Marine Mammals: Phylum Chordata. The Pollack Farm Site, in the Cheswold sands of the lower Miocene Calvert Formation, produced a fragmentary marine mammal fauna. The Pollack location yielded at least six cetaceans (whales, porpoises), a sirenian(manatee), along with one of the earliest records of a true seal (Listed below). Marine mammals have the same characteristics as all other mammals, but they have adapted or adjusted to life in the ocean. For instance, many species residing in the deep oceans have a thick layer of blubber, which holds their body temperature warm.