New evidence for reconstructing the marine faunal assemblage of the Decorah Formation (southeastern Minnesota and northeastern Iowa, USA):

A qualitative survey of microfossils

Phillip J. Varela Senior Integrative Exercise

March 11, 2009

Submitted in partial fulfillment of the requirements for a Bachelor of Arts degree from Carleton College, Northfield, Minnesota

Table of Contents Abstract Introduction............................................................................................................1 Background............................................................................................................1

Ordovician Laurentia...................................................................................1 The Great Ordovician Biodiversification Event..........................................4

Geologic Setting......................................................................................................5 Stratigraphy of the Decorah Formation......................................................5 Paleoenvironment and Depositional Setting...............................................8

Field and Laboratory Methods...........................................................................13 Microfossil Survey Results..................................................................................14

Conulariids..................................................................................................17 Chitinozoans................................................................................................20 Conodonts....................................................................................................20 Scoleconodonts............................................................................................25

Reevaluation of Faunal Assemblage...................................................................27 Acknowledgements..............................................................................................29 References Cited...................................................................................................29

New evidence for reconstructing the marine faunal assemblage of the

Decorah Formation (southeastern Minnesota and northeastern Iowa, USA): A qualitative survey of microfossils

Phillip J. Varela Carleton College

Senior Integrative Exercise March 11, 2009

Advisor: Clinton A. Cowan, Carleton College Department of Geology

ABSTRACT

Three Late Ordovician (Mohawkian) outcrops of the Decorah Formation in southeastern Minnesota and northeastern Iowa contain abundant fossils representing a diverse shallow-water marine biotic community. Although some macrofossil communities have been documented and analyzed at great depth, many microfossil communities remain virtually unknown. In order to establish a complete understanding of the paleoenvironmental conditions and dominant biologic communities, it is necessary to establish a better understanding of microfossils. A total of nine shale samples from Kenyon, MN, Rochester, MN, and Decorah, IA were analyzed for microfossil content. From this survey, I identify five groups of poorly documented or poorly understood macro- and microfossils, all of which are not easily found in hand sample and are only recoverable after processing for microfossil collection. Fossil groups include chitinozoans, conodonts, conluariids, Linguliformea (phosphatic inarticulate brachiopods) and scolecodonts. The goal of the survey is to add to the growing knowledge of the biotic community that defines the Decorah Formation and to integrate these poorly understood fossil groups with the previously established faunal assemblage. Based on the inferred lifestyles of microfossils found within the Decorah Formation, this study maintains the paleoenvironmental parameters of previously established reconstructions and adds insight into the representative biotic community. Keywords: Decorah Formation, microfossils, chitinozoan, conodont, conulariid, Linguliformea, scolecodont, paleoenvironment

INTRODUCTION

Examination of the distribution of microfossils in three outcrops of the

Late Ordovician (460.9 - 443.7 Ma) Decorah Formation, including an outcrop not

previously investigated for microfossils, helps constrain the paleoenvironment

and depositional setting in southeastern Minnesota and northeastern Iowa. The

Decorah Formation contains an abundance of well-preserved macrofossils and

trace fossils, and the biostratigraphic distribution of these fossils has been

examined in great detail (see summary in Sloan, 1987). This report does not focus

on macrofossils; rather, it surveys the microfossil content within the lower

members (Spechts Ferry, Guttenberg, Ion) of the Decorah Formation to further

constrain the known faunal assemblage and assess the paleoenvironment based on

parameters for organism life styles. Through this qualitative microfossil survey, I

attempt to provide insight for further understanding the general environments and

biological communities represented within the Decorah Formation.

BACKGROUND Ordovician Laurentia

The Ordovician Period (488.3±1.7 to 443.7±1.5 Ma; Gradstein et al., 2004)

is characterized by generally high sea levels, with global transgressions leading to

the generation of shallow epeiric seas (Barnes, 2004). Most of the large

landmasses were situated in the southern hemisphere (Achab and Paris, 2006),

with the exception of Laurentia (largely modern North America), which straddled

the equator. Paleographic reconstructions (Fig. 1; Witzke, 1980; Young et al.,

2005) place what are now most of eastern and central North America anywhere

1

EQUATOR

N30° S

Fig. 1. Late Ordovician paleogeography of Laurentia (Modified from Witzke 1980). Study area is outlined.

2

between 10° and 30° S latitudes during this time. Consequently, Laurentia is

considered to have been tropical during most of the Ordovician, with very little

continental migration or rotation (Cocks et al., 2004).

The eastern margin of Laurentia is marked by the development of a

convergent tectonic setting, resulting in subduction, microterrane collision, and

volcanism during Late Ordovician time (Mac Niocaill, 1997). This tectonic

activity was responsible for the Taconic Orogeny, a mountain building event

attributed to the genesis of the Appalachian mountain belt (Achab, 2006). K-

bentonites, widespread ash beds altered to potassium-rich clays characteristic of

volcanism along an active tectonic margin (Huff et al., 1992), are well known in

Mohawkian sequences, and Emerson et al., (2004) emphasize their stratigraphic,

paleontological, and sedimentological implications in the Decorah Formation, as

K-bentonite correlation has provided a framework for biostratigraphy.

The global climate of the Ordovician is generally considered a transition

from an early greenhouse state to a later icehouse state, terminating with

extensive glaciations and a mass extinction event (Barnes, 2004). Because

shallow epeiric seas covered much of the Ordovician world continental areas, sea

surface temperatures are important for understanding the nature of oceanographic

and climatic systems. Recent studies of the marine 18O record from conodont

apatite (Trotter et al., 2008) and calcite (Tobin et al., 2002) assume sea surface

temperatures between ~42°C and ~22°C. The lower end of these temperatures

falls within the range of present day sea surface temperatures (Trotter et al., 2008).

