Omeisauruswas a genus of long-necked sauropod dinosaurs that lived in the Middle Jurassic, approximately 165 to 160 million years ago. At 14 m (45 feet) long and 4,000 kg, it was about the weight of an adult male elephant and was not as large as some of its truly gargantuan relatives. It lived in heavily forested environments and could have fed from the treetops using its long neck. Omeisaurus appears to be part of an Asian radiation of sauropods, originating during a time when this continent was in isolation (1). They are distinguished by their exceptionally long necks, with several more vertebrae than most other sauropod species. They likely traveled in herds (2).
Omeisaurus lived approximately 165 to 160 million years ago in the Middle Jurassic period. The distribution of Omeisaurus appears to be limited to China. When it was alive, it walked through a heavily forested semitropical environment and could have foraged from the upper levels of trees (1). It is commonly found at fossil deposits for this time period in China, and it is likely that Omeisaurus was a common dinosaur in its ecosystem (2). Omeisaurus shared this environment with a rich diversity of crocodilians, pterosaurs, turtles, fish, amphibians, and other dinosaurs. Other dinosaurs it may have encountered include Sinraptor, Kaijiangosaurus, Shunosaurus, and Dianchungosaurus (3).
A bony tail club has been found in association with Omeisaurus bones. It is possible that this bony club belonged to Omeisaurus and was used for self-defense, however, it could have come from the dinosaur Shunosaurus instead (2).
Omeisaurus is a member of Sauropoda, the group that includes massive long-necked herbivorous dinosaurs. Traditionally, Omeisaurus has been classified within the family Cetiosauridae. However, this family is generally called a “waste bin taxon,” or one that contains unrelated, difficult to place, species and ultimately does not stand up to rigorous phylogenetic analysis. Omeisaurus is also a member of Eusauropoda. Considered the “true sauropods,” this group contains sauropod dinosaurs more closely related to titanosaurs than to primitive sauropods, such as Vulcanodon (1).
The closest relatives of Omeisaurus include Mamenchisaurus and Shunosaurus, other Chinese dinosaurs. Omeisaurus evolved prior to the formation of Neosauropoda, the group containing Diplodocoidea, Macronaria, and their famous group members, such as Diplodocus, Brachiosaurus, and Apatosaurus (1).
Five different species of Omeisaurus were originally described. Several more have been identified since. However, some of these are likely synonymous (2).
Omeisaurus was a high level browser in its forested environment and had many adaptations to support this lifestyle. The structure of its limbs and hips suggest that Omeisaurus may have been able to rear upon its back legs for short periods of time. This could have allowed it to reach even higher vegetation when feeding (1). Its teeth were spoon-shaped, well suited for feeding on plant material (2).
The neck of Omeisaurus is remarkable even among its other long-necked sauropod dinosaur relatives. It had a proportionally longer neck than most sauropods and it achieved this through two different adaptations. It had longer vertebrae and it had more of them. Omeisaurus had 17 neck vertebrae, only surpassed by Mamenchisaurus, a close relative. While this neck is seemingly out of balance with the rest of the dinosaur’s body, its vertebrae were lighter because they had thin walls with large holes (1).
The neck of Omeisaurus was very inflexible. It was braced by elongated cervical ribs, backwards-pointing spine-like ribs attached to the neck vertebrae. These ribs were bound and strengthened by muscles and connective tissue. Even though the neck is made up of many vertebrae, this arrangement transformed it into what is a structurally continuous unit. The ribs were so long that they extended well beyond a single vertebra, forming bundles including several cervical ribs. While this was a very strong structure, it left Omeisaurus with limited flexibility in its neck. Most movement probably occurred at the base of the neck, at the shoulders (2).
