Fractal-like features of dinosaur eggshells

We carried out measurements on serial sections through dinosaur eggshells found in the Hateg basin of Romania. The pore structure of these eggshells exhibits peculiar hierarchical self-similarity, from millimeter scale to nanoscale. At optical scale, the eggshell is built up by packing bundles of calcite carrots, well aligned with their axis perpendicular to the eggshell surface, with "gaps" (macro pores) in between. Each calcite carrot is about 0.5 mm in diameter and has the length equal to the eggshell thickness. Scanning electron microscopy (SEM) reveals that these carrots are in turn micro porous spongy calcite structure, with 1 mu m average pore diameter. The structure on an even smaller scale is studied by transmission electron microscopy (TEM) on thin sections prepared by ion milling, using methods from material science, revealing yet another layer of complexity. The observed features lead us to the conclusion that calcite crystallization leading to the carrot morphology is controlled from the nano- to micro-scale by the structure of the collagen net developed in the eggshell cells.