Reconstructions of Late Ordovician crinoids and bryozoans from the Decorah Shale, Upper Mississippi Valley

Sibo Wang Senior Integrative Exercise

March 10, 2010

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

TABLE OF CONTENTS ABSTRACT INTRODUCTION ........................................................................................................ 01 GEOLOGIC SETTING ................................................................................................ 03 Late Ordovician world ................................................................................. 03

Southern Minnesota and the Decorah Shale ............................................... 03 Benthic community ....................................................................................... 05 Marine conditions ........................................................................................ 05

CRINOIDS ................................................................................................................. 06 General background and fossil record ........................................................ 06

Anatomy ....................................................................................................... 07 Decorah Shale crinoids ................................................................................10

BRYOZOANS ............................................................................................................. 10 General background and fossil record ........................................................ 10 Anatomy ....................................................................................................... 11 Decorah Shale bryozoans ............................................................................ 13

METHODS ................................................................................................................. 13 Research area .............................................................................................. 13 Processing fossils ......................................................................................... 14

Photographing ............................................................................................. 14 Reconstructions ............................................................................................ 14 RESULTS AND RECONSTRUCTIONS .......................................................................... 16

Crinoid fossils .............................................................................................. 16 Crinoid reconstructions ............................................................................... 18 Bryozoan fossils ........................................................................................... 18 Bryozoan reconstructions ............................................................................ 21

DISCUSSION .............................................................................................................. 21 Crinoids ....................................................................................................... 21 Bryozoans ..................................................................................................... 24

Further studies ............................................................................................. 25 CONCLUSION ............................................................................................................ 26 ACKNOWLEDGEMENTS ............................................................................................ 26 REFERENCES CITED ................................................................................................ 27 APPENDICES ............................................................................................................. 31

Reconstructions of Late Ordovician crinoids and bryozoans from the Decorah Shale,

Upper Mississippi Valley

Sibo Wang Carleton College

Senior Integrative Exercise March 10, 2010

Advisors: Clinton A. Cowan, Carleton College Department of Geology Cameron Davidson, Carleton College Department of Geology

ABSTRACT Questions are prevalent among introductory geology students about the appearances and forms of the animals whose fossils are found in local formations, one of which is the Upper Ordovician Decorah Shale. The Decorah is highly fossiliferous, with abundant remains of bryozoans, crinoids, brachiopods, and other various animal groups that are often loose within the formation and thus easily collectable. Fossils from outcrops at Wang’s Corner, MN; Massey Quarry near Rochester, MN; and Country Road 9 Quarry, Decorah, IA were collected with the intention of using them as the bases for reconstructions, primarily focusing on crinoids and bryozoans. The aims of this study are to reconstruct several crinoid species (Carabocrinus dicyclicus and Pycnocrinus sardesoni) as well as both a general bryozoan colony and the animals within the colony (Stictoporellina cribrosa) to visually aid students in understanding the anatomy and form of these organisms. General background and anatomical information is also presented for crinoids and bryozoans. Keywords: Decorah Shale, reconstruction, Carabocrinus, Pycnocrinus, Stictoporellina, Crinoidea, Bryozoa

INTRODUCTION

One of the most prevalent uncertainties paleontologists face is the nature and

appearance of the organisms they study, both extinct and extant. The interest is not solely

confined to scientists, however; understanding what extinct animals look like has

interested people of all ages and backgrounds. It is therefore no surprise that

reconstructions have been a familiar sight in museum paintings and dioramas and other

media intended for public consumption for more than a century (Isham et al., 1989; Paul

& Chase, 1989). In the past 20 years, demand for reconstructions within the scientific

community itself has increased as well (Paul & Chase, 1989). Reconstructions of

hominoids and large vertebrates such as dinosaurs and extinct mammals have dominated

the public’s interest; however, reconstructions of invertebrates, though perhaps less

lucrative, are nevertheless both scientifically and educationally valuable. Reconstructions

of invertebrates have been useful in gaining an understanding of paleoenvironmental

conditions—mostly marine—(Brower and Veinus, 1978) and can also be used to provide

a more accessible pathway for the public to learn about a sometimes enigmatic group of

animals.

Many invertebrates are preferentially preserved in the rock record because of their

mineralized skeletons and the marine environment in which they live, and consequently

there is an abundance of specimens from which to work (Moore et al., 1952; Müller,

1979). Two groups of animals that have been focused on are the class Crinoidea and the

phylum Bryozoa (Ectoprocta). Although there are extant species in both groups,

questions still remain about their predecessors, particularly Paleozoic (and specifically

Ordovician) species (Boardman et al., 1983). Soft part preservation is rare, and, as is the

1

case for crinoids, whole specimen preservation is uncommon because their calcareous

endoskeletons often disarticulate after death (Sprinkle and Guensburg, 2004).

