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Revue de micropalontologie 51 (2008) 3966

Original article

Upper Ordovician microphytoplankton of the Bills Creek Shaleand Stonington Formation, Upper Peninsula of Michigan, U.S.A.:

Biostratigraphy and paleogeographic significance

Microphytoplancton du Shale de Bills Creek et de la Formationde Stonington (Ordovicien Superieur), Peninsule superieure du Michigan,

Etats-Unis : biostratigraphie et signification paleogeographique

Reed Wicander a,, Geoffrey Playford b

a Department of Geology, Central Michigan University, Mount Pleasant, Michigan 48859, USAbDepartment of Earth Sciences, The University of Queensland, Brisbane, Queensland 4072, Australia

Abstract

An abundant, diverse, and well-preserved organic-walled microphytoplankton assemblage is described from the Upper Ordovician Bills Creek

Shale and the lower Stonington Formation (Bay de Noc Member) in the Upper Peninsula of Michigan, U.S.A. Based on graptolite and conodont

evidence, the Bills Creek Shale and Stonington Formation are Richmondian (=Ashgill) in age. The assemblage is dominated by acritarchs,

which comprise 29 species (including the enigmatic palynomorphGloeocapsomorpha prisca) assigned to 20 genera. The prasinophyte phycomata

are represented by undifferentiated species ofLeiosphaeridia and Tasmanites. In addition, chitinozoans are abundant, and scolecodonts and

graptolite fragments are common. Paleontologic-palynologic and sedimentologic evidence indicates that the Bills Creek Shale was deposited in a

low-energy, shallow, nearshore marine environment. The overlying Bay de Noc Member of the Stonington Formation also accumulated in a low-

energy, normal marine environment, but in a more offshore, somewhat deeper water setting. Both formations experienced minor transgressive andregressive episodes as indicated by fluctuations in the composition of the palynoflora. The combined Bills Creek/Stonington acritarch assemblage

closely resembles those described from the Richmondian-aged Maquoketa Shale (Missouri and Kansas), Sylvan Shale (Oklahoma), and Vaureal

Formation (Anticosti Island, Quebec, Canada). The overall composition of the acritarch assemblage from these four formations reflects a distinctive,

recognizably Laurentiancharacter. Nonetheless, many of the Bills Creek/Stonington acritarchs have beenreportedfrom Upper Ordovician localities

elsewhere, providing additional evidence for Late Ordovician cosmopolitanism of the marine microphytoplankton community. Additionally, the

restricted stratigraphic range of many of the taxa further enhances their biostratigraphic application, both regionally and globally, and reaffirms the

Richmondian (=Ashgill) age of the Bills Creek Shale and Stonington Formation.

2007 Elsevier Masson SAS. All rights reserved.

Resume

Un assemblage de microphytoplanctona paroi organique abondant, diversifie et bien preserve est decrit dans lOrdovicien Superieur du Shale

de Bills Creek et de la Formation Stonington inferieure (Membre Baie de Noc) dans la Peninsule superieure du Michigan, Etats-Unis. Dapres

le contenu en graptolites et conodontes, le Shale de Bills Creek et la Formation Stonington sont dage Richmondien (=Ashgill). Lassemblageest domine par des acritarches, comprenant 29 especes (dont le palynomorpheenigmatiqueGloeocapsomorpha prisca) reparties dans 20 genres.

Les prasinophytes phycomata sont representes par des especes indifferenciees deLeiosphaeridia et Tasmanites. De plus, les chitinozoaires sont

abondants et les fragments de graptolites et scolecodontes sont communs. Les resultats sedimentologiques et paleonto-palynologiques indiquent

que le Shale de Bills Creek sest depose dans un environnement marin proximal, peu profond et de faibleenergie. Le Membre Baie de Noc de la

Formation Stonington susjacent sestegalement accumule dans un environnement marin normal de faibleenergie mais dans des eaux plus distales

et plus profondes. Les deux formations ont connu desepisodes transgressifs et regressifs mineurs, indiques par les fluctuations dans la composition

de la palynoflore. Lassemblage a acritarches de Bills Creek et Stonington ressemble a ceux decrits dans le Shale de Maquoketa (Missouri et

Corresponding author.

E-mail address:[email protected](R. Wicander).

0035-1598/$ see front matter 2007 Elsevier Masson SAS. All rights reserved.

doi:10.1016/j.revmic.2007.01.001
mailto:[email protected]://localhost/var/www/apps/conversion/tmp/scratch_4/dx.doi.org/10.1016/j.revmic.2007.01.001http://localhost/var/www/apps/conversion/tmp/scratch_4/dx.doi.org/10.1016/j.revmic.2007.01.001mailto:[email protected]

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40 R. Wicander, G. Playford / Revue de micropalontologie 51 (2008) 3966

Kansas), le Shale de Sylvan (Oklahoma) et la Formation Vaureal (Ile dAnticosti, Quebec, Canada) dage Richmondien. La composition generale

de lassemblagea acritarches de ces quatre formations reflete un caractere Laurentien distinctif et reconnaissable. Cependant, la plupart de ces

acritarchesde Bills Creek et Stoningtonont ete signales dansdautres localites de lOrdovicien Superieur,ce qui constitueune preuve supplementaire

du cosmopolitisme de la communaute microphytoplanctonique marine de lOrdovicien Superieur. De plus, la repartition stratigraphique restreinte

de la plupart des taxons renforce leur application biostratigraphiquea la fois au niveau regional et global et reaffirme lage Richmondien (=Ashgill)

du Shale de Bills Creek et de la Formation Stonington.

2007 Elsevier Masson SAS. All rights reserved.

Keywords: Acritarchs; Bills Creek Shale; Biostratigraphy; Paleoenvironment; Paleogeography; Stonington Formation; Upper Ordovician

Mots cles :Acritarches ; Shale de Bills Creek ; Biostratigraphie ; Paleoenvironnement ; Paleogeographie ; Formation de Stonington ; Ordovicien Superieur

1. Introduction

Organic-walled microphytoplankton have been widely used

for stratigraphic correlation both regionally and globally for sev-

eral decades, andtheir utility in dating Proterozoic andPaleozoic

marine sedimentary rocksis widely recognized(Playford,2003).

Additionally, thesepalynomorphs are beingapplied increasingly

in paleoenvironmentalstudies (Vecoli, 2000; Li et al., 2004), and

more recently, as they relate to global biodiversification, and to

paleoclimatic and paleogeographic changes (e.g.,Servais et al.,

2004; Vecoli and Le Herisse, 2004).

Acritarchs and prasinophyte phycomata are the major com-

ponents of Proterozoic and Paleozoic microphytoplankton

communities. Acritarchs (from the Greek akritos, uncertain,

confused; and arche, origin) are an informal and undoubtedly

polyphyletic groupof organic-walledmicrofossils of unresolved

biologic affinities. Most palynologists accept that acritarchs

are probably the cysts of various marine microphytoplanktonic

groups, and that many of them are pre-dinoflagellates (Playford,

2003).The Prasinophyceae are an extant class of marine green

algae, whose fossil record consists of the cyst-producing (orphycomata) stage in their life cycle.

Acritarchs and prasinophytes were the primary producers of

the Proterozoic and Paleozoic oceans. Thus, being at the base

of the marine food web, fluctuations in their diversity and abun-

dance would, presumably, have influenced to some extent the

evolution of the marine ecosystem. The Ordovician Period was

a time whenacritarchsexperienced theirgreatest radiation (more

than 1500 species are known from the Ordovician) and also mir-

rored the provincialism of marine invertebrates during the Early

and Middle Ordovician, as well as the cosmopolitanism of the

Late Ordovician (Vecoli and Le Herisse, 2004).

Although the Lower Ordovician has been more intensivelystudied palynologically than the Upper Ordovician, a fairly

extensive literature exists regarding the taxonomy, biostratig-

raphy, and paleogeographic distribution of Upper Ordovician

organic-walled microphytoplankton taxa. A profuse and varied

assemblage of acritarchs and prasinophytes from two Upper

Ordovician (Richmondian = Ashgill) localities in Michigans

Upper Peninsula is described here, following preliminary obser-

vations byWicander and Playford (1999). The assemblage is

compared to three other coeval Laurentian acritarch suites, and

the biostratigraphic, paleoenvironmental, and paleogeographic

significance of the assemblage is discussed in both regional and

global contexts.

2. Stratigraphy and age

2.1. Stratigraphy

The Bills Creek Shale, Stonington Formation, and Big Hill

Formation are the three uppermost Ordovician marine lithos-

tratigraphic units exposed successively in the Upper Peninsula

of Michigan, northern U.S.A. (Catacosinos et al., 2001).Hussey (1926: p. 121)named the Bills Creek beds (=Bills

Creek Shale) for thinly bedded shales and argillaceous lime-

stones exposed along the banks of Bills Creek in Michigans

Upper Peninsula and placed them in the Richmond portion of

the Cincinnatian series. Hussey (1952: p. 41) later described

an outcrop of Bills Creek beds at Haymeadow Creek, also

in the Upper Peninsula. However, he noted that the lower

Bills Creek beds, exposed at the falls on Haymeadow Creek,

may not belong to the Richmond. It now seems best to corre-

late these lower beds, at least, with the Collingwood [Middle

Ordovician]. . . (p. 43). Hussey (1952: p. 45) remarked that

these lower beds differed lithologically from the typical Bills

Creek beds at Bills Creek in being darker and not weatheringto a light gray color. He termed them the Haymeadow Creek

Member, which he stated was formerly the basal part of the

Bills Creek beds of Richmond age (Hussey, 1952: p. 13).

The Haymeadow Creek Member is not formally recognized,

the beds exposed at Haymeadow Creek being considered the

lowermost portion of the Bills Creek Shale (Catacosinos et al.,

2001).

The Bills Creek Shale consists of up to 26.8 m of mostly

thin-bedded gray shale and calcareous shale, with numerous

alternations of shale and argillaceous limestone near its top

(Hussey, 1926, 1952).It rests disconformably on limestone of

the Gross Quarry Member of the Middle Ordovician TrentonFormation (Catacosinos et al., 2001).

A disconformity also separates the Bills Creek Shale from

the overlying Stonington Formation, which is divided into two

members (Hussey, 1926: p. 132; Catacosinos et al., 2001).

The lower Bay de Noc Member comprises ca. 3.1 m of mas-

sive grayish-brown argillaceous limestone and gray calcareous

shale, succeeded by ca. 4.6m of alternating, thinly bedded gray

argillaceous limestone and gray calcareous shale. The 0.9 to

6.1 m-thick Ogontz Member conformably overlies the Bay de

Noc, and consists largely of gray to dark brown, massive and

irregularly bedded cherty limestone with some lenses of argilla-

ceous limestone.