3

The Great Ordovician Biodiversification Event

The Paleozoic Era experienced two major radiations of global

biodiversification. The first is known as the Cambrian Explosion, which saw a

rapid appearance of complex organisms and diversification at the phylum and

class levels (Droser and Finnegan, 2003). The second is the Great Ordovician

Biodiversification Event (or the Ordovician Radiation), which led to a significant

increase in organism biodiversity at lower taxonomic levels (Miller, 2001).

During the Ordovician Radiation, organism family and genus diversities increased

three- to fourfold relative to Cambrian communities (Trotter et al, 2008).

Most significant to the character of the seafloor was the diversification of

skeletal organisms such as brachiopods, bryozoans, cephalopods, conodonts,

corals, crinoids, graptolites, ostracodes, stromatoporids, and trilobites. The

dominance of these organisms greatly changed the marine environment from

trilobite-dominated to brachiopod-dominated soft substrates, and echinoderm- and

bryozoan-dominated hard substrates developed (Droser et al, 1997; Harper, 2005).

In reef ecosystems, tabulate corals and stromatoporids began to dominate over

sponges in the Ordovician and continued throughout the Paleozoic Era.

Diversification of marine organisms induced important paleoecologic

changes, such as the development of new modes of life (Harper, 2005). Droser et

al. (1997) propose the term “Bambachian megaguilds” for the adaptive strategies

of organisms originally described by Bambach (1983). During the radiation, new

megaguilds (organism life styles) developed and there was an increase in sessile

epifaunal (e.g. suspension feeders) and mobile epifaunal (e.g. detritivores,

4

herbivores, carnivores) organisms (Harper, 2005). There is a clear transition from

a primarily infaunal community in the Cambrian to a mixed infaunal and

epifaunal community in the Ordovician.

The timeframe of this biodiversification event is estimated to have lasted

25 million years (Harper, 2005), and although the chronostratigraphy for the

Ordovician is well established, there remains difficulty in accurately determining

a distinct timeframe for a global event at this magnitude, and this is a potential

complication for correlating international time slices. Despite this complication, it

is estimated that the radiation commenced during the Arenig and lasted until the

late Ashgill (Harper, 2005).

GEOLOGIC SETTING

Stratigraphy of the Decorah Formation

The Platteville Formation and Galena Group span the middle part of the

Mohawkian Series (Turinian and Chatfieldian stages) of the Upper Ordovician

(Fig. 2; Saltzman and Young, 2005). Overlying the Platteville Formation, the

basal unit of the Galena Group is termed the Decorah Formation (or subgroup)

and a distinct fossil bed of Prasopora bryozoans marks the upper boundary. The

Decorah Formation is a fossiliferous, mixed carbonate-shale succession that was

deposited in a subtidal open-marine setting (Ludvigson, 1996) within the

Hollandale Embayment, a tectonic basin containing a shallow epeiric sea locally

situated between the topographical highs of the Transcontinental Arch and the

Wisconsin Dome and Arch (Fig. 3; Sloan, 1987). The nearby Transcontinental

5

DEC

ORA

H

GA

LEN

A G

ROU

P

PLATTEVILLETURINIAN

CHATFIELDIAN

MO

HAW

KIA

N

LATE

ORD

OVI

CIA

N

Guttenberg

Spechts Ferry

Ion

Dunleith(Cummingsville)

STAGES

SERI

ES

PERI

OD

FORMATION

Fig 2. Late Ordovician timescale for the Decorah Formation (Modified from Emerson et al., 2004; Ludvigson et al., 2004; Mossler, 1987; Young et al., 2005).

6

N

Kenyon

Rochester

Decorah

Fig 3. Paleogeog-raphy of the Upper Mississippi Valley. Gray areas represent land, white areas repre-sent water. Field localities: Kenyon, MN, Rochester, MN, and Decorah, IA. Coordinate values represent present day latitude and longitude.

7

Arch served as a source of marine siliciclastic sediments from which detritus was

introduced into the Hollandale Embayment during an episode of early

Rocklandian uplift, thus depositing shale through erosional processes and

freshwater runoff (Witzke, 1980).

Paleoenvironment and Depositional Settings

Because sedimentary deposits above fair weather wave base are typically

mud-free due to constant agitation, all units within the Decorah Formation were

deposited in muddy subtidal environments between fair weather wave base and

storm weather wave base (Ludvigson, 1996). The Decorah Formation was

deposited in an expansive shallow epeiric ramp, and depositional settings along

this ramp were variable. Thus, deposition resulted in a variety of lithofacies.

Three lithofacies can be characterized along a gradient from inboard (proximal to

land) to outboard (distal to land): terrigenous shale, calcareous shale, and

carbonate.

