Hard-Shelled Mollusks

The origins and earliest evolution of this diverse group, which includes clams, snails, squid, and octopuses, remain unclear. Among the first fossil specimens were the monoplacophorans—the ancestors of many shelled mollusks alive today. Thought to be descended from annelids, monoplacophorans lacked eyes and moved on a rounded foot under a simple, limpet-like shell.

In Cambrian times, they inhabited warm, shallow seas. Monoplacophorans were believed long extinct until a living species was discovered deep in the Pacific Ocean in the 1950s.