Ostracoda and dysaerobia in the Lower Jurassic of Wales: the reconstruction of past oxygen levels

The Ostracoda are a subclass of small Crustacea which inhabit most aquatic environments; they have been recorded from the Cambrian through to the Recent. The biology of one group of marine Ostracoda, the Platycopina (Triassic-Recent), enables them to better withstand decreased levels of dissolved oxygen, in their immediate environment, than other ostracods. Study of a number of geological sections has already shown that this suborder often dominates stratigraphical intervals which are considered to be representative of dysaerobic conditions. In the light of this work the changing faunal composition of the Lower Jurassic of the Mochras Borehole, Wales is interpreted as a series of environmental changes which include fluctuating oxygen levels. The degree of dysaerobia is assessed and its effect on the rate of faunal turnover is discussed.