CONICET | Buscador de Institutos y Recursos Humanos

Juvenile sauropods are very rare in the fossil record. No extensive ontogenetic growth series exists for sauropod skulls, and consequently, the morphological changes in ontogeny are not well understood. This study explores such changes in the most complete collection of disarticulated skull bones of a single sauropod taxon, the dwarf basal macronarian Europasaurus holgeri from the Kimmeridgian of northern Germany. The skull material includes different individuals of various ontogenetic stages. Because size alone is not a good indicator to determine ontogenetic stage, we used size-independent characters to stage the bone elements. Since the original description of Europasaurus, new skull material has been found and has allowed us to create an updated skull reconstruction of an adult individual. The updated reconstruction features smaller external nares and a larger orbit as compared to the original reconstruction. Important ontogenetic changes are the decreasing ventral exposure of the jugal, which is extensive in the juvenile, and the increasingly erect nasal process of the premaxilla. This, the large orbit, and other skull parts are paedomorphic characters resulting from dwarfing. The ventral exposure of the jugal and the inclined premaxillary nasal process in the juvenile Europasaurus represents the retention of plesiomorphic characters seen in basal sauropodomorphs (e.g. Plateosaurus and Massospondylus) and basal sauropod dinosaurs (e.g. Shunosaurus). Apart from three partial braincases, only isolated skull bones have been found. This pattern is another paedomorphic character, the lack of fusion of the skull bones. By studying the skull bones in detail, we also found that the material represents two morphs of different size classes that also belong to different ontogenetic stages. One such distinctive feature is the dimorphism of the orbital margin of the frontals.

enviar mensaje