An aberrant, double-layered, fusioolithid titanosaur eggshell from the Poyos fossil site (late Campanian-early Maastrichtian, Guadalajara, Spain)

Resumen

Multi-layered eggs have been observed to occasionally occur in extant amniotes as a consequence of a stress period during its formation. The multi-layered condition has previously been described in several megaloolithid eggshells from the end-Cretaceous of Europe (South Pyrenees and Southern France), an oofamily typically associated with titanosaur sauropods. Now, an anomalous eggshell presenting two superimposed shell layers from the Campanian – Maastrichtian Poyos fossil site (Villalba de la Sierra Fm.) has been identified and prepared at thin section. While its inner layer is well-developed and shows a normal structure, the external layer presents an aberrant morphology and ornamentation. The eggshell thickness, shell unit morphology and dimensions of the inner layer match those of Fusioolithus baghensis, an ootaxon also associated with titanosaurs. The external profile of this layer is clearly demarcated by a dark layer, possibly indicating the presence of an additional membrane. Over this dark layer, only a few possible extra organic cores have been observed, located over internodal spaces of the subjacent layer. Instead, most of the shell units of the superimposed layer seem to epitaxially grow from the nodes of the subjacent layer and extend the shape of their shell units, even presenting near complete optical continuity under polarising light. All the shell units of the superimposed layer are considerably wider and end on a non-coalesced node with similar height but around twice as wide as the subjacent node from which they grow. This produces a compactituberculated ornamentation with large nodes and no internodal spaces, contrasting with other ornamentations identified at Poyos and misleadingly resembling that of a distinct ootaxon.