 All modern baleen whales or mysticetes are filter @-@ feeders which have baleen in place of teeth , though the exact means by which baleen is used differs among species ( gulp @-@ feeding within balaenopterids , skim @-@ feeding within balaenids , and bottom plowing within eschrichtiids ) . the first members of both groups appeared during the middle Miocene . Filter feeding is very beneficial as it allows baleen whales to efficiently gain huge energy resources , which makes the large body size in modern variety possible . The development of filter feeding may have been a result of worldwide environmental change and physical changes in the oceans . A large @-@ scale change in ocean current and temperature could have initiated the radiation of modern mysticetes , leading to the demise of the toothed forms . The earlier variety of baleen whales , or " archaeomysticetes " , such as Janjucetus and Mammalodon had very little baleen and relied mainly on their teeth . Generally it is speculated the four modern mysticete families have separate origins among the cetotheres . Modern baleen whales , Balaenopteridae ( rorquals and humpback whale , Megaptera novaengliae ) , Balaenidae ( right whales ) , Eschrichtiidae ( gray whale , Eschrictius robustus ) , and Neobalaenidae ( pygmy right whale , Caperea marginata ) all have derived characteristics presently unknown in any cetothere and vice versa ( such as a sagital crest ) . 
