Diversity and body size dynamics of marine invertebrate assemblages across the Pliensbachian-Toarcian crisis

The uppermost Pliensbachian- Lower Toarcian (Pli-Toa, ~183.7 Ma) interval of the Early Jurassic is known for palaeoenvironmental perturbations that affected both pelagic and benthic marine organisms. While some groups reacted by changing their abundance and diversity, others reveal, in addition, morphological changes such as body size fluctuations. Belemnites are extinct coleoid relatives of squid and, together with benthic organisms, such as foraminifera, played an important role in the Jurassic ocean ecosystem. By having calcitic structures, such as the rostrum and the external shell, they have high fossilization potential. Additionally, they were very abundant and their wide dispersal have made them widely used tools for palaeoecology and biodiversity studies. Belemnites have been mainly investigated from a geochemical point of view, as a source of palaeoclimate proxies, but very little has been done on morphology and taxonomical diversity fluctuations and their relationship with environmental, ecological and palaeogeographic changes during this interval. With an innovative method using computed tomography (CT) scanning, two German belemnite battlefields were studied in order to analyse different proxies for belemnite rostrum size such as linear measurements and volumetric approaches. The geometric mean of width, height and length revealed to be the proxy which best reflected body size changes of differently-shaped taxa, when compared with the actual volume calculated with the CT data. The Lusitanian Basin, namely the Peniche section offers the optimal conditions to study belemnite body size response to environmental conditions, from individuals to assemblage by having dense accumulations of belemnites and being the GSSP (Global Boundary Stratotype Section and Point) of the Toarcian. The quantitative analysis of belemnite body size mechanisms across the Pli-Toa boundary event revealed a decrease in adult size (Lilliput effect) in the most common species (Catateuthis longiforma, and to a lesser degree also in Passaloteuthis bisulcata). General linear modelling showed that the palaeotemperature proxy (δ18O) best explains fluctuations in size, despite the influence of carbon cycle perturbations (δ13Ccarb) and volcanic outgassing (mercury anomalies) – even when correcting for the impact of sedimentary properties (lithology and belemnite abundance). This highlights the complex interplay between environmental stressors associated with hyperthermal events in belemnite body size. For the first time, a detailed taxonomic and ontogenetic description of the belemnite assemblages from the Peniche section was presented. Additionally, a diversity analysis compared belemnite assemblages from the Euro-Boreal and Mediterranean domains in the Tethyan Ocean during the Upper Pliensbachian-Lower Toarcian interval. Rarefied diversity analyses reveal a minor decrease in taxonomic diversity at the Pli-Toa boundary event in the Lusitanian Basin and other coeval NW Tethyan basins. Ordination and cluster analyses, however, indicate that the major changes in belemnite diversity and palaeogeographic distribution happened during the Toarcian Oceanic Anoxic Event (T-OAE; base of Levisoni Zone). The extinction of taxa affected more severely the Mediterranean domain assemblages and resulted in a provincial discrepancy amongst NW European and Boreal-Artic belemnite faunas. In this context, the new species Bairstowius amaliae is described and an evolutionary relationship between B. amaliae and Catateuthis longiforma is suggested, supporting the northward range shift of this lineage resulting in endemic Toarcian Boreal-Arctic faunas. The study of benthic foraminifera in the Pli-Toa of the Lusitanian Basin was hitherto focused on the Peniche section. However, the Rabacal area, a more proximal area of the Lusitanian Basin, offers excellent outcrop conditions as well. An integrated study of micropalaeontology and geochemistry in the Maria Pares reference section allowed to assess morphological and taxonomic diversity changes in benthic foraminifera assemblages across the T-OAE. The peak phase of the crisis - Levisoni Zone - records the lowest values of foraminiferal abundance and diversity, concomitantly with the development of oxygen-depleted conditions. The immediate aftermath of the T-OAE is characterized by dominance of opportunistic forms, while the recovery and return to oxic conditions is indicated by high abundance of specialists and potentially deep infaunal foraminifera. The results obtained highlight the complexity of abundance, diversity and body size patterns of marine invertebrate during the Pli-Toa crisis. Additional quantitative data on belemnite abundance, diversity and body size are crucial to perform an interbasinal correlation in the Tethys Ocean. This will enable to identify geographic distribution patterns and, ultimately, to assess belemnite response to differences in climatic latitudinal changes during the Pli-Toa crisis.