Possible Late Cretaceous dromaeosaurid eggshells from South Korea: A new insight into dromaeosaurid oology

Among non-avian maniraptoran eggshells, oofamilies Prismatoolithidae and Elongatoolithidae are usually associated with troodontid and oviraptorosaur dinosaurs, respectively. However, dromaeosaurid eggshells are poorly known so far except for one possible Deinonychus egg associated with the gastralia of Deinonychus. Since then, some Deinonychus eggshell-like non-prismatoolithid and non-elongatoolithid maniraptoran eggshells have been reported including oogenera Reticuloolithus and Montanoolithus. Here we report a new ootaxon Reticuloolithus acicularis oosp. nov. from the Upper Cretaceous Wido Volcanics in South Korea. R. acicularis is characterized by reticulate ornamentation, an acicular mammillary layer, the continuous layer composed of two sublayers, and no external zone. EBSD analysis shows that R. acicularis has predominant low-angled grain boundaries, thereby its misorientation distribution is very similar to those of oviraptorosaurs and paleognaths. The comparisons to other maniraptoran ootaxa show that R. acicularis is morphologically very similar to Deinonychus eggshell, Paraelongatoolithus, and Nipponoolithus. The phylogenetic analysis based on a revised character matrix in this study shows that R. acicularis, R. hirschi, Paraelongatoolithus, and egg of Deinonychus make a polytomic relationship with the Elongatoolithidae. The new cladogram also raises some issues related to possible homology and homoplasy in the evolution of theropod eggshells. The possible dromaeosaurid affinity of R. acicularis is well matched with the body- and ichno-fossil record of the Cretaceous dromaeosaurids in East Asia. In addition, the low-angled misorientation of R. acicularis may imply the brooding behavior of the dromaeosaurids. Although dromaeosaurid eggshells have never been confirmed yet with embryos in ovo, the results of this study suggest that more possible dromaeosaurid eggshells can be clearly identified with EBSD analysis.