Eggshell structures exert a primary control on the distribution of secondary U and Th, so spatial characterization of key elements and careful sampling are required to produce accurate ages by 230Th/U dating. Scale bars are 1 mm. A. Thin section photomicrograph of a modern ostrich eggshell in cross section, and corresponding eggshell structures denoted by V (vertical layer), P (palisade or prismatic layer), and C (cone layer). Pores serving as oxygen pathways for incubating chicks are visible as open holes penetrating through the eggshell. B. Epoxy-mounted fragment of an ancient eggshell from Ysterfontein 1 in cross section, showing the same eggshell structures are well preserved in deep time. Analyses from laser ablation are evident along pitted lines and track concentrations of U and Th. A pore is apparent in the center of the mounted fragment. 230Th/U burial ages of eggshells from this layer are ~118 thousand years old.