Coprolites from the Eocene of Central Patagonia, Argentina: applying the actualism for interpreting the producer

KRAUSE, M.; PIÑA, C.I.

Mendoza

Congreso; 18º International Sedimentological Congress; 2010

International Associations of Sedimentologists

Biogenic structures previously attributed to coprolites are studied applying an actualistic criterion for determining a possible producer. The structures were found in the Las Flores Formation (Lower Eocene), San Jorge basin, Patagonia Argentina. The coprolitic origin is supported by fusiform to sub-cylindrical shape, constrictions, striated surface texture, sub-circular to sub-elliptical transversal sections, grey- olive to white color, phosphate cryptocrystalline groundmass, radial and concentric marks, and the presence of phytoliths. Comparison with fe -ces of extant fauna allows establishing the following relationships: (1) the morphology is similar to that of reptiles, in particular canid and felid feces; (2) surface texture would be the product of the superposition of successive layers of excrements added during the passage by the cloacae; (3) resulted from deposition of the original mass in plastic state on the substrate; (4) cracks would be formed by dehydration of the original fecal mass; (5) the color of the feces, the basal mass without inclusions, and the content of phosphates indicate carnivore producers with high digestive power. Herein, definition of reptiles as possible producers, discarding mammals, is based on the absence of inclusions. In order to test this hypothesis we have compared broad-snouted caiman feces with the coprolites. The comparison shows high similarity in shape, size, color, and elemental composition. Absence of inclusions would be the result of bone decalcification by crocodilian digestive processes. The content of phytoliths could be a consequence of secondary ingestion, common in the caiman, and other crocodilians. The assignation of a crocodilian origin for the coprolites allows inferring the presence of organisms with similar paleocological habits and needs like those of caiman. The paleontological record of the area supports this interpretation. Remains of Eocaiman, a genus accepted as the most basal of the alligatoridae Caimaninae are known for strata underlying and overlying strata bearing the coprolites.