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tional environment that is similar to the change seen in coeval rocks at Dob's Linn. The Mn and Fe concentrations in south China indicate that oxic waters were present in the area during the glacial maximum. The graptolite faunas obtained from several Ordovician-Silurian boundary sequences in south China suggest that environmental conditions favored by many graptolites persisted longer in south China than in Dob's Linn in the Late Ordovician (Mu, 1988; Berry et al., 1990). Furthermore, those waters preferred as habitats by many graptolites returned more rapidly to areas in south China than they did in southern Scotland (Berry et al., 1990).

The geochemical evidence from two sites that were distant from one another in the Late Ordovician appears to be consistent with significant deep ocean circulation and ventilation during glaciation. The geochemical data are also consistent with the hypothesis that only certain graptolites lived in near-surface and relatively oxic waters, and that many others lived at some depth close to oxygen-poor but nitrogen oxide rich waters (see Berry et al., 1987). The geochemical data indicate that graptolites disappeared from areas in which they had been plentiful during glacial maximum when the oxygen-poor, nitrogen oxide-rich waters were diminished and forced upward into locations nearer the surface than in nonglacial times. Lateral spread of oxygen-poor and possibly toxin-bearing waters across shelf-marginal sites during glacioeustatic sea-level fall may have been a factor in mass mortalities among certain benthic shell dwellers as well as graptolites living relatively low in the oceanic mixed layer.

PATTERNS IN EXTINCTION AND RERADIATION

Graptolites

The graptolite extinction and reradiation pattern is linked closely to the presence and absence of richly graptoliferous black shales. Such shales both at Dob's Linn and in south China bear Fe and Mn concentrations of cluster 3 or 4 (Quinby-Hunt et al., 1989). The most abundant and richly diverse graptolite faunas occur in sediments deposited under anoxic ocean water. Analogous conditions are found today in the eastern tropical Pacific (Berry et al., 1987). There, oxygen-poor, nitrogen oxide-rich waters are inhabited by zooplankton that may be modern ecologic analogues of many ancient graptolites (Berry et al., 1987, 1990). Sulfate-reducing bacteria have been recorded from sediment accumulating under the oxygen-poor, nitrogen oxide-rich waters (Gallardo, 1963). Stratigraphic study of Ordovician-Silurian boundary sections indicates that such conditions were diminished markedly during glacial maximum (see summaries in Cocks and Rickards, 1988). Koren (1991) pointed out that the Late Ordovician graptolite mass mortality occurred at the end of the P. pacificus zone, a time when black shales disappeared from nearly all Ordovician-Silurian boundary sequences. Commenting on the Late Ordovician graptolite mass mortality, Melchin and Mitchell (1988) pointed out that the "Late Ordovician graptolites experienced nearly total extinction," and that "post-extinction morphological radiation stemmed from only a few species, most of which had the same pattern in colony development."

Graptolite faunas during glacial maximum included mostly normalograptids (i.e., small climacograptids of the C. normalis and C. miserabilis types, glyptograptids of the G. persculptus group, and Climacograptus extraordinarius) as well as Diplograptus bohemicus and a few other rare diplograptid (biserial) species. All taxa except the climacograptids of the C. normalis and C. miserabilis type and glyptograptids of the G. persculptus group became extinct prior to the appearance of those glyptograptids characteristic of the G. persculptus zone.

Rickards et al. (1977) and Rickards (1988) drew attention to graptolite development at the onset of reradiation. Reradiation commences when black shales become more widespread during G. persculptus zone time. Species diversity is low, but a number of glyptograptids and climacograptids appear. The most significant innovation among graptolites in G. persculptus zone time is the appearance of the uniserial scadent or monograptid colony (Atavograptus ceryx and similar species). The base of the superjacent Parakidograptus acuminatus zone is characterized by the appearance of a number of biserial scandent graptolites (new genera) with a slender, elongate proximal region (initial part of the colony). The sicula in these taxa is a relatively long, slender cone, as are the first few thecae or zooidal cups. Commonly, the origin of the first zooidal cup is high enough on the conical sicula so that much of the sicula is exposed. Taxa with slender initial regions spread widely as sea-level rose and platform areas flooded. Most genera and species are new. Their appearances are relatively slow, taking most of the time duration of three graptolite zones for a fauna that is relatively species rich to redevelop. The morphological innovation that is most striking among the reradiation faunas is that of the monograptid colony organization. At least three graptolite zones elapsed; however, that new style of colony development led to many new taxa.

Shelly Faunas

Brachiopods were numerous and widespread during the latter part of the Ordovician. Mass mortalities occurred among them in two phases in the Late Ordovician. The first phase took place at or near the Rawtheyan-Hirnantian Stage boundary. At that time, sea-level was falling as



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