”When Cincinnati was in the Southern Hemisphere”
Posted On: 10/11/2017 - 1:20pm, Posted By: Brenda Hunda, Curator of Invertebrate Paleontology
During the Late Ordovician, global paleogeography was very different than from today. North America as we know it had not yet completely formed, missing most of the eastern and western seaboards of both the United States and Canada. Its predecessor, known as Laurentia, straddled the equator and was rotated 45 degrees clockwise from its present day orientation. As a result, Cincinnati was situated 20 degrees south of the equator for the entire Late Ordovician. Laurentia’s closest neighbors were Siberia and Baltica (Europe and Scandinavia) with the other major continents united as a large megacontinent called Gondwana. The Atlantic Ocean as we know it did not exist, but instead, a narrower ocean called the Iapetus Ocean separated North America from Siberia and Baltica.
Global paleogeography, 448 million years ago. Laurentia (proto-North America) is tilted on its side and located mainly in the southern hemisphere. At this time, an ocean covers most of the United States. A star locates the position of the Cincinnati region. To the upper right of Laurentia is Siberia, and to the lower right is Baltica. The large megacontinent that occupies most of the southern hemisphere is Gondwana, which consists of Africa, Antarctica, South America, Australia, and numerous other small continents. Ordovician paleogeographic map by C.R. Scotese. Permission granted by C.R. Scotese.
The climate was warm due to high concentrations of carbon dioxide in the atmosphere, with as much as an estimated 16x greater than today. There were no polar ice caps on Earth. Global sea level was high during the Ordovician, and Cincinnatian fossils and rocks provide evidence for the existence of widespread warm shallow seas (called epicontinental or epeiric seas) over most of the North American continent at this time. The nearest landmasses to the Cincinnati region were the rising Taconic mountain chain, about 300 miles to the east, produced by the collision of Laurentia with a small plate or island arc, such as the modern-day islands of Japan and the Aleutians. Jagged mountain chains resembling the modern Alps or Himalayas were lifted high above sea level. Weathering and erosion attacked these ranges, and rivers carried huge loads of fresh water, sediments, and nutrients into the sea.
Great volumes of sediment, consisting of coarse gravels, sands, silts, and muds were deposited as river deltas and redistributed by ocean currents along the coastline in the Appalachian Basin. Offshore in deeper water, only the muddy components of this sediment input remained suspended as clay particles, which were carried by currents generated by frequent hurricanes to Cincinnati. These muds were imports to the region that eventually lithified to form mudstones. In the Cincinnati area, mudstones are interbedded with limestones, which are composed of calcareous shells and skeletons of “native” marine invertebrates. Both sediments intermingled in the Cincinnati region, producing a varied and patchy sea floor that was muddy in places and shelly in others. Such a variegated bottom environment offered more potential types of living spaces for bottom-dwelling organisms and provides further reason why high diversity developed in the region.
Meyer, D. L., and Davis, R. A. 2009. A Sea without Fish: Life in the Ordovician Sea of the Cincinnati Region. Indiana University Press, Bloomington, Indiana.
Scotese, C.R., 2014. Atlas of Silurian and Middle-Late Ordovician Paleogeographic Maps (Mollweide Projection), Maps 73 – 80, Volumes 5, The Early Paleozoic, PALEOMAP Atlas for ArcGIS, PALEOMAP Project, Evanston, IL.