Functional morphology of Palaeozoic ostracods: phylogenetic implications

Recent discussions of ostracod systematics have focused on soft anatomy, both as seen in extant groups and as recorded by rare examples of special fossil preservation. The position of the fossil Palaeocopina and Leperditicopida, for which no substantial soft part evidence has yet been found, remains in the view of post-Palaeozoic workers uncertain, with some doubt as to whether they should be retained within the Ostracoda. The evolution of carapace bauplans (e.g. the development of brood pouches and lobal structures in palaeocopids as well as the development of adductor muscle scar patterns, calcified inner lamellae and carapace incisures in podocopines) is discussed in relation to presumed soft anatomy. It seems possible to distinguish between plesiomorphic (ancestral, simple) and apomorphic (derived, advanced) characters and consider their significance in ostracod systematics. Although the presumed ‘protostracod’ is not known, the combination of soft anatomy, carapace architecture and behaviour (feeding techniques, brood care) provide evidence of a general body plan which appeared (at the latest) during the Ordovician and continuously evolved towards the anatomy of modern ostracods. In parallel lineages, plesiomorphic forms have died out (leperditicopids and most palaeocopines as well as metacopines), while apomorphic lineages (‘drepanellid archetype’ of palaeocopines; resistant platycopines, podocopines and myodocopines) have survived all extinction events. The evidence supports the retention of the Palaeocopina (and probably the Leperditicopida) in the Ostracoda.