Therizinosauroid Dinosaurs From The Early Cretaceous Of Yujingzi Basin, Jiuquan Area, Gansu Province, China

Therizinosauroid dinosaurs are among the most distinctive non-avian theropod dinosaurs yet discovered. They may well have been primarily or wholly herbivorous, and the basal therizinosauroid has recently been found to possess"protofeathers"and a pygostyle-like structure. The geographic distribution of therizinosauroids is now known to extend beyond Asia, with recent discoveries from the western United States. The stratigraphic range of the group has also been expanded, with undoubted therizinosauroids now known from the Early Cretaceous and a controversial record from the Early Jurassic.Although with recent advances on study of therizinosauroids, a number of questions continue puzzling researchers. One of these is the transition from relatively small-sized Early Cretaceous therizinosauroids to large-sized and more derived Late Cretaceous forms. When did this happen? And what are the anatomical changes and associated evolutional significances during this transition?This dissertation studied two new therizinosauroid material from the Early Cretaceous of the Yujingzi Basin of Jiuquan area, Gansu Province, China. These specimens represent a new taxon, Suzhousaurus megatherioides gen. et sp. nov. Suzhousaurus represents one of the largest-known Early Cretaceous therizinosauroids, demonstrating that this clade attained considerable body size early in its evolutionary history. Suzhousaurus megatherioides is characterized by transversely expanded dorsal ends of posterior dorsal, sacral, and anterior caudal neural spines; shallow, poorly demarcated glenoid fossa with a prominent rounded and striated tumescence on the dorsomedial surface of its scapular portion; laterally deflected, thin and flat preacetabular process of the ilium; smoothly curved anterodorsal margin of the preacetabular process of the ilium; concave anterior margin of the pubic shaft. Cladistic analysis recovers Suzhousaurus as the sister taxon of Nothronychus mckinleyi from the mid-Cretaceous of western North America; together, they are basal members of the Therizinosauroidea, more derived than the Early Cretaceous Falcarius and Beipiaosaurus but less derived than Therizinosauridae.The dinosaur-bearing sediments in the Yujingzi Basin of Jiuquan area, Gansu Province are divided into three units: the lower, middle and upper units. They belong to the Lower Cretaceous Xinminpu Group, which widely developed in the Jiuquan area. The Suzhousaurus-bearing lower unit is probably mid Early Cretaceous (?Aptian) based on palynological and dinosaur data, and deposited in a lacustrine environment.