Spheroolithid eggshells in the Lower Cretaceous of Europe. Implications for eggshell evolution in ornithischian dinosaurs

Early Cretaceous spheroolithid eggshells have always been extremely scarce, unlike their putative egg layers, ornithopod dinosaurs, which are ubiquitous in Early Cretaceous ecosystems. In this paper, we re-evaluate eggshells previously assigned to the oofamily Elongatoolithidae, an oofamily related to derived theropod dinosaurs (including maniraptorans and oviraptorids), and reassign them to the oofamily Spheroolithidae, on the basis of the lack of a squamatic ultrastructure, and the presence of a prolatocanaliculate pore system and undulating extinction, typical of spheroolithid eggshells. We erect a new oogenus, Guegoolithus to include material previously reported as Macroolithus turolensis Amo-Sanjuan, Canudo and Cuenca-Bescَs 2000, and new material from several lower Barremian localities of the Maestrazgo Basin in Spain. Guegoolithus eggshells exhibit an acicular radial ultrastructure in the lower part of the shell units, which develops into a tabular radial ultrastructure in the upper part, giving an appearance of a two-layered eggshell in thin sections that is not seen in SEM pictures. These features fill the gap between the two-layered prismatic eggshell of ceratopsians and the prolatospherulitic ornithopod eggshell of derived hadrosaurs.