Nashville has a lot of fossils.  In this exercise you will learn what they are, why we find them here, and what we can learn from them.  We will consider three points:

• What are fossils?  What kinds of fossils are there?
• What fossils are found in Nashville?
• What fossils are not found in Nashville and why aren't they found here?
• What do our fossils tell us about the environment at the time these organisms were alive?

What are Fossils?

Fossils are traditionally divided into two categories:

• Body fossils:  remains of the organisms themselves
• Trace fossils:  remains of the activities of the organisms

Whole Animals	Shell Fragments
Internal Molds  (sediment which has filled up shells and then hardened to rock)	External Molds  (imprints of animals, hardened to rock)
Casts  (copies of part of the original animal made by filling external molds)	Trace Fossils

There are many good websites which can give you background on how fossils form.  I am not going to redo what they have already done well.  Take a look at the sites below to get the background you need for this exercise.

In this exercise, we will look at some fossils that we find in middle Tennessee and learn how these specific fossils formed and what we can learn about the depositional environment at the time they were alive.
 

What fossils do we find in middle Tennessee?
 

Brachiopods	Mollusks - gastropods
Mollusks - cephalopods	Mollusks - bivalves (internal mold)
Bryozoan	Bryozoan
Cnidaria - rugose corals	Cnidaria - tabulate corals
Porifora - sponges	Arthropods - ostrocods
Echinoderms - crinoids

From looking at these fossils, you should already be able to say whether this was a land or a water environment.

Now let's take a look at some specific fossils.  Most classifications distinguish between body fossils and trace fossils.  Let's start looking at body fossils, which are the remains of the hard parts of the ancient organism itself.
 

What fossils don't we find in middle Tennessee?

Sometimes it is possible to learn as much from what we don't find as from what we do find.

Why are there no dinosaurs like you find in the west?

There are no dinosaurs because their remains were never deposited.  Not everything that dies becomes a fossil.  To become a fossil, the remains of organisms have to be quickly buried to prevent the remains from disintegrating.  During the mesozoic Era, when dinosaurs were alive, sedimentary rocks were not forming in middle Tennessee.  If sedimentary rocks are not forming, no fossils will be preserved.

It is likely that that Pennsylvanian sediment was deposited in Nashville.  We know it was deposited on the Cumberland Plateau and as close as Short Mountain in Cannon County.  We don't find Pennsylvanian fossils here because between then and now, all Pennsylvanian sediment has eroded away.

Why are there no Cambrian fossils?

It is almost certain that middle Tennessee was under water during the Cambrian Period, and that Cambrian sediments, with fossils, were deposited here.  We don't find them because they are still buried.  Come back in a few million years and our Cambrian fossils will be exposed.

As we have seen, the fossil record in middle Tennessee exposes only one small sliver of the geologic column.  However, it  does not even give a complete picture of life at that time.

• Many organisms are not fossilized at all, and others leave only incomplete evidence of what they were.
• During the fossilization process, soft parts of organisms disappear, but even the hard parts are changed in ways that cause us to lose information about what the animal was like.
• Many fossils do not survive the process of weathering out of the matrix.

What organisms, which probably lived here in the Ordovician Period were not fossilized at all?

Organisms which do not have hard parts are not usually preserved as fossils.  For examples, we know from a few very rare fossils that jellyfish existed in the Cambrian Period, which means that they were certainly around in the Ordovician Period.  Jellyfish fossils are so rare that it is unlikely that we will find any here.

This is the case for most of the trace fossils which we find.  They were made by invertebrates which lived in the sediment at the bottom of the ocean, but, because we only have their burrows

How have the remains of the organisms been altered by the fossilization process?

The fossil shells that we find, simply do no look like modern shells.  This is because the the original shells have changed over time in a process called diagenesis.

The calcium carbonate are originally laid down by the animal in a series of thin layers. Almost immediately, the soft parts of the animal disappear. If the center of the shell is filled with sediment, this can form an internal mold.

Over time, these layers dissolve and reform as interlocking crystals.  This change gives fossil shells their distinctive appearance.

If If the shells entirely dissolve, and the resulting void is filled with sediment, a cast is the result.

Modern brachiopod	Fossil brachiopod
Modern river mussel	Internal mold and cast of bivalve mollusk Byssonychia

Original fossils are sometimes replaced by crystals of other chemicals.  In the case below, the Streptelasma on the right the original Calcium Carbonate has been replaced by Silicon Dioxide crystals.  The crystals started growing in many places and expanded as far as they could.  This led to what are called Beekite Rings.  Although Beekite Rings are very beautiful, they destroy  information about the the structure of Streptelasma.  Beekite rings are common in the Stones River Group, but only found occationally in other strata.
 

Solitary coral Streptelasma without Beekite Rings

Solitary coral Streptelasma with Beekite Rings

In other places, fossils are replaced with calcite crystals.  This completely distroys any details of the fossil, leaving, at best the fossil's general shape.  Fossils replaced by calcite crystals are common in the Catheys Formation of the Nashville Group and the Maysville Group, and less common elsewhere.  Again, I don't know why.
 

Large brachiopod (Rafinesquina) replaced by calcite crystals (Maysville Group)

Coral Tetradium largely replaced by  calcite crystals (Nashville Group Catheys Formation)

For nearly half a billion years middle Tennessee fossils remained underground.  Then with erosion, they are exposed.   Because the fossils have been undergoing chemical change which are different from those of the surrounding rock (matrix), sometimes the fossils are slightly more resistant than the matrix, and sometimes less.  This causes some fossils to remain in the matrix and some to weather out, and causes some fossiliferous limestone to be very solid, and others to be very loosely consolidated (called coquina)
 

Fossils in limestone matrix

Fossils weathered out of matrix

Dalmanella Coquina (Hermitage Formation) North Nashville 8th Avenue

Dalmanella Coquina (Hermitage Formation) South Nashvile  8th Avenue

What can we tell about Nashville's Ordovician ecosystem from its fossils?
 

Despite all of their problems, Nashville fossils tell us a great deal about life in Nashville

They indicate that the density of life was at times very high.

Brachiopods (Pionodema) the Stones River Group
 

Bryozoans (possibly Rhinidictya) Stones River Group
 
 

Brachopods (Platystrophia) Nashville Group
 

The communities in which we find our fossils, tell us a great deal about the animals themselves.

In the Ordovician Period, we find brachiopods and bryozoans in the same habitat as corals.  We do not find that today.  Corals, which thrive through a symbiotic relationship with photosynthesizing algae, must life in water that is shallow and well lit.  There is indipendent information that this is what Nashville was like during the Ordovician Period.  Modern brachiopods and bryozoans, however live in much deeper water.  Something significant has changed in the interim.

We find evidence from the fossils themselves about how the animals related to one another.

Bryozoans frequently grew colonies on the shells of other animals.  Two bryozoans specialized.
 

• Atactoporella, grew in single layers on the shells of small snails.

• Prasopora grew in thick large colonies on the shells of bivalve mollusks or brachiopods.

• Other bryozoans started their colonies by encrusting brachiopod or mollusk shells, and then grew up to form tree like colonies.