Carbonate preservation of dinosaur eggs in the upper Cretaceous Anacleto Formation at Auca Mahuevo, Neuquen Basin, Argentina

Abstract

Preservation of dinosaur eggs and footprints by precipitation of calcium carbonate in the Upper Cretaceous Anacleto Formation at Auca Mahuevo, Argentina represents a relatively unusual occcurence in the fossil record. Under normal condition, eggs are readily destroyed in sediments shortly after burial by physical, chemical, and biological processes. This study attempts to determine a preservational model for carbonate eggs by characterizing their mineralogical composition and microstructures using a variety of analytical instruments including petrographic microscope, cathodoluminesce (CL) microscope, X-ray powder diffraction (XRD) and field-emission scanning electron microscope (FEM) to characterize the composition and fabric of the fossilized eggs. Several textutal features have been observed in the carbonate eggs, including membrane, embryonic skin, spherulites, ooids, peloids, Microcodium, calcified filaments, and micrite. Microbial actvity is likely responsible for the formation of these microfabric features, facilitating calcium carbonate precipitation leading to exceptional preservation of eggs. Although microbial influence in the carbonate egg preservation has not been clearly elucidated, laboratory experiments by other workers provide an argument for the role of microbes in the precipitation of calcium carbonate. The rare preservation of egg contents in the Anacleto Formation have been linked to biological-mediated processes. This preservation also provides evidence for penecontemporaneous carbonate precipitation under subaerial conditions before significant burial.