Taphonomic analysis of the Late Pleistocene micromammals of Quequén Salado, Buenos Aires, Argentina

MONTALVO, CLAUDIA; TOMASSINI, RODRIGO; VISCONTI, GRACIELA; TIRANTI, SERGIO

Tübingen (Baden-Wurttemberg)

Encuentro; Taphos 2011. VI International Meeting on Taphonomy and Fossilization; 2011

Institute of Geosciences, University of Tübingen

Although it has been mentioned that the birds of prey (mainly Strigiformes) may have produced accumulations of small vertebrates in Quaternary levels from the Pampean region (Argentina), taphonomic analysis to support this affirmation are still scarce. The main goal of this study is to present results of taphonomic analysis of the micromammals remains recovered from pellets from a section exposed at the banks of Quequén Salado river (38°26´14.14´´ S, 60°40´13.29´´ W). The stratigraphic succession is interpreted as a fluvial system consisting of a lower sector made up of fine laminated siltstones with pedogenic features (rhizoconcretions and weak peds) interpreted as a floodplain environment. The upper section (channel deposits) is composed of cross-bedding sandstones overlain by massive siltstones. The succession is assigned to the Late Pleistocene based on the presence of megamammals Macrauchenia patachonica and Glyptodon reticulates. The studies assemblage consisting of remains coming from whole and disaggregated pellets recovered from the floodplain deposits. A total of 3,119 remains were recovered. Two specimens were assigned to reptiles Iguanidae, while the rest were mammals: marsupials Didelphidae (Lestodelphys halli) and rodents Octodontidae (Ctenomys sp.) and Cricetidae (Reithrodon auritus and Eligmodontia sp.), with estimated body masses ranging between 17 and 200 grams. This analysis was undertaken following the taphonomic methodology of Andrews (1990). The minimal number of elements (MNE) evaluated was 1,797 and the minimal number of individuals (MNI) was 34, estimated on the basis of mandibles. The average of relative abundance was 58.71%. The best represented skeletal elements were mandibles, humeri and femora. Molars, vertebrae, and scapulas were the skeletal elements with minor representation. Three indexes were calculated to assess the relationship among skeletal elements. The first two indexes, that give an idea about the preservation state of the cranial elements, show a deficiency of these elements. The third index shows an important loss of distal elements in relation the proximal ones. The isolated molars and empty alveoli ratio indicates loss of mandibles and maxillary. The degree of digestion was calculated for molars, incisors, proximal femora, and distal humeri, although it should be noted that more than 90% of skeletal elements show evidence of light modification. Among the postcranial elements evaluated from breakage, proximal portions predominated (37.15%), but complete elements (23.72%) were frequent. Complete skulls were scarce. More than 25% of the mandibles only lacked the articular portion and retained all their teeth. Other cranial elements had different degrees of completeness. The percentage of remains modified both by digestion and breakage is low. These features and the anatomical representation results, indicate that the studied assemblage was produced by a nocturnal bird of prey, most likely Tyto alba. This species was reported in Pleistocene deposits of Buenos Aires province. A suite of two thin sections of pellets was examined with a petrographic microscope. In both samples, the original bone microstructure was preserved intact, there were only small fractures caused by compression. Fossil bone cavities and fractures were filled by sparite, sometimes as druses, and Fe/Mn oxides. There was no evidence of bioerosion. The pellet matrix was argillaceous, including different minerals(e.g. quartz, calcite, plagioclase, Fe/Mn oxides). On the basis of EDX (Energy Dispersive X-ray) analysis performed on a fossil bone, the presence of original calcium phosphate is inferred, while the pellet matrix shows the same chemical components of bearing levels. Their degree of preservation suggests a low energy environment consistent with the floodplain conditions inferred for the bearing deposits.