The conservative structure of the ornithopod eggshell: electron backscatter diffraction characterization of Guegoolithus turolensis from the Early Cretaceous of Spain

The Spheroolithidae oospecies Guegoolithus turolensis, putatively attributed to non-hadrosauroid styracosterns was first described in the Barremian of the Iberian Basin, and later reported in the Valanginian–Hauterivian of the Cameros Basin, with both occurrences separated by a few hundred kilometres but by over 10 million years. Here we apply electron backscatter diffraction to search for crystallographic and ultrastructural differences between both occurrences of this ootaxon. Not only did not we found significant differences between eggshells from the two basins, probably laid by different non-hadrosauroid styracostern species but also, we found significant similitudes with the Late Cretaceous Maiasaura eggshells from North America. All Spheroolithidae eggshells are characterized by a profusion of low angle misorientation boundaries within larger orientation domains, that fan out from the eisospherite. A very quick reorientation of the c-axis of the crystals is observed, and explained partially via competitive growth. This conservative eggshell architecture suggests that the physical properties of the eggshell of styracosterns remain unchanged for at least 80 million years, implying similar nesting and breeding behaviours.