Short answers, pulled from the story.
Pinnipeds are a group of carnivorous, fin-footed, semiaquatic mammals that include walruses, sea lions, fur seals, and true seals. There are 34 living species and more than 50 extinct species known from fossils. They belong to the order Carnivora and are most closely related to musteloids such as weasels, raccoons, skunks, and red pandas.
Pinnipeds split from other caniforms approximately 50 million years ago during the Eocene. Early fossil relatives such as Puijila from Arctic Canada and Potamotherium from Europe lived in the Early Miocene and were found in lake deposits, suggesting seal ancestors originally adapted to fresh water before colonising the ocean. The genus Enaliarctos, from California around 24 to 22 million years ago, already closely resembled modern pinnipeds.
The smallest pinniped is the Baikal seal, which measures about 1 metre and weighs around 45 kg. The largest is the southern elephant seal, which reaches 5 metres in length and can weigh up to 3,200 kg. Pinnipeds generally tend to be larger than other carnivores.
Before a deep dive, pinnipeds exhale most of the air from their lungs and seal their nostrils and throat cartilages. Their chest muscles and alveoli can completely deflate, and remaining air shifts to the bronchioles and trachea during deep dives to prevent gas exchange and avoid decompression sickness. Deep-diving species such as elephant seals have blood volumes up to 20 percent of their body weight and large stores of hemoglobin and myoglobin for extended oxygen supply.
The Caribbean monk seal was declared extinct by the IUCN in 1996; the last reliable sighting was in 1952 at Serranilla Bank. The Japanese sea lion was last recorded as a juvenile in 1974, after its population collapsed in the 1930s from overexploitation and competition with fisheries.
Pinniped vibrissae, or whiskers, carry ten times more nerve connections than those of terrestrial mammals, allowing them to detect pressure waves generated by fish moving through water. Harbor seals can follow hydrodynamic trails left by a fish swimming minutes earlier. Walruses have the most vibrissae of any pinniped, with between 600 and 700 individual hairs.