Pennsylvanian - 290 to 323 million years ago
During the Pennsylvanian time period, Oklahoma continued to be located south of the equator but was drifting slowly north.
The sea level rose and fell several times during the Pennsylvanian. Time after time, large areas of Oklahoma were flooded by a shallow sea and then became dry land as sea level fell. Also, during this time, the Arbuckle Mountains, Wichita Mountains, Ouachita Mountains and Ozark Mountains formed.
Brachiopods, crinoids and corals were the most abundant invertebrate animals that lived in this sea. Clams and snails can also be found in some rocks but trilobites are rare. Fossils of land plants are common in coal deposits of the Pennsylvanian.
You can find fossils of Pennsylvanian age in limestone, shales and sandstones of the Arbuckle Mountains and in most of eastern Oklahoma.