The validity of Nereites saltensis (trace fossil) from the Puncoviscana Formation of NW Argentina (ediacaran-Early Cambrian

ACEÑOLAZA, G.F.

Hefei

Congreso; International Paleontological Congress; 2006

University of Science and Technology of China

Ediacaran – Early Cambrian aged rocks in the Argentine Central Andean Basin are represented by over 3000 meters of highly tectonized sligthly metamorphosed strata dominated by the Puncoviscana Formation. Widespread siliciclastics characterize this sequence, but associated limestones, volcanoclastics and lava flows are mentioned emphasizing the complexity of facies included in the basin. The Puncoviscana Formation has long been recognized as an important ichnofossiliferous unit, with a record of over thirty different ichnospecies, and only two body fossils represented by Selkirkia and Beltanelloides (Aceñolaza and Aceñolaza, 2005 with references). The geographical distribution of trace fossil assemblages display a remarkable alignment as belts, with a shallower eastwards Nereites association, and a deeper westwards Oldhamia association, that do not represent archetypal ichnofacies. This distribution is related to the morphology of the basin, the chronological record of the ichnofaunas, and may represent different temporal levels on the evolution of the Puncoviscana Sea. Recent contributions dealing with the ichnofauna of the Puncoviscana Formation have generated a controversy respect to the taxonomy and validity of Nereites saltensis, one of the most remarkable trace fossils of the unit. Nereites has been synonymized with Psammichnites, an Early Cambrian trace fossil that clearly differs from the first one (Seilacher et al., 2005).  A detailed analysis including thin sections and polished slabs of type, housed and new material of Nereites saltensis has been carried out in order to recognize conclusive elements characterizing Psammichnites, and to compare them to Nereites. As a result of it, neither the type, nor additional material has shown confidently the distinctive characters of Psammichnites, while most of it displayed the typical simpler trail system of Nereites. The first requires a pendulating siphon trace in its upper surface, an element always absent in Nereites. The pseudo-bilobed surface by sectors in Nereites saltensis reflects the collapse of the burrow, a common taphonomic character in this type of traces. In addition, 3-dimensional complete tunnels were clearly preserved, allowing a detailed diagnosis of Nereites.   References:   Aceñolaza, G. and Aceñolaza, F.G., 2005. La Formación Puncoviscana y unidades estratigráficas vinculadas en el Neoproterozoico-Cámbrico Temprano del Noroeste Argentino. Latin American Journal of Sedimentology and Basin Análisis, 12 (2): 65-88. Seilacher, A., Buatois, L. and Mangano, M., 2005. Trace fossils in the Ediacaran-Cambrian transition: Behavioral diversification, ecological turnover and environmental shift. Palaeo. Palaeo. Palaeo, 227: 323-356.