Many crinoid reconstructions have been attempted (Jaekel, 1918; Pisera and Dzik,

1979; Haude et al., 1994), although there are only a few reconstructions specific to

Ordovician species (Ubaghs, 1969; Brower and Veinus, 1978). Crinoids found in

Lagerstätten have naturally been focused on (Liddell, 1975; Brett & Liddell, 1978;

Ausich, 1999) but reconstructions of crinoids from their disarticulated parts are rarely

tackled (Brower and Veinus, 1978). Brower and Veinus (1978) did attempt such

reconstructions of crinoids from the Twin Cities area, although they are schematic at best.

Information on Ordovician bryozoans is more lacking than on Ordovician crinoids

(Boardman et al., 1983). Despite the fact that bryozoan species were diverse and

abundant in the Ordovician (Moore et al., 1952), little research has been done on them in

comparison to other phyla of the time, in part because they are not particularly useful in

biostratigraphy (Taylor and Ernst, 2004). Identification of bryozoans beyond order-level

is also difficult because it requires thin sections taken at specific angles through the

skeleton (Moore et al., 1952; Tayler and Ernst, 2004). Reconstructions of the individual

animals within a bryozoan colony are often generalized or schematic (Boardman et al.,

1983; Utgaard, 1983) because several bryozoan orders have no extant species (Moore et

al., 1952).

In this paper, I am attempting to reconstruct crinoid and bryozoan species from

the Upper Ordovician Decorah Shale. Using illustrations, I am aiming to reconstruct

whole specimens with their soft parts and in life position. The bases for the

reconstructions are fossils that have been collected from the Decorah Shale in addition to

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graphical and descriptive information from published literature. The reconstructions are

done with the intention of providing a visual aid to undergraduate introductory geology

students about the organisms that have existed locally. A general overview of crinoid and

bryozoan anatomy is also presented as a resource for students.

GEOLOGIC SETTING

Late Ordovician world

During the Late Ordovician, Earth is thought to have experienced two eustatic

highstands, the first of which occurred during the Caradoc and was the largest in the

entirety of the Phanerozoic (Hallam, 1992). A high greenhouse state and high carbon

dioxide levels led to a warm climate (Berner, 2001; Barnes, 2004a), although it is

theorized that the Earth plunged into an ice age during the terminal Ordovician which

correlated with the end Ordovician extinction event (Berry and Boucot, 1973; Jablonski,

1991; Brenchley, 2004). Increased volcanism, possibly in conjunction with a Mid

Ordovician mantle superplume, also contributed to shaping the climate (Barnes et al.,

1996; Qing et al., 1998; Barnes, 2004b).

Southern Minnesota and the Decorah Shale

Much of the Laurentian craton (North America) was submerged during the

extensive Caradoc transgression (Barnes, 2004a), and what is now southern Minnesota

was inundated by the Hollandale Embayment, part of a larger epeiric sea (Simo et al.,

2003; Fig. 1). The Hollandale Embayment was bounded by the Transcontinental Arch to

the north and west and the Wisconsin Dome to the east. The paleolatitude of the

embayment was approximately 15°S, and thus, the climate was likely tropical to

subtropical (Simo et al., 2003).

3

Transcontinental A

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Wisconsin

Dome

15˚S

N

WC

MQ

CRD

Hollandale Embayment

100 km

Figure 1. Geologic map, showing Transcontinental Arch, Wisconsin Dome, and Hollandale Embayment (modified from Witzke, 1980). Gray denotes Precambrian rock, but is not necessarily above sea-level during the Late Ordovician. Paleolatitude of 15°S from Simo et al. (2003). Note direction of paleo-north. Study locations are also shown: WC: Wang’s Corner, MN; MQ: Massey Quarry, near Rochester, MN; CRD: County Road 9 Quarry, Decorah, IA.

4

The Decorah Shale was deposited in the Hollandale Embayment, which

comprised a large stretch of what is now the Upper Mississippi Valley (Simo et al., 2003).

Abundant volcanism led to the deposition of several K-bentonite layers within the

Decorah Shale (Sloan, 1988), which were used to date the formation’s deposition to

around 454 Ma (Gradstein et al., 2004; Sadler and Cooper, 2004; Ogg, 2009). The

Decorah crops out in many locations in southern Minnesota and northern Iowa (Sloan et

al., 1988). Moving southeast across the formation, there is a decrease in shale content and

an increase in carbonates, and in southern Minnesota, most of the formation is composed

of fossiliferous shale with carbonate lenses and some beds of limestone (Mossler, 1987).

