When claims are made that certain animals known only by their fossils are "extinct," all that is really meant is that no living specimens have been found. For example, the oceans are the least explored area of this planet and undoubtedly contain many creatures that are passing into extinction without ever having been discovered. The plesiosaur may possibly be one such example. On the other hand, when species once abundant in historical times, such as the carrier pigeon, are no longer seen, it can be said with reasonable confidence that they are indeed extinct.