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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
brac
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.
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