The terrigenous shale facies is characterized by fine, dark, green-gray,

platy shale and was deposited closest to land, having experienced the heaviest

terrestrial influence. The calcareous shale facies is characterized by brown shale

mixed with thin micrite to biosparite beds and was likely deposited further from

land. The carbonate facies is characterized by an increase in micrite to biosparite

beds, with fewer interbeds of calcareous shale and was thus deposited furthest

from the land with much less terrestrial influence (Fig 4). Generally, the Decorah

Formation records an up-section change from a more shale-dominated facies to a

8

FWW

B

SWW

B

high terrigenous

inputshallow

lightpenetration

(low photic zone)

increased lightpenetration

increased lightpenetration

(high photic zone)

lowterrigenous

input

highnutrients

lownutrients

no terrigenousinput

North

northwest

Southsoutheast

high faunaldiversity

low faunal

diversity

low carbonate

productivityhigh carbonate

productivity

Terrigenous shale

Calcareous shale

Carbonates

Fig. 4. Schematic representation of a vertically exaggerated shallow

epeiric ramp upon w

hich the Decorah Form

a-tion w

as deposited. Represented here is a ‘snapshot’ in tim

e, showing variation in deposition style dependent on

proximity to land. Fluctuations in sea-level w

ill change deposition style for a particular position along the ramp.

Mixed carbonate shale units (such as the D

ecorah Formation) reflect episodes of transgression and regression, and

the Decorah Form

ation records an overall transgression.

9

more carbonate-dominated facies, thus representing a general transgressive trend,

and Simo et al. (2003) interpret the mixed carbonate-shale depositional sequence

to represent episodes of continental flooding and terrestrial weathering.

In eastern Iowa, the Decorah Formation comprises three members (in

ascending order: Spechts Ferry, Guttenberg, and Ion). The Spechts Ferry member

consists of gray-green shale with few carbonate interbeds, the Guttenberg member

is primarily limestone with brown and green shale partings, and the Ion member

consists of interbedded limestone and gray-green shale. These members lose

lithologic distinction to the northwest (into southeastern Minnesota) as the unit

becomes more shale-dominated (Ludvigson et al., 2004). Thus, some members

present in Iowa may not be present in Minnesota (Fig. 5).

Dominant skeletal organisms of the Decorah Formation include bryozoans,

brachiopods, cephalopods, crinoids, and trilobites, all of which are interpreted as

part of the Heterozoan Association (James, 1997). Heterozoan Association

sediments are abundant during late Middle Ordovician through Late Ordovician

time (James, 1997) and have significant paleoenvironmental implications for the

Decorah Formation. A cool-water environment for the Decorah Formation is

implied by the deposition on a shallow, marine, low-energy, epeiric ramp

containing calcite mineralogy and an appropriate faunal assemblage (Fig. 6).

Absolute depositional water depth is difficult to determine, but an abundant

Heterozoan Association fauna suggests a relatively shallow paleoenvironment

(Lukasik et al., 2000).

10

1 2 3 4 5 6 8 9 107 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 270

0%50%

100%%

Carbonate

ShaleM

SBPB

B

m

Outcrop begins at the top of the

Platteville Formation

Outcrop ends in vegetation

1 2 3 4 5 6 8 9 107 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 270

0%50%

100%%

Carbonate

ShaleM

SBPB

B

outcrop ends in �eld

Outcrop begins in m

arsh on the west side of H

WY 52

m

ROCH

ESTER, MIN

NESO

TA

DECO

RAH

, IOW

ADECORAH

R-933

R-935

R-953

D-974

D-975

D-976

Outcrop begins at the base

of a trench in a ditch on the side of the road

1 2 3 4 5 6 8 9 107 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 270

0%50%

100%%

Carbonate

ShaleM

SBPB

B

Outcrop ends in vegetation

KENYO

N, M

INN

ESOTAT-912

T-922

T-958

(Inaccessible Cli� Face)

(Inaccessible Cli� Face)

(Inaccessible Cli� Face)

Mbr

Fm

DUNLEITH

GrpGALENAPLATTEVILLE

StageTURINIAN CHATFIELDIANMOHAWKIAN Series

UPPER ORDOVICIAN

CUMMINGSVILLECarimona

Period

Fm

Mbr

LEGEN

D

DECORAH Spechts Ferry Guttenberg Ion

SE Minn.

NE Iow

a

PLATTEVILLECarimona

T-912Sam

ple Num

ber

Cover

Limestone

Shale

MM

icrite

SBSparse Biom

icrite

PBPacked Biom

icrite

BBiosparite

Fe Ooids

Shale Sample

Fig. 5. Lithostratigraphic columns and chronostratigraphic correlation of the �eld localities: Kenyon, MN; Rochester, MN; Decorah, IA (nomenclature from Mossler, 1987). Included are stratigraphic locations of shale samples used for microfossil analysis.

11

1

23

2

4

5

36

7

8

9

Fig.

6. P

aleo

envi

ronm

enta

l rec

onst

ruct

ion

of a

Lat

e O

rdov

icia

n se

a �o

or o

f the

Dec

orah

For

mat

ion

base

d on

evi

denc

e of

mac

ro-

foss

ils. M

arin

e fa

una

incl

ude:

1. c

rinoi

ds, 2

. bry

ozoa

ns, 3

. pel

ecyp

ods,

4. g

astr

opod

s, 5.

art

icul

ate

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hiop

ods,

6. tr

ilobi

tes,

7.

ceph

alop

ods,

8. Cho

ndrites

bur

row

s, an

d 9.

rugo

se c

oral

s.

12

FIELD AND LABORATORY METHODS

A total of nine shale samples were collected for microfossil analysis from

three well-exposed outcrops of the Decorah Formation. Three samples were

collected in Kenyon, MN, three in Rochester, MN, and three in Decorah, IA.

Wherever possible, spacing between samples did not exceed more than one

stratigraphic meter. Although two kilograms of each sample were collected, only

one kilogram of each was processed for microfossil analysis.

