The use of Ostracoda to detect kenoxic events: A case history from the Spanish Toarcian

Abstract

In such well documented kenoxic events as that of the Late Cenomanian, while other ostracods become virtually extinct, either locally or regionally, the Cytherellidae of the Platycopina survive and flourish. The same group also dominates in the oxygen minimum zone of the world's oceans at the present day. The hypothesis formulated is that the Cytherellidae are able to survive dysaerobia by virtue of being filter feeders. As such, they pass more water across the respiratory surfaces than do ostracods which feed in other ways. In oxygen reduced water, they alone will be able extract a sufficiency of the gas to survive. A preponderance of platycopids, which in some cases amounts to virtual monotypy by the family, is seen in all of the well-documented kenoxic events examined so far in the Mesozoic. Examples of the Late Cenomanian are given from southern England and the South of France and another example, from the British Liassic is also examined. In the Spanish Middle and Upper Liassic, platycopid and metacopid Ostracoda, are used to distinguish two kenoxic episodes. In the Sierra Palomera section, a distinct peak in numbers of metacopid ostracods in the Late Pliensbachian is taken to represent a dysaerobic environment, as is a similar peak in the Platycopina in the early Toarcian. The recognition of kenoxic episodes based on the predominance of filter feeding ostracods must become an important stratigraphical and palaeoenvironmental tool.