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report by Finney et al. (1999) in which data from all aspects of the collaborative study are summarized and interpreted. SECTIONS AND COLLECTIONS.
J. Paleont., 74(6), 2000, pp. 1148–1160 Copyright q 2000, The Paleontological Society 0022-3360/00/0074-1148$03.00

CONODONTS AND BIOSTRATIGRAPHY OF UPPER ORDOVICIAN STRATA ALONG A SHELF TO BASIN TRANSECT IN CENTRAL NEVADA WALTER C. SWEET Department of Geological Sciences, The Ohio State University, Columbus, Ohio 43210

ABSTRACT—Conodonts representing 38 species of 26 genera have been identified in samples from Upper Ordovician rocks at three central Nevada localities. Ranges of these species and associated graptolites are used graphically to determine correlation of the strata considered with an evolving composite standard that includes information from Ordovician strata at more than 100 localities in North America. Results indicate that the Hanson Creek Formation at Lone Mountain is latest Edenian through mid-Richmondian in age; that the Ordovician part of the Hanson Creek in the Monitor Range section spans an interval from Maysvillian through Richmondian; and that the upper 29 m of the Vinini Formation at the Vinini Creek locality is of mid-Maysvillian to late Richmondian age. Physical discontinuities in the Ordovician-Silurian boundary interval complicate correlations, but it is now clear that conodonts that range upward into, and have long been considered distinctive of the Lower Silurian, make their debut in central Nevada in an upper segment of the Upper Ordovician Normalograptus persculptus graptolite zone that may be latest Richmondian in age.

INTRODUCTION

with a comprehensive team study of Upper Ordovician and Lower Silurian rocks, I have collected conodonts from stratigraphic sections at three localities in central Nevada (Fig. 1). Studies of these fossils, combined with information from other specialists on graptolites, radiolarians, chitinozoans, acritarchs, and carbon isotopes from the same sections, have enabled synthesis of a detailed history for an important Paleozoic interval. In this report, I deal primarily with the conodonts collected from the central Nevada sections and suggest how their distribution might be used in the correlation and interpretation of those sections. Additional information on the lithic and carbon-isotope stratigraphies, and on the nature, distribution and significance of other groups of fossils collected from the same sections, is in a recent report by Finney et al. (1999) in which data from all aspects of the collaborative study are summarized and interpreted.

I

N CONNECTION

SECTIONS AND COLLECTIONS

The conodonts considered here are from stratigraphic sections at three localities in central Nevada. Location of the sections is indicated in Figure 1, and generalized columns showing the lithic and faunal sequence for each section are provided in Figures 2– 4. The following remarks consider the sections along a hypothetical shelf to basin transect. Lone Mountain section.(W1/2 unsurveyed sec. 19, T20N, R51E, Bartine Ranch, Nevada, 15-minute quadrangle.) Disconformably above the Eureka Quartzite, the Lone Mountain section (Fig. 2) exposes a succession of dolomitized, shallow-marine carbonates referred to the Hanson Creek Formation. From 18 large samples between 0 m and 79 m, Leatham (1987) extracted sizeable collections of elements that represent a variety of distinctive Late Ordovician conodont species. About 79 m above the top of the Eureka Quartzite, the conodont-bearing carbonate sequence is interrupted by a bed of cross-stratified quartz sandstone, which overlies an irregular surface cut into the subjacent carbonates and grades upward into sandy, pervasively silicified dolostones from which Leatham collected two samples that yielded a few Early Silurian conodonts. The sandstone marks the base of the Roberts Mountains Formation. Overall ranges of the 26 conodont species represented in Lone Mountain samples are charted in Figure 2. With the exception of Amorphognathus ordovicicus Branson and Mehl, Protopanderodus insculptus (Branson and Mehl), Pseudooneotodus mitratus (Moskalenko), Pseudooneotodus beckmanni (Bischoff and Sannemann), and Scabbardella altipes (Henningsmoen), which are only sparsely represented, collections are dominated by elements

of Pseudobelodina, Panderodus, Belodina, and Oulodus, all characteristic of Late Ordovician faunas of the Red River Province (Sweet, 1979; Sweet and Bergstro¨m, 1984). Rocks deposited in this broad province are dominantly burrow-mottled dolomites and other carbonates that indicate deposition on a wide, shallow shelf. Walliserodus amplissimus (Serpagli), which is represented in some abundance in Lone Mountain collections, has also been reported from Upper Ordovician rocks in the Red River Province at Avalanche Lake, in the Mackenzie Mountains (Northwest Territories) (Nowlan et al. 1988); in an outlier north of Aberdeen Lake, in the District of Keewatin (Bolton and Nowlan, l979); and from the Road River Group of the northern Yukon Territory (McCracken, 1987). Monitor Range section.(S1/2 sec. 36, T16N, R49 E, Horse Heaven Mountain, Nevada, 7.5-minute quadrangle.) Some 30 mi directly south of the Lone Mountain section, on opposite sides of Copenhagen Canyon in the Monitor Range (COP and MR on Fig. 1; Fig. 3), the Eureka Quartzite is succeeded disconformably by some 240 m of Ordovician and Silurian carbonates. In this section, the Monitor Range section of Finney et al. (1999), the lower 125 m is gray to brownish gray, thin-bedded lime mudstone thought by Dunham (1977) to have been deposited in an embayment along the platform margin. Between 125 and 146 m above the base, limestone beds are thicker than below, contain local concentrations of disarticulated megafossils (primarily trilobites), and exhibit at least one major interval of convolute bedding. Taken together, these features suggest a fairly abrupt shallowing of the depositional site. Although darker and shalier carbonates between 146 and 152 m may indicate a brief episode of deepening, the presence of thick-bedded, burrow-mottled, largely unfossiliferous dolomitic carbonates to 187 m suggests a period of substantial shallowing. At 187 m, a thin bed of iron-stained quartz sandstone drapes across an irregular surface cut into oolitic dolomitic grainstones that represent the latest phase of the subjacent shallowing record. This thin sandstone, first noted at this locality by Mullens and Poole (1972), is overlain by dark-gray wackestones a few meters thick, which yield a variety of comminuted megafossils, and a few conodonts representing species of Early Silurian aspect. These few meters of presumably shallow subtidal strata are followed upward by more than 30 m of dark, pervasively cherty lime mudstone, with few fossils but with graptolites of the Normalograptus persculptus Zone near the base. The highest part of the studied section, a few meters of chert-free limestone, is capped by a distinctive phosphorite, from which both Murphy et al. (1979) and I have assembled immense collections of conodonts representing the late Llandovery Pterospathodus

