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 (oviraptorosaur egg) and prismatoolithid (troodontid egg) features, and it may hold implications for the evolution of microstructure in non-avian maniraptoran and extant palaeognath eggshells. Considering the similarity between Protoceratopsidovum and potential dromaeosaur eggs, at least some Protoceratopsidovum may be deinonychosaur eggs. Fossil localities that yield both deinonychosaurs and Protoceratopsidovum may yield conclusive evidence for the true affinity of this ootaxon. Future discovery of the egg-layer of Protoceratopsidovum will not only improve our understanding of maniraptoran reproductive biology, but also make Protoceratopsidovum a reliable indicator of the palaeobiogeography of that maniraptoran clade.

Methods
Detailed methodologies for EBSD analysis are summarized in Choi et al. (2019, Palaeontology). All data were acquired by using AZtec software and exported into cpr and crc files. The cpr and crc files are uploaded in the Dryad repository.