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PALEONTOLOGICAL SOCIETY PAPERS, V. 8, 2002FIGURE 7—A living larger foraminiferan,Alveolinella quoyi, containing diatomendosymbionts. While foraminifera host a varietyof algal symbionts, most are dinoflagellates ordiatoms. The darker areas on the specimen aregreen or brown in life. Specimen is one cm long.(Photo by J. H. Lipps at the Motupore IslandResearch Station, Papua New Guinea.)where conditions may have been unfavorable tometazoans microbial mats grew (Fig. 6B). Otherbenthic prokaryotes and protists left few or noremains and nothing can be inferred about them.They must have been present in great diversity andconsiderable abundance.Mesozoic skeletonized nano- andmicroplankton radiated starting in the Triassic andcontinuing through the Cretaceous. Pelagiccoccolithophorids, dinoflagellates, diatoms,silicoflagellates, radiolaria, and foraminifera reachedtheir highest diversities during this era. They becameprovincial in their distributions from north to south,their taxonomic distributions generally reflecting thewater masses and sea surface circulation patterns.The continents were arrayed mostly across latitudes,forcing boundary currents along their margins anda gyre system in the mid-latitude centers of theoceans. These currents distributed fairly warm andhomogeneous waters from low to high latitudes. Theopening of the Atlantic Ocean, and thus thedecreasing width of the proto-Pacific, caused theseoceanographic patterns to be restricted. Thediversification of pelagic microorganismsthroughout the Mesozoic relates to the intensifiedand restricted surface circulation (Lipps, 1985). Asthese currents encountered continental margins,particularly on the eastern sides of ocean basins,upwelling—bringing nutrients into the surficialwaters—occurred more readily than before. Thus,the oceanic environments became more heterogeneousand separated, allowing diversificationthrough the creation of new pelagic environmentsand isolation. While this general model may accountfor the evolutionary patterns in some respects, moreinformation is required for a complete interpretation.Since much of the Mesozoic deep sea floor has beensubducted or translated to regions far removed fromits origin, collecting this information is an oneroustask. However, the nano- and microplankton are wellknown from sediments preserved on land or in afew places in the deep sea. These microfossils showthat a major shift in the sites of carbonate depositiontook place between the Paleozoic and the Mesozoic.Most Paleozoic carbonate was deposited in shallowseas by larger invertebrates, calcareous algae, andbenthic foraminifera; but in the mid- to lateMesozoic the deposition of carbonate was chieflycarried out by calcareous phytoplankton andforaminifera. Siliceous deposition in the deep seaalso became prevalent as radiolaria, diatoms, andsilicoflagellates incorporated vast quantities of silicathat was then deposited onto the sea floor.Trophic structures in these pelagic systemsbecame more complex than ever before (Table3,4).Not only were different ecosystems now in place indifferent parts of the oceans, but they also had moreelements in them. A bacterial loop must have beenpresent, primary producers included prokaryotes,and algal protists included skeletonized forms.Microcarnivores likewise had unskeletonized andskeletonized elements. The skeletonized formswere dependent on particular areas in the oceanwhere their skeletal materials were available—carbonate in widespread areas of the oceans andsilica mostly in upwelling areas in eastern boundarycurrents and along the equator. Food webs inupwelling systems were probably bimodal, aslarger phytoplankton grew rapidly and supporteddirect feeding by larger herbivores and carnivores.Smaller nano- and microplankton were presentwith their own set of herbivores and carnivores.The advent of larger plankton was exploited bypelagic fish and reptiles.84