Ordovician Fossils of the Richmond Formation, Montgomery Co., Ohio

Organized by Amy Payne
terra_x@prodigy.net

Introduction

To geologists, the Ordovician System of Ohio is probably the most famous of the state's Paleozoic rock systems. The alternating shales and limestones of the upper part of this system crop out in southwestern Ohio in the Cincinnati region and yield an incredible abundance and diversity of well-preserved fossils. Representatives of this fauna reside in museums and private collections throughout the world. Indeed, fossils from Ohio's Ordovician rocks define life of this geologic period, and the rocks of this region, the Cincinnatian Series, serve as the North American Upper Ordovician Standard. Furthermore, in the late 1800's, Ordovician rocks in the subsurface in northwestern Ohio were the source of the first giant oil and gas field in the country.

The marine invertebrate life that appeared in abundance in the Cambrian Period continued into the Ordovician, and many groups diversified into numerous species. Testimony to the abundance and success of marine invertebrates during the Ordovician is the diversity and abundance of fossils preserved in the rocks of southwestern Ohio. It has been suggested that if all of the fossils could be removed from the Ordovician rocks of the Cincinnati area, Cincinnati would be below sea level. Anyone who has examined these rocks in the field immediately notes that, volumetrically, many beds are tightly packed with fossils.

Perhaps the most common fossil remains are those of bryozoans, colonial animals that lived in branching, treelike colonies or flattened, encrusting masses on shells of other invertebrates. In some areas bryozoans litter the outcrop. Brachiopods are no less spectacularly abundant than bryozoans and are a favorite of the beginning collector.