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R. Wicander, G. Playford / Revue de micropalontologie 51 (2008) 3966 41

The Big Hill Formation is usually separated from the under-

lying Stonington Formation by a covered interval. Where

exposed, it consists predominantly of gray, argillaceous lime-

stone (Hussey, 1926).

Both the Bills Creek Shale and Stonington Formation con-

tain abundant and diverse marine faunas. In general, fossils are

plentiful and well-preserved in the limestones, and locally abun-

dant and well-preserved in the shales of the Bills Creek Shale

(Hussey, 1926, 1952). The fauna comprises graptolites, crinoids,

locally abundant bryozoans, brachiopods, bivalves, and ostra-

codes, as well as trilobites andCornulites.

The paleontologic and sedimentologic characteristics of the

Bills Creek Shale indicate deposition in a low-energy, shallow,

normal marine environment. Desiccation cracks, although rare,

are preserved in some of the calcareous shale layers, indicating

at least occasional aerial exposure. The argillaceous limestone

layers may represent more offshore conditions, farther from the

terrestrial sources for the shales.

An erosional break between the Bills Creek Shale and Ston-

ington Formation is indicated not only by their disconformablerelationship, but also by distinct differences in their faunal

content. Only a few taxa (e.g., the brachiopod Rafinesquina

alternata, bivalve Pterinae demissa, and ostracode Tetradella

regularis) occur in both formations (Hussey, 1926:p. 130). The

Bayde NocMember of theStonington Formation contains abun-

dant marine fossils, mainly in the argillaceous limestones. In

the Ogontz Member, fossils are most numerous in the cherty

limestones and include gastropod and trilobite taxa not found

in the Bay de Noc Member. Typical Stonington fossils include

crinoids, brachiopods, bivalves, gastropods, trilobites, and Cor-

nulites.

Deposition of the Bay de Noc Member was probably in alow-energy, offshore,normal marine environment, with periodic

terrigenous input as indicatedby theshales. TheOgontz Member

was very likely deposited under low-energy, offshore, normal

marine conditions.

2.2. Age

The latest global Ordovician chronostratigraphy and regional

series and stage correlations (Webby et al., 2004)indicate that

the base of theAmorphognathus ordovicicusconodont Biozone

occurs within the upper part of the Amplexograptus manitouli-

nensis graptolite Biozone, and thus slightly predates the base

of theDicellograptus complanatusgraptolite Biozone (Fig. 1).Furthermore, the base of theA. ordovicicuscondodont Biozone

is slightly higher than the base of the North American Richmon-

dian stage andthe base of theBritish Ashgill series. Accordingly,

the respective bases of the Richmondian and Ashgill are nearly

coincident (Webby et al., 2004:Figs. 2.1, 2.2).

Goldman and Bergstrom (1997: p. 980981) suggested a

possible two-part subdivision of the North American A. man-

itoulinensis graptolite Biozone, its upper part assigned to the

lower portion of the Richmondian stage (Fig. 1).Although not

formally proposing this subdivision, they thought it likely to

have biostratigraphic utility in the North American Midconti-

nent successions. Additionally,Goldman and Bergstrom (1997:

p. 981) noted that several Midcontinent successions contain

co-occurring conodonts and graptolites, thus facilitating pre-

cise correlation between the biozonal schemes based on each

of these groups. Particularly important for regional correlation

is the fact that Goldman and Bergstroms (1997) data support the

interpretation that the base of the A. ordovicicusconodont Bio-

zone is equivalent to a level within the upperA. manitoulinensis

graptolite Biozone.

The Stonington Formation was assigned by Goldman and

Bergstrom (1997: p. 980)to theD. complanatusgraptolite Bio-

zone (Richmondian = Ashgill) and theA. ordovicicusconodont

Biozone (Fig. 1).Moreover, Bergstrom (personal communica-

tion, 2001) has confirmed that his conodont collections from

shore exposures of the Stonington Formation signify a post-

Arnheim Richmondian age. The Arnheim Formation in the type

Cincinnati region is placed in the lower Richmondian stage,

which equates to the C3 Cincinnati sequence (Goldman and

Bergstrom, 1997:p. 983, Text-fig. 10).Bergstrom and Mitchell

(1986: p. 256)also noted that based on then available grap-

toloid evidence, particularly the occurrence ofGlyptograptusanacanthus, Mitchell and Bergstrom (1977: 261) concluded

that the basal Richmondian Arnheim Formation is coeval with

the upper A. manitoulinensis Zone in our 1977 study. Thus,

the Stonington Formation is attributable to the upperlower

to middle Richmondian (=D. complanatus graptolite Biozone).

Catacosinos et al. (2001: p. 37) placed the Stonington Formation

in the Upper Ordovician, Cincinnatian series, Richmond Group.

The underlying Bills Creek Shale is assigned to the upper

A. manitoulinensis graptolite Biozone, which is equivalent to

the lower Richmondian (Goldman and Bergstrom, 1997:p. 980,

Text-fig. 8). No conodonts have been described from either the

Bills Creek Shales type section or the Haymeadow Creek sec-tion. However, conodonts recovered by Votaw (1980a: p. 19)

from the upper part of the Bills Creek Shale in the Escan-

aba region of Michigans Upper Peninsula indicate attribution

to the Amorphognathus ordovicicus or possibly the uppermost

A. superbusconodont Biozone (lowest Richmondian;Goldman

and Bergstrom, 1997:p. 971): Fig. 1.

Goldman and Bergstrom (1997: p. 971) re-examined the

graptolite fauna of the Haymeadow Creek Shale (=Haymeadow

Creek Member ofHussey, 1952; lower Bills Creek Shale of

Catacosinos et al., 2001) exposed at Haymeadow Creek and

assigned it to the A. manitoulinensis graptolite Biozone. Fur-

thermore, they stated that the Haymeadow Creek section does

not contain the typical lower A. manitoulinensisgraptolite Bio-zone fauna [=upper Maysvillian stage] such as is present in the

Blue Mountain Formation of Manitoulin Island and in the Lor-

ranine Group siltstones and shales of New York (p. 971). Thus,

it appears that the Bills Creek Shale can, with reasonable con-

fidence, be placed in the lower Richmondian stage (upper A.

manitoulinensisgraptolite Biozone).

It should be noted that Goldman and Bergstrom (1997: p.

980, Text-fig. 8)showed only the upperA. manitoulinensisand

D. complanatus graptolite Biozones of the Richmondian in the

North American Midcontinent region. According toBergstrom

and Mitchell (1986: p. 259),there are no graptolite data use-

ful for classification of the middle and upper Richmondian in

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Fig. 1. Chronostratigraphic correlation of Richmondian palyniferous formations within the Cincinnatian series of North America. Correlation among the global

series, British stages, North American series and stages, graptolite and conodont biozones, and time slices is based onWebby et al. (2004).Radiometric dates based

onSadler and Cooper (2004).Biostratigraphic placement of the palyniferous Richmondian formations within the Ordovician time scale ofWebby et al. (2004)based

onJacobson and Achab (1985),Bergstrom and Mitchell (1986),andGoldman and Bergstrom (1997).

Fig.1.Correlations chronostratigraphiques des formationsfossiliferesdagerichmondienne dansles seriesduCincinnatienenAmerique duNord. Correlationsentre les

seriesglobales, lesetages Britanniques, les series et lesetages dAmerique du Nord, lesbiozonesa conodontes: dapres Webbyet al. (2004). Datations radiometriques

dapresSadler et Cooper (2004).Position biostratigraphique des formations fossiliferes dage richmondienne en rapporta lechelle chronostratigraphique deWebby

et al. (2004) :dapresJacobson et Achab (1985),Bergstrom et Mitchell (1986),etGoldman et Bergstrom (1997).

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terms of graptolite zones but indirect evidence suggests that this

interval is coeval with (part of?) theDicellograptus complanatus

Zone, and perhaps the lowermostClimacograptus inuitiZone.

Jacobson and Achab (1985: p. 168, Text-fig. 3) showed the

Dicellograptus ancepsgraptolite Biozone of Britain and Scan-

dinavia correlating to a position between the D. complanatus

andAmplexograptus inuitigraptolite Biozones of North Amer-

ica. This interval corresponds to the complexus and pacificus

graptolite Biozones of Britain and North America (Webby et

al., 2004).Therefore, the A. inuiti graptolite Biozone of North

America correlates to the Climacograptus extraordinarius grap-

tolite Biozone of Britain and Scandinavia (Jacobson and Achab,

1985), whichis approximately equivalent to the Hirnantianstage

of Britain and the Gamachian stage of North America (Webby

et al., 2004).

Fig. 1thus shows the Sylvan Shale of Oklahoma, Maquoketa

Formation of Missouri, and the Stonington Formation and Bills

Creek Shale of Michigans Upper Peninsula as occurring in the

graptolite and conodont biozones indicated by Goldman and

Bergstrom (1997).A dashed line indicates that the upper bios-tratigraphic extent of those formations within the Richmondian

is not known in accordance with the Ordovician timescale of

Webby et al. (2004). Placement of the Vaureal Formation of

Anticosti Island, Quebec, Canada, is based on Jacobson and

Achab (1985).

3. Materials and methods

Representative lithologies of the Bills Creek Shale and Ston-

ington Formation were sampled at two locations in Michigans

Upper Peninsula (Fig. 2). Eight samples of the Bills Creek

Shale (Locality 1) were collected from a beach exposure onthe east side of Little Bay de Noc; specifically, along the section

line between sec. 14 and 23, T. 39 N, R. 22 W, Delta County

(Votaw, 1980b:p. 24;Hussey, 1926:p. 126;Hussey, 1952:p.

41). Here, the 2.6 m interval of the Bills Creek Shale (Fig. 3)

consists of gray-brown, blocky shale weathering to a bluish-gray

flaggy shale (samples BC1, 3, 5, 6, 8) alternating with brown-

gray, argillaceous, fine-grained crystalline limestone weathering

bluish-gray (samples BC2, 4, 7).

