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Effects of Past Global Change on Life
Although many glaciations have occurred during the history of the Earth, the glaciation near the end of the Ordovician stands out because the environmental changes that took place then were accompanied by near extinction of a great number of organisms (Berry and Boucot, 1973; Sheehan, 1973). A massive continental ice cap centered near the South Pole covered a large part of a continent that may be called Gondwanaland for about 2 million years (m.y.) about 435 to 437 m.y. ago (Ma). During that time of maximum glaciation, sea-level fell at least 50 m and perhaps as much as 100 m. The rock record suggests that sea-level fall may have been slower than its rise following glacial melting. These glacioeustatic sea-level changes created significant changes in marine environments. Extinctions among most marine organisms living at the time appear to have been related to the environmental changes.
The features typical of continental ice sheets, including glacial pavements, striated pebbles, esker-like ridges, and glacial ice-carried dropstones have been described by Beuf et al. (1971) and Rognon et al. (1972) in Saharan Africa and Saudi Arabia (see summaries in Brenchley, 1988; Vaslet, 1990). Evidence of glaciation appears to have extended as far north from the South Pole as about 40° of south latitude (Brenchley and Newall, 1984; Brenchley, 1988). Tilloids of glaciomarine origin have been recorded from many localities in Spain, Portugal, and France (Brenchley, 1988). The presence of large dropstones that deform finely spaced sediment laminae in deposits in Spain and France indicates that large icebergs floated many miles from the shores of Gondwanaland. The spread of continental ice and the icebergs that calved off from it are analogous to the development of ice and icebergs during Pleistocene glacial maximum (Brenchley and Newall, 1984). Brenchley et al. (1991) described Late Ordovician glaciomarine diamictites in stratal sequences in Portugal and in the Prague Basin, Czechoslovakia. They concluded that "the sequence in the Prague Basin suggests that cold climates with floating marine ice developed early in the Hirnantian, before the main glacioeustatic regression, whereas in Portugal, deposition from marine ice was somewhat later and postdated the regression."
Areas that had been sites of shallow marine deposition became lands into which rivers cut channels and across which sands spread in sites of nearby terrigenous materials or karst topography formed on lime rock deposits in tropical areas (Brenchley and Newall, 1984). The rock record suggests that the features formed during glacial maximum and lowstand of sea were covered rapidly as sea-level rose during glacial melting. Postglacial environmental changes seemingly were rapid (Brenchley, 1988).
Cocks and Rickards (1988) assembled an extensive body of data concerning environmental and faunal changes in the Ordovician-Silurian boundary interval. Those summaries, arranged by region and faunal group, provide precise information on environmental changes, near extinctions, and radiations of organisms in the postglacial interval. Brenchley and Newall (1984) summarized the evidence for sea-level changes and related marine environmental changes during and after glaciation. Detailed summaries of changes in specific groups of organisms during the interval of Late Ordovician faunal change may be found in Rickards et al. (1977), Briggs et al. (1988), Fortey (1989), and Sheehan and Coorough (1990). Summaries in Barnes and Williams (1991) enhance the faunal and stratigraphic data for the Late Ordovician glacial interval. These several summaries and the data on which they are based provide the information essential to this overview of faunal changes in the Late Ordovician glacial-postglacial sequence of environmental changes.
THE TIME FRAME
Six primary divisions, termed Series, have been recognized within the Ordovician Period. Each of the Series is typified by a unique fauna. The youngest Series, the Ashgill, has been divided into four stages based on associations of brachiopods and some trilobites. The youngest Ashgill division or stage, the Hirnantian, is succeeded by the earliest Silurian stage, the Rhuddanian (see Table 2.1). The stages are characterized by fossil faunas that were benthic in life and lived in shelf sea environments. Remains of planktic organisms are rare, with certain exceptions, among the shelly faunas. Graptolites—the remains of colonial, marine plankton—occur in abundance in sequences of thinly laminated black shales and mudstones in which shelly fossils do not occur, except in debris materials derived from environments in which shelly fossils live. Accordingly, graptolite zones have been recognized as divisions of the Ordovician in black shale sequences. Graptolite zone divisions of the Ashgill are shown in Table 2.1. Time synchronous correlation of the graptolite zones with the shelly fossil stages is imprecise because the occurrence of the two types of organisms in the same deposit is so rare. Two sets of graptolite zones are shown for the early part of the Ashgill because graptolites of that time interval were distributed in two faunal provinces.
The stages typified by brachiopod-trilobite associations and the graptolite zones provide a temporal context for analysis of patterns in mass mortality and reradiation. Glacial maximum occurred during the Hirnantian Stage, although glaciation probably commenced early in the Ashgill.