 By the middle of the Paleozoic , around 400 Mya , the brachiopods were among the most abundant filter feeders in the ocean , and over 12 @,@ 000 fossil species are recognized . By the Permian – Triassic extinction event 250 Mya , bivalves were undergoing a huge radiation of diversity . The bivalves were hard hit by this event , but re @-@ established themselves and thrived during the Triassic period that followed . In contrast , the brachiopods lost 95 % of their species diversity . The ability of some bivalves to burrow and thus avoid predators may have been a major factor in their success . Other new adaptations within various families allowed species to occupy previously unused evolutionary niches . These included increasing relative buoyancy in soft sediments by developing spines on the shell , gaining the ability to swim , and in a few cases , adopting predatory habits . 