Nine samples from a 3.95m section of the Bay de Noc Mem-

ber of the Stonington Formation (Locality 2) were collected

from a bluff along the east shore of Little Bay de Noc, south of

Stonington Community Hall (SE 1/4, sec. 26, T. 39 N, R. 22 W,

Delta County; Stop 5 ofVotaw, 1980c:p. 56;Hussey, 1926:p. 132). The sampled interval (Fig. 3)comprises the following:

0.65 m of gray, argillaceous, fine- to medium-grained crystalline

limestone, with occasional stringers of gray shale (samples S0,

1); 1.8 m of gray to gray-brown, calcareous, blocky shale (sam-

ples S2-5); 0.5 m of gray, argillaceous, fine- to medium-grained

crystalline limestone, with a few stringers of gray shale (sample

S6); 0.5 m of gray, calcareous, blocky shale, weathering gray-

brown (sample S7); and 0.5 m of gray, argillaceous, fine- to

medium-grained crystalline limestone with occasional stringers

of bluish-gray, calcareous, blocky shale (sample S8).

These are all composite samples, with thicknesses ranging

from 1060 cm, designed to provide an overview of acritarch

Fig. 2. Map showing the two sampling localities in Delta County of the Upper

Peninsula of Michigan from which eight studied samples of the Bills Creek

Shale and nine of the overlying Stonington Formation were collected for this

study (Fig. 3).

Fig. 2. Localisation des deux sites dechantillonnage dans le Comte de Delta de

la Peninsule superieure du Michigan.

diversity and abundance for each lithology. Thus, major changes

in acritarch composition between successive lithologies can be

ascertained, as well as variations among similar rock types at

differing stratigraphic levels (e.g., the thicker portions of theStonington Formation and Bills Creek Shale).

Laboratory preparation of the samples followed the standard

palynologic technique of dissolving 2530 g of rock succes-

sively in cold HCl, HF, and HNO3 for respective removal

of carbonates, silicates, and sulfides. Samples were neutral-

ized with distilled H2O between acid treatments. The resultant

residues were filtered through 52m and 20m nylon screens,

thus yielding >52m, 2052m, and 52m and 2052m

fractions, and two slides each of the 52m and 2052m fraction and one slide of the

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Fig. 3. Stratigraphic sections of the Richmondian (Upper Ordovician) Bills

Creek Shale and overlying Stonington Formation, Delta County, Upper Penin-

sula of Michigan, showing lithofacies and composite intervals sampled for this

study. Samples from Locality 1 are prefixed BC (=Bills Creek Shale) in text

andFig. 4;samples from Locality 2 are prefixed S (=Stonington Formation)in text andFig. 5.

Fig. 3. Sections stratigraphiques des formations de Bills CreekShale et de Ston-

ington,dagerichmondienne (Ordovicien Superieur), ComtedeDelta,Peninsule

superieure du Michigan, montrant les lithofacies et les niveaux echantillonnes

au cours de letude. Lesechantillons provenant de la localite 1 sont indiques

avec le prefixe BC (=Bells Creek Shale) dans le texte et dans laFig. 4 ;

les echantillons provenant de la localite 2 sont indiques avec le prefixe S

(=Stonington Formation) dans le texte et dans laFig. 5.

20m fraction were allowed to dry on

microscope slides where they were then picked using a micro-

manipulator (Leffingwell and Hodgkin, 1971) and transferred to

a circular no. 1 coverslip (12 mm diameter) that was pre-coated

with a very thin adhesive. The coverslip was mounted on an

aluminum stub and gold-platinum-coated for SEM examination

and imaging. Following SEM examination, the coverslip was

permanently mounted using Eukitt mounting medium on a

microscope slide.

Light photomicrographs were taken under Nomarski differ-

ential interference contrast illumination using an Olympus BH2

microscope equipped with an automatic photomicrographic sys-

tem. Specimens were photographed using Kodak black and

white 35 mm ISO 400 film. A JEOL JSM-840A SEM instru-

ment was used for detailed examination and electronic image

capture.

All illustrated microphytoplankton specimens (Plates 14)are deposited in the Carnegie Museum of Natural History, Pitts-

burgh, Pennsylvania, U.S.A., and are assigned catalog numbers

CMNH 19086 through CMNH 19147 (see Appendix A).

4. Systematic paleontology

The vast majority of the palynomorphs encountered in the

Bills Creek Shale and Stonington Formation are acritarchs,

with minor amounts of leiospheres and tasmanitids, which are

attributed to the prasinophycean green algae.

The acritarchs recorded hereunder are arranged alphabeti-cally by genera under the informalincertae sedisgroup name

Acritarcha, and are treated as form genera and species in accor-

dance with the provisions of the International Code of Botanical

Nomenclature (I.C.B.N.;Greuter et al., 2000).A complete syn-

onymy is not provided for all species. The original binomial

name andany subsequent generic transfersare listed; and, where

appropriate, a reference to a complete synonymy is cited. Mor-

phologic terminology followsWilliams et al. (2000).

The dimensions for all species are given, and where there

are three numbers, the first number is the minimum value, the

second number in parenthesis is the arithmetic mean, and the

third number is the maximum value. The dimensions of each

species are followed by the number of specimens measured.

Plate 1. Fig. 1.Tasmanitessp. 400.Figs. 2, 6. Leiosphaeridiasp.2. 400.6. 490.Figs. 3, 7. Baltisphaeridium adiastaltumWicander, Playford and Robertson,

1999. 400.Figs. 4, 8. Baltisphaeridium perclarumLoeblich and Tappan, 1978. 400.Fig. 5. Aremoricanium squarrosumLoeblich and MacAdam, 1971. 400.

Figs. 9, 10.Dactylofusa ctenista (Loeblichand Tappan, 1978) Fensome,Williams, Barss, Freeman and Hill,1990.400. Figs. 11,12,16, 17.Dactylofusaplaynetrella

(Loeblich and Tappan, 1978)Fensome, Williams, Barss, Freeman, and Hill, 1990.11, 12. 400.16. 600.17.Showing discontinuous nature of ridges,1200.Figs.

13, 14, 15. Dorsennidium hamii (Loeblich, 1970)Sarjeant and Stancliffe, 1994.13. 400. 14. 600. 15. 400. Figs. 18, 19. Dorsennidium undosumWicander,

Playford and Robertson, 1999. 400.Figs. 20, 21.Estiastrasp. A. 400.

Planche 1.Fig. 1.Tasmanitessp. 400.Figs. 2, 6. Leiosphaeridiasp.2. 400.6. 490.Figs. 3, 7.Baltisphaeridium adiastaltumWicander, Playford et Robertson,

1999. 400. Figs. 4, 8. Baltisphaeridium perclarum Loeblich et Tappan, 1978. 400. Fig. 5. Aremoricanium squarrosum Loeblich et MacAdam, 1971. 400.

Figs. 9, 10.Dactylofusa ctenista(Loeblich et Tappan, 1978)Fensome, Williams, Barss, Freeman et Hill, 1990.400.Figs. 11, 12, 16, 17. Dactylofusa playnetrella

(Loeblich et Tappan, 1978Loeblich et Tappan, 1978) Fensome, Williams, Barss, Freeman et Hill, 1990.11, 12. 400.16. 600.17. Montrant la nature discontinue

des rides, 1200. Figs. 13, 14, 15. Dorsennidium hamii(Loeblich, 1970)Sarjeant et Stancliffe, 1994.13. 400.14. 600.15. 400. Figs. 18, 19. Dorsennidium

undosumWicander, Playford et Robertson, 1999.

400.Figs. 20, 21. Estiastrasp. A.

400.

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Plate 2. Figs. 1, 2. Excultibrachium concinnum Loeblich and Tappan, 1978. 800. Figs. 3, 4, 7. Hoegklintia sp. cf. H. radicosa (Loeblich, 1970)Jacobson and

Achab, 1985.3. 800.4. 2000.7. 2000.Figs. 5, 6. Elektoriskossp. A. 800.

Planche 2.Fig. 1, 2.Excultibrachium concinnumLoeblich et Tappan, 1978. 800.Fig. 3, 4, 7.Hoegklintiasp. cf.H. radicosa(Loeblich, 1970)Jacobson et Achab,

1985.3. 800.4. 2000.7. 2000.Fig. 5, 6. Elektoriskossp. A. 800.

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Plate 3. Figs. 1, 2.Leiofusa litotesLoeblich and Tappan, 1978. 800.Figs. 3, 4.Lophosphaeridium varumWicander, Playford and Robertson, 1999. 800.Fig. 5.

Lophosphaeridium edenenseLoeblich and Tappan, 1978. 800. Fig. 6. Multiplicisphaeridium irregulareStaplin, Jansonius and Pocock, 1965. 800. Figs. 7, 10.

Micrhystridium prolixumWicander, Playford and Robertson, 1999. 800. Figs. 8, 11. Micrhystridium hirticulumWicander, Playford and Robertson, 1999. 800.

Fig. 9.Veryhachium europaeumStockmans and Williere, 1960. 800.

Planche 3. Fig. 1, 2. Leiofusa litotes Loeblich et Tappan, 1978. 800. Fig. 3, 4. Lophosphaeridium varum Wicander, Playford et Robertson, 1999. 800. Fig.

5. Lophosphaeridium edenense Loeblich et Tappan, 1978. 800. Fig. 6. Multiplicisphaeridium irregulare Staplin, Jansonius et Pocock, 1965. 800. Fig. 7, 10.

Micrhystridium prolixumWicander, Playford et Robertson, 1999. 800.Fig. 8, 11. Micrhystridium hirticulumWicander, Playford et Robertson, 1999. 800. Fig.

9.Veryhachium europaeumStockmans et Williere, 1960. 800.

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4.1. Prasinophyte phycomata

Division CHLOROPHYTAPascher, 1914

Class PRASINOPHYCEAEChristensen, 1962

Order PYRAMIMONADALESChadefaud, 1950

Family LEIOSPHAERIDIACEAE Timofeev, 1956 nom.

corr.Madler, 1963

GenusLeiosphaeridiaEisenack, 1958a

Type species: Leiosphaeridia baltica Eisenack, 1958a; by

original designation.

Leiosphaeridiaspp.

Plate 1,Figs. 2 and 6

Measurements:Vesicle diameter 54 (63) 70 m. (5 speci-

mens).

Discussion:There are a number of specimens that have a

simple, psilate, spherical vesicle and we attribute them, nonspe-

ciated, toLeiosphaeridia.

Occurrence: Bills Creek Shale and Stonington Forma-

tion (present study). Leiosphaeridia Eisenack, 1958a is a

widely occurring and stratigraphically long-ranging genus(Proterozoic-Recent).

Order PTEROSPERMATALESSchiller, 1925

Family TASMANITACEAESommer, 1956

GenusTasmanitesNewton, 1875

Type species:Tasmanites punctatusNewton, 1875;by orig-

inal designation.