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FIGURE 1—Location in central Nevada of the Vinini Creek (VC), Lone Mountain, and Monitor Range (COP, MR) sections.

celloni Zone. The phosphorite marks the base of the Roberts Mountains Formation at this locality. From sections on the east and west sides of Copenhagen Canyon, which combine to form the Monitor Range section of Finney et al. (1999) and this report, I collected 42 large bulk samples. Acid residues of these yielded representatives of the 25 conodont species whose distribution is shown in Figure 3. In addition, I have also had for study the specimens reported by Mullens and Poole (1972) from carbonates just above the sand-blanketed erosion surface at 187 m. Conodont elements in these collections have a color-alteration index (CAI) of 2 to 2.5. Vinini Creek section.(North side Vinini Creek, 4.3 mi west of its mouth; approximately E1/2 unsurveyed sec. 9, T23N, R51E, Garden Valley, Nevada, 7.5-minute quadrangle.) About 25 mi north-northeast of the Lone Mountain section, near the eastern margin of the Roberts Mountains (Fig. 1), a 30-m succession of shales and lime mudstones in the uppermost part of the Vinini Formation (Fig. 4) is exposed in a hillside trench on the north side of Vinini Creek. These Upper Ordovician strata yield graptolites of the D. ornatus, P. pacificus, N. extraordinarius, and N. persculptus graptolites zones and are overlain disconformably by sandstones and shales of the Elder Formation, which has midLlandovery (Aeronian) graptolites at the base. From the Vinini Formation at the Vinini Creek locality, I collected 29 3- to 7-kg samples at approximately one-meter intervals. Conodonts recovered from these samples have a CAI of 1.0 and

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FIGURE 2—Lone Mountain section with ranges of conodont species identified by W. Britt Leatham and the author. Although not specifically identified, numerous specimens of Panderodus feulneri (Glenister) occur in every sample between the base of the section and 73 m above the base, and Murphy et al. (1979) reported late Llandovery Distomodus, Pterospathodus celloni, and Aulacognathus bullatus from samples 27–28 m above the 79-m level in the section shown schematically here. Lithic symbols conventional; lithologies more fully explained in text.

represent the 13 species whose distribution is charted in Figure 4. The fauna is dominantly of basinal aspect, with Amorphognathus ordovicicus Branson and Mehl, Protopanderodus insculptus (Branson and Mehl), Scabbardella altipes (Henningsmoen), Besselodus borealis Nowlan and McCracken, and an unnamed species of Periodon as its most distinctive members. However, a few elements are assignable to Belodina confluens Sweet, Gamachignathus ensifer McCracken, Nowlan, and Barnes, an indeterminate species of Plectodina, and Drepanoistodus suberectus (Branson and Mehl), which are more characteristic of shelf than basinal faunas. THE CONODONTS

A majority of the conodonts recovered from samples collected from the sections considered in this report are identifiable with species that are well known and have been thoroughly discussed in the recent literature. Thus, representative specimens are illustrated in Figures 9–12, but exhaustive systematic treatment is deemed unnecessary. Illustrated specimens are kept at The Ohio

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FIGURE 4—Vinini Creek section, with ranges of conodont species identified by the author. Lithic symbols conventional. FIGURE 3—Monitor Range section, with ranges of conodont species identified by the author and others. Mullens and Poole (1972) reported Icriodina stenolophata Rexroad from just above the sandy layer at 187 m and Murphy et al. (1979) reported species of the late Llandovery Pterospathodus celloni Zone from the phosphorite at the base of the Roberts Mountains Formation. Lithic symbols conventional.

State University and numbers prefixed with OSU on Figures 9– 12 refer to the catalog of the Orton Museum of Geology at that institution. Drepanoistodus suberectus (Branson and Mehl) (Fig. 9.23– 9.25) and Plectodina tenuis (Branson and Mehl) (Fig. 11.7–11.12) are long-ranging, well-known Mid- and Late Ordovician species and Plectodina florida Sweet (Fig. 12.5) is common in Upper Ordovician rocks. Panderodus sp. cf. P. gracilis (Branson and Mehl) (Fig. 9.3) and P. sp. cf. P. panderi (Stauffer) (Fig. 9.10) closely resemble species that have extensive ranges and are widely distributed in Middle and Upper Ordovician strata. These species, which have little biostratigraphic utility, are illustrated, but need no further discussion. Representatives of Phragmodus undatus Branson and Mehl and species of Panderodus, which have long ranges in the Lone Mountain section (Fig. 2), have been described and illustrated from virtually every Upper Ordovician locality in the Red River

