Many species of baleen whales are endangered. Birds and mammals are endothermic. Eras form an Eon, which is the largest division of geological time, lasting hundreds of millions of years. ERA Two or more geological periods comprise an Era, which is hundreds of millions of years in duration. An estuary is where a river meets the seas.
Some beluga whales visit estuaries seasonally to breed and feed.
They are baleen whales with no dorsal fin, thick blubber, and large, hairy heads. They are about 1 inch long. An exoskeleton is a tough, structural body armor made of chitin. An animal species that is extinct has died out. Most animal species that ever existed have gone extinct, including all the dinosaurs.