Tasmanitesspp.

Plate 1,Fig. 1

Measurements:Vesicle diameter 44m, 132m, 145m.

(3 specimens).

Discussion: The specimens have a simple, psilate, thick-walled, spherical vesicle, and are attributed here, without

any attempt at speciation, to Tasmanites. They differ from

Leiosphaeridia in being generally larger with a thicker

eilyma.

Occurrence:Bills Creek Shale and Stonington Formation

(present study). Tasmanites Newton, 1875 is a widely occur-

ring and stratigraphically long-ranging genus (Proterozoic-

Recent).

4.2. Acritarchs

Group ACRITARCHAEvitt, 1963

GenusAremoricaniumDeunff, 1955

Type species: Aremoricanium rigaudiae Deunff, 1955;by


Aremoricanium squarrosumLoeblich and MacAdam, 1971

Plate 1,Fig. 5

1971.Aremoricanium squarrosumLoeblich and MacAdam,

p. 44, Pl. 18, Figs. 18.

1971.Aremoricanium syringosagis Loeblich and MacAdam,

p. 44, Pl. 18, Fig. 9.

Measurements:Vesicle diameter 76m; 12 processes; pro-

cess length 38+m; process width at base 78 m; neck length

36m; neck width 20m. (1 specimen).

Discussion: We follow Playford and Wicander (2006)

and others (e.g., Jacobson and Achab, 1985) in accepting

thatLoeblich and MacAdams (1971)species Aremoricanium

squarrosum andA. syringosagis nom. corr. syringosage are con-

specific.Occurrence: Stonington Formation (present study). Previ-

ously reported from the Edenian (upper Caradoc) of Indiana

and Ohio (Loeblich and MacAdam, 1971); Richmondian

(Ashgill) of Oklahoma (Loeblich and MacAdam, 1971;

Playford and Wicander, 2006) and Anticosti Island, Quebec,

Canada (Jacobson and Achab, 1985);?CaradocAshgill of

Gaspe, Quebec, Canada (Martin, 1980);and from coeval strata

in Europe, the Middle East, and North Africa (e.g., Molyneux

et al., 1996:Pl. 2, Fig. 7;Vavrdova, 1997:Fig. 3).

GenusBaltisphaeridium Eisenack, 1958b ex Eisenack, 1959

emend.Eisenack, 1969Type species: Baltisphaeridium longispinosum (Eisenack,

1931ex O. Wetzel, 1933) Eisenack, 1959; by original desig-

nation.

Discussion: Despite being the repository of more species

than any other acritarch genus, Baltisphaeridium Eisenack,

1958bexEisenack, 1959emend.Eisenack, 1969,remains one

of the most unsatisfactorily defined acritarch genera (Wicander

et al., 1999: p. 5). Because of the continued nomenclatural

Plate 4. Figs. 1, 2,10.Leiofusa fusiformis (Eisenack, 1934) Eisenack, 1938. 1. Showingfine striations,400. 2. 400. 10. 325. Fig. 3.Lophosphaeridium acinatum

Wicander, Playford and Robertson, 1999.

400.Figs. 4, 11. Gloeocapsomorpha priscaZalessky, 1917emend.Foster et al., 1989.4.

400. 11.

800. Figs. 5, 6.Peteinosphaeridium septuosumWicander, Playford and Robertson, 1999. 400.Figs. 7, 8.Navifusasp. A. 400.Fig. 9. Orthosphaeridium rectangulare(Eisenack,

1963)Eisenack, 1968. 400. Figs. 12, 13. Peteinosphaeridium septuosumWicander, Playford and Robertson, 1999. 12. 600. 13. Showing trilaminate nature of

processes,800.Figs. 1416.Veryhachium oklahomenseLoeblich, 1970.14. 400.15, 16. Showing psilate nature of eilyma and processes, 1000.Figs. 17, 18.

Polygonium gracileVavrdova, 1966emend.Sarjeant and Stancliffe, 1996.17. 400.18. 600.Figs. 19, 23. Veryhachium trispinosum(Eisenack, 1938)Stockmans

and Williere, 1962complex. 19. 400.23.Showing psilate-microgranulate surface of eilyma, 600.Figs. 20, 24. Villosacapsula setosapellicula(Loeblich, 1970)

Loeblich and Tappan, 1976.20. 400.24. Showing excystment by epityche, 600.Figs. 21, 22. Sylvanidium paucibrachiumLoeblich, 1970. 400.

Planche 4.Fig. 1, 2, 10.Leiofusa fusiformis(Eisenack, 1934)Eisenack, 1938.1. Montrant les tres fines rides, 400.2. 400.10. 325.Fig. 3. Lophosphaeridium

acinatum Wicander, Playford et Robertson, 1999. 400. Fig. 4, 11. Gloeocapsomorpha priscaZalessky, 1917emend. Foster, Reed et Wicander, 1989. 4. 400.

11. 800. Fig. 5, 6. Peteinosphaeridium septuosum Wicander, Playford et Robertson, 1999. 400. Fig. 7, 8. Navifusa sp. A. 400. Fig. 9. Orthosphaeridium

rectangulare(Eisenack, 1963)Eisenack, 1968. 400.Fig. 12, 13. Peteinosphaeridium septuosumWicander, Playford et Robertson, 1999.12. 600.13. Montrant

la structure trilaminate des processus,800.Fig. 1416.Veryhachium oklahomenseLoeblich, 1970.14. 400.15, 16.Montrant la nature psilate de leilyma et des

processus,1000.Fig. 17, 18.Polygonium gracileVavrdova, 1966emend.Sarjeant et Stancliffe, 1996.17. 400.18. 600.Fig. 19, 23.Veryhachium trispinosum

(Eisenack, 1938)Stockmans et Williere, 1962 complex. 19. 400.23.Montrant la surface psilate-microgranulate de leilyma, 600.Fig. 20, 24. Villosacapsula

setosapellicula(Loeblich, 1970)Loeblich et Tappan, 1976. 20. 400. 24. Montrant lexcystement par epityche, 600. Fig. 21, 22. Sylvanidium paucibrachium

Loeblich, 1970.

400.

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confusion concerning this genus, we follow Wicander et al.

(1999) in regarding assignment of the baltisphaerid species

herein as only provisional.

Baltisphaeridium adiastaltum Wicander, Playford and

Robertson, 1999

Plate 1,Figs. 3 and 71999.Baltisphaeridium adiastaltumWicander, Playford and

Robertson, p. 5, 7, Fig. 4.64.9.

For additional synonymy, see Wicander et al. (1999: p. 5).

Measurements: Vesicle diameter 40 (49) 60m; 1521 pro-

cesses; process length 21 (34) 50m; process width at base 2

(3.3) 4m. (13 specimens).

Discussion:For a complete discussion of this species and

comparison to similar species, see Wicander et al. (1999: p.

7).AlthoughBaltisphaeridium adiastaltumWicander, Playford

and Robertson, 1999 andB. oligopsakium Loeblich and Tappan,

1978 are morphologically similar, and tend to co-occur, all of

our specimens can definitely be attributed toB. adiastaltum.


(present study). Previously reported from the CaradocAshgill

of Kansas (Wright and Myers, 1981);Richmondian (Ashgill)

of Missouri (Wicander et al., 1999), Oklahoma (Playford

and Wicander, 2006), and Anticosti Island, Quebec, Canada

(Jacobson and Achab, 1985).

Baltisphaeridium perclarumLoeblich and Tappan, 1978


1978.Baltisphaeridium perclarumLoeblich and Tappan, p.

1253, Pl. 6, Figs. 57.

For additional synonymy, see Wicander et al. (1999: p. 9).

Measurements:Vesicle diameter 50 (56) 70m; 69 pro-


(8.8) 12m; plug height 4m. (7 specimens).




of Missouri (Robertson, 1997; Wicander et al., 1999)and Okla-

homa (Loeblich and Tappan, 1978; Playford and Wicander,

2006); CaradocAshgill of Gaspe, Quebec, Canada (Martin,

1980); Ashgill of northeast Libya (Molyneux, 1988; Hill and

Molyneux, 1988);?LlanvirnAshgill of Iran (Ghavidel-syooki,

2001); and possibly from the CaradocAshgill of Gotland,

Sweden (Eiserhardt, 1989) and Estonia (Uutela and Tynni,1991).

GenusDactylofusaBrito and Santos, 1965

Type species: Dactylofusa maranhensis Brito and Santos,

1965;by original designation.

Discussion: The generic categorization of sculptured

fusiform acritarchs such as Dactylofusa Brito and Santos,

1965, Poikilofusa Staplin, Jansonius and Pocock, 1965, and

Eupoikilofusa Cramer, 1971 is confused and controversial

(Fensome et al., 1990; Playford and Wicander, 2006). We

followFensome et al. (1990) in attributing to Dactylofusa the

species assigned to Eupoikilofusa by Loeblich and Tappan

(1978).

Dactylofusa ctenista(Loeblich and Tappan, 1978)Fensome,

Williams, Barss, Freeman and Hill, 1990


1978.Eupoikilofusa ctenistaLoeblich and Tappan, p. 1263,Pl. 8, Figs. 8, 9.

1990.Dactylofusa ctenista (Loeblich and Tappan, 1978) Fen-

some, Williams, Barss, Freeman and Hill, p. 180.

Measurements:Vesicle length 150 (160)176m; maximum

vesicle width 27 (33) 39m. (5 specimens).

Discussion: Loeblich and Tappan (1978) named four new

species ofEupoikilofusarecovered from the Upper Ordovician

Sylvan Shale, Oklahoma. All were based on very few speci-

mens (apparently only one specimen in three of the species,

and five specimens in the other). Loeblich and Tappan (1978:

p. 1263) differentiated E. ctenista from E. parvuligranosa on

the basis of the latter possessing small grana that may be

aligned in rows (see alsoWright and Myers, 1981:p. 24, Pl.

3, Fig. H). Such a distinction based on so few specimens and

the fact that all other morphologic features, including size are

the same, appears tenuous. However, all of our specimens are

nongranulate and clearly fit the circumscription forDactylofusa

ctenista.


(present study). Previously reported from the Richmondian

(Ashgill) of Oklahoma (Loeblich and Tappan, 1978; Playford

and Wicander, 2006); Ashgill of northeast Libya (Molyneux,

1988); Ordovician/Silurian boundary of northwest Argentina

(Rubinstein and Vaccari, 2004).

