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
pears that none of the Devonian tetrapods had any sort of tadpole stage; they went directly from egg to land dweller without a water-breathing larval stage.
PLUGGING ROMER’S GAP?
The long interval supposedly without amphibians was “plugged” in 2003 by Jenny Clack with great media fanfare. While looking through old museum collections she came upon a fossil long thought to have been a fish. But more detailed examination showed it to be a tetrapod and, more than that, it was an animal with five toes and the skeletal architecture that would have allowed land life. More importantly, it was within the mysterious Romer’s Gap. The popular press reports of this finding, which was named Perdepes, would have us believe that Romer’s Gap was filled. Hardly. Perdepes may indeed have been the first true amphibian, and it did come from an interval of time within the gap: the fossil comes from the time interval between 354 million and 344 million years ago. But here is where reality sometimes escapes the news. Dating sedimentary rocks is devilishly hard. And more so for non-marine deposits. Perdepes was not an amphibian living the 10 million years from 354 to 344 million years ago. Instead, it is an admittedly (by its discoverer) short-ranging genus living sometime in that interval. Perdepes does not plug the gap—it is a small boat sailing in a vast sea of time. It does tell us that somewhere in the middle of Romer’s Gap a tetrapod did evolve the legs necessary for land life. But did it breathe air? Could it live entirely emerged from the water, even for a few minutes? That we do not know. So let’s demystify the gap, as alluded to earlier in this chapter.
Alfred Romer thought that evolution of the first amphibians came about because of the effect of oxygen. But the pathway may not have been that supposed by Romer, who considered that lungfish or their Devonian equivalents were trapped in small pools that would seasonally dry up. In his view the lack of oxygen brought about by natural processes in these pools, and the drying, was the evolutionary impetus for the evolution of lungs. His idea was that seasonally drying swamps spurred the jump to land or smaller freshwater ponds or lakes. Accord-