and part submarine—and entirely imaginary. What we might see, however, comes from two centuries of paleontological work.

We sweep down over the land surface and notice immediately that it is greener. There are still no trees or even commonly rooted plants, but mosses have spread and among them are primitive vascular plants. There is also the occasional movement of animals on the land surface, not vertebrates, but a diversity of scorpion and centipede-like arthropods. There is not yet a high diversity of them, but there are enough to convince us that an invasion has started, a beachhead has been established, and more troops are on the way. As we pass over the landscape, much is familiar: mountains and plains, great glaciers in the high latitudes, sandy beaches with sand dunes and ripples, the spray of the sea on the land. True, the lack of trees and bushes, flowers and grass, birds and even flying insects is curious enough, and there is much more bare rock than we are used to outside the desert, high mountain, or glacial regions of our own world. But there is something else about this landscape that nags, something also seen on our previous look at the land in the Cambrian world 50 million years previously. As during the Cambrian, the rivers are still braided, not meandering. There are no riverbanks, just a vast wide complex of shallow rushing water on its journey from the distant snow-capped mountains to the sea.

We return to the seashore and power downward into the sea, expecting changes and not being disappointed in this. At first, as we settle onto a shallow, warm bottom, things look superficially the same because the most common bottom dwellers are sponges. In the clear blue tropical water, we first think we are in the shallows off Jamaica or the Florida Keys—there are sponges everywhere, in myriad colors and shapes. Many are glass sponges, but the more familiar demosponges are there too. Amid them are untold numbers of bivalved creatures, looking a bit like clams of our world. But closer inspection shows them to be brachiopods, forms with articulated shells that are more advanced than the small, inarticulate brachiopods of the Cambrian world. They coat the bottom, and amid them is an animal not seen in the Cambrian—the colonial bryozoan, which builds calcareous skeletons, each being a tiny miniature replicate of the solitary brachiopods. The brachiopods look like clams from our world, but this is but



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