Microstructures and sclerochronology of the Lithiotis Facies bivalves (Lower Jurassic): Paleobiological and paleoclimatic significance and their resilience to the early Toarcian extinction

Mesozoic bivalves show some evolutionary phases of proliferation of larger, aberrant, and gregarious shells with a high rock-forming potential. The most well-known are the Upper Triassic megalodontids, the Lower Jurassic lithiotids and the Cretaceous rudists. Their extinction occurred during global crisis of biocalcifiers. The strongest demise of megalodontids occurred at the end of Triassic, the lithiotid bivalves almost disappear during the early Toarcian, while the extinction of the rudists occurred at the end of Cretaceous. Here, we analyzed bivalves of the Lithiotis Facies, which contains the most aberrant, peculiar and large shells of Cochlearites, Lithioperna, and Lithiotis and other, less common, gregarious taxa represented by the pterioid Gervilleioperna and Pachygervillia, the myalinid Pseudopachymytilus, the carditid Opisoma, and the megalodontid Pachyrisma (Durga) and Protodiceras, in order to understand their response to the early Toarcian extinction. Specimens from the Pliensbachian Rotzo Formation (Trento Platform, northern Italy) are exquisitely preserved, with most of the shell made of pristine aragonite and calcite, allowing to describe in detail their shell microstructures and to discuss it in the framework of the early Toarcian biocalcifier crisis. Due to the excellent preservation of these specimens, it was possible to perform sclerochronological and sclerochemical analyses through microscopy, stable isotope analysis and micro-X-ray fluorescence scanning, allowing to investigate seasonal patterns and their functional and paleoenvironmental significance. This analysis results in the identification of different growth rates among the different taxa and adds new tentative evidence for the paleoclimatic evolution of the Rotzo Formation. The maximum growth rate in ventral direction ranges from 20 mm/year in Lithioperna to a few mm/year in most of the other genera which have thick but smaller shells. These latter taxa showed a greater resilience to the environmental perturbances of the early Toarcian and most of them survived this crisis. The survivors also include representatives of the Fam. Pachyrismatidae (e.g., Pachyrisma, Early-Late Jurassic in age) from which the rudists had origin. Instead, bivalves mostly affected by the extinction were those with thicker and larger shells and a high annual growth rate.