Each shale sample was broken into small chunks (less than a few

centimeters in diameter each) by hand and placed on a metal tray to dry in an

oven. Each tray was dried for at least six hours at low temperatures (70 º C - 80 º

C). After drying, each sample was submerged in a bath of odorless mineral spirits

and allowed to soak overnight; this aids in shale disaggregation. Excess solvent

was drained through a very fine sieve to prevent sample contamination and kept

for future use. Hot water was then used to disaggregate the shale samples, turning

the shale into mud. Each sample was allowed to sit immersed in hot water for a

few days, and once the shale was fully disaggregated, the mud was washed

through a series of nested sieves with a variety of size fractions (63 m, 177 m,

707 m, and 1000 m) in order to separate different sized fossils.

Macrofossils such as bryozoans, articulate brachiopods, crinoids,

gastropods, pelecypod shells, and rugose corals were collected in the upper sieves,

while microfossils and fragments were collected in the lower sieves. Each sieve

tray was allowed to dry overnight, and the samples were transported to dishes for

microfossil identification. For this study, only the smallest size fraction (63 m)

13

was used for microfossil collection. Larger size fractions (>177 m) contained an

abundance of macrofossil fragments, and are thus not presently considered for

further examination. Individual microfossil specimens were picked using the tip

of a single human hair under an optical microscope and placed on carbon plates

for identification using a Hitachi S-3000N Scanning Electron Microscope. Due to

the nature of this survey and considering the size fraction for individual

specimens, overall sample size, and sampling resolution, quantitative and

systematic analysis was abandoned as it lies outside the scope of the present study.

MICROFOSSIL SURVEY RESULTS

The microfossil survey of the Decorah Formation yielded an abundance

of specimens, many of which are not easily identifiable. Most specimens are

fragmented and do not represent complete morphologies. In addition to

microfossil fragments, some microscopic fragments of macroinvertebrates were

recovered and identified. Most notable of the macrofossil fragments are

conulariids and Linguliformea (phosphatic inarticulate brachiopods), and

microfossils include chitinozoans, conodont elements, and scolecodont elements,

descriptions of which are included in the survey results.

All of the fossils described in this survey are not easily found in hand

sample and are only recoverable after processing for microfossil collection.

Presently, very few full conulariid specimens have been found worldwide, and

this survey is one of the first to document their presence in the Decorah Formation.

Likewise, Linguliformea are not usually reported from the Decorah Formation.

14

Although chitinozoans, conodonts and scolecodonts have been recognized in

Ordovician rocks from Laurentia and Baltica, these fossil groups remain relatively

poorly understood.

Eight microscopic fragments of macroinvertebrates (five conluariid, three

Linguliformea) were recovered from the shale residue (Figs. 7, 8). Each field

locality is represented in this survey, but stratigraphic distribution has yet to be

determined. Neither of these fossil groups is well known, especially not in the

Decorah Formation. The presence of these fragments at microscopic levels with

the lack of individual specimens at the macroscopic level has interesting

implications for possible preservation potential of phosphatic macroinvertebrates

in the Decorah Formation.

In a recent study of preservation potential of phosphatic

macroinvertebrates, Van Iten et al. (2006) argue for taphonomic bias (a natural or

analytical systematic skewing of information) of conulariid and Linguliformea in

the Maquoketa Formation (Upper Ordovician) of northeast Iowa (spatially and

temporally comparable to the Decorah Formation). Preservation bias against

conulariids and Linuliformea may be partly explained by minute fragmentation of

fragile skeletons by episodic storm currents (Van Iten et al., 2006). Minute

fragmentation of macroscopic skeletons makes them difficult to detect at low

magnifications, thus making it appear that they were not present in the biotic

community of the Decorah Formation. Although this study does not test any

systematic character of these two macroinvertebrates, the presence of microscopic

fragments provides an opportunity for future work.

15

200 µm100 µm

100 µm

Fig. 7. SEM micrographs of Conu-lariids (macroscopic phosphatic taxa) represented as microscopic fragments in the Decorah Forma-tion. 1-3, ?Conularia sp. fragments (D974, R933, D975). 4-5, Examples of microscopic pores (arrows) in conulariid fragments (D975, T912).

1 2

3

4 5

100 µm 100 µm

16

Fig. 8. SEM micro-graphs of Linguli-formea (macroscopic phosphatic inarticulate brachiopods) repre-sented as microscopic fragments in the Deco-rah Formation. 1-2, ?Trematis sp. fragment (D975). 3, fragment of phosphatic brachiopod (D974).

1

2

3

100 µm

100 µm

200 µm

17

Conulariids

Conulariids are an extinct group of marine metazoans of unknown

phylogenetic affinity (Ivantsov and Fedonkin, 2002; Hughes et al., 2000;

Richardson and Babcock, 2002; Van Iten, 1991; Van Iten, 2005). Although there

have been extensive debates regarding the affinity of these organisms, they are

presently classified under the Phylum Cnidaria (Van Iten, 1991; Hughes et al.,

2000; Richardson and Babcock, 2002). Conulariid morphology is poorly

established, but the organism is considered to have secreted a four- or six-sided

pyramidal exoskeleton (Richardson and Babcock, 2002; Ivantsov and Fedonkin,

2002; Van Iten, 2005) primarily made of phosphatic rods (Hughes et al., 2000).

Although holdfast soft parts are not easily preserved in the fossil record, Babcock

and Feldmann (1986) describe conulariids as having an attachment stalk,

suggesting that they were sessile animals attached to the substrate. Reconstruction

of a conulariid-like fossil depicts an inverted pyramidal conical test with

protruding tentacles (Fig. 9; Ivantsov and Fedonkin, 2002).

