Bivalves are among the oldest hard fossils, showing up in Early Cambrian times more than 500 million years ago. It is believed that a permanent change in ocean or atmospheric chemistry made it possible for organisms to secrete hard shells of calcium carbonate. This fossil clam is young, from the Pliocene or Pleistocene rocks of central California. Still, it looks just like its oldest ancestors.
For lots more detail on the bivalves, see this lab exercise from SUNY Cortland.