A unique association of fusioolithid dinosaur eggs from the Upper Cretaceous of Spain (Poyos, central system)

The oological record from the late Campanian – early Maastrichtian Poyos fossil site (Guadalajara Province) within the Villalba de la Sierra Formation at the Iberian Ranges, is described here in detail for the first time. This site yields the southernmost and possibly the oldest occurrence of Fusioolithus baghensis in Europe, a widespread oospecies mainly found in mid to uppermost Maastrichtian deposits. The record from Poyos also allows the recognition of the second fusioolithid ootaxon from Europe, herein described as Litosoolithus poyosi oogen. et oosp. nov. This ootaxon is mainly characterized by very large eggs with a exceptionally thin eggshell compared to other ootaxa attributed to titanosaur sauropods. It also exhibits an unusually low porosity, and an eggshell composed of an uneven combination of two types of shell units which differ in their shape, dimensions, fusion degree, and absence or presence of a node. This new ootaxon also presents a dispersituberculated ornamentation that contrasts with the typical compactituberculated ornamentation of the Fusioolithus oogenus. The analysis of the spherolith dimensions of Fusioolithidae and Megaloolithidae ootaxa provides statistical support for discriminating Litosoolithus poyosi from other oospecies and for recognizing the differences between its two types of spheroliths. Litosoolithus poyosi and Fusioolithus baghensis co-occur on the same stratigraphic levels in Poyos which is, unlike the co-occurrence of megaloolithids or megaloolithids with fusioolithids, a rare occurrence in the global fossil record outside India. Therefore, the record of the Poyos site provides valuable data for ongoing discussions, allowing comparison with other similarly aged Ibero-Armorican formations. • The Campanian-Maastrichtian site of Poyos represents a vast titanosaur nesting area. • Poyos records the second co-occurrence of multiple fusioolithids outside of India. • Litosoolithus poyosi has the largest eggs with the thinnest shells among titanosaurs. • Poyos records the southernmost and possibly oldest Fusioolithus baghensis from Europe. • The egg association correlates with lower Maastrichtian faunas from Ibero-Armorica.