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The Ordovician period of geological history spans from around 510 to 445 mya. This was an era of rapid marine evolution with many different experimental organism evolving and dying. The end of the Ordovician was marked by a mass extinction event brought on by changes in the paleogeography of the Earth.
During the Ordovician, Oklahoma was still covered by a shallow sea and remained located in roughly the same position as during the Cambrian. Limestones, dolomite, sandstone, and shale are the primary rocks of the period in Oklahoma. The climate was still generally warm throughout the period with a slight decrease in average temperature during the later Ordovician.
Landmass locations during Ordovician Earth
From the PaleoMap Project
The most remarkable aspect of the Ordovician period was the rapid explosion of evolution that occured in the ocean. This period saw a great increase in the number of filter-feeding organisms. This theoretically occured because the warm shallow continental seas provided a haven for microplankton to flourish. In addition, the first Corals, Bivalve molluscs, Graptolites, and Brachiopods arrived along with Bryoza, the most predominant colonial animal of the Ordovician. Nautiloid cephalopods began the process of evolving very quicly into many unique and diverse lines. The most sucessful animal during the Ordovician had to be the Brachiopods, who, with a lackluster performance during the Cambrian exploded into no less than 14 new superfamilies during the middle and later Ordovician. Brachiopod fossils are the majority of invertebrate fossils found today in Oklahoma limestones, followed by trilobites, corals, and crinoids.
Trilobites, the predominant creature of the Cambrian, evolved quite differently from their Cambrian ancestors. Some trilobites evolved huge eyes with over a thousand facets while others de-evolved their eyes completely! Some trilobite species developed necessary adaptive appendages like a shoveled snout for digging and borrowing in the muds of the shallow continental seas. During the later Ordovician, the trilobites began a steady decline in numbers.
Exposed Ordovician rocks are prevelant in the Arbuckle Uplift.
While all of this evolution and experimentation was occuring in the Earth's oceans, creeping lichens and bryophytes became the first organisms to venture onto the barren lands above water, starting a process of colonization that would eventually give way to more complex plant and animal life forms later in history.
By the end of the Ordovician, the giant continent of Gondwanna had parked itself at the South Pole causing the formation of massive glaciers which resulted in the lowering or drainage of many shallow seas. Because of this, many lifeforms went extinct very quickly. In fact it is estimated that 60% of all marine invertebrates and 25% of all families went extinct by the end of the Ordovician.
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