The most common shelled animal in the ancient seas was the brachiopod. From about 20,000 species of brachiopods, only about 300 species exist today. They are found in every Paleozoic marine layer at the canyon. Brachiopods had two asymmetrical shells, or valves, with one larger than the other. They often fossilized whole because when their muscles were relaxed, as in death, the valves were closed. They contracted their muscles to open the valves and filter feed.