Province (Sweet and Bergstro¨m, 1984). Elements at hand are typical of these conodonts so illustration of additional ones would serve no useful purpose. Cornuodus (Fig. 9.4), Hamarodus europaeus (Serpagli) (Fig. 9.1, 9.2), and Strachanognathus parvus Rhodes (Fig. 9.11) are represented by only a few specimens in the interval between 111.5 and 126.4 m in the Monitor Range section. These species are more abundantly represented in cold-water faunas from Upper Ordovician rocks in Great Britain, Scandinavia, and the Carnic Alps than in the low-latitude, warm-water faunas that characterized the Red River Province in North America (see summary in Sweet and Bergstro¨m, 1984). Hamarodus europaeus is characteristic of a Late Ordovician biofacies in the Ordovician cold-water province. The only previous record of Hamarodus in North America is of specimens from the Late Ordovician Grog Brook Group of New Brunswick identified by Nowlan (1983) as H. cf. H. europaeus. The specimens illustrated by Nowlan differ in several respects from the ones at hand. The North American record of Strachanognathus parvus includes occurrences reported from the Cobbs Arm Limestone on New World Island, Newfoundland (Bergstro¨m et al., 1974); from strata in the Diplograptus ornatus-Parorthograptus pacificus graptolite zones of the Road River Group, northern Yukon Territory, Canada (McCracken, 1987; McCracken and Lenz, 1987);

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FIGURE 6—Graphic approximation of relations between the Vinini Creek and Monitor Range sections. A line of correlation, with the equation shown is weakly controlled by bases of the Parorthograptuis pacificus, Normalograptus extraordinarius, and N. persculptus graptolite zones reported by Finney et al. (1999).

FIGURE 5—Graphic correlation of the Lone Mountain section with the Ordovician composite standard of Sweet (1995a, 1995b). Circles mark range bases; crosses mark range tops. The Line of Correlation (LOC) is controlled by the range top of Culumbodina occidentalis Sweet, and the range bases of Oulodus rohneri Ethington and Furnish and Gamachignathus ensifer McCracken, Nowlan, and Barnes.

from a mixed collection of Middle and Upper Ordovician conodonts from Markey Brook, New Brunswick (Nowlan et al., 1997); from Middle Ordovician rocks at Camel Back Mountain, New Brunswick (Kennedy et al., 1979); from Middle or Upper Ordovician strata on the Cassiar Platform in the western Canadian Cordillera (Pohler and Orchard, 1990); from the Road River Group, northern Yukon Territory (McCracken, 1991a); from Middle, or more probably Late Ordovician rocks in the Medfra area, central Alaska (Dumoulin et al., 1999) Amorphognathus ordovicicus Branson and Mehl (Fig. 10).At least a few elements of this species have been collected from nearly every sample in the Vinini Creek and Monitor Range sections, but from only two at Lone Mountain. The species is most abundantly represented in samples from 111.5 to 126.4 m above the base of the Monitor Range section. It has been discussed recently by Ferretti and Barnes (1997), who note the same degree of morphological variability we recognize in the specimens at hand. Illustrated in Figure 12.13, 12.14 are two tiny specimens identified as A. ordovicicus? Specimens like these are common in samples from the Vinini Creek and Monitor Range sections. Elements of similar conformation have been identified as representatives of Eocarniodus by several authors. The ones illustrated in Figure 12 lack the prominent mid-lateral ridges that characterize many (but not all) of the tiny elements previously referred to Eocarniodus. Because specimens like the ones illustrated occur most abundantly in collections that also include numerous elements of Amorphognathus ordovicicus, they are here regarded as the detached posterior processes of ramiform elements of that species and are illustrated in this report only for the sake of completeness. Belodina confluens Sweet (Fig. 12.12).Only a few specimens of this species have been recovered and, like the one illustrated, they are all fragmentary. Conodonts assigned to this species have been reported from rocks as old as late Mohawkian and from strata as young as latest Richmondian. Leatham (1987) assigned Lone Mountain specimens to Belodina stonei Sweet, which has not been recognized by other authors and may be only a provincial variant of the more widely distributed B. confluens. Besselodus borealis Nowlan and McCracken (Fig. 9.12, 9.13).Three collections from the upper half of the Vinini Creek section contain distacodontiform elements that closely resemble those in the type collection (Nowlan et al., 1988). However, none

of the geniculate coniform elements that are especially distinctive of this species has been recovered. The species has been reported previously from Late Ordovician rocks in the Yukon and Northwest Territories (Lenz and McCracken, 1982; Nowlan et al., 1988) and possible representatives have been recovered from the Vaure´al Formation of Anticosti Island (Nowlan and Barnes, 1981) and the Keisley and Rhiwlas limestones (Rawtheyan) of north England and Wales (Orchard, 1980). Culumbodina.Elements of both Culumbodina penna Sweet (Fig. 12.3, 12.4) and C. occidentalis Sweet occur in samples from the basal 17 m of the Lone Mountain section. These species have almost identical ranges in mid-Edenian to mid-Maysvillian strata. In addition to Leatham’s (1987) report from the Lone Mountain section, Harris (in Ross, Nolan, and Harris, 1980) illustrates (as Belodina sp. A of Sweet, Ethington, and Barnes) an element of C. penna from 2 m above the base of the Hanson Creek Formation at Spanish Mountain, in the Mountain Boy Range, and mentions occurrence of the species in a sample 10 m above the base of the type Hanson Creek in the Roberts Mountains. Drepanoistodus n. sp. (Fig. 9.8, 9.9).A collection from 126.4 m above the base of the Monitor Range section (Fig. 3) includes geniculate and nongeniculate coniform elements that appear to represent an undescribed species of Drepanoistodus. Nongeniculate forms (Fig. 9.9) have a basal segment that is slightly deflected to one side and has a flat basal margin. Geniculate forms (Fig.

FIGURE 7—Graphic correlation of the central Nevada section (a composite of range data from the Vinini Creek and Monitor Range sections) and the Ordovician composite standard of Sweet (1995a, 1995b). Range bases marked by circles, tops by crosses.

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FIGURE 8—Correlation of the Vinini Creek, Monitor Range, and Lone Mountain sections with the Ordovician composite standard of Sweet (1995a, 1995b). Vertical scale is in arbitrary composite-standard units (csu).

