MEMBRANA TESTACEA OF TITANOSAURID EGGS FROM AUCA MAHUEVO (ARGENTINA): DESCRIPTION, SIGNIFICANCE, IMPLICATIONS, AND PRESERVATIONAL INTERPRETATION

Fossil dinosaur eggs and eggshells have traditionally been classified in parataxonomic schemes and sometimes putatively assigned to dinosaur taxa on employment of disparate criteria such as proximity of adult or juvenile dinosaur remains. An exception is Auca Mahuevo in Argentina (Patagonia, Neuquén Province), where numerous eggs of Campanian age are found with diagnostic skeletal embryonic remains in ovo allowing the identification of these eggs as those of titanosaurid dinosaurs. Although fossilization biases favor the preservation of the calcium carbonate section of eggshells, five rare preservations of the membrana testacea have already been reported throughout the later Phanerozoic. In this paper I provide a detailed description of the 83.5 MY old membrana testacea which is composed of two identifiable layers as observed in extant reptiles, and compare it with the membrana testacea observed in eggs of extant reptiles and previously reported fossils. This membrane appears in two distinct morphs. One of those displays a solid calcium carbonate layer with a fine preservation of the negative imprints of shell unit nucleation centers each surrounded by discoloration characteristic of stratified bacterial communities. The other still displays the mesh-like texture of the original protein membrane. Taphonomic factors that have affected the eggshell structures are interpreted as microbial in origin. This interpretation is further supported by the presence within the voids of the membrane layer of micron-size, rod-like calcium carbonate structures identical to those precipitated by (nano) bacteria in the formation of ooids, while calcium carbonate precipitation outside this membrane consists of characteristic abiotic rhombohedric crystals. Although permineralized filamentous layers in situ have been previously reported from the Cretaceous sites, the membrana testacea from Auca Mahuevo is presently the oldest one reported for titanosaur dinosaurs. Furthermore, the preservation of this soft tissue is undoubted bacterially mediated as that of exceptional fossils from lagerstätten.