View - Kowalewski, M. - Virginia Tech

PALEONTOLOGICAL SOCIETY PAPERS, V. 8, 2002animals, and larvae) herbivory, so that they, too,developed new strategies and morphologies. TheCambrian radiation was likely due to a number ofinteracting oceanographic, sedimentologic, andbiologic factors; but microbes were definitely andfundamentally involved.The pelagic biome continued to diversifythrough the Cambrian. In the middle and laterCambrian, acritarchs (Vidal and Moczydlowska-Vidal, 1997) and radiolarians (Dong et al., 1997;Won and Below, 1999) diversified. The radiation ofthese primary producers and herbivorous/carnivorous microplankton, as well as the presenceof graptolites and conodonts, indicates that the openocean was far more complex biotically andtrophically; and very little is known about the nonfossilizablecomponents. With a large diversity ofbenthic animals, planktic larvae were likely far morecommon in the Cambrian than in earlier times.Archaeocyathids evolved shortly after thebeginning of the Cambrian and formed reefs andbioherms in low latitudes (Rowland and Shapiro,2002). The individual specimens and reefs havecharacteristics (Cowen, 1983) indicative of thepresence of microbial photoendosymbionts. Todaysponges, a group related to archaeocyathans, harborarchaeans, bacteria (including cyanobacteria), andprotist symbionts (Osinga et al., 2001), so thatmodern analogy supports the hypothesis ofphotoendosymbiosis. Dinoflagellates, a commonphotoendosymbiont group, or their ancestors havea biomarker record extending at least to theCambrian (Moldowan and Talyzina, 1998).Archaeocyathans may have been the first animalsthat hosted such symbionts but this strategy ofphotoendosymbiosis becomes quite commonthrough the subsequent geologic record.The radiations in the Cambrian made trophicstructures far more complex. Now there wereprimary producers, nano- and microherbivores,both tiny and larger zooplanktic carnivores, andmost likely meroplankton, as indicated by anincrease in diversity and turnover rate in protists(Knoll, 1994) as well as animals. A bacterial loopwas probably well-developed at this time too. Onthe sea floor, benthic prokaryotes, protists, andanimals made trophic structures even morecomplex in terms of diversity of the various trophiccomponents. Bioturbating and surface detritalfeeders, suspension feeders, herbivores, and majorand minor predators were all present. Trophicstructures in benthic and pelagic environmentsassumed a fundamentally modern aspect, althoughfurther modification by the evolution and extinctionof taxa continued.The Ordovician was a time when protists andanimals diversified even more (Knoll, 1994). Thediversification of each group tracks that of theother, indicating that evolutionary processestransgressed organizational levels, ecologicrestrictions, biogeography, and behavior. After theCambrian, prokaryotes became less obvious in thefossil record (because grazers restricted theirdistribution), but without doubt they existed in bothpelagic and benthic environments. Among protists,benthic agglutinated foraminifera diversifiedgreatly and moved into a variety of habitats. They,like their modern counterparts, were most likelygenerally omnivorous with some taxa takingdetritus, prokaryotes, and other protists, andperhaps being fed upon intentionally by juvenileand specialized invertebrates (such as scaphopodsthat first appeared in the Ordovician) andaccidentally by non-selective detritovores andgrazers (Lipps, 1983; Culver and Lipps, in press).Evidence for these kinds of trophic interactions isdifficult to assess because most of them result incomplete destruction of prey. Heterotrophic protistscontinued to move into the plankton, withradiolarians diversifying (Casey, 1993) andtintinnids first appearing (Tappan, 1993). In the lateOrdovician, a major extinction eliminated 85% ofthe marine biota; it was followed by a radiation oftaxa into similar habitats during the early Silurian(Sheehan, 2001). Protists tracked the invertebrates(Knoll and Lipps, 1993).Later in the Paleozoic, foraminifera evolvedcalcareous tests. The fusulinids originated in theSilurian, then developed heavily calcified, largetests in tropical carbonate environments in theCarboniferous and Permian (Culver, 1993). Theselikely were the first coevolved photosymbiont–82