Ana Parras

MIOCENE FOSSIL DECAPODA (CRUSTACEA: BRACHYURA) FROM PATAGONIA, ARGENTINA, AND THEIR PALEOECOLOGICAL SETTING

ABSTRACT Five previously undescribed decapod taxa have been collected from lower upper Miocene rocks of the Puerto Madryn Formation, Península Valdés 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 Pirámide 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 Pirámide, 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.

Mass Mortality Of Fossil Decapods Within the Monte LeóN Formation (Early Miocene), Southern Argentina: Victims Of Andean Volcanism

Abstract Four exposed planar surfaces within the type area of the Monte León Formation (early Miocene) of southern Patagonia, Argentina, enclose significantly different fossil assemblages positioned in close geographic and stratigraphic proximity to one another. The exposed paleosurfaces were mapped in planar view using a quadrat grid system. Precise fossil position and orientation data, stable isotope thermometry and petrographic analyses, and petrologic and taphonomic evidence suggest an inner-shelf, shallow water habitat, above storm wave base, with a well-oxygenated benthos and weak offshore bottom currents. The rate of sedimentation was generally low, interspersed with periods of non-deposition and sporadic, higher-energy pulses of sediment input. Stable isotope analyses of foraminiferans indicate bottom water temperatures consistent with a modern mid-latitude coastal setting. Two distinctly different assemblages were observed: 1) a relatively diverse, normal marine benthic fauna consisting of bivalves, gastropods, bryozoans, echinoids, and decapods; and 2) a unique assemblage consisting solely of numerous, exceptionally preserved, fully-articulated crabs. These assemblages occur in a one meter interval within the lower-most beds of the Monte León Formation. The occurrence and preservation of large numbers of decapods within the fossil record are rare, making these deposits remarkable. The crab-rich assemblage was stratigraphically positioned below the surfaces containing the normal marine assemblage. The crabs are contained within a slightly compacted, argillaceous volcanic tuff, consisting mostly of volcanic glass shards and euhedral plagioclase grains. Biogenic fragments are noticeably absent from the deposit, unlike sediments collected from the upper surfaces. Most of the crabs were preserved with their third maxillipeds in an open, gaping posture. This is consistent with postures observed in extant crabs suffering from respiratory distress. From the above evidence it is inferred that the fossil crabs were suffocated, killed, and rapidly buried. Supradjacent layers record the re-establishment of normal marine conditions. The apparent faunal disparity observed on the four paleosurfaces within the Monte León Formation is interpreted as representing the initial decimation and eventual re-establishment of the benthic marine fauna following a catastrophic volcanic event.

Elasmosaurs (Sauropterygia, Plesiosauria) from the La Colonia Formation (Campanian–Maastrichtian), Argentina

O’Gorman, J.P., Salgado, L., Varela, J., & Parras, A., 2013. Elasmosaurs (Sauropterygia, Plesiosauria) from the La Colonia Formation (Campanian–Maastrichtian), Argentina. Alcheringa 37, 257–265. ISSN 0311-5518. Elasmosaur postcranial remains from the La Colonia Formation (Campanian–Maastrichtian), Chubut Province, Patagonia, Argentina, are described. The new material has small dimensions and caudal vertebrae with parapophyses strongly projected laterally—characters shared with some Elasmosauridae indet. from the coeval Allen Formation, Río Negro Province, Argentina. These features reinforce the similarities between the plesiosaur faunas to the north and south of the Somún Curá Plateau. The small size of these elasmosaurs may be palaeoecologically related to the marginal marine depositional environment of the sedimentary host rocks. José P. O’Gorman [joseogorman@fcnym.unlp.edu.ar], División Paleontología Vertebrados, Museo de La Plata, Universidad Nacional de La Plata, Paseo del Bosque s/n., B1900FWA, La Plata, Argentina and [CONICET (Consejo Nacional de Investigaciones Científicas y Técnicas)]; Leonardo Salgado [salgadoleito@yahoo.com.ar], Instituto de Investigación en Paleobiología y Geología, Universidad Nacional de Río Negro, Isidro Lobo y Belgrano, 8332 General Roca, Argentina and [CONICET]; Julio Varela [julioadrianvarela@hotmail.com], and Ana Parras [aparras@exactas.unlpam.edu.ar], INCITAP (CONICET-UNLPam), Facultad de Ciencias Exactas y Naturales, Universidad Nacional de La Pampa, Uruguay 151, 6300 Santa Rosa, La Pampa, Argentina. Received 27.7.2012; revised 19.10.2012; accepted 27.10.2012.

Palinología del Miembro Gran Bajo de la Formación San Julián (Oligoceno Tardío) en su Localidad Tipo, Santa Cruz, Argentina: Consideraciones Paleoambientales

Abstract. PALYNOLOGY OF THE GRAN BAJO MEMBER, SAN JULIÁN FORMATION (LATE OLIGOCENE) IN ITS TYPE LOCALITY, SANTA CRUZ, ARGENTINA: PALEOENVIRONMENTAL CONSIDERATIONS. The aim of this paper is to present the palynological results from the Gran Bajo Member of the San Julián Formation in its type area (Gran Bajo de San Julián). The studied section crops out in the eastern Santa Cruz Province and has been assigned to the late Oligocène based on 87Sr/86Sr data. The sedimentological analysis suggests a shallow marine environment close to the coast barely influenced by ocean waters at the lower part of the section, which evolved toward a foreshore-shoreface environment at the middle part of the section and to a shoreface-offshore towards the upper part of the section. The palynological assemblages from Gran Bajo section are primarily conformed by terrestrial elements with high preservation stage. A detailed palynological study allowed reconstructing the structure and composition of the forest in relation to depositional environments. Changes in the depositional environments correlate with the pollen zones determined by cluster analysis. Among the arboreal elements, the Podocarpaceae dominated the middle part of the section and Nothofagaceae (mainly Nothofagidites brassii type) dominates in the upper part, associated with Araucariaceae and Proteaceae in lower proportions. Myrtaceae, Arecaceae, Sapindaceae and Anacardiaceae were scarcely represented. The understory was composed by different species of monilophytes, mainly Cyatheaceae and Dicksoniaceae that decrease towards the top of the analyzed interval.