Life advanced quickly during the Devonian Period. Forests began to cover the continents for the first time, fish populations exploded (the Devonian Period is also called the “Age of Fish”), and amphibians emerged on the land. Reef organisms made major adjustment to a changing environment.
Devonian fish fossil can have a very scary appearance. Most of these early fish lacked jaws and teeth, had no scales – rather a skin made of armored plates, and were ferocious predators. One fish, the Dunkleosteus, grew to over 30 feet in length, and its two front plates had self-sharpening “points” for slicing and dicing, for this organism was toothless. It is common to find piles of regurgitated, partially digested fish around the fossils of a Dunkleosteus, suggesting that its gluttony often resulted in indigestion. The Dunkleosteus was collected out of the black shale beds in Cleveland, Ohio by Jay Terrell in 1897.
Marine organisms of Devonian age are common in the United States, as the Kaskasian sea slowly invaded the land over a period of 50 million years. The picture below shows how much of North America was covered by the end of the Devonian 360 million years ago. The very lightly shaded regions formed limestone, the medium shading indicates where organic-rich, black mud was deposited, which usually occurs in deeper regions. The regions that are very dark are parts of the deep ocean basin, and these regions will get completely destroyed as plates move and the ocean floor gets thrust under the continents.
Some of the shallow area (light shades on the map) still had trilobites crawling around on the bottom of the sea. However, these trilobites had evolved as many as 16 long spines that are presumed to be defensive mechanisms to stop from being eaten by the growing fish population.
The premier Devonian trilobite locality in the U.S. is Black Cat Mountain in Coal County, Oklahoma. Here the trilobites are in an ancient near shore, white limestone, which makes for perfect preservation and display, although they must be carefully extracted, usually by sand blasting. This locality was featured in the 2008 Tucson Show.
The Devonian reefs also occur in the shallow, lightly shaded, limestone. The best viewing of fossil for the Devonian reef creatures is at “Falls of the Ohio State Park”, near Clarksville, Ohio (across the river from Louisville) with nearly 600 species already identified within the Park. Collecting is not allowed except in designated collecting piles located next to the parking lot behind the interpretive center. The park hosts nearly 500,000 visitors every year. It is best to go in the late summer or early fall, when the gauge on the river reads 13.5 feet or lower, indicating the lower fossil beds are exposed.
The lower bed when exposed has been extensively carved by the flowing water into many small caves, where the fossil stick out of the walls and sometimes litter the floor as black, petrified organisms against the white limestone and chert nodules. The fossils are so abundant that you cannot help stepping on them as you walk across to the cave zone. In the Devonian strata, some horn corals grew to large lengths. The horn coral Siphonophrentis giganteas, specimen was discovered in the Park, and this organism may have reached lengths of 6 feet!
There are also about 16 known species of trilobites in the Park, but you will have to look harder to find them. The Park has a virtual tour: http://www.fallsoftheohio.org/virtual_tour.html
Closer to home, in Northwest Georgia, is the Chattanooga Shale Formation, a deeper water, Devonian, mud deposit. Although the bed is loaded with organic matter from dead microscopic organisms, the presence of pyrite and the complete lack of larger marine fossils suggest the formation was deposited in a stagnant, oxygen-deficient ocean. However, this is not all bad… because the abundant micro-organisms have decayed over time and the shale bed has produced a considerable amount of oil and gas, especially in Tennessee. The gas bearing horizon barely cuts across Georgia, and although we are not currently extracting any oil or gas from it, several companies are currently prospecting for natural gas in this region of Georgia.