Trace Fossils and the Agronomic Revolution at the Neoproterozoic-Cambrian Transition in Southwest Gondwana.

ACEÑOLAZA G.F, GERMS G. Y ACEÑOLAZA F. G.

Developments in Precambrian Geology, Elsevier

Elsevier

Año: 2009; p. 10 - 498

The Ediacaran-Cambrian transition represents an eventful period in the evolution of life on Earth. At this particular time, not only did the demise of the soft-bodied Ediacaran biota occurred, but also the explosive diversification of organisms known as the ‘Cambrian explosion’ (see Chapter 9.1). During the same time period, a considerable increase in the disruption of sediments known as the ‘agronomic revolution’ occurred, when shallow marine Ediacaran ‘matgrounds’ were largely replaced by the Early Cambrian better ventilated ‘mixgrounds’ (Seilacher and Pflu¨ ger, 1994). In the shallow marine Ediacaran deposits, trace fossil diversity is relatively low and dominated by horizontal simple trails and shallow burrows that display feeding strategies related to the exploitation of microbial matgrounds (Hagadorn and Bottjer, 1997, 1999; Jensen, 2003; Porada et al., 2008). This scenario changed in the Early Cambrian, when an explosive increase in trace fossil diversity occurred, followed by the onset of vertical bioturbation and the disappearance of a matground-based ecology. This increase in extent, tiering depth and complexity of bioturbation, determined biofabrics distribution and was strongly controlled by sedimentary fabrics (Droser and Li, 2001). During the Ediacaran, animals were already living on deep-sea floors. However, the ichnofaunas were poorly diverse, with a high proportion of undermat miners (Crimes, 2001; Seilacher et al., 2005). Simple horizontal, unbranched burrows, primitive systems and rare open meandering traces are recognised as a characteristic ‘upper’ Ediacaran set, while a more diverse branching burrow systems, resting traces and furrows represent some typical features of the pre-trilobite lower Cambrian strata (Jensen, 2003; McNaughton, 2007). During the Early Cambrian a dramatic radiation of trace fossils behavioural programmes and sedimentary fabrics took place (Droser and Li, 2001). Even though the chronology of the Ediacaran-Early Cambrian transition in South America is not completely understood, an early attempt of the onshore2offshore shift is probably represented in the rich ichnofauna of the remarkable Puncoviscana Formation of NW Argentina (Aceñolaza and Aceñolaza, 2005; Seilacher et al., 2005). Although the relative roles of environment and evolution in controlling the trace fossil record has been emphasised for the Ediacaran-Cambrian transition (McNaughton, 2007), a re-evaluation of the whole significance of the occurring trace fossil associations and their palaeoecological significance is still pending. The aim of this contribution is to briefly discuss the trace fossil associations of the Puncoviscana Formation s.l. in northwest Argentina and of the Nama and Vanrhynsdorp groups in southern Africa, and to discuss their significance with regard to the ‘agronomic revolution’ trends, biodiversity and environmental shift during the Ediacaran to Early Cambrian transition.