9.8) have a subquadrate anterolateral profile that clearly distinguishes them from comparable elements in the apparatus of D. suberectus (cf. Fig. 9.23). Nongeniculate components of the apparatus of this species are reminiscent morphologically of the ‘scandodontiform’ elements that proxy for geniculate elements in the apparatus of species of Drepanodus. However, apparatuses of those species lack geniculate coniform elements. Gamachignathus ensifer McCracken, Nowlan and Barnes (Fig. 12.6–12.11, 12.16).A few of the distinctive elements of this stratigraphically important Late Ordovician species occur in collections from sections at each of the three localities considered in this report. Type material is from the Ellis Bay Formation of Anticosti Island, Quebec (McCracken et al., 1980) but it has also been reported from the Vaure´al Formation of Anticosti (Nowlan

and Barnes, 1981). Additional occurrences are discussed by McCracken (1987), who also reports four specimens from Parorthograptus pacificus-Zone strata of the Road River Group, northern Yukon. Harris (in Ross et al., 1980) reported this species (as Aphelognathus? n. sp. and Exochognathus keislognathoides Pollock, Rexroad, and Nicoll) from the Hanson Creek Formation in the Mountain Boy Range, central Nevada, and noted its occurrence also in the Ely Springs Dolomite, Funeral Mountains, Inyo County, California. Evidently, then, the species was rather widely distributed, although it is not very abundantly represented in published summaries of any of the western North American collections. Oulodus rohneri Ethington and Furnish (Fig. 11.13–11.19).P elements of O. rohneri are distinctive and not readily confused →

FIGURE 9—Digital images of representative conodonts from Upper Ordovician rocks, central Nevada. Originals of 1, 2, and 11 from 111.5 m above base of Hanson Creek Fm., Monitor Range section; 3, 10 from 60 m above base of Lone Mountain section; 4, 5, 7–9, 14–25 from 126.4 m above base of Hanson Creek Fm., Monitor Range section; 6, 12, 13, from Vinini Fm., 23 m above base of Vinini Creek section. 1, 2, Hamarodus europaeus, 1, lateral view, 368, of P element, OSU 50101; 2, lateral view, 382 of M element, OSU 50102. 3, Panderodus sp. cf. P. gracilis, lateral view, 383, OSU 50103. 4, Cornuodus sp., lateral view, 343, OSU 50104. 5, 6, Paroistodus? sp., lateral views, 3113, 398, OSU 50105, 50106. 7, Pseudooneotodus mitratus, superior view, 362, OSU 50107. 8, 9, Drepanoistodus n. sp., lateral views of geniculate and nongeniculate elements, 365, OSU 50108, 50109. 10, Panderodus sp. cf. P. panderi, lateral view, 394, OSU 50110. 11, Strachanognathus parvus, lateral view, 3101, OSU 50111. 12, 13, Besselodus borealis, 12, detail 3158 of lower half of 13; 13, lateral view, 3101, OSU 50112. 14, 15, Scabbardella altipes, lateral views, 365, 375, OSU 50113, 50114. 16-20, Walliserodus amplissimus, 16, 17, 19 lateral views, 3125, 3129, 3117, OSU 50115, 50116, 50118; 18, 20, posterior views, 3122, 146, OSU 50117, 50119. 21, 22, Protopanderodus insculptus, 21, lateral view, 338, of posteriorly adenticulate specimen, OSU 50120; 22, lateral view, 356, of posteriorly denticulate specimen, OSU 50121. 23-25, Drepanoistodus suberectus, lateral views, 3108, 379, 368, OSU 50122, 50123, 50124.

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SWEET—ORDOVICIAN CONODONTS AND BIOSTRATIGRAPHY (NEVADA) with those of associated species. Ramiform elements, on the other hand, are less diagnostic, particularly when mixed with those of several unrelated species. For this reason, it is not possible to distinguish or illustrate the Sc elements of O. rohneri from those of Plectodina tenuis, with which O. rohneri is associated in the most productive samples available. Britt Leatham (personal commun.) suggested more than a decade ago that P elements of Oulodus rohneri are more like those on which Stone and Furnish (1959) based Pristognathus bighornensis than those of most Oulodus species. There is also great similarity between the apparatuses of O. rohneri and Early Silurian Aspelundia fluegeli (Walliser), which is represented in abundance at the base of the Roberts Mountains Formation in the Monitor Range section (see McCracken, 1991b). This similarity suggests that the Late Ordovician Oulodus-Pristognathus lineage might continue into the Llandovery as Aspelundia. If so, Aspelundia might well be a junior subjective synonym of Pristognathus. This would reduce the number of disappearances attributed to the Late Ordovician ‘‘mass extinction.’’ Oulodus ulrichi (Stone and Furnish) (Fig. 11.1–11.6).This species is best represented in collections from the lower 10 m of the Lone Mountain section, but its distinctive elements have also been recovered from a sample 126.4 m above the base of the Monitor Range section. Ovlodus ulrichi was widespread in the shallow-shelf deposits of the Red River Province during the Late Ordovician, where it has a known range from latest Edenian almost to the end of the Richmondian (Sweet, 1979). Paroistodus? sp.Elements like those in Figure 9.5, 9.6 have been treated taxonomically in several ways by previous authors (e.g., Dapsilodus mutatus, Paroistodus mutatus, Paroistodus? sp. A). The species identified in these ways is not abundantly represented in the collections at hand and study of them does not resolve the debate about their generic identity. Paroistodus? sp. A of Nowlan et al. (1988) (5Paroistodus? nowlani of Zhen et al., 1999) characterizes North American collections that are comparable in age to ours, but geniculate coniform elements from central Nevada strata (the e elements of Nowlan, McCracken, and Chatterton) are less variable and differ subtly in morphology, which suggests the species may not be the same. Periodon n. sp. (Fig. 12.1, 12.2).Collections from five levels in the Vinini Creek section include the distinctive M elements of an unnamed species of Periodon. Unfortunately, those collections yield only fragments of the other element types that make up the skeletal apparatus of the species. Similar elements from Upper Ordovician rocks in eastern North America have also been described in open nomenclature by Nowlan (1981, 1983) and, most recently, by Nowlan et al., (1997). Protopanderodus insculptus (Branson and Mehl) (Fig. 9.21, 9.22).The large, distinctive elements of this species have been recovered from samples ranging from 0 to 126.4 m above the base of the Monitor Range section and from 2 to 28 m above the base of the Vinini Creek section. The species is also represented by a few specimens in samples from the lower 49 m of the Lone Mountain section. Elements have a long, bladelike posterior process and the variable disposition of prominent costae distinguishes several morphologic groups. The posterior margin of bilaterally symmetrical to slightly asymmetric forms tends also to develop a conspicuous denticle. This is the feature that especially distinguishes the holotype and the one that led Branson and Mehl

