puzzling. We have seen them before in earlier oceans, but not like this. Virtually alone among echinoderms, crinoids have no respiratory structures, but the oxygen levels of this sea are higher than at any time in Earth’s history, and this change has led to the ascendance of crinoids. They are so abundant that their stony stalks make up the majority of bottom sediment, for most are relatively large for their kind.
We pass to a reef habitat, and once again the changes are evident. Sponges have taken the place of corals as the major framework builders of the reef ecosystems. Some of these reefs extend along the shelfslope break for hundreds of miles, a habitat filled with fish and ammonoids among the giant sponges. Large as well are resident brachiopods, with two kinds in abundance: one with spines, the other like a large garbage can sitting among the other reef organisms. Both are huge for their race—testament again to the high oxygen of this world.
Oxygen reached extraordinarily high values in the time interval from about 320 million to 260 million years ago, with maximal values occurring near the end of this interval. The Carboniferous Period (in North America subdivided into the Mississippian and Pennsylvanian Periods) and the first half of the subsequent Permian Period (299 million to 251 million years ago) were the times of high oxygen, and the biota of the world at that time left clear evidence of the high oxygen. Insects from that time present the best evidence.
The Carboniferous high (and much else as well) was wonderfully described by Nick Lane in his 2002 book, Oxygen. Lane wrote about a fossil dragonfly discovered in 1979 that had a wingspan of some 20 inches. An even larger form with a 30-inch wingspan is known from fossils of this Carboniferous time—a beast aptly named Meganeura, yet another dragonfly (Mega means large). And it was not only the wings that were large. The bodies of these giants were proportionally larger, with a width of as much as an inch and a length of nearly a foot. This is about seagull size and while seagulls are never linked in any sentence with the word “giant,” an insect with a 20-inch wingspan was indeed a veritable giant. In comparison, today’s dragonflies may reach