The Upper Cretaceous outcrops of the Pyrenees yield one of the most extensive and continuous records of paleoological remains anywhere in the world. Most of eggs and eggshells have been referred to the oofamily Megaloolithidae. In this study, we present a revision of eggshell fragments from the Blasi 2 locality, lattermost Maastrichtian in age, previously assigned to aff. Megaloolithidae. The presence of a blocky extinction pattern and basal knobs supports a crocodilian affinity of these materials. We classify them as Krokolithidae indet. Three structural layers can be recognised in the Blasi 2 eggshells, a feature that is shared with other recent eggshells (e.g. Crocodylus porosus and Crocodylus niloticus) and fossil crocodylomorph eggshells (Krokolitheswilsoni), which were previously described as single layered. The new proposed affinity of the Blasi 2 eggshells reduces the Megaloolithidae oodiversity of the last few million years of the Cretaceous in the Pyrenees to only two valid ootaxa, Megaloolithusmamillare and Megaloolithusbaghensis. The lack of more complete material precludes the erection of new ootaxa based on the Blasi 2 material.

—The direct relationship between eggshell structure and eggshell formation is well established for avian eggs, but has never been studied in depth in non-avian dinosaurs. Both biological and crystallographic mechanisms take part in eggshell formation, due to its dual mineralogical and proteinaceous nature, but the exact relative contributions of these processes are still poorly known. Competitive growth has been postulated to be the general mechanism leading to the characteristic columnar construction seen in dinosaur eggshell. Here we analyze the eggshell structure of both ornithopod and non-avian theropod ootaxa with orientation contrast imaging and electron backscatter diffraction and present the first misorientation angle boundary maps of fossil eggshell, in order to ascertain whether competitive growth can explain the development of the columnar structure in non-avian dinosaur eggshell. Our results show that both eggshell types can be constructed via competitive growth, and that small changes in organic core spacing and crystal grain size, which are organically controlled, may develop into major changes in general eggshell structure, which will determine the physical properties of the egg. However, interseed distance cannot be directly correlated with organic core spacing as the competitive growth model predicted.