Mississippian Conodont Biostratigraphy of the Ste. Genevieve

This summer we researched the history of the limestone quarry in the DePauw Nature Park. The main goal was to assemble a stratigraphic column of the nature park by collecting and analyzing samples from each bed.  In order to do this, focus was directed towards an animal called a conodont(, that is very important for stratigraphy). From here, it was decided that the main objectives would be to figure out: 

• Are there conodonts in the Nature Park?
• Can we identify them and use them to determine the age of the rocks in the Nature Park?
• And do the specimens we find match the species found in previous studies of the Ste. Genevieve Limestone?

• A conodont is a soft-bodied, nektonic, primitive chordate comparable to a modern-day lamprey or hagfish. Conodonts are grouped with chordates despite the fact that they do not have vertebrae, and instead possess a notochord. A notochord is a rod that runs through an animal in the same way that a spinal column would, except it is made of cartilage rather than bone. (Notochords are found in all embryonic animals.).
• From around 500 ma (Late Cambrian) to 200 ma (Late Triassic), conodonts lived and roamed the aquatic realm. 
• The conodont element, which was the main focus of our study, is the phosphatic tooth apparatus that paleontologists use for dating purposes. Therefore, by finding teeth in beds sampled, we could get a better idea of the age of the Nature Park's rocks.

Conodonts were first described in 1856 by Christian Heinrich Pander using form-specific taxonomy. At this time, conodonts were unheard of since their importance (in stratigraphy) had yet to be uncovered. The following is a brief run-down of the history of conodonts:

1856 – conodonts were first described by Christian Heinrich Pander using form-specific taxonomy.

1952 – the acetic acid method for uncovering elements from carbonates was developed.

1971 - multi-element taxonomy was adopted as the preferred method for identifying specimens.

1983 - the first complete anatomy of a conodont was found encased in shale in Scotland.  ​

The majority of conodont elements, before a complete specimen was found, were thought to all be apart of different species of conodonts. But with the discovery of a whole fossilized conodont, we now know that they were teeth from just one conodont. The anatomy also allowed for us to get a closer idea of how the animals functioned by looking at their different types of teeth and each one's purpose.

The switch from form-specific taxonomy has been adopted slowly, making identification harder since the switch wasn't immediately adopted by scientists studying and publishing about these organisms. Most conodont specimens before the 1856 discovery were found in shales and sandstones, therefore being able to uncover specimens in carbonates opened up a new range of conodonts.

Greencastle, Indiana, our place of research, is located in the Blue River Group, also known as the Mississippian system. Within the Blue River Group are three main units: St. Louis (Limestone), Ste. Genevieve (Limestone), and the Paoli (Limestone). This area of rocks is mostly comprised of carbonate rocks, but contains small amounts of other rock types, such as the cross-bedded sandstone visible in the Spar Mountain section of the Ste. Genevieve Limestone.

We believe that we are in the Ste. Genevieve Limestone.

Within this stratigraphic column, all layers have very similar compositions, but the most distinct layer is the spar mountain member. Spar mountain has cross-beds, which are indicative of currents and current direction, as shown by the dotted lines going in different directions.

Oolites, a common occurrence in this column, indicate a high energy environment, and silty/muddy limestone indicates low energy environments. More conodont and foraminifera were found intact in oolitic and silty (/muddy) layers.

The key shown in the upper right explains what the patterns in the stratigraphic column represent. The first one is oolitic limestone = high energy, cross-bedded oolitic grain-stone = indicates the presence of currents, and silty limestone = low energy environments. In order to obtain the compositions of the layers, we came up with a method of collection and extraction of samples and their components.

