Relationship between anoxic conditions and size in Posidonotis (Bivalvia) from the Lower Jurassic of the Neuquén Basin, Argentina

ROS, S.; DAMBORENEA, S.E.; AL-SUWAIDI, A.; HESSELBO, S.; JENKYNS, H.; MANCEÑIDO, M.O.; RICCARDI, A.C

Mendoza

Congreso; 4th International Palaeontological Congress; 2014

International Palaeontological Association

The Early Toarcian extinction event is frequently linked to the Early Toarcian Oceanic Anoxic Event (T-OAE) and/or global warming. Recently, the sedimentary record of the Toarcian OAE was recognized in the Neuquén Basin, Argentina, confirming the event as a global phenomenon. The event was identified chemostratigraphically on the basis of a relative increase in marine organic carbon and a characteristic negative carbon-isotope excursion (δ13Corg) in bulk rock and fossil wood. High-resolution sampling and biostratigraphical work was carried out in the late Pliensbachian-early Toarcian interval in the Arroyo Lapa section, were benthonic species are scarce. We present size and abundance data from the Toarcian OAE interval in the Neuquén Basin for the dominant bivalve species, the paper clam Posidonotis cancellata (Leanza), and correlate these data with geochemical proxies (TOC and δ13Corg). The main results indicate that individual shell size (surface and length) shows a statistically significant increase over levels where δ13Corg was decreasing. As has been suggested for other species in similar conditions, this relationship can be connected with primary productivity. The species disappeared just before the minimum negative carbon-isotope value (OAE sensu stricto) and the genus never appeared again in the basin. Although P. cancellata was occasionally present before oxygen levels began to decrease, its abundance only increased when the rest of the benthos disappeared, indicating that it was an opportunistic bivalve adapted to low-oxygen conditions. However, when its size frequency distribution was analyzed, we noticed that it is not strictly comparable either to present-day opportunistic bivalve species, or to other species studied for the same time interval in other parts of the world, where most individuals are concentrated in the smaller sizes. Here size distributions indicate that juvenile mass mortality was not evident in any of the populations; on the contrary, in most cases mortality of mature individuals (medium sized) was higher. Therefore, population features of P. cancellata as inferred from these shell concentrations do not match strictly those of either opportunistic or equilibrium species. Yet, we can consider this taxon as basically opportunistic with populations primarily controlled by physical rather than biotic resource-limited environmental conditions, where juvenile individuals were more able to resist hostile environments with low oxygen content, as is the case with some living bivalves.