Benthic community

Brachiopods and bryozoans are abundant throughout the Decorah Shale, and it is

likely they were the primary constituents of the benthic fauna (Mossler, 1987). Crinoids

can be found in abundance in several layers, though usually in the form of disarticulated

ossicles (Brower and Veinus, 1978). Also present, but in less abundance, are gastropods,

pelecypods, cephalopods, rugose corals, receptaculities, and trilobites (Mossler and

Benson, 1999).

Marine conditions

The fossils found throughout the Decorah belong to the Heterozoan Association

proposed by James (1998). Although a Heterozoan assemblage indicates the possibility of

cool-water conditions, it does not exclusively signify such an environment (James, 1998).

There are a number of different variables that could have contributed to the found

assemblage; for instance, salinity, nutrient levels, and upwelling from colder polar waters

could all have affected the fauna, and it is uncertain how large a role each factor played

5

(James, 1998). Worldwide climate conditions and the paleolatitude of the area favor

warmer marine conditions, but neither are definitive indicators either (Simo et al., 2003).

Brower and Veinus (1978) were able to infer that the benthic fauna of the

Decorah Shale experienced strong but intermittent agitation from currents as well as

periods of quiet water. The substrate was likely unstable and not well-lithified, but also

rough and hummocky (Brower and Veinus, 1978). They also concluded that the

environment had normal marine salinity; however Barnes (2004a) argues instead for

raised salinity due to the warm climate and high evaporation rates to which fauna slowly

adapted.

CRINOIDS

General background and fossil record

Crinoids belong to the class Crinoidea of the phylum Echinodermata and, like

most other organisms within the phylum, exhibit some form of pentameral radial

symmetry when mature (Moore et al., 1952; Ubachs, 1967). They are entirely marine,

mostly sessile or slow-moving, and unlike other echinoderms, filter food from the water

column rather than from the substrate (Hess and Ausich, 1999). As larvae they are free-

floating and typically exhibit bilateral symmetry (Ubachs, 1967). Crinoid skeletons are

composed of high-Mg calcite (Sprinkle and Guensburg, 2004), and crinoids were

important reef-builders from the Ordovician to the Jurassic, with many of their skeletal

parts being found in limestones worldwide (Moore and Teichert, 1978).

Some argue that a basal crinoid existed in the Cambrian (Sprinkle and Moore,

1978); this is, however, repeatedly disputed by Ausich and Babcock (1998; 2000), who

believe the specimen is more probably an octocoral. The first unequivocal fossil crinoid

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is from the middle Early Ordovician (Ausich and Babcock, 1998; Guensburg and

Sprinkle, 2001). Nevertheless, it is thought that crinoids first evolved during the Middle

to Late Cambrian, despite the lack of fossil evidence (Guensburg and Sprinkle, 2001).

Within the Ordovician, crinoids experienced the most species-level diversity during the

Mohawkian/Caradoc Age of the Late Ordovician, rising to become the most prominent

class of echinoderms during this time (Sprinkle and Guensburg, 2004).

Anatomy

Crinoids are composed of two basic sections: the pelma (the stem and holdfast, if

any), and the crown or corona (which includes the calyx and free arms, if any) (Ubaghs,

1978). The crinoid skeleton is made up of individual ossicles, which can refer to any

segment or plate of the skeleton (Moore et al., 1952). Ossicles in the stem are called

columnals, whereas ossicles in the arms are known as brachials (Moore et al., 1952).

Moore et al. (1952) has an excellent introduction to and explanation of basic anatomy and

anatomical terminology.

Stem: The stem serves a dual purpose of anchoring the crinoid to the substrate as

well as elevating the crinoid above the seafloor so that it has a better position to filter

food from the water (Ausich et al., 1999; Hess and Ausich, 1999). The stem is composed

of numerous columnals connected to each other with ligaments and other living tissue,

and can take on a wide range of forms, varying in shape, length, and diameter depending

on the environment to which the crinoid is adapted (Ausich et al., 1999). Some crinoid

species have specialized holdfasts (Ausich et al., 1999), whereas in other species, the

distal part of the stem serves as a makeshift holdfast, curling around other objects on the

seafloor or itself propping the crinoid upright (Liddell, 1975).

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Calyx: The calyx (or cup) is generally bulb-shaped and is composed of plates that

sit atop the stem and house the crinoid’s internal organs (Hess and Ausich, 1999).

