 Brachiopods are shelled marine organisms that superficially resembled bivalves in that they are of similar size and have a hinged shell in two parts . However , brachiopods evolved from a very different ancestral line , and the resemblance to bivalves only arose because of a similar lifestyle . The differences between the two groups are due to their separate ancestral origins . Different initial structures have been adapted to solve the same problems , a case of convergent evolution . In modern times , brachiopods are not as common as bivalves . 