Conulariids often occur in dark shale and lime mudstones in which normal

marine organisms such as echinoderms and articulate brachiopods are rare. Such

incidence, along with their inferred sessile mode of life, suggests that conulariids

may have been able to live in bottom waters possibly influenced by oxygen

depletion (Van Iten et al., 1996).

18

Fig. 9. Anatom

ical reconstruction of a conulariid (M

odified from

Ivantsov and Fedonkin, 2002)

19

Chitinozoans

Common in rocks of Ordovician through Devonian ages, chitinozoans are

flask- or bottle-shaped, organic-walled microorganisms of uncertain phylogenetic

affinity (Fig. 10; Brasier, 1980). Recent studies (Paris and Nolvak, 1999; Achab

and Paris, 2006) propose that chitinozoans are egg sacks of soft-bodied marine

metazoans, although this is still under debate. They are regarded as having a

pelagic or nektonic mode of life (Paris and Nolvak, 1999). Chitinozoan walls are

unusually resistant to oxidation, thermal alteration, tectonism, and

recrystallization (Braiser, 1980) so preservation potential is high.

Although enigmatic, these microfossils are particularly useful for

biostratigraphy because of their abundance, diversity, and evolutionary patterns

(Paris and Nolvak, 1999). Bourahrouh et al. (2004) suggest that the increased

biodiversification of chitinozoans may indicate a general cooling trend in the Late

Ordovician.

Conodonts

Conodonts are extinct marine animals with a generally accepted chordate

affinity (Sweet and Donoghue, 2001). Historically, conodonts were known only

from isolated fossilized ‘conodont elements’. Conodont elements are small (0.5-

5.0 mm long), tooth-like skeletal structures of phosphatic composition, often

interpreted as part of a feeding apparatus (Fig. 11). Anatomical reconstruction of

conodonts is difficult due to the scarcity of preserved soft parts, but a few

sedimentary deposits worldwide, termed “Lagerstätte” are known to contain

20

1

5

3

4

2

70 µm

60 µm

60 µm

50 µm60 µm

Fig. 10. SEM micrographs of selected chitinozoan specimens from the Decorah Formation. All specimens are from sample D975 (Decorah, Iowa).

21

Fig. 11. SEM micrographs of selected conodont elements from the Decorah Formation. Stratigraphic location of individual elements: 1, R-953; 2, T-958; 3, R-953; 4, R-935; 5, R-935; 6, R-935; 7, R935; 8, R935.

100

µm

100

µm

200

µm20

0 µm

300

µm

100

µm20

0 µm

300

µm

12

3

45

67

8

22

fossilized remains of conodont soft parts and tissues. Most notable are the

Lagerstätte of the Soom Shale in South Africa which reveals details of preserved

musculature, feeding apparatus, and eye structure (Gabbott et al., 1995), and the

recently discovered Winneshiek Lagerstätte from the St. Peter Shale of northeast

Iowa which contains complete basal structures, a rare preservation in most

conodont assemblages (Liu et al., 2006). Anatomical reconstruction from the

remains of the large conodont species Clydgnathus windsorensis (Purnell, 1995)

and Promissum pulchrum (Aldridge et al., 1993, Aldridge et al., 1995) reveals a

physical resemblance to an eel, with a complex feeding apparatus, large eyes, and

a ray-supported fin (Fig. 12, Aldridge et al., 1995).

North American conodont biostratigraphy is well established for the

Mohawkian, and six zones are identified (in ascending order): Plectodina

aculeate, Erismodus quadridactylus, Phragmodus undatus, Plectodina tenuis, and

Belodina confluens (Sweet, 1984; Hall, 1986; Haynes, 1994; Leslie, 1995;

Richardson and Bergstrom, 2003). For this reason, conodont systematics is not a

major focus of the present study. However, the importance of conodonts as

valuable biostratigraphic indicators is useful for stratigraphic correlation due to

their relative abundance, widespread distribution, and distinct evolutionary

patterns (Brasier, 1980). In fact, conodonts are now the fossil group of choice for

biostratigraphic work in rocks dated from Late Cambrian age through the Triassic

(Sweet and Donoghue, 2001).

23

1 cm

Fig. 12. Anatomical reconstruction of a living conodont (from Aldridge et al., 1995)

24

Scolecodonts

Scolecodonts, jaws of polychaete worms, are common and diverse

microfossils in Ordovician sedimentary rocks (Hints, 2000; Eriksson and

Bergman, 2003; Eriksson et al., 2005; Eriksson and Mitchell, 2006). Fossils are

usually preserved as individual elements (much like conodont preservation) and

are regarded as parts of complex jaw apparatuses (Fig. 13; Eriksson and Mitchell,

2006). Scolecodont fossils are known from Cambrian age rocks but are most

abundant in Ordovician, Silurian, and Devonian marine sediments (Hints, 2004).

Despite the well-known presence of scolecodonts in the fossil record, few authors

have studied this group in great detail due to difficult and confusing systematics

(Hints, 2000). Therefore taxonomy, ecology, stratigraphic and geographic

distribution remains poorly understood for this fossil group.