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(1933) to include the species in Phragmodus. Although some authors have referred specimens without a posterior denticle to Protopanderodus liripipus Kennedy, et al. (1979) distribution, as well as close similarity in other features suggest that all these elements represent a single species. They are so treated in this report. Pseudobelodina.Lone Mountain and Monitor Range collections include elements referable to several species of Pseudobelodina, of which we illustrate two, P. inclinata (Branson and Mehl) (Fig. 12.17) and P. vulgaris vulgaris Sweet (Fig. 12.15, 12.18). Pseudobelodinids are especially characteristic of Late Ordovician faunas in the Red River Province of North America (Sweet, 1979; Sweet and Bergstro¨m, 1984), but two species (P. inclinata and P. vulgaris) were widely distributed in the higherlatitude Ohio Valley Province in latest Ordovician times (see Sweet and Bergstro¨m, 1984, for details). Pseudooneotodus.Pseudooneotodus mitratus (Moskalenko) (Fig. 9.7) ranges from 111.5 to 126.4 m above the base of the Monitor Range section; is represented at 28 m above the base of the Vinini Creek section; and is a component, with a few P. beckmanni, of several samples from the Lone Mountain section. Sansom (1996) gives a thorough description of the histology of elements representing several Pseudooneotodus species and provides a summary of previous opinions as to the phylogenetic position of the genus. He notes that the elements he studied are divisible into an outer lamellar layer indistinguishable histologically from vertebrate enamel, and a basal layer with features suggestive of dentine. Scabbardella altipes (Henningsmoen) (Fig. 9.14, 9.15).Large elements referable to this species are components of collections from nearly every one of the samples prepared for this study. The wide geographic distribution in the late Ordovician of S. altipes, together with its occurrence in samples from different biofacies, suggests that the species was pelagic, rather than nekto-benthic, or benthic. Orchard (1980) established the genus and provided a diagnosis of the species. Study of the specimens at hand adds no additional information. Walliserodus amplissimus (Serpagli) (Fig. 9.16–9.20).Nowlan et al. (1988) provide a detailed discussion of W. amplissimus and emphasize the features that distinguish its elements from those of closely related, Early Silurian W. curvatus (Branson and Mehl). Specimens in our collections agree closely with those illustrated by Nowlan, McCracken, and Chatterton and with those figured in a more recent report by Nowlan et al. (1997). The wide, biofacies-independent distribution of W. amplissimus, which was based initially on material from the Carnic Alps (Serpagli, 1967), suggests that the species was pelagic. Conodont faunas.Species identified in collections from the three central Nevada localities are divisible into three general groups, a pelagic contingent, and shelf and basinal associations. The pelagic contingent includes Drepanoistodus suberectus, species of Panderodus, Scabbardella altipes, Walliserodus amplissimus, and probably the species identified here as Paroistodus? sp. and Besselodus borealis. Specimens of W. amplissimus are in every sample from the lower 59 m of the Lone Mountain section, and they are also present in nearly every sample from the lower 156 m of the Monitor Range section. W. amplissimus is not represented, however, in my collections from the Vinini Creek locality, where Drepanoistodus suberectus, Paroistodus? sp., and Scabbardella altipes are the dominant pelagic forms. Panderodus

← FIGURE 10—Digital images of the elements of Amorphognathus ordovicucus from 126.4 m above base of Hanson Creek Fm., Monitor Range section. 1, Sc element, 3124, OSU 50125; 2, Sa element, 3119, OSU 50126; 3, Sb element, 3165, OSU 50127; 4, Pb element, lateral view, 3125, OSU 50128. 5, Pb element, superior view, 3116, OSU 50129; 6, 7, M elements, lateral views, 3119 and 3103, OSU 50130, 50131; 8, 11, non-bladed Pa elements, superior views, 367, 372, OSU 50132, 50133; 9, 10, bladed Pa elements, superior views, 372 and 363, OSU 50134, 50135.

