Bradleya Problem, with Descriptions of Two New Psychrospheric Ostracode Genera, <i>Agrenocythere</i> and <i>Poseidonamicus</i> (Ostracoda: Crustacea)

Abstract

The “<i>Bradleya</i> problem” is concerned with the discovery and definition of a group of fossil and Recent reticulate ostracodes, several of which are common to Cenozoic deep-sea sediments in many parts of the world ocean floor. These species have often been misunderstood and taxonomically confused with genera characteristic of the study of shallow-water forms. The present study attempts to resolve some of these misunderstandings by designation of several important type-specimens, description of new evidence and the proposal of a new classification based on the concept of the evolution of a reticulum in response to environmental change. A method of pattern analysis is used to define elements of the reticulum subject to evolutionary change. Over 40 reticulate species, which would have at one time been regarded as <i>Bradleya</i>, were examined; only 14 of these are assigned and belong to <i>Bradleya</i>. Two new genera, <i>Agrenocythere</i> and <i>Poseidonamicus</i>, are described for the reception of the others, and these are placed in the new subfamily, Bradleyinae, and placed with Thaerocytherinae Hazel in a new family (Thaerocytheridae Hazel). Twenty-seven of these species are described, including <i>Bradleya arata</i> (Brady), <i>B. dictyon</i> (Brady), <i>B. normani</i> (Brady), <i>Agrenocythere radula</i> (Brady), <i>A. pliocenica</i> (Sequenza), and <i>A. hazelae</i> (van den Bold). The diagnostic characteristics of the related genera <i>Cletocythereis, Oertliella, Jugosocythereis</i>, and <i>Hermanites</i> are discussed and illustrated. It is concluded that the psychrospheric species <i>Agrenocythere pliocenica</i>, which has been reported from outcrops in Italy and a long core from the Tyrrhenian Sea floor, is most closely related to <i>A. hazelae</i>, which became geographically widespread during the Miocene. <i>Bradleya, Jugosocythereis, Agrenocythere</i>, and <i>Cletocythereis</i>, now genera in separate families, are all thought to have been derived from a common stock of Cretaceous age.