Of the known faunas, most come from Baltica and Laurentia, notably

from the Mohawkian and Cincinnatian epochs of North America (Hints and

Eriksson, 2006) with scolecodont stratigraphy of Cincinnatian age being the best

documented (Eriksson and Bergman, 2003; Eriksson et al., 2005). Studies of

scolecodont diversity, biostratigraphy, and geographic distribution in the United

States and Estonia (Hints, 2000; Eriksson and Bergman 2003; Hints and Eriksson,

2006) conclude that jawed polychaetes are facies-controlled, and that abundance

and diversity are heavily dependent on water depth. Shallow water environments

(between fair weather wave base and storm weather wave base) provide more

suitable living conditions over deeper water environments (Hints, 2000). However,

abundance and diversity are low in very shallow supratidal and intertidal high-

25

300 µm

100 µm

300 µm

100 µm

100 µm

1 2

3

4 5

Fig. 13. SEM micrographs of selected scolecodont elements from the Decorah Formation. Strati-graphic location of individual elements: 1, R933; 2, R933; 3, R933; 4, R933; 5, R935.

26

energy environments. This can be explained in part by low preservation potential

in these environments (Eriksson and Bergman, 2003). Although water depth

seems to be the principal factor in scolecodont diversity and abundance, other

associated parameters such as nutrient availability, light, temperature, salinity,

and turbidity should be considered. However, assessment of these influences is

difficult.

REEVALUATION OF THE FAUNAL ASSEMBLAGE This survey of microfossils reveals the presence of at least five poorly

documented or poorly understood fossil taxa in the Decorah Formation:

conulariids, Linguliformea, chitinozoans, conodonts, and scolecodonts. The

addition of these taxa into the previously established faunal assemblage provides

further insight into the biotic character of the late Ordovician seafloor (Fig. 14).

The presence of microscopic fragments of phosphatic macroinvertebrates has

interesting implications for paleoenvironment (e.g. episodic storm action) and

preservation potential, and microfossil availability of conodonts, chitinozoans,

and scolecodonts within the Decorah Formation provide opportunities for further

studies of biostratigraphy.

27

1

23

2

4

5

36

7

8

9

1 cm

10

11

12

1 cm

50 µm

13

14

Fig.

14.

Pal

eoen

viro

nmen

tal

reco

nstr

uctio

n of

a L

ate

Ord

ovic

ian

sea

�oor

of t

he

Dec

orah

For

mat

ion

base

d on

new

evi

denc

e of

ad

ditio

nal m

acro

- and

m

icro

faun

a. E

xist

ing

faun

a in

clud

e: 1

. crin

oids

, 2.

bryo

zoan

s, 3.

pel

ecyp

ods,

4.

gast

ropo

ds, 5

. art

icul

ate

brac

hiop

ods,

6. tr

ilobi

tes,

7.

ceph

alop

ods,

8. Cho

ndrites

bu

rrow

s, an

d 9.

rugo

se

cora

ls. A

dditi

onal

faun

a in

clud

e: 1

0. c

onod

onts

, 11

. co

nula

riids

, 12.

inar

ticul

ate

brac

hiop

ods,

13. s

cole

co-

dont

s (p

olyc

haet

e w

orm

s),

and

14. c

hitin

ozoa

ns.

28

ACKNOWLEDGEMENTS

I offer many thanks to my advisor, Clint Cowan, for introducing this

project to me and for his guidance and enthusiasm throughout the process. I thank

Tyler Mackey for fieldwork assistance during the initial stages of this research.

My gratitude goes to Julia Anderson of the Minnesota Geological Survey who

assisted in fieldwork, and Tony Runkel of the Minnesota Geological Survey for

additional support. Thanks also to Kristin Sweeney, Sophie Williams, and Lila

Battis with whom I worked on the lithostratigraphy, and Brian Witzke and Heyo

Van Iten for offering insight into fossil identification. I would like to thank the

Carleton College Geology Department for funding my project, Cam Davidson and

Tim Vick for offering logistical support, and finally my fellow senior geology

majors for sharing this “comps experience” with me.

REFERENCES CITED

Achab, A., and Paris, F., 2007, The Ordovician chitinozoan biodiversification and its leading factors, in Munnecke, A., and Servais, T., eds., Palaeogeography, Palaeoclimatology, Palaeoecology: Amsterdam, Elsevier, p. 5-19.

Aldridge, R. J., Briggs, D. E. G., Smith, M. P., Clarkson, E. N. K., and Clark, N.

D. L., 1993, The anatomy of conodonts: Philosophical Transactions - Royal Society of London. Biological Sciences, v. 340, no. 1294, p. 405-421.

Aldridge, R. J., Purnell, M. A., Gabbott, S. E., and Theron, J. N., 1995, The

apparatus architecture and function of Promissum pulchrum Kovacs-Endrody (Conodonta, Upper Ordovician) and the prioniodontid plan: Philosophical Transactions - Royal Society of London. Biological Sciences, v. 347, no. 1321, p. 275-291.

Babcock, L. E., and Feldmann, R. M., 1986, The phylum Conulariida, in Hoffman,

A., and Nitecki, M. H., eds.: New York, Oxford Univ. Press.

29

Barnes, C. R., 2004, Ordovician oceans and climate, in Webby, B. D., Paris, F.,

Droser, M. L., and Percival, I. G., eds.: New York, Columbia University Press.

Bourahrouh, A., Paris, F., and Elaouad-Debbaj, Z., 2004, Biostratigraphy,

biodiversity and palaeoenvironments of the chitinozoans and associated palynomorphs from the Upper Ordovician of the central Anti-Atlas, Morocco, in Servais, T., and Wellman, C. H., eds., Review of Palaeobotany and Palynology: Amsterdam, Elsevier, p. 17-40.

