Phosphatization is a common way of preservation of the soft tissues of animals. Within bivalves this process ensured fossilization of gills in several lineages. The anatomy of gills (the presence of calcium concretions in them) in two related groups – Trigonida and Unionida – likely facilitate their mineralization.
Gills of the members of Unionida serve as the gas exchange and feeding organ and a parental care tool. Marsupia in gills of the females of all Recent unionoids are the place where embryos develop into parasitic larva (a likely adaptation to freshwaters). All the members of Unionida have today eulamellibrach gill anatomy - the tissue transversally connects gill filaments, what result in a basket-like construction (this facilitates parental care). Thus, one may expect this character already existed in their last common ancestor – probably before the Middle Jurassic, when advanced unionoids already existed. Gills which filaments that lack these connections (filibranch anatomy) are present in both: all known, fossil and living trigoniids (the accepted ancestors for Unionida), and early Late Triassic (Carnian) one of the oldest unionoid Silesunio parvus. The time period for evolving the eulamellibranchy (possibly followed by parental care) may thus be restricted to between early Late Triassic and Middle Jurassic.
New findings from Middle Norian and Rhaetian of Late Triassic of Poland may complete this picture. Although parts of their gills are preserved, their anatomy is unsolved (possibly due to taphonomy process). They lack typical for eulamellibranchy residues of transverse tissue connections, however they also lack observed in decaying filibranch gills fan-shaped organization of (at least some) filaments.
Acknowledgments: All the specimens are stored in Institute of Paleobiology, PAS