who are its ancestors?
Dorudon of Dorudontidae.
Cetacea
The direct ancestors of today's cetaceans are probably found within the Dorudontidae whose most famous member, "Dorudon", lived at the same time as "Basilosaurus". 
what is baleen whale? A parvorder of the infraorder Cetacea. what is the lifespan of the latter mentioned? Over 200 years. what is the head like of these species? Elongated head how intelligent are they? Known to teach, learn, cooperate, scheme and grieve. what are some environmental hazards they are susceptible to? Plastic waste flotsam, damaged ozone layer, and ocean acidification. how does its respiration work? Deliberate breathers who must be awake to inhale and exhale. what are the threats it faces? Directly from drive hunting and indirectly from pollution. how does it die? UNANSWERABLE
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Cetacea [SEP] Ancestors
The direct ancestors of today's cetaceans are probably found within the Dorudontidae whose most famous member, "Dorudon", lived at the same time as "Basilosaurus". Both groups had already developed the typical anatomical features of today's whales, such as hearing. Life in the water for a formerly terrestrial creature required significant adjustments such as the fixed bulla, which replaces the mammalian eardrum, as well as sound-conducting elements for submerged directional hearing. Their wrists were stiffened and probably contributed to the typical build of flippers. The hind legs existed, however, but were significantly reduced in size and with a vestigial pelvis connection.