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PALEONTOLOGICAL SOCIETY PAPERS, V. 8, 2002ABFIGURE 3—Modern microbial mats. A, A temperatemat flooring an ephemeral lake surrounded bySpartina and Juncus vegetation, Pea Island, NorthCarolina, USA. The mat is generally continuouswhen submerged, but becomes shrunken, furledand sometimes overturned upon drying. (Photo byS. J. Culver.) B, A tropical mat consisting ofcyanobacteria, diatoms, and other microbes builton a tidal flat at Tamae, Moorea, French Polynesia.The flat is in a tidal inlet with salinity usually rangingfrom 38–68 o / oo. These mats are thin and variable insurface texture and topography. Some are furled(as in the image), rolled-up, flat, or disrupted. (Photoby Shelene Poetker.) Measuring tape in cm.With the oceans covered with ice, primaryproduction would have been severely restricted,although phytoplankton would not necessarily havebecome extinct. The continuous record ofacritarchs can be interpreted under the ice-coveredoceans scenario as the survival of these forms inareas where the ice was cracked and fissured bytidal and current action, in melt water pools on ornear the surface of the ice, near and below holes inthe ice cover, and in regions where ice could notform because of volcanic activity. In either of theother two scenarios, primary production might havebeen constrained to lower latitudes, but it couldstill have been substantial and supportive ofcomplex trophic interactions. Benthic ecosystems,of course, would also have been affected differentlyunder each scenario because most deeper watersystems are dependent on surface productivity. Butthere is currently no record of these biotas.Shallow-water mats and stromatolites are presentin many places around the globe, presumably fromshallow areas unaffected by ice. As the ice agesended, thick deposits of laminated, algal-dominatedcarbonates were deposited. None of these algalassociations appear to have been grazed.Trophic relationships changed fundamentallyjust after 600 Ma when large animals, the Ediacarabiota, first appeared (Narbonne, 1998). This biotahas been found on all continents except Antarcticaand includes over 250 described species of softbodiedand trace fossils (Runnegar, 1992a; 1992b).Sometimes referred to as “an experiment,” theseorganisms were hardly a trial or test, for they livedfor over 25 million years, were fully functional,were quite diverse, comprised precursors to thelater metazoan record, and would be consideredvery successful had they lived later (Runnegar,1992c). Originally they were described mainly asanimals (Glaessner, 1984), but were later consideredto be other kinds of organisms based on alternativemorphologic interpretations and the perceivedtaphonomic difficulties of preserving soft bodies(Seilacher, 1989; Buss and Seilacher, 1994).Suggested alternatives were xenophyophorans, anagglutinated group of protists (Zhuravlev, 1993),lichens (Retallack, 1994), or extinct sister groups78