Ostracods of the Toarcian (Jurassic) of Peniche, Portugal: taxonomy and evolution across and beyond the GSSP interval

Abstract

With the formal recognition of the Global Boundary Stratotype Section and Point (GSSP) for the base of the Toarcian Stage at Peniche, western Portugal (Rocha et al. 2016), all stages of the Early Jurassic are now defined thus providing the essential stratigraphic framework for developing understanding of global environmental conditions and biota for that period of time (201.4-174.2 Ma, Ogg et al. 2016). The Toarcian stage represents a very special phase of Earth history when, to cite Xu et al. (2018, pp. 396-397): "The Toarcian stage (~183-174 Ma) is considered to be the warmest interval of the Jurassic period encompassing a transient temperature rise of up to ~7 C in mid-latitudes The stage is also marked by one of the most intense and geographically widespread developments of anoxic/euxinic oceanic conditions of the Mesozoic era (Jenkyns 2010). This phenomenon of extreme redox changes is therefore termed the Toarcian Oceanic Anoxic Event (T-OAE, at ~183 Ma) and is marked by large-scale organic-carbon burial in shelf-sea settings, deeper marine basins, and lakes The T-OAE was characterized by a severe perturbation to the global carbon cycle". We give this quotation at length because it is an efficient introduction to Toarcian times, which are currently the subject of a large and rapidly growing literature which it is not our purpose to summarize here. Current work links the growth of a late Pliensbachian cryosphere and its decline in the early Toarcian with climate change and sea level fluctuations, broadly Pliensbachian regression and early Toarcian transgression, and freshwater input into the oceans with greenhouse gases released into the atmosphere and reflected in the carbon isotope record This phase of the Earth history also records a widely documented mass extinction (e.g.

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