Description of the first lithostrotian titanosaur embryo in ovo with Neutron characterization and implications for lithostrotian Aptian migration and dispersion

Abstract

Although titanosaurs represent one of the most diverse radiations of non-avian dinosaurs during the Cretaceous, our knowledge of their early developmental stages was restricted to the Auca Mahuevo (Argentina) embryos in ovo. Here, we present the first complete lithostrotia titanosaur embryo in ovo. The relatively small spherical 87.07 to 91.1 mm egg was discovered at the Lower Cretaceous locality Algui Ulaan Tsav in Mongolia, and is, to date, the smallest positively identified titanosaur egg. Through taphonomic processes, the egg was transformed into a calcite geode at the bottom of which the embryonic bones settled down and are now partially exposed on the lower egg surface. Neutron tomography characterization reveals a fully developed embryo fossilized within a thin (7.6 mm–8.6 mm) calcite layer. EBSD, a SEM-based diffraction technique, which measures the complete crystallographic orientation of the crystal lattice from a submicron area on the sample surface, is used for the first time on an extinct dinosaur eggshell. Observations of the egg and its embryo combined with eggshell microcharacterizations suggest that this new embryo was a lithostrotia titanosaur with an intermediate robusticity index that shares a mosaic of skeletal characters with Diamantinasaurus matildae from Queensland (Australia) and the nemegtosaurid Rapetosaurus krausei (Madagascar) more than with any other titanosaurs. The Early Cretaceous age of Algui Ulaan Tsav implies that this specimen greatly predates the previously described lithostrotian titanosaurs from the Late Cretaceous sediments of Mongolia. In addition, the recognized amount of similar eggs that has been recovered during the last 70 years at Algui Ulaan Tsav suggests that a well-established population of lithostrotian titanosaurs used this site as a nesting site. The combined observations provide an important addition to Mongolian fossil richness and alter our understanding of the paleodispersion of this sauropod group. It now appears that lithostrotian sauropods would have reached Mongolia during the Aptian–Albian, thus suggesting the existence of a passage before the complete separation of the Laurasian and Gondwanan continents. Possibly, the lithostrotian north and eastward migration could have occurred between North Africa, Spain, and the rest of Europe prior to its fragmentation in large islands during the Cretaceous, thus justifying the presence of lithostrotians in the Cretaceous Romanian Hateg Island.