If you had been living in Chattanooga during any of the times when our rocks were just soft squishy sediments, you would have needed a very long snorkel to breathe under water. The rocks around Chattanooga were deposited in ancient seas over a very long interval of time - from the Cambrian through the Pennsylvanian time periods. These rocks record the breakup of a continent, the closure of an ocean, and the building of at least 2 mountain ranges.
Almost all these rocks were once sediments collected on the sea floor, and hardened after burial. That is why we find so many seashells cemented in limestone rocks today. There is only one local group of rocks that did not form from seafloor sediments; the Pennsylvanian sandstones and shales on the tops of Lookout and Sand mountains and Walden Ridge (Signal Mountain). Besides lots of river sand, those rocks contain coal and associated plant fossils that were preserved in ancient river swamps.
Much of the life preserved as fossils in ancient rocks is recognizable as ocean dwelling creatures similar in appearance to those we might see today. We can recognize them as “seashells”. But the habitats of today’s marine snails, clams and mussels were occupied by organisms that ate, breathed and reproduced differently in the past. Some of these common organisms include brachiopods, corals and bryozoans.
Brachiopods have two shells that resemble a clam’s two valves, but their soft body parts were very different from clams. They are not related at all. One interesting thing about brachiopods is that they are often preserved with both valves still hooked together. Clams almost always come apart after death because they only have one muscle that holds the shell closed. Brachiopods have two muscles – one to open the valves and one to close them. After a brachiopod dies, both muscles relax but this tends to keep the shells together during burial.
Corals today are in a class called Scleractinians. They are the reef-builders, and also dwell in deeper ocean waters. Corals of the ancient seas around Chattanooga belonged to two different classes – the Tabulates and the Rugosa. Tabulate corals were all colonial – they lived together in one “building”. Coral polyps lived only on the outside surface; everyone had a “window” to the sea. As the colony grew, more surface area was added (like blowing up a balloon with more air – it gets bigger around). All the chambers had flat floors to separate them; that is where the name tabulate coral originates. The other type of common coral, Rugosa, had two different lifestyles. They lived either by themselves as solitary or in groups as colonial corals. Both growth forms divided the living chamber with radiating septa, structures that are like spokes on a wheel. The outside surface was rough; that is where their name rugose coral originates. Many solitary corals looked like horns-of-plenty, or cornucopia, and are nicknamed “horn corals”.
Bryozoans are usually preserved as stick-like colonies where very tiny animals lived together in “apartment buildings”. They are named “moss animals” after the appearance of bryozoans living today in the oceans. Sometimes they encrust, or grow on top of, other organisms rather than the stick growth form. The stick-like growth form sometimes can be mistaken for colonial corals, but a close inspection of the little holes in a bryozoan colony will just be a hole; only corals will have radiating septa dividing up the hole into “pie” slices. One very peculiar bryozoan has a cork-screw shape and only occurs in Mississippian rocks (it’s a Mississippian index fossil). It is named Archimedes, after the Greek who invented a screw-type water pump.
Preserved in limestones and shales we also find the top predator of the ancient seas. What is the top predator of today’s oceans? If you answered sharks, you’d be correct. But in most of these ancient rocks around Chattanooga, there are no shark fossils. Instead we find nautiloid cephalopods. These fierce ancient predators are a type of mollusk that built chambers inside the shell. Today, the chambered nautilus is the only living genus of these hard-shelled cephalopods. We still have their relatives, the squid, octopus and cuttlefish, but the populations of shelled cephalopods is almost extinct. Cephalopods were meat-eaters and could change their buoyancy in the water by changing the density of the gas and fluid in their sealed-off chambers. They could move up or down to where the food was. They could also jet backwards (just like an octopus) using a structure called a hyponome. They were all tentacles, eyes and brains – that’s where they get their name “head-foot”.
Crinoids are also a very common fossil, especially in Mississippian-age limestones. These organisms resemble plants, but they are really animals. They grow attached the sea floor like a sea-lily. Crinoids and their relatives, the blastoids, have lots of hard parts that usually separate after death, providing lots of fossil fragments per individual. Their stems are made of columnals that look like miniature car wheels, their cups look like flowers, and they root themselves to the seafloor like plants.
The youngest rocks (around here anyway) were deposited after the seas all filled up with sediments, and rivers began to flow over the exposed limestones and shales of older oceans. The rivers flowed to the west, draining water away from the tall Appalachian Mountains. Swamps developed on river floodplains. If you’ve ever been to a swamp, you might have noticed that the water doesn’t flow fast at all and the water is dark with organic matter. When the bottom is stirred up, it smells like rotten eggs. It may not be pleasant for your nose, and it certainly isn’t pleasant for the bacteria that would love to eat the leaves and sticks, if only there was oxygen down there. The rotten-egg smell is the clue to the absence of oxygen and thus, the absence of decay of organic matter. Coal formed in these ancient swamps and there are beautiful fossils of plants that lived in these swamps. Around here, on the tops of the mountains, fossils of roots, leaves and bark of these ancient trees are preserved in stone. There is one bark pattern commonly found in Pennsylvanian age rocks –Lepidodendron. Lepidodendron is commonly known as “fish-scale”; it appears as a diamond pattern with little half-moon leaf scars in the center of each diamond. Fern and seed-fern leaves are also commonly found associated with coal deposits of this age.
For fossil collectors, the rocks around Chattanooga provide a very rich treasure trove of specimens from ancient oceans and ancient swamps. You can get started today with an egg carton and some labels. Make sure you note where you collected your fossils, and always ask permission if the land isn’t in your family or along a roadside.
Dr. Ann Holmes
Assistant Professor of Geology
She is in the process of creating a fossil identification site for fossils in the Chattanooga area. Check her webpage for its progress.
In conjunction with Dr. Jonathan Mies, Dr. Holmes also has information on Science workshops for middle school teachers.
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