Gastropod egg capsules from the Lower Cretaceous of Russia preserved by calcitization

Abstract

Small-sized (0.8–1.6mm in diameter), circular to oval, three-dimensionally preserved calcitized structures have been found embedded within ammonite body chamber moulds from the Lower Aptian (Lower Cretaceous) of Russia. The characteristic morphology, consisting of a flat attachment base and convex upper hemisphere possessing an apical, tiny, circular opening indicate that these structures represent gastropod (possibly Caenogastropoda) egg capsules. Originally, the egg capsules were attached to the empty shells of the ammonites Deshayesites and Sinzovia which later were embedded within carbonate concretions. The preservation of the egg capsules resulted from both their deposition within a suitable, cryptic habitat provided by the empty ammonite shells, and the quick cementation of the infilling sediment which not only sufficiently protected the capsules from external environment, but also created a suitable, closed microenvironment for fossilization. The calcitization of the egg capsules may have occurred under low pH conditions in an environment characterized by a very low concentration of phosphorous ions essential for phosphatization. So far, such structures are known from a few examples derived from different stratigraphic horizons and geographic locations. Those which are known have been reported in the form of pyritized, phosphatized, carbonaceous and even bioimmured fossils. The calcitized gastropod egg capsules presented here indicate, that such structures may in fact be preserved by a wide array of fossilization modes in different paleoenvironments/microenvironments. Thus, such fossils seem to be much more common in the fossil record than previously considered.