Rostroconchia

Rostroconchia was an extinct class of marine mollusks that existed from the earliest Cambrian period (about 542 million years ago) until the Cretaceous period (which ended about 65.5 million years ago). These fossils are typically small, ranging from one to five millimeters in size.1

The Rostroconchia are considered ancestral to modern bivalves and scaphopods. They possessed a mantle that enlarged the shell to enclose the soft body and an anterior foot adapted for living on sandy or muddy bottoms. The evolutionary pathway of mollusks, including the differentiation of classes like Rostroconchia, is largely inferred from comparative anatomy and molecular data, as the fossil record provides limited clues about their origins.1