The most common seashells at the beach today are bivalves: clams, oysters, scallops, and mussels. However, from the Cambrian to the Permian (542 to 252 million years ago), another group of organisms called brachiopods dominated the world's oceans. Over 12,000 fossil species of these hinge-valved organisms have been described, but only 330 species remain alive today.
Bivalves and brachiopods are both sessile filter feeders, sitting on the seafloor and filtering water for food and oxygen. Their abundance reversed at the end of the Permian, when the greatest of all known mass extinctions eliminated more than 95 percent of Earth’s ocean species. The Permian extinction involved a crisis of low oxygen in the atmosphere that favored the more muscular and actively respiring mollusks over the passively respiring brachiopods. Clams and their relatives are much more efficient at extracting oxygen from seawater, so they were more successful after the extinction.
Unlike bivalves, brachiopods are symmetrical along the midline of the shell, which inspired the Chinese name "stone butterflies." Their beauty and variety is illustrated by this selection donated to the museum by paleontology collections director Greg Retallack.
Images © Museum of Natural and Cultural History.