Structure and evolution of podocopan ostracod hinges

Abstract

A new concept of the hinge classification in the Podocopa is established based on a comprehensive description of the hinge ultrastructure. Four new types of structure are defined, according to the relative position of their ligaments and hingements (teeth elements): basic type, exterior type, intermediate type and interior type. The new classification reveals that the adont hinge, which is known as a simple structure, shares the fundamental structure (basic or exterior type) found in the Platycopida, Darwinuloidea and Bairdioidea, and that it develops not only as the fundamental structure, but also as the advanced structure (interior type) in the Cypridoidea and Cytheroidea. Additionally, the high level of morphological diversification of cytheroid hingements is explained in terms of the structural plasticity of the intermediate and interior hinge structure types. Most cytheroid hinges can develop complex arrangements of teeth (hingements) by only small changes of calcification because their hingements can develop independently from the particular restricted location of the ligaments. The evolutionary pathway of hinge structures is also examined based on a study of fossil records. The pathway indicates that the complex hinge structures of the derived taxa arose from simpler ones, but that sometimes this trend can be reversed and derived taxa can evolve simple hinge structures as adaptations to new habitats. © 2007 The Linnean Society of London, Biological Journal of the Linnean Society, 2007, 92, 41–62.