Trace fossils and the agronmomic revolution at the Neoproterozoic ? Cambrian transition in Southwest Gondwana

ACEÑOLAZA, G.F.; GERMS, G. AND ACEÑOLAZA, F

Developments in Precambrian Geology

Elsevier

Lugar: Amsterdam; Año: 2009 vol. 16 p. 339 - 347

3567-3575

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 EarlyCambrian, 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 andcomplexity 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).