In Depth
Ouranosaurus similarities and differences to Iguanodon
Although initially classed as an iguanodontid dinosaur, subsequent studies of Ouranosaurus fossils have revealed it to be a form of basal hadrosaur. Despite this, Ouranosaurus still bears some features that are similar to the more famous Iguanodon, specifically the forelimb. The hands of the forelimbs still have five digits with the three central digits being the most robust and arranged to support the weight of Ouranosaurus when it was in a quadrupedal posture. The inner digit is a single thumb spike and the outer is more flexible than the weight bearing digits.
However the forelimbs of Ouranosaurus was roughly just over half the length of the hind limbs, making them proportionately shorter than those of Iguanodon. Also not only was the thumb spike smaller, but the outer digit was reduced and underdeveloped. In Iguanodon this fifth digit is thought to have been prehensile and used for wrap around and pull down vegetation so that Iguanodon could feed on a greater abundance of plant material. Ouranosaurus’s fifth digit however was no way near as flexible and while it could still potentially be used, it would not have been as capable as Iguanodons.
At first impression it may seem strange that a herbivorous dinosaur like Ouranosaurus would lose an adaptation that should have helped it to feed upon plants. What needs to be done however is to look at the bigger picture, or in this case the whole forelimb, and environment. Ouranosaurus is thought to have lived in lowland areas such as river deltas that were unlikely to have high growing vegetation, but a large amount of rapidly growing reeds and low plants. With most if not all of the available food being nearer the ground, Ouranosaurus would have spent most of its time on all fours in a quadrupedal posture, and the shorter forelimbs would have reduced the distance between its mouth and the food it was eating. Also the lack of tall growing vegetation would mean that Ouranosaurus had no need to pull plants down to its mouth which resulted in the fifth digit becoming less flexible. An additional benefit of this digit becoming inflexible may have even been to reduce the ground pressure of the forelimbs as Ouranosaurus walked on softer water logged ground, meaning that they did not stick in as much. This all comes together to suggest that rather than being a generalist browser like Iguanodon, Ouranosaurus was a low browsing specialist. Further support for this theory can also be inferred by the presence of the sauropod Nigersaurus, also from North Africa that has a very specialised skull and mouth for browsing on low vegetation.Ouranosaurus’s Beak, Teeth & Skull
Ouranosaurus was on its way to becoming a duck billed hadrosaurid, and this can be seen by the broad beak that in life would have been covered with keratin. This beak would have been very good at pulling up the soft and leafy plants that grew on the edges of water systems, and the broad edge would have also allowed for the pulling of multiple small plants at the same time.
The beak was not the only food processing mechanism and roughly two fifths back from the beak the teeth started (the toothless gap between the end of the teeth and beak is sometimes called the diastema). These were arranged in rows for small gaps between them where small replacement teeth were emerging to fill the gaps, thus forming a constant line of teeth. The presence of teeth suggests that some of the plant material that Ouranosaurus was eating was tougher than just leafy fronds, and may have been the stems and perhaps even roots as well. It is also possible that Ouranosaurus may have sometimes fed upon tougher plants when it was unable to feed from softer vegetation.
There was probably a limit to just how plants could be until Ouranosaurus had real difficulty in eating them. The jaws do not display strong muscle attachments which suggests that Ouranosaurus had a very weak bite force. Herbivores usually do not have high bite forces unless specially adapted to eat tough fibrous vegetation, and other well-known dinosaurs such as Stegosaurus are known to also have low bite forces. The fact that Ouranosaurus had a low bite force simply means that it would have been more predisposed to eat softer vegetation. The teeth may even have been used to mash the surface of the plant material slightly so that digestive enzymes could more easily extract nutrients from it.
The nostrils of Ouranosaurus were placed high on the snout, presumably so that they did not get blocked by mud and dirt as Ouranosaurus was feeding. There are also two bony growths on either side of the skull between the nasal and eye openings which may have been a display feature or identifying characteristic of the species that allowed Ouranosaurus to better recognise each other at close range. The Neural Spines of Ouranosaurus
The most spectacular feature of Ouranosaurus is the presence of tall neural spines along the dorsal, sacral and caudal vertebrae. Popular thinking has these spines supporting either a sail or hump that rose up from the back of Ouranosaurus. The robust nature of the spines may lean towards the hump theory or at the very least a thick and fleshy sail rather than a flap of skin. As always, asking what kind of structure it was is but the first question, the second one being what was it there for?
Thermoregulation, the ability for an animal to have some control over its body temperature, either cooling or warming, is a popular but controversial theory. Herbivores tend not to need a high metabolism, something that would be provided by a higher body temperature, because they usually rely upon an extensive digestive system. Another theory is that the growth was primarily a display device to make individuals stand out to others of their kind, and may have been brightly coloured, or at least differently to the rest of the body for this purpose.
Another option which is more associated with the hump theory is that it provided food storage in times of abundant food so that Ouranosaurus could better survive times when food got scarce. In can be hard to conceive of a river delta that does not have some greenery, but deltas only exist as long as water keeps flowing into them. Should the rains fail one season, water levels would drop an even possibly disappear. Also if Ouranosaurus lived in herds, the herd may have quickly depleted the available food in an area meaning that they would have to keep traveling to find areas of fresh growth. Ouranosaurus may have even travelled to drier areas further in land to lay eggs where ground was more stable and less prone to flooding, and needed to build up food reserves for the journey as well as caring over the eggs and newly hatched juveniles, as seen in the dinosaur Maiasaura. While the precise nurturing behaviour of Ouranosaurus is still not currently know, it’s not totally inconceivable that it may have shown similar behaviour. The development of neural spines is also strongly seen in the African predators Suchomimus and Spinosaurus, and this could also point to an adaptation for an ecological factor such as an especially arid environment or the need to go prolonged periods without feeding.Potential Predators
It’s conceivable that Ouranosaurus, particularly smaller juveniles, may have come into contact with the spinosaurid predator Suchomimus. Although thought to be primarily fish eaters, bones of a juvenile Iguanodon were found inside the remains of another smaller spinosaurid named Baryonyx from England. Spinosaurids did frequent river deltas that also seem to have been the main habitat of Ouranosaurus, making predation by spinosaurids possible while scavenging of dead Ouranosaurus probable. The type genus of the Carcharodontosauridae, Carcharodontosaurus, would have also been a potential threat.
Ouranosaurus also may have come into contact with giant crocodiles like Sarcosuchus. The larger individuals of this genus approached lengths of up to twelve meters, making them a very real threat to Ouranosaurus, particularly not yet fully grown juveniles and subadults.
Further Reading
– G�ologie et Pal�ontologie du Gisement de Gadoufaoua (Aptien du Niger) [Geology and Paleontology of the Gadoufaoua Locality (Aptian of Niger)]. – Cahiers de Pal�ontologie, Centre National de la Recherche Scientifique, Paris 1-191. – Philippe Taquet – 1976. – Neural spine elongation in dinosaurs: sailbacks or buffalo-backs? – Journal of Paleontology 71: 1124-1146 – J. B. Bailey – 1997. – The Venice specimen of Ouranosaurus nigeriensis (Dinosauria, Ornithopoda). – PeerJ. 5: e3403. – F. Bertozzo, F. M. Dalla Vechia & M. Fabbri – 2017.