Calyxes can have several different plate configurations depending on order, family, etc.,

and the treatment of specific configurations is beyond the scope of this paper (see Moore

et al., 1952; Moore and Teichert, 1978; Simms, 1999).

In some species, the cup comprises the calyx and the entirety of the arms, as

plates connect all the appendages together; in other species, the cup is composed solely of

the calyx and the entire extent of the arms are free-moving (Ausich et al., 1999).

Arms: Crinoid arms comprise brachials that are connected by ligaments and other

tissue (Ausich et al., 1999). The primary function of the arms is to filter out food from the

water column (Ausich et al., 1999). Each arm ray can branch in a variety of ways (see Fig.

2). Arm sections between branch nodes are known as brachs (Moore et al., 1952). Arms

may or may not possess pinnules, which are small branches on the arms intended to

improve filtration (Ausich et al., 1999).

Soft Parts: Crinoids are composed mostly of skeleton, with little living tissue in

comparison (Ausich et al., 1999). The stem is not covered by skin; however the

columnals have numerous microscopic holes that are filled with tissue that excrete the

calcium carbonate skeleton. The tegmen that covers the calyx is composed of leathery,

tough skin, small calcareous plates, or a mixture of both (Moore et al., 1952). Towards

the center of the tegmen is the mouth opening, which is connected to a digestive tract

comprising a feeding tube, stomach, intestines and anus (Moore et al., 1952). The anal

opening is usually located on the side of the tegmen, sometimes on a very large anal tube

(Moore et al., 1952). A water-vascular system runs through the arms and ends in the tube

8

Figure 2. Two possible arm branching forms. Sections between branching points are known as brachs. The entire arm with branches is known as an arm ray. Modified from Moore et al., 1952.

9

feet that are located in grooves within the arm brachials (Moore et al., 1952; Ausich et al.,

1999). The tube feet possess cilia, which are used both to move food down to the mouth

as well as diffuse oxygen into the crinoid (Ausich et al., 1999). Crinoids possess an

extensive nervous system that also runs throughout the animal, though the nerves are

concentrated in the arms and calyx (Moore et al., 1952; Ausich et al., 1999).

Decorah Shale crinoids

Crinoid fossils are moderately abundant throughout the Decorah Shale, which

contains fourteen known species and six types of holdfasts in the Twin Cities area

(Brower and Veinus, 1978; Appendix 1). Information about crinoids within the Decorah

in areas other than the Twin Cities is relatively unknown. In the Twin Cities area, two

subclasses are represented within the Decorah: the Camerata and the Inadunata (Brower

and Veinus, 1978). Disarticulated columnals, short stem fragments, and plate fragments

are the most commonly crinoid fossils found (Brower and Veinus, 1978). Intact calyxes

and arm segments are rare (Brower and Veinus, 1978).

BRYOZOANS

General background and fossil record

In this paper, bryozoans will refer to Ectoprocta only, and Entoprocta will not be

classified within the phylum (cf. Tudge, 2000). Bryozoans have been an enigmatic

phylum throughout history, and it was only recently confirmed using nucleotide data that

the phylum was truly monophyletic (Fuchs et al., 2009). Although bryozoans appear in

the fossil record during the Early Ordovician, it is probable that, like crinoids, more

primitive and basal forms evolved prior to the Ordovician (Fuchs et al., 2009). Bryozoans

10

are unique in that they are the only phylum in which all species are colonial organisms

(Boardman et al., 1983).

Bryozoans reproduce both sexually and asexually (Moore et al., 1952). New

colonies are formed from the sexually-produced larva of old colonies, and when a larva

settles onto a hard surface, it matures into the ancestrula, or the first adult individual of a

colony (Moore et al., 1952; Boardman et al., 1983). All subsequent members of that

colony are clones of the ancestrula and created through asexual budding (Moore et al,

1952; Boardman et al., 1983). Unlike crinoids, bryozoan species have an exoskeleton,

though some species do not excrete a mineralized skeleton (Boardman et al., 1983).

Bryozoans are also filter feeders, and use a lophophore to sift food particles from the

water (Moore et al., 1952).

Anatomy

Zooid: The individual animal within a bryozoan colony is called the zooid.

Zooids are not usually larger than 1 mm in length, and are often smaller (Boardman et al.,

1983; McKinney and Jackson, 1989). Basic zooids are called autozooids, and perform

feeding and sometimes reproductive functions (McKinney and Jackson, 1989). The

simplified anatomy of autozooids comprises a lophophore that is composed of tentacles

and encircles a mouth, an esophagus, stomach and anus (Moore et al., 1952; Boardman et

al., 1983; McKinney and Jackson, 1989). The digestive tract is recurved, with the anus

located by the mouth (Moore et al., 1952; Boardman et al., 1983). Zooids can become

highly specialized forms, known as heterozooids, and can function in several different

roles (e.g. reproduction, cleaning, protection) (Boardman et al., 1983; McKinney and

Jackson, 1989). In species that secrete calcium carbonate, zooids are enclosed by an

11

Figure 3. Section of zoarium of a trepostomes bryozoan showing zooecium. Modified from Moore et al., 1952.