Dactylofusa platynetrella(Loeblich and Tappan, 1978)Fen-

some, Williams, Barss, Freeman and Hill, 1990

Plate 1,Figs. 11, 12, 16 and 17

1978. Eupoikilofusa platynetrella Loeblich and Tappan, p.

12631264, Pl. 8, Fig. 10.

1990.Dactylofusa platynetrella (Loeblich and Tappan, 1978)

Fensome, Williams, Barss, Freeman and Hill, p. 215.

Measurements:Vesicle length 99 (116) 135 m; maximum


Discussion:Dactylofusa platynetrella (Loeblich and Tappan,

1978)is characterized as having an excentric, thin-walled rel-

atively broad fusiform vesicle ornamented with discontinuousridges (Loeblich and Tappan, 1978: p. 1263). It is probably con-

specific with D. striata (Staplin, Jansonius and Pocock, 1965)

Fensome, Williams, Barss, Freeman and Hill, 1990, the only

difference seemingly being that the vesicle ofD. platynetrella

is broader and more asymmetric than D. striata. These fea-

tures were noted byLoeblich and Tappan (1978: p. 1264) and

possibly result from postdepositional compression (Jacobson

and Achab, 1985:p. 182). All of our specimens tend to be of

the broad and compressed variety as illustrated by the holo-

type (Loeblich and Tappan, 1978: Pl. 8, Fig. 10) and figured

as Eupoikilofusa striata inJacobson and Achab (1985: Pl. 2,

Fig. 2).

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tion (present study). Previously reported from Richmondian

(Ashgill) strata of Oklahoma (Loeblich and Tappan, 1978);

Anticosti Island, Quebec, Canada (Jacobson and Achab, 1985);

Ashgill of Algerian Sahara and southern Tunisia (Vecoli,

1999).

GenusDorsennidiumWicander, 1974emend.Sarjeant and

Stancliffe, 1994

Type species: Dorsennidium patulum Wicander, 1974; by


Dorsennidium hamii (Loeblich, 1970) Sarjeant and

Stancliffe, 1994

Plate 1,Figs. 13, 14 and 15

1970.Veryhachium hamiiLoeblich, p. 741, Fig. 35AF.

1994. Dorsennidium? hamii (Loeblich, 1970) Sarjeant and

Stancliffe, p. 40 (assignment provisional).

Measurements:Vesicle sides 42 (53) 60m long side and

20 (31) 40m short side; 3 major processes in same plane 18

(30) 40m long, 4 (5.5) 7m wide at base; 13 supplementary

processes 20 (30) 36m long, 4 (5) 6m wide at base (13

specimens).

Discussion: Although Sarjeant and Stancliffes (1994: p.

40)assignment ofVeryhachium hamiiLoeblich, 1970toDors-

ennidium Wicander, 1974 was only made provisionally, we

regard such assignment as appropriate for the reasons stated

byWicander et al. (1999: p. 11).




of Missouri (Miller, 1991; Wicander et al., 1999), Oklahoma

(Loeblich, 1970; Playford and Wicander, 2006), and Anti-costi Island, Quebec, Canada (Jacobson and Achab, 1985);

Mohawkian (Caradoc) and Edenian (upper Caradoc) of St.

Lawrence Lowland, Quebec and southeast Ontario, Canada

(Martin, 1983); CaradocAshgill of Czech Republic (Vavrdova,

1988),Morocco (Elaouad-Debbaj, 1988),and Gotland, Sweden

(Eiserhardt, 1992).

Dorsennidium undosumWicander, Playford and Robertson,

1999


1981.Veryhachium hamiiauct. nonLoeblich, 1970.Wright

and Meyers, p. 2930, Pl. 3, Figs. J, N, Pl. 8, Fig. G only.1999. Dorsennidium undosum Wicander, Playford and

Robertson, p. 11, 13, Fig. 7.17.4.


cesses; process length 22 (27) 34 m; process width at base 2





of Missouri (Wicander et al., 1999)and Oklahoma (Playford

and Wicander, 2006). Li et al. (2006)recordedDorsennidium

cf. D. undosum from the Caradoc of Xinjiang, northwestern

China.

GenusElektoriskosLoeblich, 1970

Type species:Elektoriskos auroraLoeblich, 1970;by origi-

nal designation.

Elektoriskossp. A


Description:Vesicle originally spherical, outline subcircu-

lar. Eilyma psilate, ca. 0.5m thick. Numerous (30+), discrete,

generally evenly distributed, elongate, spine-like, homomor-

phic, straight to curving, solid, psilate processes; proximal

contacts orthogonal to slightly curved at base; processes

tapering gently to acuminate tips. No excystment structure

observed.

Measurements:Vesicle diameter 30 38m, 2432m;

process length 1218m, 1416m; process width at base ca.

1m. (2 specimens).

Discussion: This species superficially resembles Elek-

toriskos aktinotosWicander, Playford and Robertson, 1999 but

has a larger vesicle and longer processes. Because the ratio of

process length to vesicle diameter is nearly the same for both

species, it is possible that these specimens are just larger. How-ever, because only two specimens were found, we prefer to leave

them in open nomenclature.

Occurrence:Stonington Formation (present study).

Genus Estiastra Eisenack, 1959 emend. Sarjeant and

Stancliffe, 1994

Type species:Estiastra magna Eisenack, 1959;by original

designation.

Estiastrasp. A


Description:Vesicle outline stellate. Vesicle formed by the

confluence of 57 broad-based, hollow processes, not in the

same plane. Processes open into and freely communicate with

vesicle interior. Processes taper to acuminate tip. Eilyma and

process walls thin,

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GenusExcultibrachiumLoeblich and Tappan, 1978emend.

Turner, 1984

Type species: Excultibrachium concinnum Loeblich and

Tappan, 1978;by original designation.

Excultibrachium concinnum Loeblich and Tappan, 1978


1978.Excultibrachium concinnum Loeblich and Tappan, p.

12671268, Pl. 9, Figs. 36.

1979.Ordovicidium gracileColbath, 1979,p. 23, Pl. 8, Figs.

47.

1984.Excultibrachium oligocladatum Turner, p. 110, Pl. 9,

Figs. 7, 9.



(3.6) 4m; furcation length 4 (6) 8m. (13 specimens).

Discussion:As noted byWicander et al. (1999: p. 13),only

minor morphologic differences exist betweenExcultibrachium

concinnum Loeblich and Tappan, 1978, E. oligocladatum

Turner, 1984, and Ordovicidium gracile Colbath, 1979, such that

they should be considered conspecific. Our specimens, whilefalling within the range of published measurements have, on

average, fewer and shorter processes, but otherwise fit the cir-

cumscription ofE. concinnum.


(present study). Previously reported from the Edenian (upper

Caradoc) of Indiana (Loeblich and Tappan, 1978; Colbath,

1979); Richmondian (Ashgill) of Missouri (Wicander et al.,

1999) and Anticosti Island, Quebec, Canada (Jacobson and

Achab, 1985); CaradocAshgill of Labrador Sea, Canada

(Legault, 1982);Caradoc (Harnagian-Actonian) of Shropshire,

England (Turner, 1984); LlanvirnCaradoc (Viruan) of Got-

land, Sweden (Gorka, 1987);Caradoc and Ashgill of Gotland,Sweden (Eiserhardt, 1992); possibly from LlanvirnAshgill

of Estonia (Uutela and Tynni, 1991); Caradoc of Xinjiang,

northwestern China (Li et al., 2006).

Genus Gloeocapsomorpha Zalessky, 1917 emend. Foster,

Reed and Wicander, 1989

Type species:Gloeocapsomorpha priscaZalessky, 1917;by

monotypy.

Gloeocapsomorpha prisca Zalessky, 1917 emend. Foster,

Reed and Wicander, 1989


1917. Gloeocapsomorpha prisca Zalessky, p. 814, Figs.

13, 69.

1989. Gloeocapsomorpha prisca Zalessky, 1917 emend.Fos-

ter, Reed and Wicander,p. 743745, Pl. 1, Figs. 112; Pl. 2, Figs.

116; Text-figs. 2, 57.1, 7.3, 7.5, 7.6.

Measurements:The size of the cells, colonies, and super-

colonies varies, depending on the number of cell divisions and

whether they have been broken during palynologic processing.

Discussion: Gloeocapsomorpha prisca Zalessky, 1917

emend.Foster et al., 1989is an enigmatic palynomorph whose

biological affinities are still unclear. Until its systematic position

is resolved, we provisionally place it with the acritarchs.


tion (present study). Recorded worldwide from Ordovician

sequences, but most commonly in Middle Ordovician strata

(Wicander et al., 1996).

GenusHoegklintiaDorning, 1981

1981.HogklintiaDorning, p. 192.

1988. Hoegklintia Dorning nom. corr. Eley and Legault,

1988,p. 58, 63.

Type species: Hoegklintia visbyensis (Eisenack, 1959)

Dorning, 1981;by original designation.

Hoegklintia sp. cf. H. radicosa (Loeblich, 1970)Jacobson

and Achab, 1985

Plate 2,Figs. 3, 4 and 7

1970. cf.Multiplicisphaeridium radicosumLoeblich, p. 730,

Fig. 23AE.

1985. cf. Hoegklintia radicosa (Loeblich, 1970) Jacobson

and Achab, p. 183, Pl. 4, Fig. 2.

Description: Vesicle subcircular in outline. Eilyma thin

(

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1938.Leiofusa fusiformis(Eisenack, 1934)Eisenack, p. 28.



Discussion: Our specimens include those whose psilate vesi-

cle is both symmetrical and slightly asymmetrical around the

long axis and bears equal processes, together with those

that have essentially the same morphologic range but feature

just-visible discontinuous longitudinal striae that are probably

induced by compression along the long axis.

This range of morphology encompasses a variety of species

assigned to Leiofusa; viz., L. fusiformis as illustrated by

Jacobson and Achab (1985), Eupoikilofusa striatifera as illus-

trated by Cramer (1971) and Jacobson and Achab (1985),

Dactylofusa striatiferaas illustrated byMolyneux et al. (1996),

L. elenae as illustrated by Cramer (1971), andL. asymmetrica as

illustrated by Colbath (1979). It is beyond the scope of this study

to determine the amount of morphologic variation acceptable in

circumscribing these species.

Thereis a morphologic continuum (i.e., between symmetrical

and slightly asymmetrical vesicle shape, and psilate to faintlystriate eilyma) that is probably reflective of variation found in

natural populations. Accordingly, we assign our specimens to

L. fusiformis (Eisenack, 1934) Eisenack, 1938, which fits the

morphology most commonly encountered in our samples.


