title: Combined Use of Electron and Light Microscopy Techniques Reveals False Secondary Shell Units in Megaloolithidae Eggshells.
abstract: Abnormalities in the histo- and ultrastructure of the amniote eggshell are often related to diverse factors, such as ambient stress during egg formation, pathologies altering the physiology of the egg-laying females, or evolutionarily selected modifications of the eggshell structure that vary the physical properties of the egg, for example increasing its strength so as to avoid fracture during incubation. When dealing with fossil materials, all the above hypotheses are plausible, but a detailed taphonomical study has to be performed to rule out the possibility that secondary processes of recrystallization have occurred during fossilization. Traditional analyses, such as optical microscopy inspection and cathodoluminescence, have proven not to be enough to understand the taphonomic story of some eggshells. Recently, electron backscatter diffraction has been used, in combination with other techniques, to better understand the alteration of fossil eggshells. Here we present a combined study using scanning electron microscopy, optical microscopy, cathodoluminescence and electron backscatter diffraction of eggshell fragments assigned to Megaloolithus cf. siruguei from the Upper Cretaceous outcrops of the Cameros Basin. We focus our study on the presence of secondary shell units that mimic most aspects of the ultrastructure of the eggshell mammillae, but grow far from the inner surface of the eggshell. We call these structures extra-spherulites, describe their crystal structure and demonstrate their secondary origin. Our study has important implications for the interpretation of secondary shell units as biological or pathological structures. Thus, electron backscatter diffraction complements other microscope techniques as a useful tool for understanding taphonomical alterations in fossil eggshells.
authors.affiliation: Geobiotec, Departamento de Ciências da Terra, Faculdade de Ciências e Tecnologia, FCT, Universidade Nova de Lisboa, Caparica, Portugal.|Museu da Lourinhã, Lourinha, Portugal.|Grupo Aragosaurus-IUCA, Área de Paleontología, Facultad de Ciencias, Universidad de Zaragoza, Zaragoza, Spain.
authors.forename: Miguel
authors.identifier: 0000-0002-7210-1033
authors.initials: M
authors.lastname: Moreno-Azanza
authors.affiliation: Área de Mineralogía, Facultad de Ciencias, Universidad de Zaragoza, Zaragoza, Spain.
authors.forename: Blanca
authors.initials: B
authors.lastname: Bauluz
authors.affiliation: Grupo Aragosaurus-IUCA, Área de Paleontología, Facultad de Ciencias, Universidad de Zaragoza, Zaragoza, Spain.
authors.forename: José Ignacio
authors.initials: JI
authors.lastname: Canudo
authors.affiliation: Grupo Aragosaurus-IUCA, Área de Paleontología, Facultad de Ciencias, Universidad de Zaragoza, Zaragoza, Spain.|CONICET-Museo Olsacher, Zapala, Neuquén, Argentina.
authors.forename: José Manuel
authors.initials: JM
authors.lastname: Gasca
authors.affiliation: Fidel Torcida Fernández-Baldor Colectivo Arqueológico-Paleontológico de Salas, Museo de Dinosaurios, Salas de los Infantes, Burgos, Spain.
authors.forename: Fidel
authors.initials: F
authors.lastname: Torcida Fernández-Baldor
mesh_terms: D000818:Animals; D004528:Egg Shell; D004583:Electrons; D005580:Fossils; D005843:Geography; D049449:Luminescence; D008853:Microscopy / Q000379:methods*; D008854:Microscopy, Electron / Q000379:methods*