Calcified shell membranes in fossil vertebrate eggshell: Evidence for preburial diagenesis

ABSTRACT

Examples of mineralized organic layers of vertebrate (crocodiles, dinosaurs, and birds) eggshells are described, documented and compared to equivalents in modern avian eggs. Taphonomic studies show that burial of intact eggs prevents or delays microbial decay of the organic material and erodes the calcareous shell. However, in modern bird eggs exposed subaerially for six months, extensive decay (desulphurication, ammonification) of the yolk, with rapid creation of high alkaline conditions occurred. Between the organic fibrils of the Membrana testacea the development of a calcareous layer can be observed in these eggs. Occurrences of a mineralized Membrana testacea in fossil eggs is here regarded as evidence for preburial diagenesis under subaerial conditions.