(present study). Depending on which species are considered

synonymous, the range ofL. fusiformis is Upper Ordovician

through Silurian.

Leiofusa litotes Loeblich and Tappan, 1978


1978.Leiofusa litotesLoeblich and Tappan, p. 1271, Pl. 12,Figs. 1, 2.





of Kansas (Wright and Myers, 1981);Richmondian (Ashgill) of

Missouri (Miller, 1991)and Oklahoma (Loeblich and Tappan,

1978).

Provisionally reported (as Leiofusa aff. L. litotes) by Hill

and Molyneux (1988)andMolyneux (1988)from Well E1-81,

Libya, in rocks dated as Ashgill. Elaouad-Debbaj (1988: p.

237) cited a questionable and also reputedly Ashgill occurrence(Leiofusa litotes?) from the Anti-Atlas of Morocco, but did

not illustrate any specimens. Rubinstein and Vaccari (2004)

recorded Leiofusa cf. L. litotes from the Ordovician/Silurian

boundary in northwest Argentina.

GenusLophosphaeridiumTimofeev, 1959exDownie, 1963

Type species:Lophosphaeridium rarumTimofeev, 1959;by

subsequent designation ofDownie (1963).

Discussion:We followColbaths (1990: p. 116)suggestion

that Lophosphaeridium Timofeev, 1959 exDownie, 1963 be

restricted to species with simple processes that are less than

twice as long as wide at base, thus facilitating distinction of

that genus from Gorgonisphaeridium Staplin, Jansonius and

Pocock, 1965.

Lophosphaeridium acinatum Wicander,Playford and Robert-

son, 1999

Plate 4,Fig. 3

1999.Lophosphaeridium acinatum Wicander, Playford andRobertson, p. 15, Fig. 8.68.11.

Measurements: Vesicle diameter 33m, 50m, 54m;

grana height ca. 1.1m; grana width 11.5m. (3 specimens).

Occurrence: Stonington Formation (present study). Previ-

ously reported from the Richmondian (Ashgill) of Missouri

(Wicander et al., 1999).

Lophosphaeridium edenenseLoeblich and Tappan, 1978

Plate 3,Fig. 5

1978.Lophosphaeridium edenenseLoeblich and Tappan, p.

12721273, Pl. 14, Figs. 4, 5.

Measurements: Vesicle diameter 34m, 42m; granaheight ca. 0.8m; grana width ca. 0.6m. (2 specimens).


(present study). Previously reported from the Caradoc and

Ashgill of Kansas (Wright and Myers, 1981); Richmondian

(Ashgill) of Missouri (Wicander et al., 1999) and Anticosti

Island, Quebec, Canada (Jacobson and Achab, 1985);Edenian

(upper Caradoc) of Kentucky and Indiana (Loeblich and

Tappan, 1978);Caradoc of Tarim Basin, China (Li and Wang,

1997)and Xinjiang, northwestern China (Li et al., 2006).

Lophosphaeridium varum Wicander, Playford and Robert-

son, 1999Plate 3,Figs. 3 and 4

1999. Lophosphaeridium varum Wicander, Playford and

Robertson, p. 1516, Fig. 9.19.5.

Measurements: Vesicle diameter 32 (37) 44m; grana

height ca. 11.5m; grana width ca. 0.81m. (4 specimens).



(Ashgill) of Missouri (Wicander et al., 1999).

GenusMicrhystridiumDeflandre, 1937

Type species: Micrhystridium inconspicuum Deflandre,

1937;by original designation.Micrhystridium hirticulumWicander, Playford and Robert-

son, 1999


1999. Micrhystridium hirticulum Wicander, Playford and

Robertson, p. 17, Fig. 9.69.8.

Measurements:Vesicle diameter 22 (25) 30m; >30 pro-

cesses; process length 10 (11) 14 m; process width at base ca.

1.5m. (9 specimens).



(Ashgill) of Missouri (Wicander et al., 1999)and Oklahoma

(Playford and Wicander, 2006).

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Micrhystridium prolixum Wicander, Playford and Robertson,

1999


1999. Micrhystridium prolixum Wicander, Playford and

Robertson, p. 17, Figs. 9.13, 10.7.


cesses; process length 16 (19) 21m; process width at base

23m. (5 specimens).



(Ashgill) of Missouri (Wicander et al., 1999) and Oklahoma

(Playford and Wicander, 2006); Caradoc of Xinjiang, north-

western China (Li et al., 2006).

GenusMultiplicisphaeridium Staplin, 1961 emend. Sarjeant

and Vavrdova, 1997

Type species:Multiplicisphaeridium ramispinosumStaplin,

1961emend.Sarjeant and Vavrdova, 1997;by original designa-

tion.

Multiplicisphaeridium irregulare Staplin, Jansonius and

Pocock, 1965

Plate 3,Fig. 6

1965. Multiplicisphaeridium irregulare Staplin, Jansonius

and Pocock, p. 183, Pl. 18, Fig. 18 (non Fig. 17).


cesses; process length 12 (15) 19m; process width at base

23m. (7 specimens).

Discussion: Playford and Wicander (2006) discussed the

differences betweenMultiplicisphaeridium bifurcatumStaplin,

Jansoniusand Pocock, 1965 andM. irregulare Staplin, Jansonius

and Pocock, 1965, which are morphologically similar and typ-

ically occur together. The main difference is that M. irregulareexhibits a greater degree of process heteromorphy thanM. bifur-

catum. Despite the fact these two species tend to co-occur, all of

our specimens from the Stonington Formation exhibit a high

degree of process heteromorphy (not the near-homomorphic

processes with regular first-order distal bifurcation character-

izingM. bifurcatum).

Occurrence: Stonington Formation (present study). See

Wicander et al. (1999) for a listing of most previous occur-

rences. Molyneux et al. (1996) considered this species to

be cosmopolitan in basal Caradoc through upper Ashgill

strata.

Genus Navifusa Combaz, Lange and Pansart, 1967 ex

Eisenack, 1976

Type species: Navifusa navis (Eisenack, 1938) Eisenack,

1976;by original designation.

Navifusasp. A


Description:Vesicle navicular, sides straight and paral-

lel, ends rounded to broadly rounded. Eilyma thin, psilate. No

excystment structure observed.



Occurrence:Bills Creek Shale (present study).

Discussion: Navifusa sp. A superficially resembles N.

indianensis Loeblich and Tappan, 1978 and Leiovalia teretis

(Loeblich, 1970) Loeblich and Tappan, 1978. N. indianensis

was described as ornamented with a dense cover of fine,

variably spaced punctae (Loeblich and Tappan, 1978: p.

1277), whereas the present specimens are psilate. However,

the punctae are not obvious on the high-magnification figure

of the eilyma of N. indianensis (Loeblich and Tappan, 1978:

Pl. 13, Fig. 4). Furthermore, only one measurement was given

(holotype), and our specimens are larger than the holotype.

Navifusa sp.A is comparable in size to L. teretis as well as

having a psilate eilyma. Loeblich and Tappan (1978: p. 1272)

transferred teretis from Navifusa to Leiovalia because of the

elongate-ovate vesicle and absence of parallel sides in the

vesicle of the 19 specimens observed. Our specimens have

straight, parallel sides and resemble the specimens illustrated

byLoeblich (1970)andLoeblich and Tappan (1978).However,

because of the scarcity and somewhat poor preservation of our

specimens, we are leaving them in open nomenclature, rather

than making a definite attribution.

GenusOrthosphaeridiumEisenack, 1968

Type species: Orthosphaeridium rectangulare (Eisenack,

1963)Eisenack, 1968;by original designation.

Orthosphaeridium rectangulare(Eisenack, 1963)Eisenack,

1968

Plate 4,Fig. 9

1963. Baltisphaeridium rectangulare Eisenack, p. 211, Pl.

20, Figs. 13, 10.

1968. Orthosphaeridium rectangulare (Eisenack, 1963)

Eisenack, p. 92, Pl. 25, Fig. 1.

1970. Orthosphaeridium inflatum Loeblich, p. 733734, Fig.

29ae.

For additional synonymy, see Wicander et al. (1999: p.

1921).

Measurements: Vesicle diameter 52 (59) 66 m; 4 pro-

cesses; process length 62 (75) 106m; process width 46m;

process basal plug 56m thick (9 specimens).

Discussion: All specimens encountered ofOrthosphaerid-

ium rectangulare (Eisenack, 1963) Eisenack, 1968 are either

fragments or half-vesicles, having broken apartalongthe median

excystment split.


(present study). Previously reported from the Richmondian(Ashgill) of Missouri (Miller, 1991; Wicander et al., 1999)

and Oklahoma (Loeblich, 1970; Playford and Wicander,

2006);RichmondianGamachian (Ashgill) of Anticosti Island,

Quebec, Canada (Jacobson and Achab, 1985; Martin, 1988);

LlanvirnCaradoc (Viruan) of Gotland, Sweden (Kjellstrom,

1971);?LlanvirnAshgill of Iran (Ghavidel-syooki, 2001,2003);

Caradoc and Ashgill of Gotland, Sweden (Eisenack, 1968;

Eiserhardt, 1985); and Ashgill of Estonia (Uutela and Tynni,

1991),Morocco (Elaouad-Debbaj, 1988),and Jordan (Keegan

et al., 1990).Molyneux et al. (1996: Text-fig. 8)described this

species as cosmopolitan with a stratigraphic range of basal

Caradoc through upper Ashgill.

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GenusPeteinosphaeridium Staplin, Jansonius and Pocock,

1965 emend. Playford, Ribecai and Tongiorgi, 1995

Type species: Peteinosphaeridium bergstroemii Staplin,

Jansonius and Pocock, 1965 emend. Playford, Ribecai and Ton-

giorgi, 1995; by original designation.

Peteinosphaeridium septuosum Wicander, Playford and

Robertson, 1999

Plate 4,Figs. 5, 6, 12, and 13

1981.Baltisphaeridiumsp. b of Wright and Myers, p. 22, Pl.

5, Figs. AC.

1999. Peteinosphaeridium septuosum Wicander, Playford

and Robertson, p. 21, 23, Figs. 11.511.9, 12.2, 12.3.

Measurements:Vesicle diameter 48 (63) 75m; 14 (15)

18 processes; process length 6 (8) 11m; process width at

base 2m. (23 specimens). Pylome diameter 14 (15) 18 m.

(9 specimens).


(present study). Previously reported from the Caradoc and

Ashgill of Kansas (Wright and Myers, 1981); Richmondian

(Ashgill) of Missouri (Wicander et al., 1999)and Oklahoma(Playford and Wicander, 2006).

