Microstructural description of the maniraptoran egg Protoceratopsidovum

Since their discovery in the 1920s, the affinity of asymmetric, elongated dinosaur eggs from the Late Cretaceous of Mongolian has been controversial, with previous hypotheses supporting either a ceratopsian or maniraptoran affinity. Recent technical advancements in palaeontology provide a novel approach to diagnose maniraptoran eggs, and the discovery of soft ceratopsian eggs makes the controversy a timely issue worth revisiting. Here, we analysed Protoceratopsidovum eggshell from southern Mongolia with electron backscatter diffraction (EBSD) and the results were compared with East Asian and North American maniraptoran and ornithischian eggs. The microstructure and crystallography of Protoceratopsidovum shows diagnostic features of maniraptoran eggs that are absent in ornithischian (e.g. hadrosaur) eggs. Additional characters, such as egg shape, ornamentation, egg pairing and clutch structure all support a maniraptoran affinity. Protoceratopsidovum has both elongatoolithid (oviraptorosa...