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

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 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 representing a diverse shallow-water marine biotic community. Although some macrofossil communities have been documented and analyzed at great depth, many 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. 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

1

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 2

EQUATOR

N 30° S

Fig. 1. Late Ordovician paleogeography of Laurentia (Modified from Witzke 1980). Study area is outlined. 3

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).

4

The Great Ordovician Biodiversification Event

The Era experienced two major radiations of global biodiversification. The first is known as the 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, 5

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 Young etal.,2005). from Emersonetal., 2004; Ludvigsonetal.,Mossler, 1987; Fig 2.LateOrdovician timescalefortheDecorahFormation(Modified LATE ORDOVICIAN PERIOD

MOHAWKIAN SERIES CHATFIELDIAN TURINIAN STAGES

GALENA GROUP FORMATION PLATTEVILLE (Cummingsville) DECORAH Dunleith Guttenberg Spechts Ferry Ion 6 7

N

Fig 3. Paleogeog- raphy of the Upper Mississippi Kenyon Valley. Gray areas represent land, Rochester white areas repre- Decorah sent water. Field localities: Kenyon, MN, Rochester, MN, and Decorah, IA. Coordinate values represent present day latitude and longitude. 8

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 North South northwest southeast

high low nutrients nutrients high low carbonate high carbonate terrigenous productivity (low photic zone) productivity (high photic zone) input shallow light penetration FWWB increased light increased light penetration low penetration terrigenous Terrigenous shale input no terrigenous input low faunal Calcareous shale SWWB diversity high faunal Carbonates diversity

Fig. 4. Schematic representation of a vertically exaggerated shallow epeiric ramp upon which the Decorah Forma- tion was deposited. Represented here is a ‘snapshot’ in time, showing variation in deposition style dependent on proximity to land. Fluctuations in sea-level will change deposition style for a particular position along the ramp. Mixed carbonate shale units (such as the Decorah Formation) reflect episodes of transgression and regression, and the Decorah Formation records an overall transgression. 9 10 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).

KENYON, MINNESOTA DECORAH, IOWA % Carbonate NE Iowa SE Minn. 0% 50% 100% % Carbonate 0% 50% 100% Shale M SB PB B ROCHESTER, MINNESOTA Fm Mbr Period Series Stage Grp Fm Mbr Shale M SB PB B 27 % Carbonate 27 26 0% 50% 100% 26 Shale M SB PB B 25 Outcrop ends in vegetation 27 25 24 outcrop ends in eld 26 24 23 25 23 22 24 22 21 23 21 20 22 20

19 DUNLEITH Outcrop ends in vegetation CUMMINGSVILLE 21 19 18 (Inaccessible Cli Face) 20 18 17 (Inaccessible Cli Face) 19 17 16 (Inaccessible Cli Face) 16 18 15 15 17 14 T-958 14 16

GALENA 13 13

CHATFIELDIAN 15 12 12 14 R-953 11

MOHAWKIAN 11 T-922 13 10 UPPER ORDOVICIAN 10 12 Ion 9 9 11 8 8 10 7 7 9

6 DECORAH 6 DECORAH DECORAH 8 5 5

4 7 Guttenberg R-935 4 D-976 6 3 3 5 2 2 D-975 4 1 1 D-974 T-912 Ferry Spechts 3 0 R-933 0m Outcrop begins at the base 2 Outcrop begins at the top of the of a trench in a ditch on the Platteville Formation side of the road 1 TURINIAN 0 m Carimona

Outcrop begins in marsh on the west side of HWY 52 Carimona PLATTEVILLE PLATTEVILLE

LEGEND

Shale Sample Cover M Micrite SB Sparse Biomicrite T-912 Sample Number Limestone PB Packed Biomicrite Fe Ooids Shale B Biosparite Fig. 5. Lithostratigraphic columns and chronostratigraphic correlation of the eld localities: Kenyon, of the eld localities: Kenyon, correlation and chronostratigraphic columns 5. Lithostratigraphic Fig. 1987). Included stratigraphic are Mossler, from IA (nomenclature MN; Decorah, MN; Rochester, analysis. microfossil for of shale samples used locations 11 7

3 9 1 4 6 5 2 2 3 8

Fig. 6. Paleoenvironmental reconstruction of a Late Ordovician sea oor of the Decorah Formation based on evidence of macro- fossils. Marine fauna include: 1. crinoids, 2. bryozoans, 3. pelecypods, 4. gastropods, 5. articulate brachiopods, 6. trilobites, 7. cephalopods, 8. Chondrites burrows, and 9. rugose corals. 12 13

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) 14 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. 15

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. 16

1 2

100 µm 200 µm

3 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).

100 µm

4 5

100 µm 100 µm 17

1 3

100 µm 100 µm

2 Fig. 8. SEM micro- graphs of Linguli-

200 µm 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). 18

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).

Fig. 9. Anatomical reconstruction of a conulariid (Modified from Ivantsov and Fedonkin, 2002) 19 20

Chitinozoans

Common in rocks of Ordovician through 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 21

1 2

60 µm

70 µm 4

3

60 µm 60 µm 50 µm 5

Fig. 10. SEM micrographs of selected chitinozoan specimens from the Decorah Formation. All specimens are from sample D975 (Decorah, Iowa). 22 3 8 300 µm 300 µm 7 2 5 200 µm 200 µm 100 µm 4 1 6 100 µm 200 µm 100 µm

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. 23

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).

1 cm et al.,1995) of alivingconodont(from Aldridge Fig. 12. Anatomical reconstruction 24 25

Scolecodonts

Scolecodonts, jaws of 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, , 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 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- 26

1 2

300 µm 100 µm

3

Fig. 13. SEM micrographs of selected scolecodont elements from 100 µm the Decorah Formation. Strati- graphic location of individual elements: 1, R933; 2, R933; 3, R933; 4, R933; 5, R935.

4 5

100 µm 300 µm 27

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.

Fig. 14. Paleoenvironmental reconstruction of a Late Ordovician sea oor of the Decorah Formation based on new evidence of 1 cm 10 additional macro- and microfauna. Existing fauna 50 µm include: 1. crinoids, 2. bryozoans, 3. pelecypods, 4. 14 gastropods, 5. articulate brachiopods, 6. trilobites, 7. cephalopods, 8. Chondrites 1 cm burrows, and 9. rugose corals. Additional fauna 7 include: 10. conodonts , 11. conulariids, 12. inarticulate 13 11 brachiopods, 13. scoleco- donts (polychaete worms), and 14. chitinozoans. 3 9 1 4 6 5 12 2 2 3 8 28 29

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.

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