Jurassic bivalves, continental crises, and biostratigraphy in southern Gondwanaland

DAMBORENEA, S.E.

Napier

Congreso; Geosciences 2018; 2018

GNS Science

Bivalves are abundant and diverse Jurassic macroinvertebrate fossils worldwide, and as such they are key to understanding paleoenvironmental crises, either as source of significant data or indirectly as correlation tools. The Toarcian Oceanic Anoxic Event (T-OAE) was one of the major environmental perturbations of the Mesozoic, and it has been chemostratigraphically identified in southern Gondwanaland in the Neuquén Basin (Argentina) in bulk rock and fossil wood samples. At the same time, the variations in abundance and body size of the dominant bivalve species during that interval, the paper clam Posidonotis cancellata (Leanza), were analysed. The species disappeared just beforethe minimum negative carbon-isotope value (OAE sensu stricto). In fact, the probably related Late Pliensbachian?Early Toarcian biotic crisis, with a peak at the tenuicostatum/falciferum ammonite zone boundary, was particularly evident for marine bivalve species, and meant the extinction of both previously successful taxa and survivors from the Triassic?Jurassic extinction event. In southern Gondwanaland it involved many bivalve species and several genera (e.g., Ryderia, Asoella, Posidonotis, Agerchlamys, Weyla, Cardinia, Kalentera, and probably Pseudaucella). Additionally, although bivalves are not generally regarded as biostratigraphically useful due to their long species turnover rate, they have proven to be especially valuable in southern Gondwanaland for recognition, correlation and dating of local stratigraphic units and continental crises. In New Zealandmost of the bases of local Jurassic stages and sub-stages where originally defined (and are still characterized) by the lowest occurrence of certain bivalve species. In the Neuquén Basin the proposed regional biostratigraphic zonation based on bivalves is closely linked to a detailed ammonite zonation soundly correlated with international standard zones. Some of the key bivalve species are shared between these two regions, and thus they become significant for discussing the adjustment of the chronological equivalence of events along the southern Gondwanaland margin.