 For a long time , bivalves were thought to be better adapted to aquatic life than brachiopods were , outcompeting and relegating them to minor niches in later ages . These two taxa appeared in textbooks as an example of replacement by competition . Evidence given for this included the fact that bivalves needed less food to subsist because of their energetically efficient ligament @-@ muscle system for opening and closing valves . All this has been broadly disproven , though ; rather , the prominence of modern bivalves over brachiopods seems due to chance disparities in their response to extinction events . 
