Ouranosaurus

Kingdom: Animalia

Phylum: Chordata

Class: Reptilia

Order: Ornithischia

Family: Iguanodontidae

Genus:Ouranosaurus

Species:Ouranosaurus nigeriensis

Introduction

Ouranosaurus was a large herbivorous iguanodont living in Early Cretaceous Africa. Although its exact classification is the matter of some dispute, according to phylogenetic studies, the species is an early branch of the lineage that gave rise to the well-known hadrosaurs or “duck-billed” dinosaurs, one of the most successful groups in the Cretaceous period. Ouranosaurus is one of the few members of this lineage known to appear in the Southern Hemisphere.

In the Triassic period, the continents of the earth were united in a single land mass known as Pangaea. Dinosaurs evolved during this period and spread across the earth's surface, populating every continent. In the Jurassic and Cretaceous, Pangaea split into two supercontinents, Gondwana in the south and Laurasia in the north. Though some dinosaur lineages appeared in both Gondwana and Laurasia, others species evolved on one or the other after Pangaea split apart.

Ouranosaurus first appears in the fossil record at the beginning of the Cretaceous, as Africa and South America were beginning to separate. Iguanodonts are known to both Laurasia and Gondwana, and Ouranosaurus is thought to be part of a more ancient lineage that gave rise to the southern hadrosaurs, known mostly from Laurasia.

Classification

Scientists use two primary methods to place organisms into different categories. Linnaean taxonomy groups organisms according to overall physical similarity, while an alternate method called “cladistics,” or phylogenetic nomenclature, attempts to create groups that reflect the evolutionary relationships between species.

Under the Linnaean system, Ouranosaurus is placed in the order Ornithischia because of the “bird-like” orientation of its pelvis (though this longstanding division in dinosaur classification has been challenged by some scientists). From there, the species is placed into the suborder Cerapoda and in the infraorder Ornithopoda, or the “bird-footed” group, which were generally herbivorous dinosaurs sharing skull and leg bone features. Ouranosaurus is further placed in the Iguanodontia group, a broad category that includes the large herbivorous dinosaurs with “duck-like” bills known as hadrosaurids. More specific classification varies and is subject to debate, though the species was traditionally placed in the Iguanodontidae family.

The cladistic system also places Ouranosaurus in the clade Ornithopoda, based on features of the skull and the teeth. Again, more specific classification often varies between systems and researchers, especially as new evidence emerges. Some place the species in the clade Styracosterna, which contains the hadrosaurs and is partially defined by the presence of large nares or nasal chambers, spiked thumbs, and hoof-like claws on the other digits. Ouranosaurus likely split from this group before the division of the later Laurasian hadrosaurs into the clades Hadrosaurinae and Lambeosaurinae. Others consider Ouranosaurus within the clade Iguanodontidae, which along with the Hadrosauroidea is placed the larger group of Hadrosauriformes.

Anatomy

Ouranosaurus was a large herbivore, reaching at least 7 meters (22.9 ft) in length and weighing a maximum of 2.7 to 2.9 metric tons (5,952–6,393 lbs). The species had longer rear legs than front and is believed to have been partially bipedal, though it may also have occasionally used quadrupedal movement.

The animal had a stiff, muscular tail, presumably used for balance. The toes of both the hands and feet had hoof-like claws, with the exception of the thumbs on the hands, which terminated in spiked claws. The animal had a long skull with a relatively narrow bill and a pair of bony bumps just above the eyes.

Ouranosaurus had a number of elongated vertebral shafts along its back, which were originally thought to be remnants of a “sail-like” structure, though different in form to those seen in Triassic reptiles like Dimetrodon. However, many paleontologists now believe that instead of a sail-like feature, the enlarged spinal structures may have supported a fleshy hump, as seen in some modern mammals like buffalo and camels.