12

individual skeleton, called the zooecium (Fig. 3; McKinney and Jackson, 1989).

Colony: The skeleton of the entire colony is known as the zoarium (Moore et al,

1952; Boardman et al., 1983). Colonies can be encrusting, erect, free-living, or rooted,

and several forms have been preferentially evolved throughout geologic time by different

species in what is known as iterative evolution (McKinney and Jackson, 1989).

McKinney and Jackson (1989) determined five basic growth forms that most colonies

take on. Colony forms can be determined by species, environment, or a combination of

both (Boardman et al., 1983).

Decorah Shale Bryozoans

Bryozoan fossils are abundant throughout the Decorah Shale (Karklins, 1988).

The bryozoan fossils found in the Decorah are primarily of the orders Trepostomata (32

species) and Cryptostomata (18 species); bryozoan fossils of the order Cystoporata are

rare and represented by one species, Bythotrypa laxata (Karklins, 1988; Appendix 2).

Within the trepostomes, genera Homotrypa, Heterotrypa, Batostoma, Parvohallopora,

and Eridotrypa are the most prevalent; within the cryptostomes, the genera that are most

common are Stictopora, Trigonodictya, and Pseudostictoporella (Karklins, 1988).

METHODS

Research area

Fossils were collected in the fall of 2009 during several field trips for the

Sedimentology and Stratigraphy course at Carleton College, as well as on independent

return outings to Wang’s Corner, MN; Massey Quarry near Rochester, MN; and County

Road 9 Quarry in Decorah, IA (Fig. 1). In general, fossils were removed from 1)

weathered shale, 2) poorly-lithified shale, or 3) surface to subsurface soil, depending on

13

location; no large scale excavations were attempted and no formal sampling methods

were used.

Processing fossils

Crinoid and bryozoan fossils were separated from other irrelevant fossils and

organized by location. The fossils were initially cleaned with water spray to remove large

amounts of shaley material, soil, and other debris. They were then more thoroughly

cleaned with a Bransonic UL-452E ultrasonic cleaner. Depending on their size, fossils

were placed into either a 50 ml or 250 ml Pyrex beaker with soap and tap water and then

placed in the cleaner between 30 and 60 seconds. The cleaner itself was filled

approximately three-quarters full with water, although the water level was adjusted

depending on the size of the beaker being used. Fossils were completely immersed in

solution if possible. Some small scale iron oxidation occurred after cleaning, but did not

significantly obscure important anatomical details. Although not used in this study,

cleaning with distilled water is recommended.

Photographing

Representative fossils were photographed to show the assemblages present at each

location using a Panasonic Lumix DMC-LZ7 camera. A Nikon SMZ-10 stereozoom

microscope was used to magnify the details on fossils to aid in the reconstructions. A

Nikon Coolpix 990 digital camera was used in conjunction with the microscope in order

to further zoom in on fossil details for the reconstructions.

Reconstructions

Reconstructions were created using Adobe Photoshop CS4 Extended with a

Wacom Intuos2 6x8 graphics tablet. Illustrations, photographs, and reconstructions from

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Figure 4. Crinoid fossil assemblages. A. Assemblage of Wang’s Corner, MN; 1-7, 9-11, Pycnocrinus sp.?; 8, stem of unknown crinoid; 12, 13, Isotomocrinus tenuis? Cincinnaticrinus sp.?; 14-29, 31-33, individual columnals of several Pycnocrinus sp.? 30, Iocrinus sp.? B. Assemblage of Massey Quarry, near Rochester, MN; 1-5, Pycnocrinus sp.; 6, 8, Pycnocrinus sp.?; 7, 9, 10, Carabocrinus sp.?

15

the literature were used in addition to the fossils collected to create the most accurate

reconstructions possible.

RESULTS AND RECONSTRUCTIONS

Crinoid fossils

No whole specimens were found in the field; most crinoid fossils were in the form

of fossil debris (Fig. 4). Additionally, no recognizable crinoid calyxes or arm brachials

were found. However, numerous columnals were collected, as well as a few stem

fragments that had not disarticulated.

