Contemporary and Fossil Chelonian Eggshells

Abstract

Although fossil turtle eggs were first reported over 100 years ago, their shell structure is still poorly understood. The calcareous layer of turtle eggshells, unlike those of any other amniote, is composed of aragonite. While contemporary chelonian eggshells vary from relatively flexible to rigid, the fossil record is probably limited to the most easily preserved rigid-shelled eggs. These shells have either an unaltered aragonitic structure or an aragonitic layer which has been partially or completely replaced by calcite. Unaltered or partially altered chelonian fossil eggshells can be recognized by the spherulitic basic units composed of needle-like aragonite crystallites. Under polarized light these units display an extinction cross; in completely altered specimens the basic units are not spherulitic and extinguish totally. In nonchelonian eggs the individual columns or wedges of the basic units rather than the entire unit become extinct under polarized light and thus have no resemblance to the calcitic shell structure of nonchelonian eggs.