Brasier, M. D., 1980, Microfossils, George Allen & Unwin Ltd , 193 p. Cocks, L. R. M., and Torsvik, T. H., 2004, Major terranes in the Ordovician, in

Webby, B. D., Paris, F., Droser, M. L., and Percival, I. G., eds.: New York, Columbia University Press.

Droser, M. L., Bottjer, D. J., and Sheehan, P. M., 1997, Evaluating the ecological

architecture of major events in the Phanerozoic history of marine invertebrate life: Geology, v. 25, no. 2, p. 167-170.

Droser, M. L., and Finnegan, S., 2002, The Ordovician Radiation: a follow-up to

the Cambrian explosion?, in Annual Meeting of the Society-for-Integrative-and-Comparative-Biology, Anaheim, California, p. 178-184.

Emerson, N. R., Simo, J. A., Byers, C. W., and Fournelle, J., 2004, Correlation of

(Ordovician, Mohawkian) K-bentonites in the Upper Mississippi Valley using apatite chemistry; implications for stratigraphic interpretation of the mixed carbonate-siliciclastic Decorah Formation, in Pope, M. C., and Harris, M. T., eds., Palaeogeography, Palaeoclimatology, Palaeoecology: Amsterdam, Elsevier, p. 215-233.

Eriksson, M., and Bergman, C. F., 2003, Late Ordovician jawed polychaete

faunas of the type Cincinnatian region, U.S.A: Journal of Paleontology, v. 77, no. 3, p. 509-523.

Eriksson, M. E., Leslie, S. A., and Bergman, C. F., 2005, Jawed polychaetes from

the upper Sylvan Shale (Upper Ordovician), Oklahoma, USA: Journal of Paleontology, v. 79, no. 3, p. 486-496.

Eriksson, M. E., and Mitchell, C. E., 2006, Stratigraphic origin of Hinde's (1879)

Ordovician scolecodonts inferred from associated graptolites: Journal of Paleontology, v. 80, no. 2, p. 396-399.

30

Gabbott, S. E., Aldridge, R. J., and Theron, J. N., 1995, A giant conodont with preserved muscle tissue from the Upper Ordovician of South Africa: Nature, v. 374, no. 6525, p. 800-803.

Gradstein, F. M., Ogg J.G., Smith, A.G., 2004, A Geologic Time Scale:

Cambridge, Cambridge University Press, p. 589. Hall, J. C., 1986, Conodonts and conodont biostratigraphy of the Middle

Ordovician in the western overthrust region and Sequatchie Valley of the Southern Appalachians [Doctoral thesis]: Ohio State University.

Harper, D. A. T., 2006, The Ordovician biodiversification; setting an agenda for

marine life: Palaeogeography, Palaeoclimatology, Palaeoecology, v. 232, no. 2-4, p. 148-166.

Haynes, J. T., 1994, The Ordovician Deicke and Millbrig K-bentonite beds of the

Cincinnati Arch and the southern Valley and Ridge Province: Special Paper - Geological Society of America, v. 290, p. 80.

Hints, O., 2000, Ordovician eunicid polychaetes of Estonia and surrounding areas;

review of their distribution and diversification, in Servais, T., and Paris, F., eds., Review of Palaeobotany and Palynology: Amsterdam, Elsevier, p. 41-55.

Hints, O., Eriksson, M., Hogstrom, A. E. S., Kraft, P., and Lehnert, O., 2004,

Worms, wormlike and sclerite-bearing taxa, in Webby, B. D., Paris, F., Droser, M. L., and Percival, I. G., eds.: New York, Columbia University Press.

Hints, O., and Eriksson, M. E., 2007, Diversification and biogeography of

scolecodont-bearing polychaetes in the Ordovician, in Munnecke, A., and Servais, T., eds., Palaeogeography, Palaeoclimatology, Palaeoecology: Amsterdam, Elsevier, p. 95-114.

Huff, W. D., Bergstrom, S. M., and Kolata, D. R., 1992, Gigantic Ordovician

volcanic ash fall in North America and Europe; biological, tectonomagmatic, and event-stratigraphic significance: Geology, v. 20, no. 10, p. 875-878.

Hughes, N. C., Gunderson, G. O., and Weedon, M. J., 2000, Late Cambrian

conulariids from Wisconsin and Minnesota: Journal of Paleontology, v. 74, no. 5, p. 828-838.

Ivantsov, A. Y., and Fedonkin, M. A., 2002, Conulariid-like fossil from the

Vendian of Russia; a metazoan clade across the Proterozoic/Palaeozoic boundary: Palaeontology, v. 45, no. 6, p. 1219-1229.

31

James, N. P., 1997, The cool-water carbonate depositional realm, in James, N. P.,

and Clarke, J. A. D., eds., Special Publication - Society for Sedimentary Geology, vol. 56: Tulsa, SEPM (Society for Sedimentary Geology), p. 1-20.

Leslie, S. A., 2000, Mohawkian (Upper Ordovician) conodonts of eastern North

America and Baltoscandia: Journal of Paleontology, v. 74, no. 6, p. 1122-1147.

Liu, H. P., McKay, R. M., Young, J. N., Witzke, B. J., McVey, K. J., and Liu, X.,

2006, A new Lagerstaette from the Middle Ordovician St. Peter Formation in northeast Iowa, USA: Geology, v. 34, no. 11, p. 969-972.

Ludvigson, G. A., Jacobson, S. R., Witzke, B. J., and Gonzalez, L. A., 1996, Carbonate component chemostratigraphy and depositional history of the Ordovician Decorah Formation, Upper Mississippi Valley: Special Paper - Geological Society of America, v. 306, p. 67-86.

