Environmental Changes During The Early Paleozoic

The time interval considered here is from the Early Cambrian (Waucoban) through the Early Ordovician (Arenig, Late Ibexian), comprising the Sauk Sequence of Sloss

(1963). The Cambrian period began long after the Varangerian glaciation and breakup of the supercontinent Rodinia. Global environmental shifts at this time can be related to the early diversification patterns of echinoderms and to the biosphere in general. Modeling of Cambrian ocean circulation patterns supports a global warming trend (Golonka et al. 1994). Sea levels rose, with interruptions, throughout the Cambrian-Early Ordovician, resulting in widespread and increasingly extensive inundation of cratons (James et al. 1989), potentially enhanced by isostatic and/or thermal subsidence of continental margins. Generally, configuration of shallow shelves changed from narrow belts with inner detrital, carbonate bank, and outer detrital zones to broad carbonate ramps that extended well into continental interiors (Cook 1989; James et al. 1989). Siliciclastic terrigenous sediments dominate Early to Middle Cambrian sequences, but carbonates compose the majority by the Early Ordovician (see Seslavinsky and Maidanskaya, this volume). This change probably resulted from gradual constriction of emergent sediment source areas by rising sea level. Evidence of slowed sedimentation during Late Cambrian time includes widespread glauconite formation; some Early Ordovician phosphatic-rich sediments have similar implications. Seawater chemistry also changed during this time. Carbonate deposition of the Early to Middle Cambrian appears to have been dominated by metastable aragonite, which later altered to calcite (Sandberg 1983). There is little evidence that encrusting organisms exploited lithified or firm sea floors at that time. In contrast, Late Cambrian to Early Ordovician carbonates were dominantly formed in a primary calcite cementation regime, fostering the formation of widespread hardgrounds or lithified substrates (Palmer and Palmer 1977; Wilson et al. 1992; Rozhnov 1994). These conditions offered ideal habitats for slow-growing (low-metabolic), calcite-secreting, epifaunal organisms such as echinoderms, and they were among the first skeletonized metazoans to exploit these habitats. The first really widespread encrinites, or echinoderm grain-stones, are associated with both intraformational conglomerates and cryptalgal buildups that served as substrates for hardground formation, although a few echinoderm grainstones have been reported in association with late Early Cambrian reefs ( James and Klappa 1983). Multiplated echinoderm skeletons were rapidly reduced by postmortem taphonomic processes to concentrations of durable clasts; these significantly increased the volume of sediment available for cementation (Wilson et al. 1992). Their porous construction and high-magnesium calcite composition were ideal nu-cleation sites for marine cements in the form of syntaxial overgrowths, thus leading to rapid lithification and formation of hardgrounds. This resulted in a self-perpetuating cycle whereby subsequent generations of echinoderms literally built upon the disarticulated remains of their ancestors.

Paleogeographic reconstructions of the Early Cambrian depict Laurentia, Baltica, and Kazakhstan (in part) separated from Gondwana and other continental masses (Golonka et al. 1994; Ruzhentsev and Mossakovsky 1995). Virtually all landmasses were concentrated in the Southern Hemisphere, with Laurentia and parts of Gond-

wana closest to the equator. Cratonic seas were widely distributed but covered only continental margins. These landmasses retained their identity throughout the Cambrian and Early Ordovician, and echinoderm faunas remained separate and distinctive on these continental blocks (Smith 1988; Sprinkle 1992). Baltica, Kazakhstan, and Laurentia moved slightly farther north into the tropics, then gradually converged (Golonka et al. 1994). Baltica and Laurentia collided with the closing of the Iapetus during the Middle Ordovician. The reconstructions support merging or linkage of faunal provinces for several continental blocks during the Middle to Late Ordovician, and the echinoderms reflect this greater interchange.

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