Landmasses stretched almost from pole to pole during the Permian Period, about 290-250 million years ago, and were encircled by a vast ocean. Shallow seas covered the interior of Laurentia, which forms most of present-day North America.

About 270 million years ago, during the Permian Period, shallow seas covered many places that have since become dry land. Here, in a sea overlaying present-day Texas, invertebrates including brachiopods, bryozoans, sponges and microbial mats dominate a reef scene. Ocean life at this time was diverse—until a mass extinction at the end of this period wiped out animals both on land and in the seas, including some 90 percent of marine invertebrates.

	© Daniel W. Gotshall / Visuals Unlimited, Inc.

Brachiopods somewhat resemble clams but actually belong to an entirely different phylum. During the Paleozoic Era, about 550-245 million years ago, these invertebrates were more common than mollusks. Many brachiopods went extinct at the end of that period—but some species persisted and still survive today.

Although unfamiliar, brachiopods still exist today, living in marginal habitats such as submerged caves. Brachiopods are most closely related to bryozoans, both of which feed by filtering microorganisms from seawater.