Ludvigson, G. A., Witzke, B. J., Gonzalez, L. A., Carpenter, S. J., Schneider, C.

L., and Hasiuk, F., 2004, Late Ordovician (Turinian-Chatfieldian) carbon isotope excursions and their stratigraphic and paleoceanographic significance, in Pope, M. C., and Harris, M. T., eds., Palaeogeography, Palaeoclimatology, Palaeoecology: Amsterdam, Elsevier, p. 187-214.

Lukasik, J. J., James, N. P., McGowran, B., and Bone, Y., 2000, An epeiric ramp;

low-energy, cool-water carbonate facies in a Tertiary inland sea, Murray Basin, South Australia: Sedimentology, v. 47, no. 4, p. 851-881.

Mac Niocaill, C., van der Pluijm, B. A., and Van der Voo, R., 1997, Ordovician

paleogeography and the evolution of the Iapetus Ocean: Geology, v. 25, no. 2, p. 159-162.

Miller, A. I., 2004, The Ordovician radiation; toward a new global synthesis, in

Webby, B. D., Paris, F., Droser, M. L., and Percival, I. G., eds.: New York, Columbia University Press.

Paris, F., and Nolvak, J., 1999, Biological interpretation and paleobiodiversity of

a cryptic fossil group; the "chitinozoan animal", in Gayet, M., and Otero, O., eds., Geobios: Departement de Geologie, Universite Claude Bernard, p. 315-324.

Purnell, M. A., 1995, Large eyes and vision in conodonts: Lethaia, v. 28, no. 2, p.

187-188.

32

Richardson, J. G., and Babcock, L. E., 2002, Weird things from the Middle Ordovician of North America interpreted as conulariid fragments: Journal of Paleontology, v. 76, no. 2, p. 391-393.

Richardson, J. G., and Bergstrom, S. M., 2003, Regional stratigraphic relations of

the Trenton Limestone (Chatfieldian, Ordovician) in the eastern North American Midcontinent: Northeastern Geology and Environmental Sciences, v. 25, no. 2, p. 93-115.

Saltzman, M. R., and Young, S. A., 2005, Long-lived glaciation in the Late

Ordovician? Isotopic and sequence-stratigraphic evidence from western Laurentia: Geology, v. 33, no. 2, p. 109-112.

Simo, J. A., Emerson, N. R., Byers, C. W., and Ludvigson, G. A., 2003, Anatomy

of an embayment in an Ordovician epeiric sea, Upper Mississippi Valley, USA: Geology, v. 31, no. 6, p. 545-548.

Sloan, R. E., 1987, Tectonics, biostratigraphy and lithostratigraphy of the Middle and Late Ordovician of the Upper Mississippi Valley: Report of Investigations - Minnesota Geological Survey, v. 35, p. 7-20.

Sweet, W. C., 1984, Graphic correlation of upper Middle and Upper Ordovician

rocks, North American Midcontinent province, U.S.A, in Bruton, D. L., ed., Paleontological Contributions from the University of Oslo, vol.295: Paleontologisk Museum, University of Oslo, p. 23-36.

Sweet, W. C., Donoghue, P. C. J., and Adrain, J. M., 2001, Conodonts; past,

present, future: Journal of Paleontology, v. 75, no. 6, p. 1174-1184. Tobin, K. J., and Bergstrom, S. M., 2002, Implications of Ordovician ( nearly

equal 460 Myr) marine cement for constraining seawater temperature and atmospheric pCO (sub 2): Palaeogeography, Palaeoclimatology, Palaeoecology, v. 181, no. 4, p. 399-417.

Trotter, J. A., Williams, I. S., Barnes, C. R., Lecuyer, C., and Nicoll, R. S., 2008,

Did cooling oceans trigger Ordovician biodiversification? Evidence from conodont thermometry: Science, v. 321, no. 5888, p. 550-554.

Van Iten, H., 1991, Evolutionary affinities of conulariids, in Simonetta, A. M.,

and Conway Morris, S., eds.: Cambridge, Camb. Univ. Press. Van Iten, H., Fitzke, J. A., and Cox, R. S., 1996, Problematical fossil cnidarians

from the Upper Ordovician of the north-central USA: Palaeontology, v. 39, no. 4, p. 1037-1064.

Van Iten, H., Lichtenwalter, M., Leme, J. M., and Guimaraes, M., 2006, Possible

Taphonomic Bias in the Preservation of Phosphatic Macroinvertebrates in

33

the Uppermost Maquoketa Formation (Upper Ordovician) of Northeastern Iowa (North-Central USA): Journal of Taphonomy, v. 4, no. 4, p. 207-220.

Van Iten, H., Vyhlasova, Z., Zhu, M., and Yi, Q., 2005, Widespread occurrence of

microscopic pores in conulariids: Journal of Paleontology, v. 79, no. 2, p. 400-407.

Witzke, B. J., 1980, Middle and Upper Ordovician paleogeography of the region

bordering the Transcontinental Arch, in Fouch, T. D., and Magathan, E. R., eds.: Rocky Mount. Sect., Soc. Econ. Paleontol. Mineral.

Young, S. A., Saltzman, M. R., and Bergstrom, S. M., 2005, Upper Ordovician

(Mohawkian) carbon isotope (delta (super 13) C) stratigraphy in eastern and central North America; regional expression of a perturbation of the global carbon cycle: Palaeogeography, Palaeoclimatology, Palaeoecology, v. 222, no. 1-2, p. 53-76.

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