Histovariability and growth in the basal ceratopsian dinosaur Psittacosaurus mongoliensis from the lower Cretaceous Khulsangol Formation, central Mongolia
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Large datasets are required in order to study the details of growth in non-avian dinosaurs. The Cretaceous dinosaur sites in Mongolia offer tremendous opportunities to collect a sufficient data for such study. This dissertation introduces a brief history of dinosaur research in Mongolia. As a result of many expeditions since the 1920s, numerous new dinosaurs were named and many new dinosaur localities have been discovered. Whereas the majority of studies of Mongolian dinosaurs have focused on the fossils of Late Cretaceous taxa, dinosaur fossils are also abundant in the Early Cretaceous dinosaur sites. One of the richest Early Cretaceous dinosaur sites is Builst Khudag (BLK) in central Mongolia. Sedimentologic and taphonomic studies of the BLK site suggest that the depositional environment of the locality indicates the occurrence of a debris flow and sheet-flood in an alluvial setting. The fossils of the parrot-beaked, herbivorous dinosaur Psittacosaurus is the most common taxon from the BLK site. A Psittacosaurus aggregation consisting of juveniles reflects a catastrophic mortality event. Previously, the humerus, femur, tibia, and fibula were suggested to represent the ideal bones for growth determination studies. Since various elements grow in different developmental regimes, it is necessary to further determine an ideal element for Psittacosaurus growth assessment. Associated femora, tibiae, and fibulae from four individuals were examined. The femur and tibia show comparable bone tissues in all individuals; the tibia and fibula present comparable numbers of growth lines. Based on this finding, the tibia appears to provide the most comprehensive view of growth in Psittacosaurus. The Psittacosaurus growth strategy was further assessed using 58 tibiae collected from the BLK site. The Psittacosaurus growth rate reaches its peak (3.19 microns/day) after passing the threshold size at the age of two. During the ages of 10-13, the body size reaches 170.3 cm in length and the growth rate is considerably slowed down. This study confirms that Psittacosaurus growth fits in a sigmoidal shaped curve.