Growth mechanisms in dinosaur eggshells: an insight from electron backscatter diffraction

Abstract

ABSTRACT—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.

ACKNOWLEDGMENTS

This paper forms part of the projects CGL2010-16447 and CGL2009-07574, subsidized by the Ministry of Science and Innovation, the “Gobierno de Aragon” and European Social Fund “Grupos consolidados”, and “Dirección General de Patrimonio Cultural”. M.M-.A. is supported by the Spanish Ministry of Science and Innovation, grant FPI BES-2008-005538. C. Shaw, A. Rodriguez-Navarro and the editor Randall Irmis revised the paper and greatly improved our work. M.M.-A. wants to thank O. Kalin for his inspiring comments and references in the preliminary phase of this work. J. Gómez and F. Jackson made helpful comments that improved the paper in its final form. R. Glasgow edited the text in English. The cf. Maiasaura eggshells were donated by F. Sweeney.

Handling editor: Randall Irmis