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SWEET—ORDOVICIAN CONODONTS AND BIOSTRATIGRAPHY (NEVADA) is abundantly represented in samples from the Lone Mountain section and, although not indicated in Figure 3, its elements have been recovered from samples above 111.5 m in the Monitor Range section. Panderodus dominates collections from Upper Ordovician rocks throughout the western Midcontinent of North America (Sweet, 1979). These distributional facts suggest that Drepanoistodus and Panderodus were the most widespread pelagic forms and that Walliserodus and Scabbardella may have been major components of the pelagic contingent only on the outer shelf and shelf margin. The basinal association is dominated by Amorphognathus ordovicicus and Protopanderodus insculptus. It is well represented in samples from the basinal Vinini Formation at the Vinini Creek locality and in those from the lower 100 m or so of the Monitor Range section. The shelf association characterizes Lone Mountain samples, which are dominated by specimens of Panderodus, but also include representatives of Culumbodina, Pseudobelodina, Belodina, Oulodus, and Plectodina. Species of all these genera are best known, most abundant, and most widely distributed geographically in the shallow-shelf carbonates that are the characteristic Upper Ordovician deposits of western North America (cf. Sweet, 1979). The presence of abundant Walliserodus amplissimus in Lone Mountain samples, and the complete absence of this distinctive species at shelf localities farther inboard on the western North American Ordovician shelf suggests that Hanson Creek strata at Lone Mountain accumulated toward the margin, rather than at a more central site on the shelf. Above 111 m in the Monitor Range section, lithologies indicate progressive shallowing of the depositional site, and in the interval between 111.5 and 126.4 m, the basinal association of conodont species is diluted by an incursion of species typical of the shelf biotope. Above 126 m, species diversity dwindles rapidly and only species characteristic of the shelf biotope remain. Single specimens of shelf-association species also occur with more numerous representatives of the basinal association in samples between 8 and 28 m in the Vinini Creek section. These occurrences support the conclusion that this segment of the North American margin became progressively more shallowly submerged from early in the Parorthograptus pacificus Zone to within the Normalograptus persculptus Zone. Thus range terminations of shelfassociation conodont species were not synchronous in central Nevada, but became younger toward the shelf margin as the shelf biotope migrated in that direction. CORRELATION OF SECTIONS

Except for Pseudooneotodus beckmanni, all the conodont species represented in samples collected from the Vinini Creek section and from sub-sand portions of the Monitor Range and Lone Mountain sections are known elsewhere only from rocks of Middle and Late Ordovician age. Elements of Ozarkodina oldhamensis?, Walliserodus curvatus, Dapsilodus obliquicostatus, Decoriconus fragilis, and Panderodus unicostatus, as well as the one identified as Icriodina stenolophata in a report by Mullens and Poole (1972), from samples just above the sand layer in the Monitor Range section would probably be regarded as Early Silurian

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(Llandoverian) in age were it not that they are associated with, or in strata just below, graptolites that indicate the upper part of the latest Ordovician Normalograptus persculptus Zone. Detailed correlation of the central Nevada sections has been effected in three stages. The Lone Mountain section, with conodonts dominantly of western Midcontinent type, is readily correlated graphically with the Composite Standard section briefly outlined by Sweet (1995a, 1995b). That correlation, shown in Figure 5, is constrained by the occurrence near the base of the section of Culumbodina penna Sweet and C. occidentalis Sweet, which characterize a distinctive late Edenian–early Maysvillian interval of the Composite Standard; by the first occurrence of Oulodus rohneri at 49 m; and by the first occurrence of Gamachignathus ensifer at 60 m. A line joining these points separates first occurrences (plotted as open circles) from highest occurrences (plotted as crosses) and its equation (1.58 LM 1 1120) indicates that the lower 3 m are Edenian in age, the next 42 m Maysvillian, and the remainder of the section, to the base of the sand, early Richmondian. The Monitor Range and Vinini Creek sections have yielded graptolites characteristic of widespread Late Ordovician zones and both preserve the record of a distinctive d13C excursion. In Figure 6, points representing the bases of the P. pacificus, N. extraordinarius, and N. persculptus graptolite zones are used to suggest, in a very generalized way, relations between the Monitor Range and Vinini Creek sections. The resulting graph (Fig. 6) confirms the regional field observation that, in the time interval considered, rock accumulation at the basinal Vinini Creek locality was at a substantially slower rate than at the Monitor Range locality. That is, graphic analysis indicates that during the time 5.7 m of rock accumulated at the Monitor Range locality, only 1.0 m formed at the Vinini Creek site. Using 5.7 VC 1 39, the equation of the LOC drawn in Figure 6, Monitor Range equivalents were determined of the ranges of conodont species identified in the Vinini Creek section. Following this, ranges from the two sections were combined into a composite section by selecting the lowest of the first occurrences and the highest of the last occurrences in the two data sets and utilizing the Monitor Range section as base. Ranges in this composite section, designated the Central Nevada Composite, were then plotted against those in the Midcontinent Composite Section described by Sweet (1995a, 1995b), with the results indicated in Figure 7. Because both the Central Nevada Composite and the Lone Mountain sections have been correlated with the same Midcontinent Composite Standard, they are then correlated with each other, even though they have yielded conodonts that represent rather different depositional environments. Correlation of the Vinini Creek, Monitor Range, and Lone Mountain sections with the Midcontinent Composite Standard is shown schematically in Figure 8, in which I have also included the extent in central Nevada of the Parorthograptus pacificus, Normalograptus extraordinarius, and N. persculptus graptolite zones. As indicated in Figure 8, the Midcontinent Composite Standard also includes information from the eastern North American type areas of the Late Ordovician Edenian, Maysvillian, and Richmondian stages. As noted by Finney et al. (1999), the upper few meters of the

← FIGURE 11—Digital images of conodonts from 126.4 m above base of Hanson Creek Fm., Monitor Range section. 1-6, Oulodus ulrichi, 1, Pa element, 3130, OSU 50136; 2, Pb element, 3116, OSU 50137; 3, M element, 368, OSU 50138; 4, Sc element, 386, OSU 50139; 5, Sb element, 396, OSU 50140; 6, Sa element, 391, OSU 50141. 7–12, Plectodina tenuis, 7, Pa element, 3109, OSU 50142; 8, Pb element, 394, OSU 50143; 9, M element, 392, OSU 50144; 10, Sc element, 391, OSU 50145; 11, Sb element, 3127, OSU 50146; 12, Sa element, 3103, OSU 50147. 13– 16, 18, 19, Oulodus rohneri; 13, Pa element, 373, OSU 50148; 14, Pb element, 388, OSU 50149; 15, Pb element, 366, OSU 50150; 16, M element, 384, OSU 50151; 18, Sb element, 380, OSU 50153; 19, Sa element, 3103, OSU 50154. 17, Specifically indeterminate Sc? element, 383, OSU 50152.

