Lucía E. Gómez Peral

Ediacaran discs from South America: probable soft-bodied macrofossils unlock the paleogeography of the Clymene Ocean

The origin, affinity and paleoecology of macrofossils of soft-bodied organisms of the terminal Ediacaran Period have been highly debated. Previous discoveries in South America are restricted to small shelly metazoans of the Nama Assemblage. Here we report for the first time the occurrence of discoidal structures from the Upper Ediacaran Cerro Negro Formation, La Providencia Group, Argentina. Specimens are preserved in tabular sandstones with microbially-induced sedimentary structures. Flute marks and linear scours at the base of the sandstone layers indicate deposition under high energy, episodic flows. Stratigraphic, sedimentologic, petrographic and taphonomic analyses indicate that the origin of these structures is not related to abiotic process. Preservational and morphological features, as invagination and the presence of radial grooves, indicate that they resemble typical morphs of the Aspidella plexus. The large number of small-sized individuals and the wide range of size classes with skewed distribution suggest that they lived in high-density communities. The presence of Aspidella in the Cerro Negro Formation would represent the first reliable record of Ediacaran soft-bodied organisms in South America. It also supports the paleogeographic scenario of the Clymene Ocean, in which a shallow sea covered part of the southwest Gondwana at the end of the Ediacaran.

The Glaciations in South America

Neoproterozoic successions in South America are recorded in many areas of Brazil, Paraguay, Bolivia, Uruguay and Argentina. Some of these units show glaciogenic formations like those represented in the Puga and Serra Azul formations in the Northern Paraguay Belt (Brazil) in agreement with their accumulation in a Snowball Earth context. However, in other cases, tillites or other glaciomarine deposits are absent, which may indicate a distant position (tropical) regarding the ice cap, as occurs in the Tandilia System (Argentina) related to a “Phantom glacial” context. In this contribution we show the comparison between direct and indirect evidence of glaciations in the Neoproterozoic successions of South America. The Puga and Serra Azul formations in the Paraguay Belt and Sierras Bayas Group in the Rio de la Plata Craton have been chosen to describe the “Snowball Earth” and “Phantom glacial” models, respectively. By means of multiproxy analysis it is also possible to indicate changes in paleoclimate conditions in both cases. The presence of tillites is considered to be direct evidence of the extreme climatic conditions during deposition during a glaciation. Meanwhile, more subtle evidence such as regional unconformities related to drastic sea level changes, trends in δ13C, events of phosphogenesis, constitute, among others, the tools to indicate the influence of Neoproterozoic global glaciations during the deposition of the sedimentary units.