Genus Polygonium Vavrdova, 1966 emend. Sarjeant and

Stancliffe, 1994

Type species:Polygonium gracileVavrdova, 1966;by orig-

inal designation.

Polygonium gracile Vavrdova, 1966 emend. Sarjeant and

Stancliffe, 1996


1966.Polygonium gracileVavrdova, p. 413, Pl. 1, Fig. 3, Pl.

3, Fig. 1.

1996. Polygonium gracile Vavrdova, 1966emend. Sarjeant

and Stancliffe, p. 359360.

For complete synonymy, seeSarjeant and Stancliffe (1996:

p. 359360).




Discussion: The Bills Creek Shale and Stonington For-

mation specimens fit the species emendation of Sarjeant

and Stancliffe (1996). The specimens are, however, some-

what variable in vesicle diameter, number of processes, and

process length. As discussed by Wicander et al. (1999: p.

23), this represents a morphologic continuum that evidentlyreflects morphologic variation within a population. If only

the end members of the continuum were observed, they

might be considered as specifically distinct, but viewed as

a whole, the entire population must be considered a single

species.



(Ashgill) of Missouri (Wicander et al., 1999), Oklahoma

(Playford and Wicander, 2006),and Anticosti Island, Quebec,

Canada (Jacobson and Achab, 1985).Upper Cambrian through

Devonian strata globally elsewhere (Playford and Wicander,

2006).

GenusSylvanidiumLoeblich, 1970

Type species:Sylvanidium paucibrachiumLoeblich, 1970;

by original designation, monotypy.

Sylvanidium paucibrachiumLoeblich, 1970


1965. Acritarchous hystrichosphere of Staplin, Jansonius and

Pocock, p. 185, Pl. 19, Fig. 15.

1970. Sylvanidium paucibrachium Loeblich, p. 737, Fig.

32AF.

Measurements:Vesicle length 54 (69) 80 m; vesicle width

44 (59) 74m; 46 processes; process length 14 (22) 34 m;

process width at base 2 (4.1) 6 m. (11 specimens).

Discussion: Jacobson and Achab, (1985: p. 193) commented

on the superficial resemblance between Sylvanidium pau-

cibrachium Loeblich, 1970 and Dorsennidium hamii, and on

their tendency to co-occur.Playford and Wicander (2006)also

noted the similarity in morphology and conjectured that the two

species may intergrade morphologically. We have observed the

same similarity in morphology between the two species, and

follow Playford and Wicander (2006) in separating them pri-marily on vesicle shape; that is, bell-shaped for D. hamii, and

bean-shaped forS. paucibrachium.

Occurrence: Stonington Formation (present study). Previ-

ously reported from the Richmondian (Ashgill) of Oklahoma

(Loeblich, 1970; Playford and Wicander, 2006) and Quebec,

Canada (Staplin et al., 1965; Martin, 1980; Jacobson and

Achab, 1985).According toMolyneux et al. (1996: Text-fig. 8),

S. paucibrachium is restricted to themiddle Ashgill of Laurentia.

GenusVeryhachiumDeunff, 1954exDownie, 1959

Type species: Veryhachium trisulcum (Deunff, 1951) ex

Deunff, 1959;by subsequent designation ofDownie (1959).

Veryhachium europaeumStockmans and Williere, 1960

Plate 3,Fig. 9

1960. Veryhachium europaeum Stockmans and Williere, p.

3, Pl. 2, Fig. 25.

Measurements:Vesicle diameter 48m,50m; 3 processes

drawn out from plane of vesicle, 23 supplementary processes

arising from plane of vesicle; process length 14 m, 22m. (2

specimens).


(present study). Veryhachium europaeum Stockmans and

Williere, 1960 is a cosmopolitan species with a previously

recorded stratigraphic range of Silurian through Devonian(Wicander and Wood, 1981).

Veryhachium oklahomenseLoeblich, 1970

Plate 4,Figs. 1416

1970.Veryhachium oklahomenseLoeblich, p. 742743, Fig.

36F, G.

1985. Veryhachium lairdii (auct. non Deflandre) Deunff ex

Downie, 1958.Jacobson and Achab, p. 195, Pl. 9, Fig. 2.

Measurements:Vesicle length 14 (28) 42 m; vesicle width

14 (34) 48m; process length 8 (22) 30m; process width at

base 4 (4.5) 6m. (6 specimens).

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(present study). See Wicander et al. (1999) for a complete listing

of previous occurrences, all but one of which are Upper Ordovi-

cian, and that one (Uutela and Tynni, 1991 ArenigLlanvirn)

we consider to be questionable. Ghavidel-syooki (2001,

2003) has recorded the species from Iranian strata dated as

LlanvirnAshgill, andRubinstein and Vaccari (2004)noted its

occurrence from the Ordovician/Silurian boundary of northwest

Argentina.

Veryhachium trispinosum(Eisenack, 1938)Stockmans and

Williere, 1962complex


1938.Hystrichosphaeridium trispinosum Eisenack, p. 14, 16,

Text-figs. 2, 3.

1954. Veryhachium trispinosum (Eisenack, 1938) Deunff,

p. 306. [combination invalid: ICBN, Article 33.2].

1962. Veryhachium trispinosum (Eisenack, 1938) Stockmans

and Williere, p. 4647, Pl. II, Figs. 25, 26, Text-fig. 1.

1981. Veryhachium trispinosum (Eisenack, 1938) Deunff,

1954complex. Wicander and Wood, p. 6771.

For additional synonymy, see Wicander and Wood (1981:

p. 6770).

Measurements: Vesicle diameter 28 (37) 40m; process

length 14 (22) 26m. (14 specimens).

Discussion:The Bills Creek Shale and Stonington Forma-

tion specimens form a morphologic continuum in which the

vesicle sides are straight to weakly convex, the spine-like pro-

cesses are drawn out from the vesicle corners, and the eilyma

and process wall are thin and psilate or scabrate under light

microscopy, and psilate to microgranulate under scanning elec-

tron microscopy. When viewed as a whole population, it isreasonable to regard these morphologically simple specimens

as part of a complex rather than speciating them on the

basis of minor morphologic variations (Wicander and Wood,

1981).


(present study). This species complex has a cosmopolitan

distribution and ranges from Ordovician through Permian

(Wicander and Wood, 1981:p. 71).

GenusVillosacapsulaLoeblich and Tappan, 1976

Type species: Villosacapsula setosapellicula (Loeblich,

1970)Loeblich and Tappan, 1976;by original designation.Villosacapsula setosapellicula (Loeblich, 1970) Loeblich

and Tappan, 1976


1970. Veryhachium setosapellicula Loeblich, p. 743, Figs.

36a,b, 37a,b.

1971.Veryhachium calandraeCramer, p. 106, Pl. 6, Fig. 99,

Text-fig. 29a.

1976.Villosacapsula setosapellicula (Loeblich, 1970)Loe-

blich and Tappan, p. 307.

Measurements: Vesicle diameter 28 (32) 40m; process

length 12 (20) 30m; process width at base 2 (2.5) 3m. (9

specimens).

Discussion: As noted by Wicander et al. (1999: p. 27),

Cramers (1971) description and line-drawing of a specimen

from the Maysville Formation (Upper Ordovician: Caradoc)

of Ohio indicate that it probably belongs to Villosacapsula

setosapellicula (Loeblich, 1970) Loeblich and Tappan, 1976.

Cramer (1971)assigned the specimen to Veryhachium calan-

draeCramer, 1971but omitted to designate a holotype for that

species, which accordingly is invalid.


tion (present study). Previously reported from the Edenian

(upper Caradoc) of Indiana (Colbath, 1979);CaradocAshgill

of Kansas (Wright and Myers, 1981);Richmondian (Ashgill) of

Missouri (Miller, 1991; Robertson, 1997; Wicander et al., 1999)

and Oklahoma (Loeblich, 1970; Playford and Wicander, 2006);

Mohawkian (Caradoc) and Edenian (upper Caradoc) of Quebec

and Ontario, Canada (Martin, 1983); upper Arenig-Llanvirn

and Caradoc-?Ashgill of Algerian Sahara (Jardine et al., 1974);

Ashgill of Algerian Sahara and southern Tunisia (Vecoli, 1999);

Caradoc of Shropshire, England (Turner, 1984)and northwest

Libya (Deunff and Massa, 1975); CaradocAshgill of CzechRepublic (Vavrdova, 1988)and Jordan (Keegan et al., 1990);

Ashgill of northeast Libya (Molyneux and Paris, 1985; Hill and

Molyneux, 1988); Ordovician/Silurian boundary of northwest

Argentina (Rubinstein and Vaccari, 2004).Probably also from

the upper Caradoc of Ohio (Cramer, 1971;see above synonymy

and discussion). The reputedly pre-Caradoc occurrence by

Jardineetal.(1974) lacksdescriptive and illustrative documenta-

tion, in addition to a questionable palynological age attribution,

and is accordingly considered unsubstantiated (Vecoli, 1999).

Ghavidel-syooki (2001, 2003)recorded this species from Iranian

strata he dated as LlanvirnAshgill.

5. Composition of the palynoflora

All 17 sampled intervals from the Bills Creek Shale (eight

samples) and Bay de Noc Member of the Stonington Formation

(nine samples) yielded a generally diverse and advantageously

preserved palynomorph assemblage (Figs. 4 and 5).The major-

ity of samples yielded abundant specimens based on total

prasinophyte phycomata and acritarch counts from the three

>52m slides and three 2052m slides counted per sample.

No counts were made from the

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Fig. 4. Occurrence of prasinophyte and acritarch species in the studied samples

of the Bills Creek Shale. All specimens present on the first three slides of the

>52m and2052m fractions were counted to determinerelative abundances

of species per sample. The term very abundant (=va) applies to those species

represented by >100 specimens; abundant (=a) for 51100 specimens; common

(=) for 1050 specimens; and rare (=r) for 100 specimens were encountered.

Fig. 4. Presence des especes dacritarches et de prasinophytes dans les

echantillons preleves dans la Formation de Bill Creek Shale. Le calcul delabondance relative des especes aete effectue sur la base de tous les specimens

presents dans les trois premieres lames palynologiques des fractions >52m et

2025m. Cle des abreviations: va = >100 specimens ; a= 51100 specimens ;

c = 1050 specimens ; r =

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6. Paleoenvironmental interpretation

As stated earlier, sedimentologic and paleontologic evi-

dence indicates that the Bills Creek Shale was deposited in

a low-energy, shallow, normal marine environment, with at

least occasional aerial exposure as indicated by rare desicca-

tion cracks within some of the calcareous shale layers. The

interbedded argillaceous limestones may represent more off-

shore conditions resulting from intermittent transgressions. The

Bay de Noc Member of the Stonington Formation accumulated

in a low-energy, offshore,normal marine environment, with peri-

odic regressions resulting in terrigenous input as evidenced by

the shales. The palynologic assemblages recovered from these

two formations generally supports the paleoenvironmental inter-

pretation suggested by the sedimentologic and paleontologic

evidence.

