Recovery of postLate Ordovician extinction graptolites a western North American perspective

WILLIAM B. N. BERRY Department of Geology & Geophysics, University of California, Berkeley, CA 94720, USA

Abstract: Late Ordovician to Early Silurian graptolite-bearing sequences have been recognized in three primary western North American areas: southeastern Alaska, northern Canadian Cordillera and the Great Basin (Nevada-Idaho). A spectrum of marine environments is represented in strata in these sequences, ranging from those of volcanic islands and deep oceans to those of shelves and shelf margins. During the time of glacial maximum, the time of the extraordinarius zone, sea-level lowered and oxic conditions prevailed on the ocean floor. The prominent Late Ordovician near-extinction among graptolites denotes the pacificus-extraordinarius zone boundary. Normalograptids appear to be the only taxa in the extraordinarius zone. They are the prominent taxa in the superjacent persculptus and acuminatus zones. Normalograptids are joined by a few diplograptids in the persculptus zone, and by a few more diplograptids as well as Parakidograptus and Cystograptus in the acuminatus zone. The first monograptids appear in the atavus zone in western North America. The atavus zone fauna is primarily diplograptids. Significant sea-level rise and transgression across the shelf following after deglaciation did not occur in western North America until the acinaces zone. At that time, new monograptids, including Lagarograptus, Pristiograptus and Pribylograptus, appeared as did many new diplograptid taxa. Stratigraphic occurrence data indicate that the major post near-extinction reradiation took place in western North America as shelf seas expanded and shelf sea habitats became increasingly available. That pattern of new taxa appearing during transgression continued into the gregarius zone.

In their discussions of the Late Ordovician graptolite near-extinction, both Melchin & Mitchell (1991) and Koren (1991) indicated a need to know precise patterns of graptoloid occurrences in the extinction interval on local and regional scales. Melchin & Mitchell (1991, p. 147) drew attention to different stratigraphic range patterns among Late Ordovician graptolites observed in different areas of the world. Recent field work in central Nevada by S. C. Finney and his students and by Finney and the author together has revealed new graptolite occurrences relevant to the pattern of near-extinction and reradiation after that biotic crisis in that area. The Late Ordovician biotic crisis observed in studies in the Selwyn Basin, northwestern Canada has been summarized by Wang et al. (1993). Melchin reported on graptolite occurrences before and after the near-extinction in the area covered by that study (see Melchin in Wang et al. 1993). This new information, some of it unpublished, and certain latest Ordovician-Early Silurian graptolite collections obtained from precisely measured stratigraphic sections in southeastern Alaska (Churkin & Carter 1970), southcentral Nevada (Berry in Mullens 1980), central Idaho (Dover et al. 1980), and the

Northern Canadian Cordillera (Lenz 1982) comprise the data set for this review of graptolite recovery after the Late Ordovician near-extinction from the western North American perspective. This review presents data from recent collecting and aims at a regional synthesis of the biotic recovery among graptolites seen in the context of tectonic controls on graptolite-bearing strata. Cocks & Scotese (1991) discussed global Silurian geography, indicating that the area under discussion in this review lay close to a subduction zone. That proximity to an area in which one plate was being subducted under another may be reflected in occurrences of Early Silurian graptolites in western North America.

The physical setting

The rock sequences bearing western North American Late Ordovician Early Silurian graptolites developed on or close to the margin of the Laurentian plate (Cocks & Scotese 1991; Fig. 1). That plate margin lay within a tropical, westerly-flowing open ocean current system (Wilde et al. 1991). Wilde et al. (1991) suggested that an ocean current (the North Subpolar Current of Wilde et al. 1991, figs 3 & 4) flowed south

From Hart, M. B. (ed.), 1996, Biotic Recovery from Mass Extinction Events, Geological Society Special Publication No. 102, pp. 119-126

Graptolites North America

Fig. 1. Llandovery (Early Silurian) palaeogeography. Map provided by C. R. Scotese. Dark stipple indicates probable highlands. Light stipple indicates land areas of low relief. Light areas on plates adjacent to lands were sites of shelf marine environments. Lines with teeth indicate likely subduction zones. Note proximity of Laurentian plate (North America) margin to subduction zone. Latest Ordovician into Early Silurian localities discussed are 1, southeastern Alaska; 2, Northern Canadian Cordillera; 3, the Great Basin (Nevada and Idaho).

