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.

An additional two skeletons were discovered 7 km (4.3 mi) southeast of Elrhaz in the "Camp des deux Arbres" locality, which were given the field numbers GDF 300 and GDI 381.

[3] While the third expedition did not turn up additional iguanodontian material, the fourth in January–March 1970 uncovered a nearly complete and partially articulated skeleton lacking the skull, 4 km (2.5 mi) south of the "niveau des Innocents" site, and was also given the field number GDF 381.

[1][3] Following a subsequent Italian-French expedition led by Taquet and Italian palaeontologist Giancarlo Ligabue that turned up a potential additional iguanodontian specimen, Ligabue offered to donate the nearly complete specimen and a skull of Sarcosuchus to the Municipality of Venice, which accepted the offer and subsequently mounted the skeleton in 1975 at the Museo di Storia Naturale di Venezia.

[1] 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.

[1] Ouranosaurus was a relatively large iguanodontian, measuring 7–8.3 metres (23–27 feet) long and weighing 2.2 tonnes (2.4 short tons).

Anteriorly, the premaxillae flare gently laterally into a rugose surface for a beak, like other iguanodontians, although dissimilar from Iguanodon and similar to hadrosaurids the nares are entirely visible from above.

The maxilla bears faces for articulation with the premaxilla in front, lacrimal above, ectopterygoid, vomer, palatine and possibly pterygoid internally, and jugal to the rear.

[1] The dental edge of the maxilla is slightly arced, and above the toothrow is a shallow depression bearing nutrient foramina, also known as the buccal emargination that is diagnostic of Ornithischia.

The jugal below and behind the orbit bears the same shape as in hadrosaurids, with a high rear process, and articulated with the quadratojugal and quadrate that are also very similar to more derived taxa.

The bones are unfused suggesting mobility, and at their ends on the top of the skull are rounded domes, which were described by Taquet (1976) as distinct and rugose "nasal protuberances".

The most conspicuous feature of Ouranosaurus is a large "sail" on its back, supported by long, wide, neural spines, that spanned its entire rump and tail, resembling that of Spinosaurus, a well-known meat-eating dinosaur also known from northern Africa[6] as well as those of the modern Crested chamaeleon (Trioceros cristatus) which has a moderate sail on its back, and to a lesser extent the sail-backed temnospondyl Platyhystrix rugosus.

The first four dorsal vertebrae are unknown; the fifth already bears a 32-centimetre-long spine (1.05 ft) that is pointed and slightly hooked; Taquet presumed it might have anchored a tendon to support the neck or skull.

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.

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.

However, although it shares some similarities with Iguanodon (such as a thumb spike), Ouranosaurus is no longer usually placed in the iguanodontid family, a grouping that is now generally considered paraphyletic, a series of subsequent offshoots from the main stem-line of iguandontian evolution.

Ouranosaurus appears to represent an early specialized branch in this group, showing in some traits independent convergence with the hadrosaurids.

[8][9] Rhabdodontidae Tenontosaurus Dryosauridae Camptosaurus Cumnoria Uteodon Hippodraco Theiophytalia Cedrorestes Dakotadon Iguanacolossus Lanzhousaurus Iguanodon Mantellisaurus Ouranosaurus Hadrosauroidea The jaws were apparently operated by relatively weak muscles.

Ouranosaurus had only small temporal openings behind the eyes, from which the larger capiti-mandibularis muscle was attached to the coronoid process on the lower jaw bone.

The back of the skull was rather narrow and could not compensate for the lack of a greater area of attachment for the jaw muscle, that the openings normally would provide, allowing for more power and a stronger bite.

A lesser muscle, the musculus depressor mandibulae, used to open the lower jaws, was located at the back of the skull and was connected to a strongly projecting, broad and anteriorly oblique processus paroccipitalis.

Remodeling is present in the subadult paratype, and high vascular density and circumferential arrangement of the microstructure suggests fast growth.

[11] It has been claimed that the iguanodontian Lurdusaurus and the rebbachisaurid Nigersaurus were the most numerous megaherbivores in the Elrhaz formation,[11] however this runs counter to Taqet and Russel,[12] who state that Ouranosaurus was the "dominant element in the assemblage."