Ordovician strata are characterized by numerous and diverse trilobites and conodonts (phosphatic fossils with a tooth-like appearance) found in sequences of shale, limestone, dolostone, and sandstone. In addition, blastoids, bryozoans, corals, crinoids, as well as many kinds of brachiopods, snails, clams, and cephalopods appeared for the first time in the geologic record in tropical Ordovician environments. Remains of ostraderms (jawless, armored fish) from Ordovician rocks comprise some of the oldest vertebrate fossils.
Despite the appearance of coral fossils during this time, reef ecosystems continued to be dominated by algae and sponges, and in some cases by bryozoans. However, there apparently were also periods of complete global reef collapse due to global disturbances.
The major global patterns of life underwent tremendous change during the Ordovician. New platform trilobites appeared. Perhaps this was because of the peak separation of continental plates at this time, and the reresulting vicariant distribution of trilobite groups. In addition, the first planktonic graptolites evolved, though some species of graptolites became extinct.
In the Arenig, the diversity of conodonts decreased in the North Atlantic Realm, but new lineages appeared in other regions. Seven major conodont lineages went extinct, but were replaced by nine new lineages that resulted from a major evolutionary radiation. These lineages included many new and morphologically different taxa. Sea level transgression persisted causing the drowning of almost the entire Gondwana craton. By this time, conodonts had reached their peak development.
Although fragments of vertebrate bone, and even some soft-bodied vertebrate relatives, are now known from the Cambrian, the Ordovician is marked by the appearance of the oldest bony vertebrates whose appearance is completely known. These were jawless, armored fish informally called ostracoderms, but more correctly placed in the taxon Pteraspidomorphi. Typical Ordovician fish had large bony shields on the head, small, rod-shaped or platelike scales covering the tail, and a slitlike mouth at the anterior end of the animal. Such fossils come from nearshore marine strata of Ordovician age in Australia, South America, and western North America.
Perhaps the most eventful occurrence of the Ordovician was the colonization of the land. Remains of early terrestrial arthropods are known from this time, as are microssils of the cells, cuticle, and spores of early land plants.
The Lake Témiscamingue Fossil Centre has an excellent exhibit on life in the Orovician and Silurian.
The Fossils of Kentucky website includes many pictures of marine invertebrate fossils.
Read about the Ordovician Mass Extinction at the Hooper Virtual Paleontology Museum.
Find out more about the Ordovician paleontology and geology of North America at the Paleontology Portal.