 Within <unk> , <unk> and Warren erected two major clades : <unk> and <unk> . <unk> include large <unk> <unk> like <unk> with flat heads and eyes near the back of the skull . <unk> include a diversity of <unk> , including large marine <unk> , aquatic <unk> , <unk> that survived into the Cretaceous , and <unk> with eyes near the front of their heads . In 2000 , paleontologists <unk> <unk> and Andrew <unk> named a third major clade of <unk> , the <unk> . This group included more primitive <unk> that could not be placed in either <unk> or <unk> , and included groups like <unk> , <unk> , and <unk> . While <unk> and <unk> are still widely used , <unk> is not often supported as a true clade in recent analyses . <unk> and <unk> are now grouped with <unk> , but <unk> are still considered to be a primitive family of <unk> . 