Fig. 1. Llandovery (Early Silurian) palaeogeography. Map provided by C. R. Scotese. Dark stipple indicates probable highlands. Light stipple indicates land areas of low relief. Light areas on plates adjacent to lands were sites of shelf marine environments. Lines with teeth indicate likely subduction zones. Note proximity of Laurentian plate (North America) margin to subduction zone. Latest Ordovician into Early Silurian localities discussed are 1, southeastern Alaska; 2, Northern Canadian Cordillera; 3, the Great Basin (Nevada and Idaho).

westerly and westerly across the western side of the Laurentian plate. Such a current would have created upwelling conditions along the plate, and continental shelf margin during the Late Ordovician and Early Silurian. If upwelling did take place along the western margin of the Laurentian plate, then rock sequences that formed there under the upwelling waters should be relatively highly fossiliferous. The very species-rich Llandovery graptolite faunas described by Lenz (1982) in stratigraphie sections in the Northern Canadian Cordillera and the coeval rich graptolite faunas in the Canadian Arctic described by Melchin (1989) are consistent with accumulation of the organic-rich rocks bearing them under oceanic upwelling conditions.

Sea level changes during the Silurian were reviewed and described by Johnson et al. (1991). That review documents a significant global drop in sea-level in the Late Ordovician. That sea-level drop may be correlated with southern hemisphere glaciation on Gondwanaland. Johnson et al. (1991) indicated that sea-level rose relatively rapidly in the Early Llandovery, during the Parakidograptus acuminatus into Coronograptus cyphus zone interval. The Late Ordo-vician sea-level fall would have resulted in the presence of those waters in which most graptolites lived over the outermost part of the

Laurentian plate shelf and its slope. Sea-level rise in the Early Silurian would have expanded graptolite habitats as waters spread across shelf areas that had been exposed or had been sites of shallow marine environments.

Western North America Late Ordovician-Early Silurian graptolites lived in environments that changed significantly as a consequence of relative fall then rise of sea-level and as a consequence of the proximity of the Laurentian plate margin to a subduction zone. Preservation of the rock sequences bearing these graptolites also reflects changes resulting from sea-level fall and rise and tectonism.

Late Ordovician-Early Silurian graptolites have been found in two primary areas in the Great Basin region of the western United States. One of the two is in a black, organic-rich shale and argillite succession in central Idaho that formed close to the plate margin. The second includes two types of occurrences in rock suites in central Nevada. One of the two is in fault-bounded thrust slices. These rocks appear to have been deposited in oceanic settings at some distance from the Laurentian plate. The second type is in rocks that formed in platform environments on the plate. Coeval graptolites found in southeast Alaska seem to have lived in waters close to volcanic islands that were situated in the open ocean at a significant distance from the shelf portion of the plate. Northwest Canadian Late Ordovician-Early Silurian graptolites occur in dark shales and thinly-bedded limestones that accumulated in plate platform and slope environments.

The Great Basin

Late Ordovician and Early Silurian graptolites occur in the Phi Kappa Formation in central Idaho (Berry in Dover et al. 1980). Ross & Berry (1963) described Late Ordovician graptolites found in dark argillites and Berry (in Dover et al. 1980) identified Atavograptus atavus and possible Coronograptus cyphus zone faunas from stratigraphically higher shales.

Glyptograptus (or possibly Normalograptus) persculptus and possible Atavograptus atavus zone graptolites found in dark, organic-rich limestones in the Copenhagen Canyon succession in central Nevada were described by Berry (1986). Mullens (1980) and Murphy (1989) discussed Late Ordovician-Silurian strata in Copenhagen Canyon in the Monitor Range, central Nevada. The latest Ordovician-Early Silurian graptolite-bearing limestones there seem to have developed in relatively deep-water platform environments. Murphy (1989, fig. 128) pointed out that sandstones deposited in shallow marine environments underlie the G. persculptus zone strata. Murphy (1989) suggested that these shallow water sandstones were deposited during lowstand of sea-level related to Late Ordovician glaciation. The latest Ordovician-Early Silurian graptolites in the Monitor Range and other central Nevada localities (see Mullens 1980) occur in thinly-bedded dark grey limestones interbedded with cherts and dark bioclastic limestones that are debris flow deposits.

