The Ordovician period (500 to 440 million years ago) comes after the Cambrian in the early Paleozoic era. The period is named for a Celtic tribe named the Ordovices who once lived in the area of Wales (in Britain) where the rocks were first studied. Ordovician limestones are over 6.4 kilometers (4 miles) thick in places and are found on all continents except Antarctica. The uniformity and thickness of the bed indicates a long period of warm and stable climate that allows them to develop.

In fact, the Ordovician period was as remarkable for the diversity of its species as the Cambrian period was for the appearance of most major phyla. A burst of evolutionary creativity in shape, size, and function tripled the number of marine species that appeared. Specialization became the dominant theme of life, with new forms filling every possible niche.

The appearance of highly efficient predators such as the...