Rostroconchs began their life as a small, bilaterally symmetrical, univalved protoconch planktonic larva. The bilateral shell grew into two valves as the rostroconch entered adulthood. Adult rostroconchs differ from bivalves because they have no functional hinge. Unlike the shell of a bivalve, which was able to move or articulate, the shell layers of a rostroconch — the layers of rigid calcite— continue across the whole dorsal area of the rostroconch. The two valves would have been rigidly fixed in place, and would have to have been broken periodically to allow the rostroconch shell to grow.