Scanning electron microscopy of early dinosaur egg shell structure: a comparison with other rigid sauropsid eggs.

Fossil eggs attributable to dinosaur (probably prosauropod) parentage that have been recovered from the early Jurassic Elliot Formation sediments at the Rooidraai locality possess shells that are similar to those of birds and crocodilians, and distinctly unlike those of chelonians and gekkonids. The preserved shell is very thin, and distinct mammillary processes are lacking, although the inner surface displays an undulating contour. The absence of these processes may be attributable to the inner portion of the shell having been at least partially decalcified during incubation and not preserved in the fossil state. The shells appear to be composed of broadly wedge-shaped, albeit ill-defined calcareous units, and they are similar to those of birds and other dinosaurs in the pattern of cleavage shown by the tabular calcite crystals of the palisade layer, and in the absence of the dominant horizontal lamellae that characterize crocodilian shells. The differential resemblance of these early Jurassic shells to the eggs of other closely related sauropsid taxa may be pertinent to questions concerning the evolution of egg shell structure within this clade.