#palass

*Breandán Anraoi MacGabhann1,2,3, Paul D. Ryan3 and John Murray3

1Department of Geography, Edge Hill University 2School of GeoSciences, University of Edinburgh 3Earth and Ocean Sciences, National University of Ireland, Galway

The Cambrian Explosion was accompanied by a fundamental shift in the nature of marine substrates. In shallow marine environments, characteristic Neoproterozoic microbial matgrounds gave way to typical Phanerozoic mixground substrates, concordant with the evolution of widespread bioturbation. In deep marine environments, the 'agronomic revolution' appears to have been delayed, as suggested by strata such as the microbially-bound middle Cambrian contourites of the Booley Bay Formation, southeastern Ireland.

Two sets of non-mineralised discoidal structures occur in this unit; millimetre-scale scratch circles, and centimetre-scale partial discs preferentially orientated in the palaeocurrent direction. We investigated the hypothesis that the larger discs were holdfasts of frond-like organisms, preferentially orientated by current pressure on their upper parts, numerically modelling the flexure of a microbial mat around such a holdfast. While highly speculative, the models predict profiles consistent with field observations. We conclude that one admissible interpretation is that the Booley Bay Formation preserves evidence of large stalked frond-like organisms, living in a contour-current environment, anchored to the microbially-bound substrate by discoidal holdfasts. The ecological similarity to deep marine Ediacaran environments such as Mistaken Point suggests that not just Ediacaran matgrounds, but Ediacaran community palaeoecology, survived in deep marine settings until at least the Middle Cambrian.

Sören Jensen1 and Graham. E. Budd2

1Universidad de Extremadura, Badajoz 2Department of Earth Sciences, Uppsala University

The decision to base the Cambrian GSSP on trace fossils was controversial at the time of its ratification in 1994, and has remained so to date. Recently efforts have been set in motion to evaluate if the definition of the base of the Cambrian should be re-opened. We would be more enthusiastic if there were clearly superior globally correlatable levels, but we do not see this being the case. Rather, there are advantages with the current deep placement of the Cambrian GSSP, and we believe its deficiencies have been overstated. It has considerable conceptual value as it undoubtedly approximates a time of biotic radiation and major changes in benthic ecology. It is readily correlatable into most areas, yet so deep that most questions of inter-regional correlation become intra-Cambrian. Claimed problems of correlation into Siberia do not consider a growing body of trace fossil data from northern Siberia. Efforts are certainly needed on advancing knowledge of the Fortune Head GSSP section, including studies to evaluate the sub-GSSP Treptichnus pedum. Work in progress shows that much remains to be done on organic-walled microfossils in this section, and the possibilities to add further chemostratigraphical data should be explored.

Anette E.S. Högstrom1, Jan-Ove Ebbestad2, Sören Jensen3, Teodoro Palacios3, Guido Meinhold4, Wendy L. Taylor5, Linn K. Novis1, Heda Agic6 and Ma?gorzata Moczyd?owska6

1Tromsø University Museum, Natural Sciences 2Museum of Evolution, Uppsala University 3Universidad de Extremadura, Badajoz 4Geoscience Center of the University of Göttingen 5Department of Geological Sciences, University of Cape Town 6Department of Earth Sciences, Uppsala University

In Scandinavia the evolutionary events across the Ediacaran?Cambrian transition can only be studied in continuous section on the Digermul Peninsula, northern Norway, in the siliciclastic Stáhpogiedde Formation. This roughly 500-m-thick unit comprises, in ascending order, the Lillevannet, Innerelva and Manndraperelva members. Trace fossils, including Treptichnus pedum, and organic-walled microfossils, including Granomarginata prima, position the base of the Cambrian in the upper part of the Manndraperelva Mbr. Some 20 years ago discoidal Ediacara-type fossils were found in the middle part of Innerelva Mbr. Recent field seasons have provided abundant new material of Aspidella-type fossils and extended their stratigraphical range to within about 15 meters above the Lillevannet Member. The exclusive presence of discoidal forms may reflect a taphonomic bias and/or be evidence of a greater age than that of the more diverse Ediacaran assemblages. That the latter may be the case is indicated by the stratigraphic proximity of the lowest occurrences of Aspidella to the Mortensnes diamictite, recently tentatively considered a Gaskiers glaciation equivalent (ca 580 Ma). This raises the question of hitherto unrecognised breaks in sedimentation in the Stáhpogiedde Formation. In order to explore this question we have sampled the succession for organic-walled microfossils, detrital mineral geochronology and sediment geochemistry.