Unusual Megaloolithus siruguei eggshells from the Villalba de la Sierra Fm. (Upper Cretaceous, Cuenca, Spain)

Resumen

Megaloolithus siruguei is to date the single megaloolithid oospecies recognized at two Campanian–Maastrichtian sites of the Villalba de la Sierra Fm. (Central Iberian Ranges): Portilla and Zafra de Záncara. This oospecies has also been reported at similarly aged sites in Southern France and the Tremp Fm. (Southern Pyrenees), and as M. aff. siruguei at the coeval Sierra Perenchiza Fm. (Southeastern Iberian Ranges). Interestingly, specimens from Villalba de la Sierra Fm show an uncommon combination of eggshell thickness and node shape that may represent extreme cases among this oospecies variability. The morphometric data obtained from measuring published thin sections from France, Southern Pyrenees, and Southeastern Iberian Ranges suggest that, overall, the eggshell units from specimens of the same site or formation show a similar range of spherolith width and height (and therefore eggshell thickness). Additionally, although its nodes can be bell-shaped, wave-shaped or dome-shaped, the latter morphology is seemingly the most common. The Portilla specimens, however, show a range of thickness variation comparable to all the considered French sites altogether, and the thickness of the Zafra de Záncara specimens is below the reported thickness range of the oospecies. The dome-shaped nodes are notably less frequent in the Villalba de la Sierra Fm. specimens, and bell-shaped nodes are the dominant morphology in Zafra de Záncara. The morphological data obtained from the specimens of this Formation may contribute to the understanding of the variability of this widespread European ootaxon and its relationship with biotic and abiotic factors.