Nereites saltensis (Trace fossil): a taxonomical re-evaliuation of type and additional material from the Puncoviscana Formation of Northwest Argentina (Ediacaran - Early Cambrian).

ACEÑOLAZA, G. Y ACEÑOLAZA, F.

Punta del Este, Uruguay

Simposio; V South American Symposium on Isotope Geology; 2006

Facultad de Ciencias, Universidad de La República

Ediacaran – Early Cambrian strata in the Argentine Central Andean Basin are represented by over 3000 meters of highly tectonized and metamorphosed siliciclastics of the Puncoviscana Formation. Widespread siliciclastics characterize this sequence, but associated limestones, volcanoclastics and lava flows have also been mentioned emphasizing the complexity of facies included in the basin. The Puncoviscana Formation is recognized as a important ichnofossiliferous unit, with a record of over thirthy different traces, and only two body fossils represented by Selkirkia and Beltanelloides. 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 Puncoviscana Formation in NW Argentina. Nereites has been synonimized with Psammichnites, an Early Cambrian trace fossil that clearly differs from the first one. A detailed analysis of type, housed and new material of Nereites saltensis has been carried out (including thin sections and polished slabs), in order to seek after the morphological elements that characterize Psammichnites and compare it with the first one. As a result of it, neither the type material non fourteen additional analyzed samples have shown confident elements characterizing the complex Psammichnites; while most of it displayed the typical simpler trail system that characterizes Nereites. The existence of 3-dimensional marks deeping into the sediments (reflecting the supposed pendulating siphon) was never found in Nereites, as needed for Psammichnites. Nereites saltensis displays only in certain opportunities a pseudo-bilobed surface by sectors of the trace, reflecting a collapse of the burrow, a common taphonomic signature in this type of traces. In some other cases, the complete 3-dimensional tunnel was preserved without any of the morphological elements that characterize Psammichnites. Finally, it is necessary to highligth that the re-assignation of Nereites to Psammichnites was done on the basis of scarce samples as stated by the reviewer authors (Seilacher et al., 2005: "It was only through the specimen shown in Figs. 21 and 22D that this assignment could be corrected" sic.).