Microstructures and sclerochronology of exquisitely preserved Lower Jurassic lithiotid bivalves: Paleobiological and paleoclimatic significance

Lithiotids are enigmatic, large-sized bivalves that formed an important biotic component of tropical shallow marine environments during the Early Jurassic. Lithiotis problematica and Cochlearites loppianus are the most peculiar lithiotids, characterized by stick-like shells of predominantly aragonite which is generally calcitized or replaced by sparry calcite. Uniquely preserved specimens of these two species from the upper SinemurianPliensbachian (Lower Jurassic) Rotzo Formation (Trento Platform, northern Italy), containing large parts of pristine shell (based on SEM, cathodoluminescence, and mu XRF analysis), were selected for a sclerochronological and sclerochemical study, allowing to describe in detail the lithiotid microstructures, to decipher seasonal patterns and to investigate their functional and paleoenvironmental significance.We show that the outer shell layer of lithiotids, rarely preserved, consists of a calcitic simple prismatic microstructure with an asymmetrical thickness between the two valves, whereas the inner layer is aragonitic and is mainly composed of a fibrous irregular spherulitic prismatic fabric, which allowed a very fast shell growth. The latter microstructure is currently unknown in other mollusc shells. We recognized diurnal, fortnightly and annual growth increments, documenting a maximum annual growth of about 25 mm/yr in ventral direction. Stable isotopes show a clear annual periodicity suggesting seasonal changes in the paleoenvironment, which also affected the shell microstructures. During the warm season, first-order prisms are very elongated and show a massive structure without growth breaks, whereas during the cold season prisms are short and with growth cessations. Our results highlight the unique adaptation of lithiotid bivalves that allowed them to dominate the tropical shelf seas during the Early Jurassic.