Crinoid fossils found at Wang’s Corner were mostly in the form of disarticulated

columnals or short (<2 cm) stem sections (Fig. 4A). The largest individual columnal had

a diameter of ~11 mm, while the smallest columnal section had a diameter of ~2 mm.

Most of the crinoid fossils that were found likely comprised the genus Pycnocrinus. Two

stem fragments appeared similar to an Isotomocrinus tenuis identified by Brower and

Veinus (1978), although they admitted that the identification was tentative, at best.

However, the stem fragments as well as the specimen identified by Brower and Veinus

(1978) bear a striking resemblance to a Cincinnaticrinus sp. found in an approximately

coeval formation from New York (Brower, 2005). An individual columnal (30) was also

found that is tentatively being identified here as Iocrinus sp. (Fig. 4A; Meyer et al., 2002);

its occurrence in the Decorah has never previously been mentioned.

Much longer columnal sections were found at Massey Quarry, with the longest

being about 4.1 cm (Fig. 4B). The crinoid fossils found likely all belonged to either the

genus Pycnocrinus or the genus Carabocrinus. No crinoid fossils were found at the

County Road 9 Quarry; this was, however, the least surveyed location, and thus, lack of

16

Figure 5. Reconstruction of the crinoid Carobocrinus dicyclicus. Note lack of pinnules and short stem. Calyx is approximately 4 cm in height.

17

recovered crinoid fossils should not be an indication that there were no crinoids present.

Crinoid reconstructions

Carabocrinus dicyclicus was reconstructed primarily using photographs by

Brower and Veinus (1978; Fig. 5). The crinoid is illustrated as having three primibrachs;

arms branch a maximum of five times, both isotomously and heterotomously (Fig. 2).

Tube feet, or the feeding apparatuses, are shown on the most distal arm from the

foreground in orange. The crinoid is shown cemented directly to the substrate with a

basal plate.

Pycnocrinus sardesoni was also reconstructed primarily using photographs by

Brower and Veinus (1978; Fig. 6) as well as using a Pycnocrinus dyeri reconstruction by

Ausich (1999). The distal stem and holdfast are excluded.

Bryozoan fossils

Many bryozoans of the orders Trepostomata and Cryptostomata were collected

from each field site (Fig. 7). At Wang’s Corner, two Prasopora insularis were found

cemented onto on mass composed of shale and fossil debris, with one colony growing on

top of the other (Fig. 7A). Two other Prasopora fossils were found, one being identified

as Prasopora simulatrix, and the other too small to identify the species. Several bryozoan

fossils were likely Batostoma sp., and one (6) was Stictopora sp.

Fossils recovered from Massey Quarry were largely of the genus Batostoma, with

two Stictoporellina cribosa fossils, two Stictopora sp. fossils, and a Hallopora

multitabulata that was still cemented to other fossil debris (Fig. 7B). Several fossils were

unable to be identified, largely due to their small size and incomplete nature. Bryozoans

collected from the County Road 9 Quarry were mostly from the genus Prasopora, with

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Figure 6. Reconstruction of crinoid Pycnocrinus sardesoni. Neither form of the distal stem nor holdfast is known. Note pinnules and lack of plate ridges. The section illustrated here is approximately 6 to 7 cm in length.

19

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20

one fossil tentatively placed in the genus Batostoma (Fig. 7C).

Bryozoan reconstructions

A Stictoporellina cribrosa colony was reconstructed (Fig. 8) based on the fossil

fragments found at Massey Quarry in addition to illustrations by Sardeson (1901) and

models of erect fan growth (McKinney and Jackson, 1989).

Stictoporellina cribrosa autozooids were illustrated within the living chambers of

the zoarium (Fig. 9). Autozooid forms are schematic. Zooecium forms were based off of

thin section photographs (Karklins, 1983) and an illustration by Sardeson (1901).

Schematic autozooid form based off of general zooid illustrations (Boardman et al., 1983;

Utgaard, 1983). Autozooid anatomical features that are illustrated are the tentacles of the

lophophore, mouth, cardia, caecum, pylorus, intestine, and anus.

DISCUSSION

It should be noted that the colors used in the reconstructions are pure conjecture,

and used only to enhance a more realistic view of specimens. Because the species being

reconstructed are extinct, it is inevitable that more imagination and speculation are

involved than is optimal.

Crinoids

A full crinoid reconstruction was difficult without an entire specimen; calyx and

arm configuration and appearance were essential to understanding the basic form, and

because no calyxes or arm segments were recovered, crinoid species were chosen based

on the amount of graphical information present in the literature. Brower and Veinus

(1978) had the most comprehensive information on Decorah Shale crinoids, and thus, the

species best represented in their compilation were chosen for reconstruction (i.e.,

21

Figu

re 8

. Rec

onst

ruct

ion

of th

e br

yozo

an S

ticto

pore

llina

cri

bros

a. C

olon

y is

app

roxi

mat

ely

15 c

m w

ide.

