Abstract
The Ordovician Period was approximately 80 m.y. in duration, the longest of any Paleozoic period and equivalent in span to the Cretaceous. The period probably began 510-515 m.y. ago and ended close to 435 m.y. ago. The period witnessed great volcanicity (notably in active belts border ing tectonic plates), widespread inundations of continents, and explosive organic evolution. Despite the extinction of many taxa at the end of the Cambrian, trilobites remained abundant and increasingly diverse. There was a great increase in numbers of diverse nautiloid, conodont, and bivalve taxa. The appearance and expansion of graptolites among the proto chordates was paralleled by that of clitambonitid, triplesiid, strophomenid, pentamerid, rhynchonellid, and spiriferid brachiopods. The Ordovician witnessed not only the beginnings of rugose and tabulate corals, but also the flourishing of bryozoans and three major types of stromatoporoids. As noted by James (1982), with these frame builders' came structures that could be called reefs.
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