Staplin (1961)first demonstrated a relationship between dif-

fering acritarch morphotype assemblages and distance from

Upper Devonian reefs in Alberta, Canada. Later studies have

shown that qualitative changes in microphytoplankton morpho-types and diversity and abundance fluctuations can assist in

determining nearshore to offshore trends (e.g.,Jacobson, 1979;

Dorning, 1981; Al-Ameri, 1983; Vecoli, 2000; Li et al., 2004;

Stricanne et al., 2004; Vecoli and Le Herisse, 2004).

However, just as interrelated physical and ecological factors

are responsible for changes in the composition and distri-

bution patterns of modern microphytoplankton assemblages,

such as dinoflagellates (Dale, 1996; Marret and Zonneveld,

2003; Rochon and Marret, 2004), these same factors are

also most likely responsible for the distribution of Pale-

ozoic microphytoplankton (Colbath, 1980; Strother, 1996).

These various complex interrelating factors must thereforebe taken into consideration before any firm relationships can

be established between the preserved palynomorph assem-

blage and the paleoenvironment. Nonetheless, compositional

changes in the palynomorph assemblages preserved through-

out a stratigraphic section can contribute to paleoenvironmental

syntheses.

The paleoenvironmental interpretation based on the paly-

nologic composition of the Bills Creek Shale complements

that deduced from sedimentologic and paleontologic studies.

The Bills Creek Shale palynomorph assemblage represents a

nearshore to inner offshore environment based on the assump-

tions and criteria ofDorning (1981),Vecoli (2000),andLi et al.

(2004).However, the Bills Creek palynomorphs do not readilyfit into a specific environmental category based on Jacobsons

(1979)form-class assignments. This is partly because leiofusid

acritarchs were not included in his classes, and this category

comprises a large proportion of the Bills Creek palynologic

assemblage.

According toDorning (1981), the Eupoikilofusa striatifera

andLeiofusa estrecha complex (i.e., leiofusid acritarchs) appear

to prefer an inshore environment, although they do range from

nearshore to deep water. Inshore and nearshore species ofVery-

hachiumcommonly have three or four processes on a planar

vesicle, whereas offshore forms commonly have four to six

processes on an inflated vesicle.

Li et al. (2004) stated that the nearshore marine environ-

ment of the Yangtze Platform, South China, was dominated by

fusiform, leiosphaerid, and polygonomorph acritarchs. Surpris-

ingly, they did not mention any veryhachid components. They

observed that both generic and specific diversity increased from

nearshore to offshore, a trend also noted by Dorning (1981),

Vecoli (2000), and Stricanneet al.(2004), and in studies focusing

on the distribution of extant microphytoplankton.

The two most abundant palynomorphs of the Bills Creek

assemblage are Leiofusa fusiformis and members of the Very-

hachium trispinosum complex, followed by Villosacapsula

setosapellicula. In addition, the leiofusid acritarchs Leiofusa

litotes, Dactylofusa ctenista, and D. playnetrella are rare to

common, as are such taxa as Peteinosphaeridium septuo-

sum, Polygonium gracile, and the veryhachids Veryhachium

europaeum and V. oklahomense. Except for D. ctenista and D.

playnetrella, all of the aforementioned species occur in all or

seven of the eight Bills Creek Shale samples. Additionally,

Leiosphaeridia spp. andLophosphaeridium varum arepresent in

all samples, andLophosphaeridium edenense occurs in six sam-ples. Taking into account the composition of the palynomorph

assemblage, as well as species diversity and abundance for

each sample (Fig. 4), the palynologic evidence also points to

a nearshore marine environment for the Bills Creek Shale, with

transgressive episodes resulting in an innermost offshore envi-

ronment.

The palynomorph assemblage of the Bay de Noc Member of

the Stonington Formation is more diverse and abundant than that

of the Bills Creek Shale, and is not dominated by a few species.

Based on those palynologic observations alone, the Stonington

Formation represents a more offshore marine environment than

the Bills Creek Shale (cf.Dorning, 1981; Vecoli, 2000; Li etal., 2004; Stricanne et al., 2004).

The Stonington Formation assemblage conforms to Jacob-

sons (1979) offshore, open-marine suite. This suite comprises

the baltisphaerid-veryhachid-Polygonium-micrhystridid form-

classes, all of which commonly occur throughout the sampled

Stonington. As noted earlier, whereas Leiosphaeridia and

related psilate sphaeromorphs are indicative of Jacobsons

nearshore environment, they are also commonly associated with

diverse acritarch assemblages in offshore situations.

The Silurian offshore shelf assemblage of Dorning (1981)

has high diversity and moderate abundance per sample, with no

single taxon dominating. Although theEupoikilofusa striatifera

andLeiofusa estrecha complex is more abundant nearshore, itis still very common in the farther offshore region of the shelf.

The veryhachids also are common from the nearshore to off-

shore environment, with forms having four to six processes on

an inflated vesicle (like Dorsennidium hamii) more common

offshore.

As stated by Vecoli (2000), Li et al. (2004), and others,

diversity increases along a nearshore to offshore transect. In

addition, Li et al. (2004) indicated that the acanthomorph genera

Baltisphaeridium and Peteinosphaeridium, as well as poly-

gonomorph genera such as Polygonium, were the dominant

elements of the EarlyMiddle Ordovician Yangtze Platform

offshore acritarch assemblage. Whereas Baltisphaeridium adi-

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astaltum and B. perclarum are rare to abundant, they do

occur consistently throughout the sampled Stonington section.

Furthermore, Peteinosphaeridium septuosum and Polygonium

gracileare likewise abundant and persistent elements.

Thus, the Stonington Formations paleoenvironment based

on palynologic evidence mirrors that of the sedimentologic and

paleontologic data, by indicating a marine, offshore shelf envi-

ronment. Furthermore, the Stonington Formation (Bay de Noc

Member) is considered to have accumulated farther offshore

than the underlying Bills Creek Shale.

7. Comparison with other acritarch assemblages

Thenumerous publications dealing with Caradoc andAshgill

acritarch/prasinophyte palynofloras are mainly from Laurentia,

Baltica, Avalonia, the northern Gondwana margin, and South

China (e.g.,Molyneux et al., 1996; Servais et al., 2004; Vecoli

and Le Herisse, 2004). Unfortunately, despite a fairly large

database of taxa for the Upper Ordovician, many of the publi-

cations lack definitive stratigraphic information or independentage control, particularly in relation to graptolite, conodont, or

chitinozoan biozones.

Because the Bills Creek Shale and Stonington Formation are

assigned to the upper Amplexigraptus manitoulinensis (Bills

Creek Shale) andDicellograptus complanatus(Stonington For-

mation) graptolite Biozones and the upper Amorphognathus

superbus and lowerA. ordovicicus conodont Biozones, we focus

our comparisons to other acritarch assemblages on those dated

reliably as Richmondian (North American terminology) or the

equivalent, globally recognized Ashgill (Fig. 1).

7.1. Laurentia

As indicated by Wicander et al. (1999), Wicander (2004),

andPlayford and Wicander (2006),knowledge of Cincinnatian

(including Richmondian) palynofloras from Laurentia is by no

means comprehensive. However, the majority of them are well-

documented in terms of stratigraphic and paleontologic control.

When considering palynomorph assemblages of exclusively

Richmondian (=Ashgill) age, definitive comparisons can be

made to the Maquoketa Shale of Missouri and Kansas, the Syl-

van Shale of Oklahoma, and the Vaureal Formation of Anticosti

Island, Quebec, Canada (Figs. 1 and 6). Martin (1980) described

an acritarch palynoflora from part of the Whitehead Forma-

tion (CaradocAshgill) of the Perce region, Gaspe Peninsula,Quebec, Canada, but only four of the 15 acritarch species she

listed also occur in the Bills Creek/Stonington assemblage (viz.,

Aremoricanium squarrosum,Baltisphaeridium perclarum,Mul-

tiplicisphaeridium irregulare,and Sylvanidium paucibrachium).

A total of 16 genera and 25 named or cf. species comprise

the present organic-walled microphytoplankton assemblage

recovered from the Bills Creek Shale and Bay de Noc Mem-

ber of the Stonington Formation (excluding Leiosphaeridia,

Tasmanites, andGloeocapsomorpha prisca):Fig. 6.Of the 25

species, 76% also occur in the Maquoketa Shale of Kansas and

Missouri, 72% in the Sylvan Shale of Oklahoma, and 60% in

the Vaureal Formation of Anticosti Island, Quebec, Canada. In

Fig.6. Comparison of theBillsCreek Shale andStoningtonFormationacritarch

assemblage withpublished age-equivalent (Richmondian) palynofloras fromthe

Maquoketa Shale of Missouri and Kansas (Wright and Myers, 1981; Miller,

1991; Wicander et al., 1999),the Sylvan Shale of Oklahoma (Loeblich, 1970;

Loeblich and MacAdam, 1971; Loeblich and Tappan, 1978; Playford and

Wicander, 2006), andthe VaurealFormation of AnticostiIsland,Quebec,Canada

(Staplin et al., 1965; Jacobson and Achab, 1985).

Fig. 6. Comparaison des assemblages a acritarches des Formations de Bills

Creek Shale et de Stonington avec des associations palynologiques publiees

dage richmondienne : Maquoketa Shale du Missouri et Kansas (Wright etMyers, 1981; Miller, 1991; Wicander et al., 1999), Sylvan Shale de lOklahoma

(Loeblich, 1970; Loeblich et MacAdam, 1971 ; Loeblich et Tappan, 1978 ;

Playford et Wicander, 2006),et la Formation de Vaureal de lle dAnticosti,

Quebec, Canada (Staplin et al., 1965;Jacobson et Achab, 1985).

addition, 32% of the Bills Creek/Stonington assemblage are

common to all three of the other Laurentian Richmondian-age

formations (Maquoketa Shale, Sylvan Shale, and Vaureal For-

mation), and 48% of the Bills Creek/Stonington assemblage

occur in at least two of the three aforementioned formations

(Fig. 6).

Wicander

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