In the course of mapping a large area in central Nevada, the Cortez Quadrangle, Gilluly & Masursky (1965) recognized two graptolite-bearing rock suites bounded by thrusts. One of them, the Fourmile Canyon Formation, includes cherts, argillites and siltstones that bear a fauna of Early Silurian normalograptids with a possible Glyptograptus (Berry and Ross & Berry in Gilluly & Masursky 1965, table 2). The second, the Elder Sandstone, is predominantly felds-pathic sandstones and siltstones that bear monograptids suggestive of the C. cyphus zone (Berry and Ross & Berry in Gilluly & Masursky 1965, table 2).

The most impressive sequence of Ordovician graptolite faunas in the Great Basin has been recorded by Finney (Finney & Perry 1991; Finney & Ethington 1992). These faunas have come from a wide spectrum of rock types that collectively are termed the Vinini Formation in central Nevada (see Finney & Perry 1991; Finney & Ethington 1992). Finney's studies have resulted in recognition of a relatively complete succession of Ordovician graptolites in a stratigraphic section in the Roberts Mountains. Collecting in that section by Finney and Finney & Berry together since the cited publications has resulted in recognition of Paraortho-graptus pacificus zone faunas followed, in superjacent strata, by faunas of the Normalograptus extraordinarius and Glyptograptus persculptus zones. The zonal faunas are characterized by presence of the name-bearers. P. pacificus zone taxa occur in dark shales and argillites whereas the normalograptids that characterize the superjacent zones are found in calcareous shales and thin bedded limestones. Argillites and limestones in the N. extraordinarius zone bear evidence of bioturbation as well as sole markings suggestive of benthic organism activity. Vinini Formation strata occur in fault-bounded slices that were piled on top of shelf carbonates during mid-Palaeozoic tectonism. The Vinini Formation rock suite appears to have accumulated in oceanic environments at some, perhaps not too great, distance from the Laurentian plate. As Finney has pointed out (see Finney & Perry 1991; Finney & Ethington 1992), debris flows derived from shallow shelf environments occur in part of the Vinini Formation interbedded with autochthonous shales. At least some of the Vinini Formation rocks appear to have accumulated in oceanic environments on the margins of the Laurentian plate.

Northern Canadian Cordillera, northwest Canada

Latest Ordovician and Early Silurian (Llandovery) graptolites and the strata bearing them have been discussed by Lenz (1979, 1982). Lenz (1982, p. 1) stated that 'graptolite-bearing strata encompassed at least in part within the Road River Formation are widespread throughout the northern Canadian Cordillera'. The lithologic aspects of the Road River Formation, Lenz (1979, p. 141) pointed out, 'vary greatly, ranging from almost entirely chert with a few shale interbeds to almost totally dark shale, with minor chert. In still other areas, the unit consists of interbedded shales and thin band of evenly bedded limestones'. Environments of deposition of the strata included within the Road River Formation appear to range from those of the Laurentian plate continental slope to those on the continental shelf. In some areas, the Road River Formation strata pass laterally into carbonates that accumulated in shallow marine environments (Lenz 1979, p. 141). Lenz (1982, p. 3) noted that although the G. persculptus zone has not been recognized with certainty in the northern Canadian Cordillera, no obvious stratigraphie discontinuity has been seen between strata bearing P. acuminatus zone faunas and those bearing P. pacificus zone faunas. As described by Lenz (1979, 1982) this interval between Late Ordovician and Early Silurian graptolite-bearing strata appears to be relatively thin and indicative of a condensed stratigraphie section. The condensed strata succession suggests low stand of sea level at the time of accumulation. Recently, Melchin (in Wang et al. 1993, p. 1879) cited the occurrence of Normalo-graptus cf. N. persculptus, N. normalis, Glypto-graptusl laciniosus and Glyplograptus n. sp.? in strata 1 m above those bearing Dicellograptus sp. aif. D. minor in Road River Formation shales exposed in the Selwyn Basin, northwest Canada. The N. extraordinarius zone position is represented by about 0.5 m of strata in this strati-graphic section. This condensation of the stratigraphie thickness in this interval is consistent with a lowered sea-level at the time of the N. extraordinarius zone. Wang et al. (1993) indicated that the depositional environment at the time was relatively shallower than it had been previously and they suggested that Cerium concentrations in this stratigraphie interval indicated 'a short period of basin ventilation in the otherwise anoxic Selwyn Basin'. They (Wang et al. 1993) discussed other geochemical features of the same stratigraphie interval, including changes in the concentration of C13.

