Origin of the Ostracoda and their maxillopodan and hexapodan affinities

There are Cambrian fossils attributed to the Ostracoda but the extant subclasses Podocopa and Myodocopa do not appear until the Ordovician. At this time the morphologically similar, free-living ancestors of the now sedentary Thecostraca (Ascothoracida, Acrothoracica and Cirripedia) may have still been extant, and from an ecological point of view it seems likely that, by and large, ostracods replaced them. However, living ostracods have an abbreviated, direct development, and some key aspects of their morphology, such as the nature of the maxillary segment and abdomen, are conjectural. Thus the affinities between these and related taxa remain uncertain; e.g., while some contemporary carcinologists place Ostracoda as a taxon coordinate with the Branchiopoda, Remipedia, Cephalocarida, Maxillopoda, Malacostraca, others tentatively or unequivocally ally them with the Maxillopoda (generally Mystacocarida, Copepoda, Tantulocarida and Thecostraca, and sometimes Branchiura and Pentastomida). Others, largely involved with fossils, have stretched the definition of the Maxillopoda even further, to the point where it seems even less likely a monophyletic taxon. Until recently cladistic analyses utilizing genetic (largely 18S rDNA) as well traditional morphological characteristics have given confusing results regarding the affinities between these taxa, and an important one suggested the Ostracoda might even be diphyletic. Furthermore, a very recent genetic study utilizing protein encoding genes places a podocopine ostracod among the most primitive of the extant crustaceans (Branchiopoda, Cephalocarida Remipedia and Mystacocarida), and then generally at the base of a lineage leading to the Malacostraca, a lineage giving rise to copepods and cirripeds along the way. This indicates these so-called maxillopodan taxa evolved independently from a malacostracan-like ancestor, and if so they are convergent. And finally, from genetic studies it is not only becoming well documented the Crustacea rather than Myriapoda gave rise to the Hexapoda, but it appears the Hexapoda stem from among the lower rather than the higher crustaceans, possibly even from the Ostracoda. Whether there were terrestrial ostracods at the time hexapods appeared in the Lower Ordovician is unknown, but the modest diversity of terrestrial ostracods today are podocopines which also first appeared in the Lower Ordovician. Thus, if current interpretations of living ostracodan and fossil hexapodan body plans are largely correct, it can be hypothesized the Ostracoda are close to the ancestor of the Hexapoda.