Intelligence

Animal intelligence can be estimated by a measure known as encephalization quotient (EQ), based on the relative size of the braincase and body. Scientists have calculated such a value for several species of ornithopods and found that they generally had an EQ rating of between 1.4 and 1.6, placing them at the middle to upper end of the intelligence range for herbivorous dinosaurs. Ouranosaurus may have been similar in intelligence to a small bird or a large reptile but was probably not as intelligent as large herbivorous mammals.

Reproduction and Population

Paleontologists have not uncovered eggs or nests from Ouranosaurus, but nesting sites from similar species, such as hadrosaurs, indicate that the animals laid up to twelve or more eggs in a nest. Because the hadrosaurs were heavy, they were probably not brooding animals but may have covered the nests with vegetation to insulate the eggs.

The unusual bony bumps above the eyes of Ouranosaurus may have served a function in reproduction, helping to attract mates, or they were possibly used in head butting competitions, similar to the horns on such herbivorous mammals as elk and bison. Some paleontologists have suggested that the bumps may represent a form of sexual dimorphism, meaning they were exclusive to either males or females, allowing members of different sexes to recognize one another.

Diet

Judging from the animal's dentition, paleontologists believe that Ouranosaurus was an herbivore, like other iguanodonts. The animal's bill would have been used to crop the leaves of gymnosperms, like conifers and cycads, and pteridophytes, such as ferns. Similar to other iguanodonts (and later hadrosaurs), Ouranosaurus had an array of teeth located on the sides of the jaws, allowing the animal to chew plant matter before swallowing to speed digestion. The evolution of cheek teeth is thought to be one of the major milestones that led to the dominance of the hadrosaurs in the Cretaceous.

Behavior

The unique hump-like structure on Ouranosaurus's back may have functioned similarly to the humps on buffalo and camels, storing nutrients in the form of rich fat deposits, which could provide nutrition to the body in times of food shortage. Alternatively, the hump might have functioned to discourage predators by making the animal appear larger and more threatening. If it was indeed a sail rather than a hump it may have served a similar purpose, functioned as a display feature, or even played a role in thermoregulation. In either case the structure most likely served more than one purpose.

The thumb spike of Ouranosaurus and other iguanodonts is generally believed to have been used as a weapon to ward off potential predators. Some paleontologists have suggested that the spike might also have been used in feeding, helping the animal to hook branches of trees and to draw vegetation to its beak.

Ouranosaurus is known only from isolated specimens, but other similar species have been found in fossil beds containing multiple adult individuals, suggesting that the species sometimes gathered into groups or herds. Ouranosaurus might also have displayed some social behavior, though further evidence is needed to confirm this hypothesis.

Habitat and Other Life Forms

Early Cretaceous Africa was a diverse habitat containing rich gymnosperm and fern forests, open scrub plains, and broad flood plain habitat. There were a variety of small reptiles, amphibians, and small mammals living alongside dinosaurs in these habitats.

Predatory dinosaurs in Early Cretaceous Africa included the large theropod Afrovenator and a variety of spinosaurids, large predators that specialized in semi-aquatic environments, including genera like Suchomimus.

Africa was home to a wide variety of herbivorous dinosaurs, including relatives of the sauropods, large herbivores that dominated the terrestrial environment in the Jurassic. The sauropods Malawisaurus and Jobaria have been discovered from Early Cretaceous fossil sites. Nigerasaurus was a smaller relative of the sauropods that possessed unusual cranial characteristics, including a shovel-shaped mouth and a pneumatic cranial structure. Another relative of the hadrosaurs, Lurdusaurus, was also present in the same fossil sites that contained Ouranosaurus skeletons.

Research

The first specimen of Ouranosaurus was uncovered in Niger's Echkar (or El Rhaz) formation in 1966 and was named by French paleontologist Philippe Taquet. In the late 1990s, paleontologist Jack Bowman Bailey contributed to the description of Ouranosaurus with a study indicating that the enlarged spinal column supported a fleshy hump rather than a sail, as was previously believed. Since the 1990s, the phylogenetic status of Ouranosaurus has been frequently revised through the continuing effort to determine the relationship of dinosaurs in the iguanodontid and hadrosaur groups.

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