cies, they seem to show high disparity—the number of different body plans. Such was the case during the Cambrian Explosion, a time of relatively low species-level diversity but of many kinds of body plans relative to the number of species. So too with the interval of time from 365 million to perhaps 360 million years ago, with many new evolutionary experiments being tried out. Ichthyostega was one of these, and, judging from its geological record, a not too successful one. The fossil record shows that soon after its first appearance, it and the other pioneering tetrapods disappeared.

But were Ichthyostega and the two or three allied forms found with it even land-dwelling organisms? The bones of this first amphibian have been reexamined in detail by Cambridge paleontologist Jenny Clack. What she and other anatomists discovered was unexpected. Taken together, the anatomy of Ichthyostega does not seem appropriate for life on land: Ichthyostega would have been very inefficient on land, if it could walk in air at all. This creature was pretty much a fish with legs, rather than an amphibian in the sense of how we know them today. And if it were the first amphibian, we would expect this great evolutionary breakthrough to be soon followed by an adaptive radiation, the rapid proliferation of new species using the breakthrough morphology. But this did not happen. There was a long gap before more amphibians appeared. This gap has perplexed generations of paleontologists and it came to be known as Romer’s Gap, after the early twentieth-century paleontologist Alfred Romer, who first brought attention to it. The expected evolutionary radiation of amphibians did not take place until about 340 million to 330 million years ago, making Romer’s Gap at least 20 million years in length. This radiation took place at a time when oxygen had again risen to, or above, present-day levels, and that did not happen until later than 355 million years ago. A 2004 summary by John Long and Malcolm Gordon similarly interpreted the tetrapods living 370 million to 355 million years ago, the time of a great oxygen drop, as entirely aquatic—essentially fish with legs—even though some of them had lost gills. Respiration took place by gulping air, in the manner of many current fish, and by oxygen absorption through the skin. None were amphibians as we know them today, species that can live their entire adult lives on land. And it ap-