View - Kowalewski, M. - Virginia Tech

LIPPS AND CULVER—TROPHIC ROLE OF MARINE MICROORGANISMSboth protist and prokaryote diversity likelyincreased without leaving a record. The trophicroles of these new additions were probably similarto the preexisting types. Benthic ecosystems alsodiversified and became more complex, withdiversity and abundance greater in shallow- thanin deep-water settings (Javaux et al., 2001).Microbial diversity continued to increase acrossthe boundary into the Neoproterozoic (Sereev, etal., 1997). A single trace fossil occurrence mayrepresent metazoan-like organisms moving on mudmore than 1.2 Ga (Rasmussen et al., 2002).Neoproterozoic: 1.0 to 0.545 Ga.—During thefirst half of the Neoproterozoic, benthic and pelagictrophic structures were much like those of theearlier Proterozoic. Although protists continued todiversify and build complex ecosystems in bothbenthic and pelagic environments, the totaldiversity remained low compared to later times(Knoll, 1994). Small, early multicellular stemmetazoans may have been widespread, butevidence is lacking (Lipps et al., 1992). EarlyNeoproterozoic pelagic trophic structures(Tables3, 4) surely included a bacterial loop, whichby then encompassed phytoplankton, their organicproducts, possible herbivorous protists, carnivores,and consumers. In the benthos, mats andstromatolites were abundant.Snowball Earth II, proposed to account forglacial evidence in low latitudes between 750 and600Ma (Harland, 1964; Kirschvink, 1992;Hoffman et al., 1998), comprised several glacialperiods, each of which would have had significantimpact on pelagic and benthic ecosystems and theirtrophic structures, depending on the geographicextent of the glaciations. Snowball Earthpossibilities include complete, global glaciationsand paving of the oceans with ice, partialglaciations reaching low latitudes but with an openequatorial ocean, and glaciations restricted tomedium-to-high latitudes. The fossil record ofplankton continues through this time interval withvariations in diversity but evidently with littleextinction (Vidal and Moczydlowska-Vidal, 1997).FIGURE 2—Modern stromatolites, Cargla Point, Shark Bay, Australia. The stromatolites (50–60 cmhigh here) grow in hypersaline waters that exclude herbivorous invertebrates and fish from the area.Other algal-rich forms are intermingled with the stromatolites including microbial mats (foreground),oolites, and floating clumps of bacteria. (Image by J. H. Lipps and S. Culver, Feburary, 2002.)77