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So now, the Devonian time scale.
One thing to understand about the time scales and dates I’m using in these podcasts – they all have some error bars on them. Even when I say, for example, “the Silurian ended 416 million years ago,” that’s not an absolute. The timing of the boundary between the Silurian and the Devonian is even less specific than some of the others, and it’s something like plus or minus 3 or 4 million years. So you might see or hear of the Devonian starting about 419 million years ago while the Silurian ended 416 million years ago. Don’t worry about it – it’s within the error bars of our knowledge. Or it might mean that I’ve checked a time scale that’s a little out of date, but I really have checked. And all these dates are close enough for our purposes, I think. If you have the original book, the History of the Earth Perpetual Calendar, you’ll find that a lot of the dates have been refined somewhat since I wrote it in 1994, usually by some international agreement, and the podcasts do reflect those changes unless I screw up.
So the Devonian began about 416 to 419 million years ago. Its formal subdivisions are simple: Early, Middle, and Late epochs. The epochs are divided into a total of seven ages, three in the Early Devonian and two each in the Middle and Late Devonian. The names of a lot of the ages, such as Famennian, Frasnian, Givetian, Eifelian, come from places in France and Belgium where Devonian rocks are well exposed.
The Devonian lasted until about 359 million years ago, giving it a time span of about 58 million years – more than twice the length of the Silurian. I think we can fill a month with Devonian happenings.
—Richard I. Gibson
Devonian time scale image from Wikipedia. Scale in millions of years ago.