Fossils of the Inland Basin

Introduction

Much of the Inland Basin is located in the Northeastern United States, but the parts extending into the Midwest span nearly 100 million years of the Paleozoic Era. A band of rocks running from extreme northeastern to central southern Ohio are from the Devonian period, while most of the Inland Basin, including eastern Ohio, and the southern portions of Indiana and Illinois, is composed of younger Mississippian and Pennsylvanian rocks. The youngest bedrock of the region, from the Permian period, is located in the southeasternmost part of Ohio.

When the rocks of this region were formed, much of central North America was covered by a relatively shallow, tropical, inland sea. The late Devonian- and early Carboniferous-aged rocks of the Inland Basin contain the fossils of a diverse marine ecosystem. Younger Carboniferous rocks show the environment transitioning from the shallow sea to an environment of extensive deltas and swamps. These rocks contain the fossils of organisms from freshwater and forest ecosystems.

Paleozoic fossils

Ordovician fossils

Ordovician fossils are found in all seven Midwestern states, primarily in the Central Lowland region, but some Ordovician rocks outcrop in the Inland Basin as well. In the Ordovician, great reefs were formed by organisms like bryozoans, corals, algae, and sponges. These reefs played a major role in forming the ecosystems that were also home to straight-shelled cephalopods, trilobites, bivalves, gastropods, crinoids, graptolites, brachiopods, and fish.

Devonian fossils

Devonian marine invertebrate fossils

During the Devonian period, what is now the Midwest was just south of the equator. Communities of corals, crinoids, bryozoans, brachiopods, and mollusks thrived in the warm sea that covered most of the Inland Basin. Numerous types of coral fossils and other invertebrates can be found in the region around Falls of the Ohio on the Indiana-Kentucky border.

Devonian fossil sharks

Sharks trace their lineage to the Ordovician, more than 420 million years ago, and they became increasingly diverse throughout the remainder of the Paleozoic Era. Ohio’s Cleveland Shale preserves specimens of the well-studied primitive shark Cladoselache. This shark was relatively common during the late Devonian and possessed an interesting blend of primitive and derived characteristics. Superficially, its body looked like that of a modern shark, but the 1.2-meter-long (4-foot-long) Cladoselache had a terminal mouth (rather than a mouth somewhat under the “nose” as in modern sharks), almost no scales, and no claspers (leaving scientists to wonder how they reproduced).

Devonian placoderm fish

By the Devonian, fish had become increasingly prominent and diverse. The Devonian saw fishes explode in diversity, from tiny scavengers to huge hunters. The 9-meter-long (30-foot-long) Dunkleosteus did not have a bony internal skeleton, but its head was covered in bony armor. While remains of its soft parts, including its unarmored rear two-thirds, have yet to be discovered, the plates of numerous specimens have been found in the Cleveland Shale of Ohio, and the Cleveland Museum of Natural History houses the largest known specimens. The plates that formed the jaw functioned as huge blades, rubbing against each other to stay sharp as the fish chomped its prey: sharks, other fish, and large invertebrates. Dunkleosteus is thought to have had one of the most powerful bites in the history of life, and it was the apex predator of the late Devonian seas. Despite its apparent advantages, it and all of its its placoderm kin became extinct at the end of the Devonian.

Carboniferous fossils

Carboniferous invertebrate fossils

During the Mississippian and Pennsylvanian, the expansion and contraction of glaciers far to the south caused sea levels to fluctuate. In the ocean, brachiopods, trilobites, and reef builders were decimated by the mass extinction at the end of the Devonian. The Mississippian is sometimes known as the “Age of Crinoids” because of the increase in the abundance and diversity of crinoids, as well as starfish, edrioasteroids, urchins, and other echinoderms during this time. 

Carboniferous fossil plants

The oldest known fossils of land plants are isolated spores dating to the Ordovician Period. By the late Silurian, fossils of simple, leafless stems just a few centimeters tall are found in sediments that accumulated in low, wet, coastal environments. The first trees and the first seeds date to the Devonian period. By the end of the Devonian, extensive forests containing many different kinds of trees and other plants had developed.

During the Carboniferous (and especially the Pennsylvanian), what is now the Interior Basin region of the Midwest was a broad, low coastal plain near the Equator. It was home to extensive swamp forests. These ancient ecosystems are sometimes called “coal swamps” because the stagnant, wet environments in which they thrived prevented large volumes of vegetation from decomposing. As the plant matter accumulated for tens of millions of years, it formed thick, extensive deposits of coal underlying much of the Central Lowland as well as the parts of the Inland Basin.

The trees found in these forests did not belong to the familiar groups that dominate modern forests, such as conifers and flowering broadleaf plants. Instead, these early forests were made up of groups of plants that are either extinct or much less conspicuous today. These groups were the lycophytes (club mosses and relatives), the sphenophytes (horsetails and relatives), the marattioid ferns, seed ferns (seed-producing plants with fern-like leaves), and the cordaites (a group related to conifers). Lycophytes and horsetails survive today, but often small, non-woody plants. Marattioid ferns are restricted to the tropics. Cordaites and seed ferns are extinct.

In some areas of the Inland Basin, such as in the Steubenville, Ohio, anatomically preserved Carboniferous plants can be found in coal balls. Coal balls are nodules of peat (accumulated plant matter) preserved in calcium carbonate. The plant remains inside a coal ball are often exquisitely preserved, showing cells, tissues, and other microscopic structures. 

Carboniferous vertebrate fossils

At the end of the Devonian, fluctuations in sea level caused the water to retreat from portions of the Inland Basin. By the beginning of the Carboniferous, the landscape was dominated by deltas and swampy forests, similar to what occurred in the Central Lowland region at this time. In addition to plants, the fossils of freshwater fish, sharks, early amphibians, and arthropods can be found in coal beds from the late Carboniferous. At the beginning of the Permian, Ohio was a terrestrial environment where lake and river deposits preserved horsetail and fern fossils. These are the youngest fossils found in the Inland Basin’s bedrock.