The Silurian Period as it is now defined spans the time from 443 million years ago – the end Ordovician extinction – to 416 million years ago. The error bars on those dates are a million or so years. The Silurian covers just 27 million years, the shortest period we’ve defined.
Where the Cambrian and Ordovician were more or less divided into early, middle, and late epochs, the Silurian has four major subdivisions: Llanderovy, Wenlock, Ludlow, and Přídolí. The first three are named for rocks found in Wales, Scotland, and England, respectively, and the Pridoli is from the Czech Republic. Each epoch is separated from the next by breaks in the rock record, some of which coincide with relatively small extinction events, but the real definition of the epochs is based on specific graptolite zones. You can hear more about graptolites in the podcast for March 8.
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Charles J. Vitaliano, a geology professor at Indiana University, one of my own college professors, was born April 2, 1910, in New York City. He specialized in igneous and metamorphic rocks and mineralogy as well as field geology.
—Richard I. Gibson
Time column from Wikipedia