Reconciling diagnostic traits in living and fossil taxa: The taxonomy and evolution of the genus Microceratina (Crustacea, Ostracoda, Cytheruridae).

The original diagnosis of the genus Microceratina Swanson is re-examined with the aim of elucidating the evolutionary history of this living and fossil genus. The different systematic placements of this genus in one of three alternative suprageneric taxa, family Bythocytheridae Sars, family Cytheruridae G.W. Müller (subfamily Eucytherurinae Puri, emend. Maddocks & Steineck) and/or family Loxoconchidae Sars are reviewed. We provide details of a special type of tiny pores, named Loophole Sieve-type Pore Canals, and show how their morphology and position on the valve differ from the typical Sieve-type Pore Canals present in other superfamily Cytheroidea groups, especially members of the Loxoconchidae. A comparative analysis of the valve ornamentation, especially the structure of the anterior peripheral area, and posterior margin morphology, between selected taxa of the three subfamilies of the Cytheruridae (Cytherurinae, Cytheropterinae, Eucytherurinae) demonstrates that Microceratina species belong to a special phylogenetic lineage of Eucytherurinae, which differs from another lineage represented by the genus Xylocythere Maddocks & Steineck. This proposal is strengthened by examination of the limb traits of Microceratina martensi Namiotko et al., currently the only living species of the genus for which both valves and limbs are described; the description is extended herein. Key diagnostic traits of the genus Microceratina are presented, a new Early Jurassic age species is described, and two new combinations are proposed. The homeomorphic valve shapes of M. andreui sp. nov., M. amfibola and ?S. rectum (Loxoconchidae) support the value of the approach adopted herein: analysis of subtle morphological details with high-resolution microscopy.