Lenz (1982) cited the stratigraphie occurrences of all Llandovery graptolites found in the northern Canadian Cordillera and described many of the taxa. He (Lenz 1979,1982) reviewed the zonal distributions of the Llandovery graptolites in the area. His data indicate that this succession of Llandovery graptolites is one of, if not the, richest in terms of number of species in the world. The rocks bearing the richest of the Early Llandovery faunas were deposited in a suite of shelf environments that were spread widely across a margin of the Laurentian plate.

Southeastern Alaska

Churkin & Carter (1970) reviewed latest Ordo-vician-Early Silurian graptolites and their strati-graphic positions in southeastern Alaska. The faunas occur in the Descon Formation. Churkin & Carter (1970) indicated that the Descon Formation is composed of coarse greywackes, conglomerates, some black chert and siliceous shale and basaltic volcanic rocks. The volcanics include pillow basalts and flow breccias that are, at least locally, interbedded with graptolite-bearing shales. The graptolite faunas indicate that 'repeated volcanism occurred from different centres during Early Ordovician through Early Silurian time' (Churkin & Carter 1970, p. 3). Clearly, the depositional environments in the area of accumulation of these graptolite-bearing strata were oceanic and in the environs of long-term, although sporadic, volcanism. The prominence of graptolite taxa closely similar to those found in coeval strata in the Great Basin and the northern Canadian Cordillera suggests that the volcanism took place in oceanic settings relatively close to the Laurentian plate margin. Churkin & Carter (1970, fig. 4) indicated that volcanic flow rocks intervene in strata bearing Late Ordovician, P. pacificus zone, taxa and those bearing P. acuminatus zone taxa.

Latest Ordovician into Early Silurian graptolite-bearing strata in the northwestern part of North America accumulated in a spectrum of plate margin (shelf seas and slope) environments as well as in a variety of open ocean basinal settings. The range in depositional environments may be broader than in other areas of comparable size and age.

The evidence from the Great Basin and northern Canadian Cordillera indicates that sea-level fell and the sea floor was ventilated coincident with the most prominent extinction of Late Ordovician graptolites. That extinction marks the top of the P. pacificus zone. The Selwyn Basin and Copenhagen Canyon localities provide data indicating that the time of maximum sea-level lowering and basinal ventilation was that of the N. extraordinarius zone. In Copenhagen Canyon, strata subjacent to those identified by Murphy (1989, fig. 128) as having accumulated at the time of glaciation, bear P. pacificus zone taxa, including P. pacificus, in abundance. This collection was made by the author subsequent to the collection of N. (or G.) persculptus zone taxa from strata superjacent to those deposited during glaciation (Berry 1986). Vinini Formation N. extraordinarius zone strata show evidence of marine benthic activity whereas subjacent strata, those bearing P. pacificus zone taxa, do not. Wang et al. (1993) discussed the consequence of ocean ventilation at the same time interval.

Volcanic activity took place in southeastern Alaska in the latest Ordovician-Earliest Silurian (Churkin & Carter 1970). Structural evidence in the Phi Kappa Formation latest Ordovician-earliest Silurian strata suggests possible tecton-ism in the depositional environment at that time.

