A minute ostracod (Crustacea: Cytheromatidae) from the Miocene Solimões Formation (western Amazonia, Brazil): evidence for marine incursions?

A huge wetland (the ‘Pebas system’) covered western Amazonia during the Miocene, hosting a highly diverse and endemic aquatic fauna. One of the most contentious issues concerns the existence, potential pathways and effects of marine incursions on this ecosystem. Palaeontological evidences (body fossils) are rare. The finding of a new, presumably marine ostracod species (<i>Pellucistoma curupira</i> sp. nov.) in the upper middle Miocene Solimões Formation initiated a taxonomic, ecological and biogeographical review of the genus <i>Pellucistoma</i>. We demonstrate that this marine (sublittoral, euhaline), subtropical–tropical taxon is biogeographically confined to the Americas. The biogeographical distribution of <i>Pellucistoma</i> largely depends on geographical, thermal and osmotic barriers (e.g. land bridges, deep and/or cold waters, sea currents, salinity). We assume an Oligocene/early Miocene, Caribbean origin for <i>Pellucistoma</i> and outline the dispersal of hitherto known species up to the Holocene. <i>Pellucistoma curupira</i> sp. nov. is dwarfed in comparison to all other species of this genus and extremely thin-shelled. This is probably related to poorly oxygenated waters and, in particular, to strongly reduced salinity. The associated ostracod fauna (dominated by the eurypotent <i>Cyprideis</i> and a few, also stunted ostracods of possibly marine ancestry) supports this claim. Geochemical analyses (δ¹⁸O, δ¹³C) on co-occurring ostracod valves (<i>Cyprideis</i> spp.) yielded very light values, indicative of a freshwater setting. These observations point to a successful adaptation of <i>P. curupira</i> sp. nov. to freshwater conditions and therefore do not signify the presence of marine water. <i>Pellucistoma curupira</i> sp. nov. shows closest affinities to Caribbean species. We hypothesize that <i>Pellucistoma</i> reached northern South America (Llanos Basin) during marine incursions in the early Miocene. While larger animals of marine origin (e.g. fishes, dolphins, manatees) migrated actively into the Pebas wetland via fluvial connections, small biota (e.g. <i>P. curupira</i> sp. nov.) were phoretically freighted and developed freshwater tolerance over long timescales.http://zoobank.org/urn:lsid:zoobank.org:pub:886C6476-393D-4323-8C0E-06BB8BD02FD9