Normal and multiple eggshells in batagurine turtles and their implications for dinosaurs and other reptiles

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Abstract
The fine structure of normal and abnormally thick eggshells of the turtles Melanochelys trijuga and Rhinoclemmys areolata (Emydidae: Batagurinae) are described through the use of scanning electron microscopy. Eggshells of R. areolata differ from those reported for other turtles in having complex pores with basal openings partially occluded with crystallites. Each of the abnormally thick eggshells has the usual membrane and mineral layer encased within a second membrane and mineral layer and, in one case, within yet a third membrane and mineral layer. Information on oviducal function and the structure of these and most previously reported multiple-shelled turtle eggs allow the hypothesis that such eggs arise from oviducal retention of normally shelled eggs of one clutch that become reshelled during shelling of eggs ovulated to form the next clutch. This is likely only in species that lay more than one clutch per season. Our evidence suggests that dietary inadequacy causes the retention. We propose that dinosaurs that laid multiple-shelled eggs also had two or more clutches per season and that this multiple shelling was produced also by abnormal retention of eggs which, in turn, may have been induced by changes in diet.

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