The physical evidence from latest Ordovician-earliest Silurian rocks in the northwestern part of North America reveals not only changes in sea-level and ocean chemistry but also tectonism potentially linked to plate motions. That plate motion-related tectonism may be suggestive of proximity to a subduction zone as indicated in Cocks & Scotese (1991; Fig. 1). The changes in sea-level, ocean ventilation and ocean chemistry altered marine habitats in the latest Ordovician. The western North American graptolite occurrence data indicate that these physical environmental changes were linked to major graptolite extinctions in the Late Ordovician. Graptolite recovery from that extinction was linked to rising sea-level and re-establishment of many habitats over the continental shelf and shelf margin during the early part of the Silurian. That recovery may be seen in review of the prominent developments among graptolites collected from stratigraphic sections in northwestern North America, especially those from the Road River Formation.

Post-P. pacificus zone extinction graptolite recovery

Post-P. pacificus zone graptolite occurrences (Fig. 2) are seen primarily in stratigraphic sections in southeastern Alaska (Churkin & Carter 1970) and the northern Canadian Cordillera (Lenz 1982). N. extraordinarius zone graptolites have been found only in the Vinini Formation, and persculptus zone faunas have been recovered from three sites. Persculptus zone taxa are known in the Selwyn Basin, Canadian Cordillera (Melchin in Wang et al. 1993), and Copenhagen Canyon sequence and the Vinini Formation, both in Nevada.

N. extraordinarius zone

N. extraor dinar ius zone taxa include only normalograptids. Although the zone is characterized by the presence of the name-bearer, N. miserabilis is more common.

N. persculptus zone

The presence of the name-bearer permits recognition of the zone. The zonal fauna includes almost entirely normalograptids. The relationships of Glyptograptus laciniosus Churkin & Carter (1970) are uncertain as are small specimens of Glyptograptus ? sp. found in the zone. Possibly, glyptograptid species join normalograptids in the zonal fauna.

Graptolites North America

Fig. 2. Diagrammatic sketches of selected latest Ordovician-Early Silurian graptolites illustrating general features of colony form among post-Late Ordovician extinction graptolites. A, B, Normalograp-tus extraordinarius; C, Normalograptus miserabilis; D, Normalograptus normalis; E, Glyptograptus laciniosus; F, Glyptograptus ? persculptus group form; G, Para-kidograptus acuminatus; H, Atavograptus atavus; I, Dimorphograptus confertus swanstoni; J, Coronograptus gregarius; K, Pristiograptus fragilis; L, Lagarograptus acinaces. All except L are xl.6; L is x3.3.

Fig. 2. Diagrammatic sketches of selected latest Ordovician-Early Silurian graptolites illustrating general features of colony form among post-Late Ordovician extinction graptolites. A, B, Normalograp-tus extraordinarius; C, Normalograptus miserabilis; D, Normalograptus normalis; E, Glyptograptus laciniosus; F, Glyptograptus ? persculptus group form; G, Para-kidograptus acuminatus; H, Atavograptus atavus; I, Dimorphograptus confertus swanstoni; J, Coronograptus gregarius; K, Pristiograptus fragilis; L, Lagarograptus acinaces. All except L are xl.6; L is x3.3.

Parakidograptus acuminatus zone

The P. acuminatus zone is denoted by the appearance of the name-bearer in an association rich in normalograptids, glyptograptids and diplograptids. Lenz (1982) cited more than 20 taxa from only a few localities in the northern Canadian Cordillera. Cystograptus vesiculosus and Orthograptus eberleini appear for the first time in the zone. The zonal fauna is represented in southeast Alaska by P. acuminatus, Normalograptus triflis and other normalograptids and a possible Orthograptus (Churkin & Carter 1979). The species and numerical richness of the zonal fauna are seen in the platform localities in the northern Canadian Cordillera. Although both Parakidograptus and Cystograptus are new phyletic elements of the zone fauna, the fauna is numerically predominantly species in the

Table 1. Prominent developments among western North American latest Ordovician-Early Silurian graptolites

Zone

Prominent features

Gregarius Appearance of coronograptids and monograptids with lobate thecae

Acinaces Influx of diverse monograptids - Lagarograptus, Pristiograptus, Pribylograptus.

