SCLEROCHRONOLOGY OF THE EARLY JURASSIC LITHIOTID BIVALVES: SEARCHING FOR SYMBIOSIS

The Lithiotid bivalves (Lithiotis, Cochlearites and Lithioperna) were an aberrant bivalve group which arose during the Pliensbachian stage of the Early Jurassic, following a 10 million year post-Triassic extinction “reef gap”. During this period of the near-complete eradication of reef-building corals, the Lithiotids became the dominant calcifiers in shallow tropical settings. Cochlearites and Lithiotis have been proposed to harbor photosymbionts because of their large size, flattened morphology, tropical distribution and aggregate lifestyle. We have gathered shells of each genus from the Trento Platform in Northern Italy and used stable isotopic sclerochronology to attempt to determine whether Lithiotis shows a photosymbiotic vital effect similar to the one displayed by known symbiotic groups such as the giant clams. If Lithiotis was photosymbiotic, we should see a δ¹³C enrichment in summer months when its proposed symbionts would have been most active, as they preferentially fixing light ¹²C through photosynthesis. This would manifest as a negative correlation between δ¹³C and δ¹⁸O sampling along a transect in shell carbonate (as δ¹⁸O values are lower during times of high summer temperatures). Preliminary results suggest that Cochlearites does not display this negative relationship and does not have a highly accelerated growth rate, showing 2.5 cm/yr of growth, more in line with heterotrophic oysters than photosymbiotic giant clams. We have collected a longer isotopic transect in order to determine whether a phase lag is present between the C and O isotope results, and whether growth is faster in the vertical direction. When enough data is gathered, we will be able to determine whether Lithiotis, previously considered a strong candidate for symbiosis, actually displays a sclerochronological signal confirming this hypothesis.