Numerous megaloolithid eggs of Early Campanian age have been found in Auca Mahuevo in Argentina (Patagonia, Neuquén Province) with diagnostic skeletal embryonic remains in ovo allowing the identification of these eggs as those of titanosaurid dinosaurs. Although fossilization favors the preservation of the eggshell, organic compounds that are often destroyed are commonly recovered in these eggs.A detailed description is provided of the 79.5—83.5 Ma-old membrana testacea found in great abundance and in situ in titanosaurid eggs. The fossilized membrana testacea is uniquely preserved and appears in two distinct morphs, each composed of two identifiable layers as observed in extant reptiles and birds. This membrane also shares great similarities with those previously reported occurrences in post-Jurassic dinosaurs. One morph displays a solid calcium carbonate layer with fine preservation of the negative imprints of shell unit nucleation centers each surrounded by coloration reminiscent of stratified microbial communities. The other still displays the mesh-like texture similar to the morphology protein membranes of extant reptiles.Taphonomic factors that have affected the eggshell structures and the membrana testacea are here interpreted as microbial in origin. This interpretation is further supported by the presence within the voids of the membrane layer of micron-sized, rod-like calcium carbonate structures identical to those precipitated by bacteria in the formation of ooids, while calcium carbonate precipitation outside this membrane consists of characteristic abiotic rhombohedric crystals. The periodic pulse of sedimentation (obrution and stagnation) from overbanking paleorivers in Auca Mahuevo coupled with the presence of organic compounds inside the buried eggs may have produced conditions similar to those observed in conservation Lagerstätten, thus invoking a Lagerstätte status for Auca Mahuevo. Furthermore, the patterns resulting from biomediation observed at this site might serve as a template for investigating other soft-tissue preservation in the fossil record.