Metaclimacograptus appears Atavus Appearance of first monograptids, Atavograptus, and of Dimorphograptids.

Primarily diplograptid fauna Acuminatus Primarily diplograptid fauna.

Normalograptids prominent. Persculptus Normalograptids, some glyptograptids

Extraordinarius Normalograptids

Extinctions

Pacificus

Paraorthograptus pacificus, Dicellograptus ornatus group.

Orthograptids of the O. truncatus group, climacograptids ? of the hastatus and supernus groups, Arachniograptus.

genera Normalograptus, Glyptograptus and Dip-lograptus.

Atavograptus atavus zone

Rich Atavograptus atavus zone faunas have been recovered from only a few localities in the Road River Formation in the northern Canadian Cordillera. The number of species and the prominence of diplograptids in the fauna remains about the same as in the P. acuminatus zone. The new feature is, of course, the appearance of the first monograptids in the western North American successions. Three species of Atavograptus mark the introduction of the monograptids in the succession where they are joined by the first species of Dimorphograptus to appear. Atavograptus occurs not only in the platform succession but also in the rocks deposited in open ocean settings. Species of Normalograptus, Glyptograptus and Diplograptus remain the numerically abundant taxa.

Lagarograptus acinaces zone

The zonal fauna is characterized by a marked increase in new taxa of monograptids. In addition to Lagarograptus, species of Pristiograptus and Pribylograptus appear in both platform and oceanic depositional settings. Metaclimacograptus makes its initial appearance in the western North American successions as do new species of Glyptograptus and Dimorphograptus. Although collected from only a few western North American localities, the fauna of the zone is characterized by at least forty-five taxa of which about half are new to the succession. The prominent features of the zone fauna are the new monograptid genera and speciations among glyptograptids. Normalograptid prominence wanes in the zone. The zone fauna has been found in more platform environments than the faunas of the prior post-extinction zones.

Coronograptus gregarius zone

The C. gregarius fauna is recognized by the sudden appearance of a new monograptid rhabdosome form, that of the curved coronograptids and of the proximally curved M. revolutus. Proximal thecae in the latter species are hooked or lobate. Species of diverse monograptids become numerically more prominent in the C. gregarius zone and the diplograptids are significantly less prominent. Both normalograptids and glyptograptids occur but in less abundance than in prior post-extinction zones. By this, the last zone within the Early Llandovery, the monograptid diversification is in full development and diplograptids have waned. The number of taxa in the zone fauna seen in western North America remains about the same as in the L. acinaces zone. The collections bearing the greatest number of species are those in shales formed in platform environments.

Major features of post-/*. pacificus zone recovery

Collecting from a spectrum of ocean basin and shelf environments has resulted in certain general conclusions (Table 1). One of them involves a difference between occurrences in strata deposited in ocean basins and those deposited on the Laurentian plate continental shelf margin. Even though the number of specimens of a given taxon may be the same in collections from basin and platform strata, the number of taxa is far richer in the platform rock suites than in ocean basin strata. The collections in which the number of different taxa present is great and the number of specimens of each taxon present is high came from strata that formed close to the platform margin. The number of taxa present in ocean basin strata may be enhanced by the presence of gravity flow rocks in the basinal suites. Gravity flow rocks may bear numbers of different taxa that have been swept along by the flow from shelf environments to be deposited in the basin.

The marked radiations among the postextinction faunas are seen in strata that developed at the time of post-glaciation sea-level rise and resultant spread across the shelf margin. Only a few taxa were present before sea-levels appear to have begun to rise significantly. Those taxa are mostly normalograptids.

The occurrences of atavograptids on the Avalonian plate two zones prior to occurrences of atavograptids in western North America suggest that a period of about 3-4 Ma may have been necessary for the atavograptids to be transported around the Laurentian plate to western North America.

