A PRELIMINARY INTERPRETATION OF THE TAPHONOMY AND PALEOENVIRONMENTAL CONDITIONS OF A LATE PLEISTOCENE NON-MARINE INVERTEBRATE AND MICROVERTEBRATE ASSEMBLAGE IN OAXACA, SOUTHERN MEXICO

Fluvial deposits are common in the Late Pleistocene record. There are several fossiliferous outcrops in the northwestern part of Oaxaca, in southern Mexico. Here we report some taphonomic considerations of the invertebrate and microvertebrate assemblage in two geographically close (14.3 Km) localities as well as the environmental conditions inferred by the habitat preferences of the faunal assemblage. The fossiliferous outcrops discordantly overlie volcanic units of Oligocene age in both areas. In both localities, the inferred main mode of accumulation of the microvertebrates and freshwater molluscs is by flowing water. There are significant differences between the taphonomic signals of the two localities.

In locality Oax-6 Cañada del Misterio, near San Antonio Acutla town, the specimens are in exposed river bars, in sand-size sediments. Three families of terrestrial gastropods are identified (Urocoptidae, Polygiridae and Helicinidae), as well as one family of freshwater gastropods (Planorbidae) and one of freshwater bivalves (Pisidiidae). The bivalves and terrestrial gastropods show microscopic evidence of erosion in their ornamentation. The microvertebrate bones are teeth, fragmented jaws and fragmented large bones with signs of some water transport as rounded edges. By the abundance of Helicinidae and Urocoptidae snails, rhizolites, and the presence of a diverse assemblage consisting of Squamata, Rodentia and Lagomorpha, the environment could be interpreted as an open forest with some seasonal water bodies.

In locality Oax-7 Río Tejupam, near Villa Tejupam de la Unión, the fossils are founded in silt to sand-size sediments of overbank and floodplain deposits. The invertebrate specimens, mainly Pisidiidae bivalves and Planorbidae snails, do not show erosion or long transport signals. Neither borings nor signs of abrasion were found on any of the shells, even the fragile valves of bivalves are founded articulated and not fragmented. Besides, it is common to found complete large bones of microvertebrate specimens. A fast burial probably occurred in this area. By the presence of Ostracoda, Ambystomidae, Charophytae and pisces indet. in locality Oax-7, we infer that, these water bodies were permanent.