April 30. Not with a bang

It’s the end of the Silurian. You’re probably expecting a big extinction event like the ones we had at the end of the Cambrian and Ordovician – but I have to disappoint you. The last of the relatively small extinction events we discussed the other day, the Lau event, was about 8 million years before the end of the Silurian, and there’s really nothing notable to mark the end of the Period. 

So how do we pick it? All these time periods – eras, periods, epochs and so on – are of course arbitrary constructions by humans to make it easier to refer to periods of geologic time. It took an international committee to define the boundary between the Silurian and the Devonian Periods, and their proposal was adopted in 1977. It’s defined by the occurrence of some particular fossils – index fossils, which are characteristic of a particular, narrow time interval. For this boundary, the base of the Devonian is defined as the first appearance of one particular type of graptolite species. Graptolites (see March 8) were floating colonial animals that created a wide variety of distinctive fossils, so they are very useful in relative age dating. This position, the base of the Devonian, is strengthened by the presence of some particular trilobites and conodonts too. It’s such a characteristic and specific assemblage of fossils it’s called a “golden spike,” and although it’s used to define the base of the Devonian, that also defines the top, the end, of the Silurian Period. On the whole, it appears that there was pretty much uninterrupted and continuous deposition of limestones and shales from the Silurian into the Devonian, so there really isn’t anything dramatic to mark the boundary.

Silurian-Devonian boundary monument at Klonk

By international agreement, there’s a particular place – called the type section – which is the official “best” example or at least the official reference example, of the rocks that define the boundary. It’s at Klonk, a village in the Czech Republic about 35 km from Prague. It’s called the global stratotype for the Silurian-Devonian boundary, and there’s a monument there to commemorate it.

So with that, at 416 million years ago, we’re done with the Silurian, and the last of the Silurian invertebrates. Mark Twain cited a Mississippi river boatman who had a library consisting of one book, Charles Lyell’s Principles of Geology, which he used as a source for what he perceived as profanity, to use against his roustabouts to stir them into action…. Twain reports the man would rail at the roustabouts and charge them “with being Old Silurian Invertebrates out of the Incandescent Anisodactylous Post-Pliocene Period and damn the whole gang in a body to perdition.” (From Twain’s Autobiography.)

So no more Silurian invertebrates for us. The Devonian starts tomorrow.

—Richard I. Gibson

Photo by Huhulenik via Wikipedia under Creative Commons license.