Comparison of the rich Llandovery graptolite faunas described in the Cape Phillips Formation in the Canadian Arctic by Melchin (1989) with those in western North America reveals marked similarities among them. The Arctic faunas include a number of taxa described from Siberia that have not been found in western North America. Potentially, the Canadian Arctic part of the Laurentian plate lay more directly in the path of ocean currents that flowed westerly past the Siberian plate towards the Laurentian.

Post-extinction recovery was marked by developments among normalograptids initially. Diverse glyptograptids and possibly certain species of Diplograptus appeared. Diplograptid faunas developed significantly. They were joined first by Parakidograptus and Cystograptus, and secondly by the first monograptids, atavograptids. Not until the L. acinaces zone did mono-graptid diversification appear. At the same time, however, the diplograptid fauna remained prominent. With the appearance of the curved and proximally coiled monograptids in the C. gregarius zone where they joined monograptids continuing from the A. atavus and L. acinaces zones, the monograptid diversification was well developed. At that time, the diplograptids that had been prominent in the post-extinction faunas began to disappear. The first steps in post-extinction recovery were among normalo graptids and other diplograptids. Not until monograptids entered the western North American Llandovery environments and became diversified in them did they become more prominent than the diplograptids.

Normalograptids demonstrably tolerated a wide range of environmental conditions for they have been found in rock suites that formed under a range of shelf and ocean basin settings. They even occur in rocks in the Vinini Formation in which benthic organisms left traces of their activities, suggesting that the rhabdosomes found were not food for some benthic organisms. Graptolites are seen to become more diverse when those depositional environments returned that led to anoxic bottom conditions and the preservation in dark, organic rich mud rocks.

The patterns in post-extinction recovery appear to reflect changes in oceanic habitats following deglaciation and establishment of a well-developed oxygen-poor water mass that moved with rising sea-level across the outer part of the Laurentian plate shelf. Environmental changes at the time of maximum or near-maximum glaciation seem to have so altered habitats in which graptolites lived that they nearly became extinct. Wilde et al. (1990) suggested certain mechanisms that could result in extinctions among organisms that result from vertical advection of toxic waters from the main ocean pycnocline. Upward advection of toxic waters could have been a significant influence on Late Ordovician graptolite extinctions. Possibly only those taxa living highest in the oceanic water column survived for they lived above that part of the ocean into which toxic waters were advected. Wang et al. (1993) also drew attention to ocean circulation and chemical changes in oceanic habitats as factors influencing Late Ordovician graptolite extinctions.

Conclusion

Western North America post-Late Ordovician graptolite near-extinction reflects ocean ventilation and chemical changes in graptolite habitats at the time of glaciation. Tectonism at the Laurentian plate margin influenced accumulation and preservation of the rock suites that bear latest Ordovician-Early Silurian graptolites. The presence of several taxa that seem to be endemic to the area (many of these are diplograptids described initially by Churkin & Carter 1970) suggests that the area was relatively isolate from major ocean circulation systems until about the Atavus or Acinaces zones. Changes in ocean circulation following from lowered sea-level and plate margin tectonism both may have contributed to regional endemism. The western North American regional pattern of graptolite recovery after the post-Late Ordovician near-extinction differs in certain features (primarily presence or absence of certain species in some zones) from that seen in other regions. Inter-regional syntheses are needed to understand the post-near extinction pattern of recovery more fully.

References

Berry, W. B. N. 1986. Stratigraphic significance of Glyptograptus persculptus group graptolites in central Nevada, U.S.A. In: Hughes, C. P. & Rickards, R. B. (eds) Palaeoecology and Bio-stratigraphy of Graptolites. Geological Society, London, Special Publication, 20, 135-143 Churkin, M. Jr & Carter, C. 1970. Early Silurian Graptolites from southeastern Alaska and their correlation with graptolitic sequences in North America and the Arctic. United States Geological Survey, Professional Paper, 653. Cocks, L. R. M. & Scotese, C. R. 1991. The Global Biogeography of the Silurian Period. In: Bassett, M. G., Lane, P. D. & Edwards, D. (eds) The Murchison Symposium. Proceedings of an International Conference on the Silurian System. Special Papers in Palaeontology, 44, 109-122. Dover, J. H., Berry, W. B. N. & Ross, R. J. Jr 1980. Ordovician and Silurian Phi Kappa and Trail Creek Formations, Pioneer Mountains, Central Idaho -Stratigraphic and structural revisions, and new data on graptolite faunas. United States Geological Survey, Professional Paper, 1090. Finney, S. C. & Ethington, R. L. 1992. White-rockian graptolites and conodonts from the Vinini Formation, Nevada. In: Webby, B. D. & Laurie, J. R. (eds) Global Perspectives on Ordovician Geology. Balkema, Rotterdam, 153-162.

