Los vertebrados del Neogeno de la costa sur del Perú: ambiente sedimentario y condiciones de fosilización

Abstract

Abstract Fossil marine vertebrates are very abundant in the Pisco and Caballas Formations from the southern coast of Peru [selachians, teleosteans, reptiles, birds, mammals (cetaceans, carnivores and edentates)]. Those fossils have contributed to establish a stratigraphy as they are distributed in seven horizons (six in the Pisco Formation and one in the Caballas Formation), the age of which spans the Early Miocene to Late Pliocene interval. The vertebrates are generally very well preserved and complete or sub-complete skeletons are very common. Sedimentological and paleontological (taphonomical and biological) studies show that the vertebrates were deposited in a beach-environment. Three types of sea shore conditions are defined: • protected beaches where coquinas are deposited that contain scarcely broken shells and sometimes lay in life position. Skeletons are generally either complete, with their various pieces still connected, or slightly dissociated; • beaches with agitated water documented by coquinas containing broken shells and bones; • reefs where the shells and bone are always broken. The vertebrate bearing beds do not display ebb and flow structures, and therefore would indicate low-range tides. The depositional environment was shallow and corresponded to supra-and intertidal zones. The presence of various connected skeletons demonstrates a very rapid burial process of carcasses that probably lasted only a few days. It is probably due to the burial action of the waves, rather than to a high sedimentation rate. In the second case, skeletons would have been disconnected by beasts of prey. The biology of whales (which generally bread close to the shore), that of abundant carnivores and birds (which are strictly depending upon the shore for reproduction) confirm the sea-shore environment. Furthermore, the edentate of the Pisco Formation was probably a poor swimmer and was feeding with seaweeds, as the peruvian coast was desert during the Neogene. Depending upon land for locomotion and sea for alimentation, this animal was forced to live on the sea shore. To conclude, the vertebrates of the Pisco and Caballas Formations were fossilized in a very littoral environment either stranded on the beach, or deposited in the intertidal zone in shallow waters.