 the emergence of the Antarctic circumpolar current or the closing of the Isthmus of Panama) – is more likely, with the pseudotooth birds as remnants of the world's Paleogene fauna ultimately failing to adapt.In that respect it may be significant that some lineages of cetaceans, like the primitive dolphins of the Kentriodontidae or the shark-toothed whales, flourished contemporary with the Pelagornithidae and became extinct at about the same time.Also, the modern diversity of pinniped and cetacean genera evolved largely around the Mio-Pliocene boundary, suggesting that many ecological niches emerged or became vacant.