If you’ve been following along, you know that Laurentia, the core of North America (NA in map below), and Baltica, the Scandinavian heart of Europe (E), fused together during the long tectonic episode called the Caledonian Orogeny. The assembled continent is sometimes called Laurasia, or Larussia. But don’t think of it as a perfect, or complete union – there were still stringers and islands and other bits of continents impacting, and Gondwana was still coming.
|Late Devonian map by Ron Blakey via Wikipedia, used under Creative Commons Attribution license|
Another block that was on its way was Siberia (S on the map). Siberia, together with various complications like island arcs and microcontinents, was north of the combined North America-Europe continent of Laurasia. Laurasia was pretty much along the Devonian equator for much of the period, which meant that any shallow seas would likely be warm and would support plenty of diverse life, and that’s what the fossil record shows in those areas.
There was plenty of variation over the 50 million years and more of the Devonian. At times, much of what is now the interior of North America was above sea level, but by the middle to late part of the Devonian, extensive shallow seas covered most of what is now the United States. It wasn’t a perfectly smooth, uniform shallow sea – troughs like the Appalachian Basin and deeper zones like the Michigan Basin made for plenty of variety in the marine environments. Maybe that variety, in North America and Europe, with warm tropical seas, contributed to the relatively rapid evolution and diversification of the fishes. But remember, life was definitely present on land during the Devonian, and there were many many niches there for life to adapt to.
—Richard I. Gibson
Globe image by Ron Blakey via Wikipedia, used under Creative Commons Attribution license.