December 1972 in a book; only the book bore any images associated with the name, and none of the earlier mentions had a diagnosis to make the name valid. The holotype and paratype specimens were suggested to belong to subadults by Bertozzo et al. in 2017, although they would have been close to adult size. MSNVE 3714 is long as mounted, although a few caudals are missing, and is roughly 90% the length of the holotype, which would be long. The variation between the sizes fits within the range of variation between adult individuals of Iguanodon, so there is a chance that the larger holotype and smaller paratype were in the same ontogenetic stage. 20 teeth are preserved in the maxilla, although the anterior end of the toothrow is broken and Taquet (1976) predicted the total number to be 22. the modern crested chameleon (Trioceros cristatus), and to a lesser extent the sail-backed temnospondyl Platyhystrix rugosus from the Carboniferous of the United States. These tall neural spines did not closely resemble those of sail-backed synapsids such as Dimetrodon as these supporting spines become thinner distally, whereas in Ouranosaurus (as well as the afformentioned sail-backed reptiles and amphibians) the spines actually become thicker distally and flatten. The posterior spines were also bound together by ossified tendons, which stiffened the back. Finally, the spine length peaks over the forelimbs.

The first four dorsal vertebrae are unknown; the fifth already bears a that is pointed and slightly hooked; Taquet presumed it might have anchored a tendon to support the neck or skull. The tenth, eleventh and twelfth spines are the longest, at about . The last dorsal spine, the seventeenth, has a grooved posterior edge, in which the anterior corner of the lower spine of the first sacral vertebra is locked. The spines over the six sacral vertebrae are markedly lower, but those of the tail base again longer; towards the end of the tail the spines gradually shorten.

thumb|Dorsal vertebrae

The dorsal "sail" is usually explained as either functioning as a system for thermoregulation or a display structure. An alternative hypothesis is that the back might have carried a hump consisting of muscle tissue or fat, resembling that of a bison or camel, rather than a sail. It could have been used for energy storage to survive a lean season.

The axial column consisted of eleven neck vertebrae, seventeen dorsal vertebrae, six sacral vertebrae and forty tail vertebrae. The tail was relatively short.

The front limbs were rather long with 55% of the length of the hind limbs. A quadrupedal stance would have been possible. The humerus was very straight and large, measuring in proximodistal (top bottom) length. The hand was lightly built, short and broad. On each hand Ouranosaurus bore a thumb claw or spike that was much smaller than that of the earlier Iguanodon. The second and third digits were broad and hoof-like, and anatomically were good for walking. To support the walking hypothesis, the wrist was large and its component bones fused together to prevent its dislocation. The last digit (number 5) was long. In related species the fifth finger is presumed to have been prehensile: used for picking food like leaves and twigs or to help lower the food by lowering branch to a manageable height. Taquet assumed that with Ouranosaurus this function had been lost because the fifth metacarpal, reduced to a spur, could no longer be directed sideways.

The hindlimbs were large and robust to accommodate the weight of the body and strong enough to allow a bipedal walk. The femur was slightly longer than the tibia. This may indicate that the legs were used as pillars, and not for sprinting. Taquet concluded that Ouranosaurus was not a good runner because the fourth trochanter, the attachment point for the large retractor muscles connected to the tail base, was weakly developed. The foot was narrow with only three toes and relatively long.

Classification

thumb|left|[[Paleoart|Life restoration of Ouranosaurus]]

Ouranosaurus was assigned to the family Iguanodontidae in 1976, however it is rarely recovered in larger phylogenetic analyses of recent years. However, Norman (2004) argued that it was more basal and at a grade in-between the genera Iguanodon and Mantellisaurus but less derived than genera like Eolambia and Protohadros. McDonald and colleagues (2011) found Ouranosaurus to be a basal member of Hadrosauriformes, the clade containing Hadrosauroidea and several basal hadrosauriformes. In Norman (2015), Ouranosaurus was recovered as a basal styracosternan sister to the clade containing "Iguanodontoids" and Hadrosauriformes. Using data retrieved from the Venice Specimen, Italian paleontologist Filippo Bertozzo and colleagues performed an updated phylogenetic analysis using McDonald and colleagues (2011)'s matrix. In contrast to studies which found it to be outside of Hadrosauroidea, Bertozzo and colleagues recovered it in as a basal hadrosauroid at a grade more derived than the British genera Mantellisaurus and Hypselospinus, but more basal than Altirhinus, Jinzhousaurus, and Ratchasimasaurus.thumb|right|upright|Paratype MSNVE 3714 in front viewIn the 2025 description of the British styracosternan Istiorachis by Lockwood and colleagues, their phylogenetic analysis recovered Ouranosaurus as the sister taxon to the taxon within the ankylopollexian clade Styracosterna, diverging immediately before the Hadrosauriformes. These results are displayed in the cladogram below: the wide beak on the other hand indicates a specialisation in eating large amounts of low quality fodder. Ouranosaurus lived in a river delta.

Histology

Ouranosaurus bears more similarities to other derived iguanodonts than more basal ornithopods. Remodeling is present in the subadult paratype, and high vascular density and circumferential arrangement of the microstructure suggests fast growth. Faster growth occurs in the same phylogenetic groups as higher body size, although their relationship is unclear. Ouranosaurus is a similar size to more basal Tenontosaurus which has slow growth, so either faster growth is caused by body size or Tenontosaurus is the maximum size of an ornithopod with a slow growth rate. Ouranosaurus is from the upper portion of the formation, probably Aptian in age. Other herbivores from the same formation include Elrhazosaurus and an unnamed titanosaur. It also lived alongside the theropods Kryptops, Suchomimus, Eocarcharia, and Afromimus. Crocodylomorphs like Sarcosuchus, Anatosuchus, Araripesuchus, and Stolokrosuchus also lived there. In addition, remains of a pterosaur, chelonians, fish, a hybodont shark, and freshwater bivalves have been found.

Notes

References

Bibliography

  • Ouranosaurus on Nature
  • Ouranosaurus (with picture)