Article: Recent and early Palaeozoic myodocope ostracodes: functional morphology, phylogeny, distribution and lifestyles
Jean Vannier and Katsumi Abe
Myodocope ostracodes are represented worldwide in a variety of modern environments as benthic, nektobenthic or exclusively planktonic organisms. Their Silurian ancestors form one component of the early Palaeozoic marine crustacean fauna. Aspects of biology (body plan, functional morphology, behaviour relevant to the interpretation of the fossil record) newly investigated by, for example, video-recordings t described. A high degree of specialization of appendages and paired visual organs known in modern (this paper) and Mesozoic myodocopes presumably occurred very early (Silurian) in the evolution of the group. External bulbs of typical Upper Silurian bolbozoid myodocopes are interpreted as having housed possit visual receptors comparable to modern analogues (e.g. planktonic forms). A phylogeny of Myodocopa (Low Silurian to Recent), based on cladistic analysis, is proposed for higher taxa. The two major components of to Recent myodocope fauna, Myodocopida and Halocyprida, were probably already differentiated during the Silurian. The modern myodocopids may have originated from late Palaeozoic forms (Superfamily nov. A) via the cypridinacean lineage (Lower Triassic to Recent). This analysis of the distribution, morphology and lifestyles of Recent myodocopes supports the hypothesis previously presented by Siveter and Vannier that representatives of Silurian myodocopes were truly pelagic. Upper Silurian myodocopes are considered as acti swimmers, capable of vertical migrations and feeding on detrital organic material sinking out from the pho zone. Their vertical distribution may have preferentially ranged from 100-200 m down to 400-500 m. Th probably lived in relatively dark environments with possibly highly sensitive visual organs as an adaptive response to dim conditions. They may have been one of the pioneer crustacean faunas to fill vacant mid-water niches during the early Palaeozoic.