Miocene Fossil Decapoda (Crustacea: Brachyura) from Patagonia, Argentina, and Their Paleoecological Setting09/30/2005
Five previously undescribed decapod taxa have been collected from lower upper Miocene rocks of the Puerto Madryn Formation, Peninsula Valdes region, Chubut Province, Patagonia, Argentina. New species include Osachila valdesensis, Rochinia boschii, Romaleon parspinosus, Panopeus piramidensis, and Ocypode vericoncava. Chaceon peruvianus and Proterocarcinus latus are also reported from the unit, in addition to two indeterminate xanthoid species. Assignment of fossil taxa to genera within the Panopeidae Ortmann, 1893, is difficult due to the marked similarity in dorsal carapace characters among several genera. Panopeus whittenensis Glaessner, 1980, is herein referred to Pakicarcinus Schweitzer et al., 2004. The Puerto Madryn Formation exposed near Puerto Piramide contains three distinct Facies Associations (1-3), each associated with specific paleoecological and paleoenvironmental conditions, and which recur throughout the section and represent trangressive systems tract (TST) deposits and highstand systems tract (HST) deposits. Within Facies Association 1, near the base of the section at Puerto Piramide, three paleosurfaces containing invertebrate fossils in life position are exposed and have been carefully mapped in plan view. Because of their sedimentologic, stratigraphic, taphonomic, and paleoecologic features, these three paleosurfaces are considered to be obrution deposits, each preserving a slightly different paleoenvironmental regime in terms of water depth and position with respect to wave base. Paleosurfaces were formed during the waning stages of the transgressive systems tract.