INTRODUCTION.

The article informs about the diversity of sea mammals, encompassing 137 living species worldwide, classified into at least seven distinct evolutionary lineages that returned to the sea. Topic include the majority are cetaceans, divided into toothed whales and baleen whales, while other groups include pinnipeds, sirenians, a polar bear, sea and marine otters, and extinct species like desmostylians, aquatic sloths, and the oyster bear Kolponomos.