22

Figure 9. Generalized reconstruction of Stictoporellina cribrosa autozooids w

ithin the living chambers of

the zoarium. C

ross-section view of colony perpendicular to plane of zoarium

. Zooids are approximately 1

mm

long.

23

Carabocrinus dicyclicus and Pycnocrinus sardesoni).

Carabocrinus dicyclicus (Fig. 5) is illustrated as having a very short stem due to

the large bulk of the calyx and the fact that strong agitation from currents would more

easily detach the calyx from a longer stem (Brower and Veinus, 1978). The tegmen is

only partially known, and thus not very well detailed in the reconstruction (Brower and

Veinus, 1978). The species only has two to three primibrachs and relatively short arms

(Brower and Veinus, 1978). Total arm length is likely a little too long in the

reconstruction; in adults, the calyx is approximately 4 cm tall, while the arms are

approximately 5 cm in length (Brower and Veinus, 1978). The holdfast is disk-shaped,

but its structure is hard to make out in the photographs of the fossil counterparts by

Brower and Veinus (1978), and so the holdfast is not very well articulated in the

reconstruction.

Pycnocrinus sardesoni (Fig. 6) is depicted without the distal stem segment or the

holdfast because the forms are unknown (Brower and Veinus, 1978). However, it is likely

that the species did not have a holdfast, instead using the distal stem to curl around

objects, such as branching bryozoan species, to anchor itself to the substrate, much like

Pycnocrinus dyeri (Ausich, 1999). Arms are pinnulated, and the plates of the calyx lack

plate margins (Brower and Veinus, 1978).

Bryozoans

When possible, I tried to identify the bryozoan fossils at the genus or species level.

Zoarium forms were relatively species-specific, although generalized forms are relatively

known at the genus level. The zoarium of many species is more or less preserved, while

the forms of the zooids are unknown, and thus, zooid reconstructions are more

24

scientifically interesting and useful.

Stictoporellina cribrosa was chosen for reconstruction because of the relative

straightforwardness of the zoarium form. Only small sections of the zoarium were found

in the field (Fig. 7B), and so the fossils were used to extrapolate the form of the entire

zoarium.

The reconstruction of the zooid of Stictoporellina cribrosa (Fig. 9) was difficult

because the forms of Ordovician zooids are almost completely unknown. Twitchell (1934)

even argues that the Trepostomata, an extinct order of Bryozoa, should be reassigned to

the Entoprocta, and though today it is widely accepted that trepostomes are Ectoprocta

(Boardman et al.), it illustrates the uncertainty of soft part anatomy of extinct bryozoan

orders. Therefore, a generalized and schematic reconstruction was attempted, more to

show zooid placement within the zoarium than for anatomical accuracy. Thin sections of

longitudinal sections of the zoarium were used as a guide for zooecium form (Karklins,

1983), as well as modeled growth forms of fan-shaped bryozoans (McKinney and

Jackson, 1989).

Further studies

In order to aid in more accurate reconstructions, detailed observations of pertinent

crinoid and bryozoan collections at museums, universities, and colleges should be

completed. More thorough and systematic collections of fossils from the field should be

also implemented, especially at different times during a week or month, as weather had a

significant effect on fossil-collecting conditions. Reconstructions of other bryozoan and

crinoid species of the Decorah Shale should also be illustrated because there was a

relatively high diversity of species within the Decorah, and knowing complete forms

25

would be educationally useful. A reconstruction of the benthic community would also be

valuable, especially to illustrate how the benthic fauna interacted with one another.

CONCLUSIONS

Undergraduate introductory geology students often come away from fossiliferous

outcrop excursions with questions about the form and living appearance of the fossilized

animals that they discover. This study was completed with the intention of providing a

basic introduction into two groups of animals that are found in the Upper Ordovician

Decorah Shale, the Crinoidea and the Bryozoa. A general overview of crinoid and

bryozoan anatomy is discussed with the hopes of providing a useful introductory resource

for students. Fossils were collected from three outcrops of the Decorah and used in

conjunction with published literature to reconstruct the likely forms of 1) the crinoids

Carabocrinus sardesoni and Pycnocrinus dicyclicus and 2) the bryozoan Stictoporellina

cribrosa colony and zooids as they appeared when living. There was some difficulty

encountered while doing the reconstructions, and collected fossils from the field were

less useful than previously anticipated. Although it is inevitable that certain aspects of the

reconstructions are the product of speculation and imagination, the intended goal in

creating the reconstructions is to provide a visual aid to students to further their

understanding, and I hope that this has been accomplished.

