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Ordovician Period—485.4 to 443.8 MYA

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stone slab with fossil brachiopods
Ordovician age fossil brachiopods, Mississippi National River and Recreation Area, Minnesota.

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Introduction

The naming of the Ordovician Period is tangled with the Cambrian Period. Suffice it to say that a Welsh tribe—Ordovices—inspired the name of this geologic period. The Ordovician System rounded out the threefold division of early Paleozoic rocks (i.e., Cambrian, Ordovician, and Silurian), which are all named for Welsh tribes. Recognizing the Ordovician between the Cambrian and Silurian ended a 40-year controversy, eliminated an “overlapping system,” and created a new interval of time in its own right.

Significant Ordovician events

Beginning in the Ordovician Period, a series of plate collisions resulted in Laurentia, Siberia, and Baltica becoming assembled into the continents of Laurussia by the Devonian and Laurasia by the Pennsylvanian (also see Cambrian Period). Meanwhile, the southern remains of Rodinia (i.e., Gondwana) rotated clockwise and moved northward to collide with Laurasia. The eventual result was the supercontinent Pangaea (“all land”), stretching from pole to pole by Permian time.

During the Ordovician, many new species replaced their Cambrian predecessors. In addition, primitive plants called lycophytes began to move onto land, which was barren until then. Later, in the Devonian, other types of plants colonized terrestrial habitats. Flowering plants, the most prolific type today, appeared even later, during the Cretaceous Period. Also, during the Late Ordovician, massive glaciers formed on Gondwana at the South Pole, causing shallow seas to drain and sea level to drop, which may be a factor in the period ending with a mass extinction that affected many marine communities.

Learn more about events in the Ordovician Period

Though less famous than the Cambrian explosion, marine fauna increased fourfold during the Ordovician, resulting in 12% of all known Phanerozoic marine fauna (Dixon et al. 2001). The sea teemed with life different from its Cambrian predecessors such as bivalves, gastropods (snails), bryozoans (moss animals), and crinoids (sea lilies). Bryozoans first appeared in the Ordovician and comprise an important group of colonial, marine organisms that still exist today. The first coral reefs also appeared during the Ordovician, though solitary corals date back to at least the Cambrian. Additionally the Ordovician is marked by a sudden abundance of trace fossils.
Rocks from the Ordovician Period contain evidence that plants began colonizing dry land at this time. Most experts agree that the ancestors of land plants first evolved in a marine environment, then moved into a freshwater environment and finally onto land.

Movement of life onto land was a major evolutionary step by both plants and animals. Development of land plants was accompanied by migration of modified forms of arthropods onto the land, which were apparently the first forms of animal life to leave the ocean (Rogers 1993).
The Appalachian Mountains are part of a collisional mountain range that includes the Ouachita Mountains of Arkansas and Oklahoma, and the Marathon Mountains of western Texas. The mountain-building event, or orogeny, that created these ranges started in mid-Ordovician time and continued through the Pennsylvanian Period. The tectonic history of the Appalachian Mountains involves the opening of an ancient ocean along a divergent plate boundary, the closing of the ocean during plate convergence, and then more divergence that opened the Atlantic Ocean (Lillie 2005).

As sea level rose during the Cambrian Period and into the Ordovician, the coastline in eastern North America gradually receded to the west. Thick sequences of sediment, especially carbonate rocks such as limestone, were deposited along the edge of the continent. At this time the East Coast was a passive margin. A change in plate motion during the middle of the Ordovician Period set the stage for the ensuing mountain-building event. Moreover, the ocean to the east, which geologists call the Iapetus Ocean, began to close through the process of subduction. The once-quiet Appalachian passive margin changed to a very active plate boundary when a neighboring oceanic plate collided with and began sinking beneath the North American craton. The process of subduction not only destroys the sinking plate, but leads to volcanic activity in the overriding continental plate, and also results in any areas of non-oceanic crust on the sinking plate (such as islands) being “scraped off” and attached to the continental plate. With the birth of this new subduction zone, the early Appalachians were born.

Detailed studies of the southern Appalachians indicate that the formation of this mountain belt was more complex than once thought. Rather than forming during a single continental collision, the Appalachians resulted from several distinct episodes of mountain building that occurred over a period of nearly 300 million years. The final orogeny occurred about 250 million years ago when Africa and Europe collided with North America. The Valley and Ridge physiographic province (Lutgens and Tarbuck 1992), which is present in Shenandoah National Park and or Blue Ridge Parkway, highlights this mountain-building event.
The extinction that occurred at the end of the Ordovician Period devastated marine communities. This extinction is the first major extinction event recorded in the rock record. An estimated 25% of the known (marine) taxonomic families were lost, including the disappearance of one-third of all brachiopod and bryozoan families, as well as numerous groups of conodonts (eel-like animals related to vertebrates), trilobites, and graptolites (colonial worm-like animals).

Geologists have theorized that the extinction at the end of the Ordovician was the result of a single event—the glaciation of the supercontinent Gondwana. Evidence for this glaciation is provided by glacial deposits in the Saharan Desert. When Gondwana passed over the South Pole, continental-size glaciers formed, which resulted in a lowering of sea level because large amounts of water became tied up in ice sheets. In conjunction with the cooling caused by the glaciation, the fall in global sea level, which reduced prime habitat on continental shelves, are likely driving forces for the Ordovician extinction.

Visit—Ordovician Parks

Every park contains some slice of geologic time. Below, we highlight selected parks associated with the Ordovician Period. This is not to say that a particular park has only rocks from the specified period. Rather, rocks in selected parks exemplify a certain event or preserve fossils or rocks from a certain geologic age.

More about the Paleozoic

Last updated: September 28, 2020