NEMAKIT-DALDYNIAN ICHNOFAUNAS: EVIDENCE OF A PHYLOGENETIC FUSE TO THE CAMBRIAN EXPLOSION?

The trace fossil record provides an independent line of evidence to calibrate the Cambrian explosion. Although ichnologic data have been advocated as evidence for a deep Precambrian cladogenesis prior to the Cambrian explosion, critical analysis of Precambrian-Lower Cambrian ichnofaunas does not support this view. Ediacaran ichnofaunas are of low diversity and limited behavioral complexity, essentially very simple small fodinichnia and nonspecialized pascichnia (e.g. Helminthoidichnites), together with rasping traces produced by Kimberella and resting traces of dickinsonids. This situation changed dramatically in the Nemakit-Daldynian with the appearance of more diverse and complex ichnofaunas. Nemakit-Daldynian elements include arthropod repichnia (e.g. Diplichnites) and cubichnia (Rusophycus), and moderate-size to large, horizontal, shallow pascichnia (e.g. guided meandering Psammichnites) and fodinichnia (e.g. several ichnospecies of Oldhamia, complex Treptichnus, radial traces) produced by deposit feeders or undermat miners. Degree and depth of bioturbation remain low. The significant expansion in behavioral complexity during the Nemakit-Daldynian can not be simply explained by progressive diversification of Ediacaran faunas. There is a conspicuous absence of vertical domichnia of suspension feeders (e.g. Skolithos piperocks) in Nemakit-Daldynian rocks. The presence of multiple trophic guilds and a well-established suspension feeding infauna (Skolithos, Diplocraterion, Arenicolites) is recorded by abundant pipe rocks in Tommotian-Atdabanian strata. Body fossils also record the emergence of new tiers in the trophic pyramid, coincident with the Cambrian explosion. This key ecological innovation may have resulted in a more efficient coupling between plankton and benthos, and a significant increase in the delivery of nutrient-rich particles into the sediment, triggering the most significant change in the history of benthic ecology: the shift from matgrounds to mixgrounds (agronomic revolution). However, ichnologic evidence indicates that the presence of metazoa able to exploit the endobenthic environment preceded the establishment of a modern endobenthic ecologic structure, suggesting a time of phylogenetic fuse during the earliest Cambrian.