Skeletal Characteristics Of Mamenchisaurus Youngi And The Systematics Of Mamenchisuarids

Previous opinions on the phylogenetic position of Mamenchisaurus, the most famous sauropod from China, varied because of the lack of the reliable skull and its complete description. This disadvantage has seriously affected and postponed scientific understanding of the phylogenetic history of the entire Sauropoda. Fortunately, the specimen best preserved known up to date of mamenchisaurs, Mamenchisaurus youngi, turned to be available to this study, so that a detailed description of its complete skeleton framework, specially of its skull, has become feasible. The results indicate that Mamenchisaurus has a camarasaurid-type skull and characteristic postcranials, which are different not only from those of camarasaurids-branchiosaurids, but also from those of diplodocuds-dicraeosaurids. Thus, it is suggested that Mamenchisaurus and some similar long-necked genera, such as Omeisaurus and Euhelopus, should be recognized as a distinct sauropoda branch, Mamenchisauridae. The skull of this family is characterized by its lighter structure with large opening, spatulate crown and thin mandibular. The characteristics of its postcranial skeleton include a very long neck, counting 17-19 cervicls associated with long and striaght cervical ribs. The tail is shorter than the neck and the chevrons in the middle caudals are forked. The posterior cervical and anterior dorsal spines are not bifurcated and the anterior caudals are slightly amphicoelous or platycoelous in primitive forms, while in the advanced forms they have the bifurcated spines both on their posterior cervicals and anterior dorsals, and the precoelous centrum on their proximal caudals.Visual descriptions for the sclerotic ring and the fossilized skin of sauropods, which are rarely discovered, are presented here in this thesis. The computerized tomography (CT) scanning technology was used to investigate the cavity structures of the brain and tooth of the Mamenchisaurus youngi. The result displays that this dinosaur had a relatively small brain cavity (less than 80 ml), but with relatively advanced structural divisions, and that its teeth replacement was continuous in life. The absorption of an old tooth root was accompanied with the growth of a new tooth. The more the absorption amounts of the roots, the more prominent the worn facets on the crown. The developed worn facets of teeth also show that the food of...