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SWEET—ORDOVICIAN CONODONTS AND BIOSTRATIGRAPHY (NEVADA) Vinini Creek section yield graptolites diagnostic of the lower part of the Normalograptus persculptus Zone, whereas Normalograptus persculptus and Glyptograptus laciniosus, indicative of the upper part of that zone, have been identified (Berry, 1986; Finney et al., 1999) in collections from 9 and 13 m above the sandmantled discontinuity in the Monitor Range section. Conodonts typical of the Late Ordovician Amorphognathus ordovicicus Zone occur with N. persculptus at the Vinini Creek locality, but the few conodonts reported from beds just above the sand-blanketed discontinuity in the Monitor Range section represent species that would ordinarily be regarded as earliest Silurian in age (Mullens and Poole, 1973; Murphy et al., 1979; Finney et al., 1999). Consequently, although graphic correlation of the Vinini Creek section locates the lower part of the N. persculptus Zone with some precision, correlation of the upper part, known only from the Monitor Range section, cannot be accurately determined. In Figure 8, I suggest in the Monitor Range column that an equivalent of the lower part of the N. persculptus Zone is missing just above the sand-blanketed discontinuity; that the upper N. persculptus Zone includes 13 m or so of the Hanson Creek Formation above the discontinuity; and that minimum separation on the discontinuity is the projected thickness of the lower part of the zone at Vinini Creek. Interestingly, but probably only fortuitously, this raises the Hanson Creek level from which N. persculptus group graptolites have been reported (Berry, 1986) to a point in the Composite Standard section (Sweet, 1995a, 1995b) closely approximating the top of the Richmondian Stage in its type area. CONCLUSIONS

Conodonts from the Hanson Creek and Vinini formations at three localities in central Nevada confirm the Late Ordovician age for these strata determined by previous authors (e.g., Murphy et al., 1979; Ross et al., 1980; Leatham, 1987). Correlations achieved in this study indicate that depositional sites on the shelf (Lone Mountain) and shelf-margin (Monitor Range) began to shallow in late P. pacificus-Zone (mid-Richmondian) time and that this gradual emergence proceeded through N. extraordinarius-Zone (late-Richmondian) time to a culmination in mid-N. persculptus-Zone time. Subsequent resubmergence, which began in mid-N. persculptus-Zone time, was accompanied, at least in central Nevada, by the first representatives of conodont species more diagnostic of the Early Silurian than the Late Ordovician. Melchin et al. (1991) document a similarly mixed, or ‘‘transitional’’ fauna in sections on Cornwallis and Truro islands in Arctic Canada, and Armstrong’s (1995) graphic assembly of conodont and graptolite range data from those and other localities in North America also identifies a late Ordovician interval in which species with long Ordovician ranges mingle with those of Silurian aspect. In a very general way, this interval is charted in Armstrong’s report as the lower, Ordovician, part of the Oulodus? nathani Zone, which has

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been regarded, with justification, as a mixed, or transitional, interval by many previous authors (e.g., McCracken and Barnes, 1981; Goodfellow et al., 1992). ACKNOWLEDGMENTS

Stanley C. Finney, California State University, Long Beach, arranged and supervised field work, which was made all the more productive and enjoyable through the assistance of W. B. N. Berry, John Cooper, and students from California State University campuses at both Long Beach and Fullerton. John Mitchell, SEM technician at The Ohio State University, prepared and recorded the digital images assembled by the author in Figures 9–12. Anita Harris and A. D. McCracken reviewed the manuscript and provided suggestions and corrections that have substantially improved the final report. Carrie Wilson, graphics specialist, and Karen Tyler, faculty draftsperson in the Department of Geological Sciences, The Ohio State University, dropped everything at the last minute to re-draft and repair Figures 1–8; their assistance is greatly appreciated. Field and laboratory work were funded by National Science Foundation Grant EAR-9628167. REFERENCES

ARMSTRONG, H. A. 1995. High-resolution biostratigraphy (conodonts and graptolites) of the Upper Ordovician and Lower Silurian—Evaluation of the Late Ordovician mass extinction. Modern Geology, 20:41–68. BERGSTRO¨M, S. M., J. RIVA, AND G. M. KAY. 1974. Significance of conodonts, graptolites, and shelly faunas from the Ordovician of western and north-central Newfoundland. Canadian Journal of Earth Sciences, 11:1625–1660. BERRY, W. B. N. 1986. Stratigraphic significance of Glyptograptus persculptus group graptolites in central Nevada, U.S.A. In C. P. Hughes and R. B. Rickards (eds.), Palaeoecology and Biostratigraphy of Graptolites. Geological Society Special Publication, 20:135–143. BOLTON, T. E., AND G. S. NOWLAN. 1979. A Late Ordovician fossil assemblage from an outlier north of Aberdeen Lake, District of Keewatin. Geological Survey of Canada Bulletin, 321:1–26. BRANSON, E. B., AND M. G. MEHL. 1933. Conodonts from the Maquoketa-Thebes (Upper Ordovician) of Missouri. University of Missouri Studies, 8:121–132. DUMOULIN, J. A., D. C. BRADLEY, A. G. HARRIS, AND J. E. REPETSKI. 1999. Lower Paleozoic deep-water facies of the Medfra area, central Alaska. In K. D. Kelley (ed.), Geologic Studies in Alaska by the U.S. Geological Survey, 1997. United States Geological Survey Professional Paper, 1614:73–103. DUNHAM, J. B. 1977. Depositional environments and paleogeography of the Upper Ordovician, Lower Silurian carbonate platform of central Nevada. In J. H. Stewart, C. H. Stevens, and A. E. Fritsche (eds.), Paleozoic paleogeography of the western United States. Pacific Coast Paleogeography Symposium 1. Pacific Section, Society for Sedimentary Geology (SEPM), 157–164. FERRETTI, A., AND C. R. BARNES. 1997. Upper Ordovician conodonts from the Kalkbank Limestone of Thuringia, Germany. Palaeontology, 40:15–42. FINNEY, S. C., W. B. N. BERRY, J. D. COOPER, R. L. RIPPERDAN, W. C.

