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.
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