Smithsonian National Museum of Natural History Geologic Time The Story of a Changing Earth
Presented by the Department of Paleobiology.
The Ordovician
Contents
Eon Overview
Reef Ecosystems and the Paleozoic Fauna
New Animals as Biostratigraphic Tools
Ordovician Climate and Plate Tectonics
Evidence
Ordovician trilobite
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sample
Starfish
references and links
Foundational Concepts
Dating Methods
Earth Processes
Life Processes
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New Animals as Biostratigraphic Tools
Another important change in the Ordovician was the evolution of deeper-water faunas, as many forms of life moved out of their early shallow-water environments. With this change came the appearance of new groups of animals. One of the most important among these was the graptolites, so named because their fossils resemble writing on rocks. Graptolites are hemichordates, relatives of modern acorn worms and pterobranchs and distant relatives of chordates and vertebrates. For many years, their exact biological nature was a mystery to paleontologists, but their rapid rate of evolution made them useful nonetheless. Because each graptolite species only existed for only a short time, their fossils could be used to identify the age of a particular rock quite specifically. This process—determining the age of a rock by the fossils contained within it—is called biostratigraphy. For more information on the concept of biostratigraphy, please visit the Foundational Concepts page.

Another important biostratigraphic group are the conodonts, which also first appeared during the Ordovician. Conodonts were also enigmatic until very recently, because most conodont fossils consist only of jaws, their only hard parts. Recent discoveries of conodonts with well-preserved soft tissues demonstrated that they were actually early marine vertebrates. Like graptolites, conodonts also evolved rapidly, thriving until their demise at the end of the Triassic.




Overview | Reef Ecosystems and the Paleozoic Fauna | New Animals as Biostratigraphic Tools
Ordovician Climate and Plate Tectonics



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