FOSSIL CROCODILIAN EGGS FROM THE EOCENE OF COLORADO

Abstract

All contemporary crocodilian eggs are rigid-shelled. The crystalline layer of this shell is composed of calcium carbonate in the form of calcite organized into tightly abutting basic units with wedge-like structures. Small variations between eggshell structures of crocodilian species exist, but as a group they are different from any other rigid-shelled amniote egg. Fossil crocodilian eggs, in particular eggshell fragments, are not always easily distinguished from other amniote eggs, especially those of birds, due to diagenesis and erosion. The extinction pattern of calcite crystals composing wedges of crocodilian eggshells and columns of avian eggshells are often not distinct enough, especially in fossils, to justify positive identification. The scanning electron microscope, however, reveals clearly that basic units in crocodilian eggshell arise from basal plate groups, whereas in avian eggshells these units originate from a central core. It is difficult to mistake the fine aragonitic crystals of chelonian eggshells, which also arise from central cores, or the jagged calcitic columns of the rigid-shelled gekko eggs for crocodilian eggshell structure. Previous to this study, fossil avian, chelonian and dinosaurian eggs had been described, but crocodilian ones had not. Krokolithus wilsoni, a new genus and species of crocodilian egg, is based on four fossil eggs and other eggshell material from the DeBeque Formation (Eocene) of Colorado.