Taphonomy and evolution of an extraordinarily exposed upper Pliensbachian - lower Toarcian succession of “Lithiotis” beds at Mercato San Severino (Salerno, southern Apennines)

Lower Jurassic Tethyan and Panthalassan shallow water successions are characterized by aberrant bivalves belonging to the Lithiotis Fauna (also named lithiotids). Their widespread occurrence, often in rock-forming abundance, represents a global event, mostly restricted to the Pliensbachian – early Toarcian. Despite their global distribution and abundance, their biogeographic and stratigraphic distribution, and their evolutionary history are obscure, mostly because the classification at lower taxonomical rank (generic or specific level) is frequently not available in the literature. In the Apennine Carbonate platform of southern Italy, the Lower Jurassic succession is represented by the Palaeodasycladus Limestone, overlain by the Oolitic-oncolitic Limestone. The former lithostratigraphical unit yields, in the upper part, the Orbitopsella Limestone and the Lithiotis Member. The latter, about one hundred metres thick, is characterized by the abundant occurrence of the lithiotid bivalves, which disappear in the lower beds of the Oolitic-oncolitic Limestones, at the onset of the early Toarcian anoxic event. The Lithiotis Member and the lower Oolitic-oncolitic Limestones are beautifully exposed in a quarry west of Mercato San Severino. Here, the Lithiotis Member consists mainly of meter-thick ― lithiotid biostromes, alternating with coarse peloidal-intraclastic grainstones and rudstones with abundant remains of Palaeodasycladus mediterraneus. Sometimes, the limestone beds are overlain by thin and discontinuous green marls. In the quarry succession more than 40 bivalve concentrations are present, the most part of which are represented by autochthonous monospecific to paucispecific assemblages, frequently with individuals preserved in life position. The taxonomic composition, biofabric and geometry of the accumulations have been examined. The bivalve assemblages are dominated, among the Lithiotis Fauna, by Mytiloperna, Cochlearites and Lithioperna. The occurrence of Lithiotis is not proved. Other bivalves are represented by megalodontids and Opisoma. The biogenic concentrations exhibit generally a tabular shape. Lens-shaped accumulations (bivalve mud-mounds) are rare and low, not thicker than one meter. The high abundance of the calcareous green alga Palaeodasycladus and of autochthonous shells of Mytiloperna, a marker of upper subtidal conditions, the tabular shape of the shell accumulations and the facies succession suggest a very shallow marine environment, with recurrent and prolonged emersions phases. This environmental setting is also supported by the absence of marine biota indicative of open platform /deep lagoon environments (e.g., brachiopods) and of infaunal bivalves (e.g. Pholadomya, Gresslya) typical of muddy substrates below the normal wave base. The frequent preservation of the sessile epi- semi-infaunal bivalves still in life position suggests high sedimentation rates favoured by the stick- and spoon-shaped shells of the Lithiotis Fauna, through the prolific production (fecal pellets) and efficient trapping of the carbonate sediments. The absence of thick bivalve-mounds, the occurrence of autochthonous biota of very shallow marine environment, the highly fossiliferous content in a relatively thin succession (about one hundred meters) suggest a carbonate platform with a low subsidence rate but with rapid sedimentation processes typical of the lithiotid habitat.