← FIGURE 12—Digital images of Upper Ordovician conodonts, central Nevada. 1, 2, Periodon n. sp., lateral views, 3104, 3167, of M elements from 2.4 and 23 m above base of Vinini Creek section, OSU 50155, 50156. 3, 4, Culumbodina penna, lateral views, 3108, 3103, of the two elements of this species, from base of Hanson Creek Fm., Lone Mountain section, OSU 50157, 50158. 5, Plectodina florida, posterior view, 392 of Sb element from 59 m above base of Lone Mountain section, OSU 50159. 6–11, 16, Gamachignathus ensifer, 6, lateral view of Pb element, 3138, from 60 m above base Lone Mountain section, OSU 50160; 7, M element, lateral view, 393, from 126.4 m above base Monitor Range section, OSU 50161; 8, Sa element?, posterior view, 3115, from 111.5 m above base Monitor Range section, OSU 50162; 9, 10, 11, Sc, Pa, Sb elements, 3118, 3139, 3165, from 60 m above base of Lone Mountain section, OSU 50163, 50164, 50165; 16, Pb element, lateral view, 3105, from 126.4 m above base Monitor Range section, OSU 50170. 12, Belodina confluens, lateral view, 381, from 15 m above base of Vinini Creek section, OSU 50166. 13, 14, Amorphognathus ordovicicus?, lateral views of posterior-process fragments of the sort that have been identified as Eocarniodus by other authors, 3138, 3116, from 111.5 m above base of Monitor Range section, OSU 50167, 50168. 15, 18, Pseudobelodina vulgaris vulgaris, lateral views 398, 3155, from 126.4 m above base Monitor Range section, OSU 50169, 50172. 17, Pseudobelodina inclinata, lateral view, 386, from 126.4 m above base Monitor Range section, OSU 50171.

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1979. Late Llandovery unconformity in central Nevada. Brigham Young University Geology Studies, 26:21–36. NOWLAN, G. S. 1981. Late Ordovician-Early Silurian conodont biostratigraphy of the Gaspe´ Peninsula—a preliminary report. In P. J. Lesperance (ed.), Subcommission on Silurian Stratigraphy, Ordovician-Silurian Boundary Working Group, Field Meeting, Anticosti-Gaspe´, Quebec, Vol. II, Stratigraphy and Paleontology,257–291. NOWLAN, G. S. 1983. Biostratigraphic, paleogeographic, and tectonic implications of Late Ordovician conodonts from the Grog Brook Group, northwestern New Brunswick. Canadian Journal of Earth Sciences, 20: 651–671. NOWLAN, G. S., AND C. R. BARNES. 1981. Late Ordovician conodonts from the Vaure´al Formation, Anticosti Island, Quebec. Geological Survey of Canada Bulletin, 329(Part 1):1–49. NOWLAN, G. S. A. D. MCCRACKEN, AND B. D. E. CHATTERTON. 1988. Conodonts from Ordovician–Silurian boundary strata, Whittaker Formation, Mackenzie Mountains, Northwest Territories. Geological Survey of Canada Bulletin, 373, 99 p. NOWLAN, G. S., A. D. MCCRACKEN, AND M. J. MCLEOD. 1997. Tectonic and paleogeographic significance of Late Ordovician conodonts in the Canadian Appalachians. Canadian Journal of Earth Sciences, 34:1521– 1537. ORCHARD, M. J. 1980. Upper Ordovician conodonts from England and Wales. Geologica et Palaeontologica, 14:9–44. POHLER, S. M. L., AND M. J. ORCHARD. 1990. Ordovician conodont biostratigraphy, western Canadian Cordillera. Geological Survey of Canada Paper, 90–15, 37 p. ROSS, R. J., Jr., T. B. NOLAN, AND A. G. HARRIS. 1980. The Upper Ordovician and Silurian Hanson Creek Formation of central Nevada. U.S. Geological Survey Professional Paper, 1126-C:C1–C22. SANSOM, I. J. 1996. Pseudooneotodus: a histological study of an Ordovician to Devonian vertebrate lineage. Zoological Journal of the Linnean Society, 118:47–57. SERPAGLI, E. 1967. I conodonti dell’Ordoviciano superiore (Ashgilliano) delle Alpi Carniche. Bolletino della Societa Paleontologica Italiana, 6: 30–111. STONE, G. L., AND W. M. FURNISH. 1959. Bighorn conodonts from Wyoming. Journal of Paleontology, 33:211–228. SWEET, W. C. 1979. Late Ordovician conodonts and biostratigraphy of the western Midcontinent Province. Brigham Young University Geology Studies, 26:45–86. SWEET, W. C. 1995a. A conodont-based composite standard for the North American Ordovician: progress report. In J. D. Cooper, M. L. Droser, and S. C. Finney (eds.), Ordovician Odyssey: Short Papers for the Seventh Internationl Symposium on the Ordovician System. Pacific Section, Society for Sedimentary Geology (SEPM), 15–20. SWEET, W. C., 1995b. Graphic assembly of a conodont-based composite standard for the Ordovician System of North America. In K. O. Mann, H. R. Lane, and P. A. Scholle (eds.), Graphic Correlation. Society for Sedimentary Geology (SEPM), Special Publication, 53:139–150. SWEET, W. C., AND S. M. BERGSTRO¨M. 1984. Conodont provinces and biofacies of the Late Ordovician. Geological Society of America Special Paper, 196,69–87. ZHEN, Y-Y, B. D. WEBBY, AND C. R. BARNES. 1999. Upper Ordovician conodonts from the Bowan Park Group, New South Wales, Australia. Geobios, 32:73–104. ACCEPTED 29 JUNE 2000