reefs themselves (not just the coral that makes them) maximizes water movement over the millions of small animals living there—and in so doing brings in more oxygenated water than would otherwise be present. Back in the Ordovician the same may apply, for the reef shapes (but not the corals making them) are very familiar indeed.

But is it really the same? It takes time to record all the impressions, to make the connections of similarity and difference. There is oddness here—and then it hits home: no fish.

The swarming schools of reef fish, the amazing color and diversity in our world’s reefs, are nowhere to be seen back in these Ordovicianaged seas. There are a few swimmers, plenty of trilobites around, and boundless numbers of other arthropods busily going about their business of life. Many bear a cornucopia of upward-pointing spines, bearing witness to the dangers in this world. These are defensive adaptations, all, and we immediately start searching for the familiar top carnivore of the older world, the anomolocarids that so effectively ruled the top of the Cambrian ecological pyramid (producers eaten by grazers, in turn eaten by carnivores) for so long. But they are nowhere to be seen and in fact are by now long extinct. Two things happened. There is now more oxygen in the Ordovician, and a new scourge of a predator has appeared in the seas. We look again into the blue waters, and among the swimming arthropods we readily see an entirely different kind of animal up in the water column. It is a body plan that was not present at all in the early and mid-Cambrian, one that first appeared only at the very end of the Cambrian in fact, at the same time that the mass extinction of trilobites and other Cambrian arthropods was under way. We look more closely at these strange swimming predators and see an animal type that is virtually unknown in our world. These are shelled creatures, but unlike the body armor of the arthropods, which is composed of many segments, here the conelike shells are the most striking feature, some straight, some gently curved, some entirely coiled. Then we see something very similar to the chambered nautilus of our world and realize that we are seeing an amazing variety of chambered cephalopods. In only a few minutes of watching their activities, a new conclusion is reached. These nautiloids have toppled the arthropods as the top carnivores.

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