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IMPACT OF LATE ORDOVICIAN GLACIATION-DEGLACIATION ON MARINE LIFE 43 glaciers expanded. Ocean surface water circulation and chemistry changed as well. Sheehan and Coorough (1990, p. 184) point out that ''. . . of the 211 genera recorded in the early-middle Ashgill, 92 (44 percent) did not survive into the Hirnantian. A substantial portion of these genera probably died during the early-middle Ashgill rather than at the base of the Hirnantian." Of 124 brachiopod genera known from the Hirnantian Stage, only 21 originated during that stage (Sheehan and Coorough, 1990, p. 185). Approximately one-half the Hirnantian brachiopod genera become extinct by the end of the Stage; thus, the Hirnantian extinctions comprise the second phase of Late Ordovician brachiopod mass mortality. Taken together, approximately two-thirds of the early-middle Ashgill brachiopods became extinct by the end of the second phase of mass mortality. Apparently, the shelf sea brachiopod fauna typified by members of the genus Hirnantia spread widely during glacial maximum conditions. Many members of the fauna may have been adapted to somewhat cooler ocean surface water temperatures than were present either before or after glacial maximum. Oxygen isotope determinations made from brachiopod shells in Sweden, which lay within the tropics in the Late Ordovician, indicate that surface water temperatures were cooler in the Hirnantian than earlier (Marshall and Middleton, 1990). Marshall and Middleton (1990) also note that the carbon isotope values from the same shells suggest "enhanced deposition of organic carbon, a process which would have decreased" the partial pressure of CO2 in both the ocean and the atmosphere, contributing to rapid global cooling. Sheehan and Coorough (1990) note that of 130 genera recorded from Early Silurian strata, about 40% are new and 60% range into the Silurian from the Ordovician. Cocks (1988) noted that the earliest Silurian brachiopod faunas have fewer species and fewer individuals in similar samples than Hirnantian Stage faunas, based on a precise review of Ordovician- Silurian boundary interval brachiopods. Sheehan (1982) pointed out that the Early Silurian brachiopod communities changed in species composition relatively rapidly. The time interval of the most rapid change in community organization was during sea-level rise and surface ocean water warming. After the transgression of seas across formerly emergent platforms had stabilized and surface waters warmed, brachiopod community organization stabilized as well. Late Ordovician trilobite mass mortality took place during and at the Rawtheyan-Hirnantian Stage boundary (Briggs et al., 1988). Hirnantian trilobites lived on the outer parts of marine shelves and were highly provincial (Lesperance, 1988). Shelf sea-dwelling trilobites generally had high rates of generic extinction as waters cooled with the onset of glaciation (Fortey, 1989). Survivors are those trilobites that either lived among reefs or "reef-like calcareous habitats, or were commoning inshore clastics around Ordovician Gondwana" (Fortey, 1989, p. 106). Lesperance (1988, p. 359) noted that Early Silurian trilobites appear to be holdovers from or survivors of the mass mortality at the end of the Rawtheyan with a few new taxa. Early Silurian originations were rare, with the result that Early Silurian trilobite faunas include primarily relatively long-ranging taxa with broad environmental tolerances. Trilobites adapted to particular habitats disappeared in the Late Ordovician mass mortality, and no taxa adapted to discrete environments appeared until the latter part of the Early Silurian. Barnes and Bergstrom (1988) summarized conodont faunas in the Ordovician-Silurian boundary interval, noting striking differences between Ordovician and Silurian faunas. The position of most intense conodont faunal turnover seems to be within the Glytograptus persculptus zone, at a position about coeval with the beginnings of graptolite reradiation. The stratigraphic position coincides with the onset of rising sea-level coincident with the commencement of deglaciation. Late Ordovician conodont mass mortality "was not a sudden catastrophic event although only a few species survived into the Silurian; rather, during the Ashgill there was a gradual disappearance involving many characteristic and long- established stocks and the new taxa that appeared were considerable fewer than those that became extinct" (Barnes and Bergstrom, 1988, p. 334). Clearly, origination rates of conodont species were reduced markedly during the Late Ordovician and earliest Silurian. The extinction rate seemingly increased during the Hirnantian Stage and the early part of the G. persculptus zone (Barnes and Bergstrom, 1988, Figures 4 and 5). Demonstrably, conodonts survived the environmental changes coincident with glaciation that had so profound an impact on extinction and origination rates among other organisms. Interestingly, the environmental changes related to sea-level rises and global warming had the greatest influence on conodont mass mortality and the origination of new taxa. Late Ordovician chitinozoans are primarily taxa with long stratigraphic ranges. Most chitinozoans in Ordovician strata disappear near the end of the Rawtheyan (Grahn, 1988). Chitinozoans are rare in the Hirnantian Stage-G. persculptus zone interval. New chitinozoan taxa appear in the Early Silurian, with reradiation commencing near the base of the Silurian (Grahn, 1988). ENVIRONMENTAL-ORGANISMAL CHANGES: A SUMMARY Late Ordovician glaciation involved several significant environmental changes, including the following: a lowering and then a rising sea-level; marked deep and midocean