About 450 million years ago, Earth's continents and oceans bore little resemblance to the familiar shapes of today. A sea covered much of the landmass that forms the center of present-day North America.

This ocean bottom teemed with invertebrate life about 450 million years ago, during the Late Ordovician Period. The animals seen here, living in a sea that covered present-day Ohio, are very different from today's marine species, although they belong to the same phyla, or groups. A nautiloid, a squidlike mollusk with tentacles and a long shell, appears in the foreground, while at the lower left is a trilobite, an early type of arthropod.

Trilobites are marine arthropods named for their three-lobed bodies. Now extinct, these animals flourished during the Paleozoic Era, times, with more than 10,000 species living in different parts of the world's seas. The global distribution of trilobite fossils has helped scientists learn more about the shifting locations of the Earth's ancient oceans and continents.