Figure 8.1 Pattern of diversity through the Cambrian-Tremadoc. A, Diversity curve for metazoan genera (stippled area shows archaeocyath diversity). B, Diversity curve for calcimi-crobe genera. C, Diversity curve for acritarch species. D, Plot of total trace fossil diversity (modified after Crimes 1992a, 1994). E, Phos phorite abundance curve (modified after Cook 1992). F, 87Sr/86Sr plot (compiled from Donnelly et al. 1990; Derry et al. 1994; Saltzman et al. 1995; Montanez et al. 1996; Nicholas 1996). NEM = Nemakit-Daldynian; D = Dat-sonian; T = Tremadoc.

to the early Botoman (Bowring et al. 1993; Shergold 1995; Landing and Westrop 1997). This is short relative to subsequent Phanerozoic radiations, and the per taxon rate of diversification was much higher (Sepkoski 1992).

The general intensity of extinction in the oceans has declined through the Phan-erozoic (Sepkoski 1994). Cambrian intensities are quite high. Detailed field biostra-tigraphy resolves some of this into three extinction events during the Early Cambrian and four extinction events during the Middle-Late Cambrian, including that at the former Cambrian-Ordovician boundary (Saukia-Missisquoia boundary) (Palmer 1965, 1979; Stitt 1971, 1975; Brasier 1991, 1995a; Zhuravlev and Wood 1996). The latter were recognized first by Palmer (1965, 1979), who called them biomere extinctions.

Quantitative analysis of global generic diversity reveals striking changes through the Cambrian. If extinction rates are plotted separately, they exhibit no additional characteristics (Zhuravlev and Wood 1996: figure 1). First, diversity decline occurs in the mid-Tommotian (Brasier 1991). However, the scale of this extinction is likely, in part, to reflect taxonomic oversplitting of scleritome taxa. More striking are two further extinction events noted in the mid-Early Cambrian: in the middle and late Botoman. The later of these events was predicted by selected data (Bognibova and Shcheg-lov 1970; Newell 1972; Burrett and Richardson 1978; Sepkoski 1992; Signor 1992a; Brasier 1995a) and is related to the well-known Hawke Bay Regression (Palmer and James 1979) or to the "Olenellid biomere event" that affected trilobites at about that time (A. Palmer 1982). Ecologic Evolutionary Unit I of Boucot (1983) was terminated by this extinction (Sheehan 1991). A more pronounced extinction occurred in the middle Botoman (approximately at the micmacciformis/Erbiella—gurarii zone boundary) and has been named the Sinsk event (Zhuravlev and Wood 1996). It was responsible for a major disturbance of the Early Cambrian Biota, after which many groups composing the Tommotian Fauna either disappeared or became insignificant. Metazoans attained their highest generic diversity of the Cambrian during the early Botoman, and contrary to Sepkoski's (1992) calculation, this was not exceeded until the Arenig. Archaeocyaths were not the principal group contributing to this pattern (figure 8.1A). At the generic level, they compose only 24 percent rather than about 50 percent (contra Sepkoski 1992) of total early Botoman generic diversity and 18 percent of extinct genera. These differences in data may be explained by the coarser strati-graphic scale and the smaller database that were used by Sepkoski (1992). In comparison, trilobite genera contribute 27 percent and 16 percent, respectively. This decline is well expressed at the species level on all major continents and terranes of the Cambrian world (Zhuravlev and Wood 1996: figure 2). Calcified cyanobacteria (31 genera) and acritarchs (470 species) show a similar decline in diversity (figures 8.1B,C). A slight fall in trace fossil diversity is observed during the Middle and Late Cambrian (figure 8.1D), followed by a steady rise through the Ordovician, resulting from an increase in deep-water trace fossil diversity (Crimes 1992a); the levels of Early Cambrian diversity were not reached again until the Early Ordovician (Crimes 1994). In general outline, this pattern resembles the diversification of body fossils across the same interval.

Four extinction events during the Middle-Late Cambrian are confirmed by global data but are most pronounced among trilobites (figure 8.1A). However, the latest of them affected cephalopods and rostroconchs too. Both rostroconchs and cephalopods produced their first diversification peak in the Datsonian (Pojeta 1979; Chen and Teichert 1983).

The dynamics of three additional indices is quantified for the Nemakit-Daldynian-early Tremadoc interval. These are (1) average monotypic taxa index (MTI), (2) average geographic distribution index (AGI), and (3) average longevity index (ALI). These are calculated for genera in each zone (Zhuravlev and Riding, this volume: tables 1.1 and 1.2, Arabic numerals; and figures 8.2A-C herein). Initially, average indices were determined for each taxonomic group separately. Then average indices were counted for each of the following biotas: Tommotian Biota (anabaritids, sabelliditids, coelo-scleritophorans, helcionelloids, orthothecimorph hyoliths, and minor problematic sclerital groups); Early Cambrian Biota (archaeocyath sponges, radiocyaths, cribri-cyaths, coralomorphs, paragastropods, hyolithomorph hyoliths, bradoriids, anomalo-caridids, tommotiids, hyolithelminths, cambroclaves, mobergellans, coleolids, para-carinachitiids, salterellids, and stenothecoids); Middle-Late Cambrian Biota (trilo-bites, lingulates, calciates, echinoderms, and lightly skeletonized arthropods); and combined Paleozoic-Modern Biota (rostroconchs, cephalopods, gastropods, tergo-myans, polyplacophorans, pterobranchs, graptolites, paraconodonts, and eucono-donts). These biotas display broadly congruent fluctuations of the indices for most of the Cambrian. Principal deviations from this common pattern will be emphasized below.

The last two indices usually display a similar coherent pattern because the wider the spectrum of conditions under which a genus is able to survive, the wider is its area and the longer it exists (Markov and Naimark 1995; Markov and Solov'ev 1995). AGI is calculated as follows. An appearance of a genus on a single craton is accepted arbitrarily as 1 unit; an appearance of genus in several regions of the same province is scored as 5 units; a global distribution is scored as 10 units. (As has been shown by Markov and Naimark [1995], the change of unit value does not influence the general pattern of the geographic distribution.) The Early Cambrian provinces are confined to Avalonia, Baltica, Laurentia (including Occidentalia), East Gondwana (Australia-Antarctica, China, Mongolia-Tuva, Kazakhstan), West Gondwana (southern and cen-

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