Ward, Peter. "5 The Silurian-Devonian: How an Oxygen Spike Allowed the First Conquest of Land." Out of Thin Air: Dinosaurs, Birds, and Earth's Ancient Atmosphere. Washington, DC: The National Academies Press, 2006.
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Out of Thin Air: Dinosaurs, Birds, and Earth’s Ancient Atmosphere
than we have today. The land changed from a place of “thin air” to a world where oxygen was almost a free commodity—which would have been a wonderful and necessary thing for an early insect with a still nearly worthless, newly evolved lung or an early land plant trying to coax oxygen into one of its clumsy, inefficient, newly evolved roots, for while plants above the ground need little oxygen for their leaves, their roots, deep in soil that may have too little oxygen for growth, are another matter. Was this coincidence or cause and effect? In this chapter let’s look at how this great rise in atmospheric air paved the way for the first real colonization of land.
Let’s travel back in time to the last time interval of the Silurian Period, a time some 420 million years ago, to get a snapshot of the middle of this long Silurian-Devonian interval. It has been some 50 million years since our last look at Earth, and the changes that have taken place are nothing short of remarkable.
The land is green, and everywhere we see the stems of low plants not only at water’s edge of swamps and lakes but also in uplands. There are true vascular plants, still low in size but now common and diverse: an enormous variety of forms are seen. And they are not alone on the land. Crawling among the plants is a small diversity of insects, also very small in size. They look like today’s springtails and the odious silverfish, and none have wings. Among them are even smaller mites. There are no land-dwelling vertebrates of any kind—yet.
We move to a small lake and find it filled with fish. For the first time we see fish with jaws amid the same jawless ostracoderms that were present in the earlier Ordovician. The jawed fish are heavily armored and upon closer examination seem very foreign looking. They are the placoderms, and some have gotten quite large and are destined to become larger yet in the upcoming Devonian Period. They use their jaws to good use, making meals of the algae-eating and jawless ostracoderms. There are a few minnow-like fish that seem to look like the bony fish of our time and perhaps some fish that may become sharks, but these are rare. The bony armor seems like a ludicrous adaptation