1. Extract about 5 kg of sample from NP layer.

2. Crush about 2 kg into pea sized pieces.

3. Use dry sieve mesh sizes 18-20 to sort out finer pieces and minimize excess limestone reacting with acetic acid.

4. Dissolve 450 g of gravel with 3 gallons of 10% acetic acid.

5. Pour partially dissolved sample into wet sieve and then pour that into a beaker and place in heater to dry.

6. Brush contents into petri dish and examine under microscope.

Conodont "elements" are the teeth lining the mouth of conodonts. As a reminder, these organisms lacked jaws, so these teeth acted as rotating mouth parts. There are several groups in which these elements fall into. The first group is P group elements, known as pectiniforms. P group elements are the ”grinders,” similar in function to human molars, and are positioned in the back of the conodont's throat. These are are ideal for identifying specimens due to there only being a few p group elements in each animal. They each have much more distinctive features distinguishing themselves from other specimens.

The second group are the M (makelliforms) and S (symmetry transition series) group elements, which are called ramiforms. Ramiforms are more commonly found, especially the long bar type ones that resemble an ice pick. The purpose of these is for latching onto prey and pulling them into the conodont's mouth.

HOW TO DISTINGUISH P ELEMENTS FROM S AND M ELEMENTS:

It seems difficult to differentiate such small fossils, but there are key factors that help in distinguishing these groups from one another. You can tell that something is a pectiniform when you see a flat plate or platform supporting the structure when you flip the element/tooth so that the denticles are facing up. Lucky for us, ramiforms lack this feature. Some aspects that we look at to differentiate one specimen from another are:

-Bar angles (is it more acute or obtuse if bent?)

-Size and closeness of denticles (teeth)

-Element is symmetrical or not

-Cusp (biggest tooth) and its positioning in relation to the basal cavity or other denticles

P elements vs S and M elements

Note: Our findings are based on single form taxonomy rather than multi-element taxonomy since most of our sources were found before the method switch in the 70s.

The results we found were mostly inconclusive, but still follow the expectation that the Nature Park outcrop belongs to the Ste. Genevieve Limestone. Certain specimens that were only thought to be in the St. Louis Limestone were found in our outcrop. This could be interpreted in a few ways:

1. The Nature Park is partially in both the Ste. Genevieve and the St. Louis Limestones.
2. The specimen was identified wrong, therefore the results could be indicating incorrect placement.

Note: Our findings are based on single form taxonomy rather than multi-element taxonomy since most of our sources were found before the method switch in the 70s.

The results we found were mostly inconclusive, but still follow the expectation that the Nature Park outcrop belongs to the Ste. Genevieve Limestone. Certain specimens that were only thought to be in the St. Louis Limestone were found in our outcrop. This could be interpreted in a few ways:

1. The Nature Park is partially in both the Ste. Genevieve and the St. Louis Limestones.
2. The specimen was identified wrong, therefore the results could be indicating incorrect placement.

The results that we found are very preliminary and more research would be required before making any final conclusions about the Nature Park quarry. Along with this, larger sample sizes and a larger amount of samples taken would help to come to a definite conclusion about where exactly we are in the Blue River Group. More strategic planning of sampling harder to access layers up along the quarry wall would also be highly beneficial, if possible. A list of changes includes:

• Larger and more frequent sampling

•Use formic acid to minimize damage to conodonts

•Find better tools for handling conodonts to minimize damage

• Get more experience practicing using the SEM before analyzing more conodonts with the instrument.

This research, if continued, will give great insight into the DePauw Nature Park's geologic history and location in the Blue River Group. With the data, samples, and information already gathered, we hope to provide beneficial groundwork for future studies. This research opportunity has taught us a plethora of skills that we are very grateful for and are excited to have in our toolboxes moving forward.

Thanks to the Asher and Norton Endowed Fund in the Sciences, that allowed us to have the resources to do our research; Tim Cope, for guidance and stratigraphic knowledge throughout research; Caroline Gibson, the librarian who helped us find sources, as well as helping us navigate through different databases; Ken Brown and Wendy Williams, for instructing us through use of the SEM, as well as allowing us access to the SEM; Jim Brack, for allowing us onto his property to see a potential outcrop of the Ste. Genevieve for further sampling; Alyssa Bancroft, the conodont expert who helped us understand what conodonts are, how to identify them, and provided us with excellent sources of information.