A NEW SPECIES OF OPISTHODACTYLUS AMEGHINO, 1891 (AVES, RHEIDAE), FROM THE LATE MIOCENE OF NORTHWESTERN ARGENTINA, WITH IMPLICATIONS FOR THE PALEOBIOGEOGRAPHY AND PHYLOGENY OF RHEAS

A new species of rheid, Opisthodactylus kirchneri, sp. nov., is erected on the basis of associated elements of both hind limbs from the late Miocene in northwestern Argentina. The new species extends the biochron of Opisthodactylus from early Miocene to late Miocene and its distribution from Patagonia to northwest Argentina. Cladistic analysis recovered an Opisthodactylus-Pterocnemia clade as sister to a Rhea americana clade. The Opisthodactylus-Pterocnemia clade would have inhabited the most southern, central, and western regions of southern South America throughout the early-middle Neogene, whereas the Rhea stock would have had a north-northeastern or Brazilian ancestral distribution in the lowlands of the continent. The similar biogeographic patterns of living and fossil rheids, cariamids, and tinamids seem to roughly reflect the environmental shift from closed to open habitats that took place at the southern end of South America during the Neogene and Pleistocene, and at least in the former two families the effects of isolation produced by the 'Paranaense' sea. Closed-habitat taxa of these three families are recorded at early Miocene localities in Patagonia (O. horacioperezi, O. patagonicus, Noriegavis santacrucensis, and Crypturellus reai), whereas open-habitat taxa come from late Miocene-early Pliocene sites at central (Pterocnemia sp. and Eudromia sp.), northwestern (O. kirchneri and Pterocnemia cf. mesopotamica), and northeastern (Pterocnemia mesopotamica) regions in Argentina.