-& Perry, B. D. 1991. Depositional setting and paleogeography of Ordovician Vinini Formation, central Nevada. In: Cooper, J. D. & Stevens, C. H. (eds) Paleozoic Paleogeography of the Western United States - II. Pacific Section Society of Economic Paleontologists and Mineralogists, 67, 747-766.

Gilluly, J. & Masursky, H. 1965. Geology of the Cortez Quadrangle, Nevada. United States Geological Survey, Bulletin, 1175. Johnson, M. E., Kaljo, D. & Rong, J.-Y. 1991. Silurian Eustasy. In: Bassett, M. G., Lane, P. D. & Edwards, D. (eds) The Murchison Symposium. Proceedings of an International Conference on the

Silurian System. Special Papers in Palaeontology, 44, 145-163.

Koren, T. N. 1991. Evolutionary crisis of the Ashgill graptolites. In: Barnes, C. R. & Williams, S. H. (eds) Advances in Ordovician Geology. Geological Survey of Canada, paper 90-9, 157-164.

Lenz, A. C. 1979. Llandoverian graptolite zonation in the Northern Canadian Cordillera. Acta Palaeon-tologica Polonica. 24, 137-153.

- 1982. Llandoverian graptolites of the Northern

Canadian Cordillera: Petalograptus, Cephalograp-tus, Rhaphidograptus, Dimorphograptus, Retio-litidae, and Monograptidae. Life Sciences Contributions, Royal Ontario Museum, 130.

Melchin, M. J. 1989. Llandovery graptolite biostrati-graphy and paleobiogeography, Cape Phillips Formation, Canadian Arctic Islands. Canadian Journal of Earth Sciences, 26, 1726-1746.

- & Mitchell, C. E. 1991. Late Ordovician extinction in the Graptoloidea. In: Barnes, C. R. & Williams, S. H. (eds) Advances in Ordovician Geology. Geological Survey of Canada, paper 90-9, 143-156.

Mullens, T. E. 1980. Stratigraphy, petrology, and some fossil data of the Roberts Mountains Formation, North-Central Nevada. United States Geological Survey, Professional Paper, 1063.

Murphy, M. A. 1989. Central Nevada. In: Holland,

C. H. & Bassett, M. G. (eds) A Global Standard for the Silurian System. National Musem of Wales, Geological Series, 9, 171-177.

Ross, R. J. Jr & Berry, W. B. N. 1963. Ordovician graptolites of the Basin Ranges in California, Nevada, Utah and Idaho. United States Geological Survey, Bulletin, 1134.

Wang, K., Chatterton, B. D. E., Attrep, M. Jr & Orth, C. J. 1993. Late Ordovician mass extinction in the Selwyn Basin, northwestern Canada: geochemical, sedimentological, and paleontologi-cal evidence. Canadian Journal of Earth Sciences, 30, 1870-1880.

Wilde, P., Berry, W. B. N. & Quinby-Hunt, M. S. 1991. Silurian oceanic and atmospheric circulation and chemistry. In: Bassett, M. G., Lane, P.

D. & Edwards, D. (eds) The Murchison Symposium. Proceedings of an International Conference on the Silurian System. Special Papers in Palaeontology, 44, 123-143.

1990. Vertical advection from oxic or anoxic water from the main pycnocline as a cause of rapid extinction or rapid radiations. In: Kauff-man, E. G. & Walliser, O. H. (eds) Extinction Events in Earth History. Springer, Berlin, Lecture Notes in Earth Sciences, 85-98.

Was this article helpful?

0 0

Post a comment