Parsons is 90 miles from Nashville, but well worth the trip.  The exposure is from the early Devonian Period (about 400 million years ago, or about 50 million years after the fossils you find in the Nashville Basin.

Directions:  Drive west on I-40 until you cross the Tennessee River.  Exit on Rt 69.  Drive south 11 miles.  The Parsons Quarry will be on your left.

Fossils are most common in the lower Devonian Ross Formation

During the early Devonian Period, what is now North America was still largely under water and close to the equator.  However, the continents which were to form Pangea at the end of the Paleozoic Era were much closer.  Follow this link for a map of the Early Devonian Period.

Thanks to Michael Gibson of UT Martin for his help in identifying the fossils.
 

Brachiopods

Brachiopods are marine invertebrate shellfish which superficially resemble bivalve mollusks, but which are very different internally.  They feed by means of a strip of muscle called a lophophore, the same feeding mechanism used by bryozoans.  Brachiopods are extremely common in the Ross Formation. You will find individual shells and complete animals.

The white patches you often see on the fossils (see the picture of Camarotoechia)are places where the original calcium carbonate has been replaced by silicon dioxide.  (See links on taphonomy below).

Camarotoechia	Ancillotoechia	Constellirostra
Discomyorthis	Gypidula	Leptaena
Levenea	Meristella	Obturamentella
Pseudoatrypa	Rhynchospirina	Uncinculus
Schuchertella		Sphaerirhynchia
Kozlowskiellina

Favosites

Streptelasma

There are a great many species of crinoids found at Parsons.  Unfortunately, what we commonly find are pieces of stem, which do not give enough information for identification.

Crinoid Stem

Crinoid Calyx

Crinoid Stem

Ribbony bryozoans

Fenestrate bryozoan

Large flat bryozoan - probably encrusting

Dr. Michael Gibson from UT Martin has done a great deal of research reconstructing the early Devonian environment in Decatur County.  These studies were published in the book Upper Silurian-Lower Devonian Biotas and Paleoenvironments of the Western Tennessee Shelf, edited by Thomas W Broadhead and Michael A Gibson.  UT Department of Geological Sciences Studies in Geology 25. Material for this section is adapted from his articles. References in the text below are to this volume unless otherwise specified.   Many thanks to Dr. Gibson.

Evidence that the sea floor was soft

The Brachiopod Genera found
In our exercise reconstruction brachiopods, we saw that some brachiopods stand erect on a muscle called a pedicle.  Others lie on or half in the sediment, while others burrow in the sediment.

The ones that stand erect need a firm seafloor (substrate), rock or shell to which to attach.  Brachiopods which lie on the bottom are better adapted to live on a soft substrate.  Most of the commonest brachiopods found in Parsons are adapted for soft substrate.

Many or Strophomenids.  These brachiopods are concavo-convex, their shells like two spoons stacked on top of one another.  Lying on the mud, concave side up, like bowls, they could keep their mantle edges out of the mud. The two commonest Strophomenids, Leptaena and Schuchertella had large surface areas (for a brachiopod), which kept them from sinking into the mud.

Leptaena

Schuchertella

Epibionts
As you are collecting, you will notice that there are other animals growing on a large number of brachiopod shells.  These are called epibionts.  Most of the epibionts you will find are bryozoan colonies, although you will also find crinoids, other brachiopods, or gastropods.  Although the most common hosts are brachiopods, we also find epibionts on crinoids, gastropods and bryozoans.

Brachiopods, bryozoans, and crinoids need a firm base on which to grow. When there are few spots on the bottom to which they can attach, they grow on each other.
 
 

A Crinoid on Leptaena	B Bryozoan on Pseudoatrypa	C Bryozoans on Discomyorthis
D	E Bryozoan on Pseudoatrypa

We can also learn something about the relationship between the host and the epibiont by examining where the epibionts are growing.  For example.  In specimens B-D, the bryozoans are growing on only one of the two shells of the brachiopod.  These could have attached while the brachiopod was alive or after it died.  In specimen E, the bryozoan covers both shells, and, therefore, could only have grown after the brachiopod died.

Evidence of the fossil formation process

The study of fossil formation is called taphonomy.

Introductory tutorial on taphonomy from SUNY at Cortland - part of its excellent paleontology site.
 
 
 
 
 

Finding the horizonal
When you are looking at a rock face which has not be disturbed, it is useful to be able to tell what was horizonal at the time of deposition.  Occasionaly we find what are called geopetal indicators which let us know.  These are usually voids, half-filled with sediment, which act as nature's levels.

Go to these links for more introductions to geopetal structures

• Images of geopetal structures
• Geopetal infilling of a crynoid calyx

In the Ross Formation we have an animal which serves the same function. If you collect crinoid stems, you will find that some of them are smooth on one side, but have bumps on the other.  This is the remains of a rooted trailing stem (stoloniferous holdfast) diagrammed in the picture on the right.

Because the stoloniferous holdfast ran along the sea floor, it marked the horizontal in the early Devonian Period.  Obviously this is only the case if you find the holdfast in an undisturbed rock face.

Because this geopetal structure was created by a living organism, it is called a biogeopetal structure.

Learning from disarticulated brachiopods

If you are interested in taphonomy, broken brachiopods can be more informative than whole ones.

When bivalve mollusks die, their shells open, which means that when you find mollusks they are generally separated from their mates (disarticulated).

When brachiopods die, in contrast, their shells close, so that in the absence of disruption, they are generally found together (articulated).

In Nashville, for example, disarticulated brachiopods are relatively rare.  In Parsons, however, they are very common.

So, be sure to collect both articulated and disarticulated brachiopods.  The percentage of disarticulated brachiopods is an index of environmental disruption.
 

Articulated	Disarticulated
Pseudoatrypa	Pseudoatrypa - both shells from the inside
Discomyorthis	Discomyorthis - from the inside

Moreover, brachiopods differ in the degree to which their shells break apart.  For example, Pseudoatrypa is almost always found articulated, while Discomyorthisis almost always found disarticulated.
 

Learning from crushed brachiopods

After remains are buried, they are subjected to pressure in the rock-forming process called diagenesis.  Many fossils are damaged in this process.  Because the pressure during diagenesis comes down from the top, the pattern of damage provides information on the post burial orientation.

In his article "Paleoecology of the Birdsong Shale and Rockhouse Limestone Members of the Ross Formation", Michael Gibson gives the following guidelines for interpreting shell deformation.

Here are some of the deformed brachiopod shells I have found at Parsons.  Can you tell their post burial orientation.