ACKNOWLEDGEMENTS

I would like to thank my advisors Clint Cowan and Cam Davidson—Clint, for

inspiring my interest in paleobiology, providing me with ideas for this paper, and

supplying invaluable resources; and Cam, who gave incredibly useful advice, critical

commentary, and encouragement. I would also like to extend my gratitude to the Carleton

26

College Geology Department for use of their resources; to Stuart Sweeney Smith for

doing several edits, offering general advice, and providing emotional support; and to all

my fellow geology senior majors for their encouragement and support throughout the

process.

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APPENDICES APPENDIX 1. CRINOID FOSSILS OF THE DECORAH SHALE (FROM BROWER AND VEINUS, 1978). Subclass Camerata Wachsmuth and Springer, 1885 Order Diplobathrida (Moore and Laudon, 1943) Family Archaeocrinidae Archaeocrinus sp. Order Monobathrida Suborder Glyptocrinina Family Glyptocrinidae

Glyptocrinus tridactylus (Brower and Veinus, 1978) Pycnocrinus multibrachialis (Brower and Veinus, 1978) Pycnocrinus sardesoni (Brower and Veinus, 1978) Periglyptocrinus spinuliferus (Brower and Veinus, 1978)

Subclass Inadunata Order Cladida Suborder Dendrocrinina Family Cupulocrinidae

Cupulocrinus jewetti (E. Billings) Cupulocrinus canaliculatus (Brower and Veinus, 1978)

Family Dendrocrinidae Grenprisia billingsi (Springer)

Suborder Cyathocrinina Family Carabocrinidae

Carabocrinus dicyclicus (Sardeson) Carabocrinus magnificus Sardeson

Family Palaeocrinidae Palaeocrinus angulatus (E. Billings)

Family Porocrinidae Porocrinus pentagonius Meek and Worthen Order Disparida Superfamily Cincinnaticrinacea Family Cincinnaticrinidae

Isotomocrinus tenuis (E. Billings) Superfamily Homocrinicae Family Calceocrinidae

Cremacrinus punctatus Ulrich Attachment devices Lichenocrinid holdfasts Lobate and digitate holdfasts cemented to bryozoans Tree stump-like cirrus root Massive conical attachment-disk

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APPENDIX 2. BRYOZOAN FOSSILS OF THE DECORAH SHALE (FROM KARKLINS, 1988). Order Trepostomata Family Amplexoporidae Amplexopora minnesotensis Amplexopora winchelli Family Trematoporidae Batostoma humile Batostoma increbescens Batostoma varium

Family Atactotoechidae Cyphotrypa cf. C. pachymuralis Cyphotrypa acervulosa Family Diplotrypidae Diplotrypa sp. Family Aisenvergiidae

Eridotrypa minor Family Dittoporidae

Hemiphragma irrasum Family Heterotrypidae

Heterotrypa* pauca Heterotrypa praenuntia echinata Heterotrypa praenuntia simplex Heterotrypa trentonensis Stigmatella crenulata Stigmatella spinosa

Family Mesotrypidae Homotrypa cf. H. curvata Homotrypa dickeyvillensis Homotrypa exilis

Homotrypa similis Homotrypa pauperata Homotrypa subramosa Prasopora insularis Prasopora simulatrix

Family Monticuliporidae Monticulipora grandis

Family Halloporidae Parvohallopora arguta Parvohallopora cf. P. tolli Parvohallopora incontroversa Tarphophragma ampla Tarphophragma multitabulata = Hallopora multitabulata

Unassigned Trepostome genera Nicholsonella ponderosa Nicholsonella sp.

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Order Cryptostomata Suborder Ptilodictyina

Family Rhinidictyidae Athrophragma foliata Stictopora cf. S. neglecta Stictopora exigua Stictopora lita Stictopora minima Stictopora mutabilis Stictopora paupera Trigonodictya acuta Trigonodictya conciliatrix Trigonodictya elegans Trigonodictya fimbriata Trigonodictya pumila

Family Stictoporellidae Pseudostictoporella angularis Pseudostictoporella dumosa Pseudostictoporella frondifera

Stictoporellina cribrosa Family Escharoporidae

Escharopora subrecta Graptodictya simplex = Arthropora simplex?

Order Cystoporata Suborder Fistuliporina Family Anolotichiidae

Bythotrypa laxata *Heterotrypa = Dekayella

33