A new, fully dated total-evidence phylogeny of baleen whales (Mysticeti) shows that evolutionary phases correlate strongly with Caenozoic modernization of the oceans and climates, implying a major role for bottom-up physical drivers. The phylogeny of 90 modern and dated fossil species suggests three major phases in baleen whale history: an early adaptive radiation (36–30 Ma), a shift towards bulk filter-feeding (30–23 Ma) and a climate-driven diversity loss around 3 Ma. Evolutionary rates and disparity were high following the origin of mysticetes around 38 Ma, coincident with global cooling, abrupt Southern Ocean eutrophication and the development of the Antarctic Circumpolar Current (ACC). Subsequently, evolutionary rates and disparity fell, becoming nearly constant after approximately 23 Ma as the ACC reached its full strength. By contrast, species diversity rose until 15 Ma and then remained stable, before dropping sharply with the onset of Northern Hemisphere glaciation. This decline coincided with the final establishment of modern mysticete gigantism and may be linked to glacially driven variability in the distribution of shallow habitats or an increased need for long-distance migration related to iron-mediated changes in glacial marine productivity.

No abstract

From Big Fish to Big Whales Whales are the largest animals today, and many feed on the abundant plankton, particularly diatoms, in the oceans. Whales arose and diversified in the Cenozoic, about 30 to 40 million years ago (see the Perspective by Cavin ). Marx and Uhen (p. 993 ) show that their diversity parallels the diversity of diatoms and changes in ocean temperature. Whether there were large predators of plankton before whales has been enigmatic, because the fossil record during the Mesozoic (245 to 65 million years ago) is sparse. Friedman et al. (p. 990 ) now show that a group of large fish filled this role for nearly 100 million years in the Mesozoic. Although not as large as whales, these globally distributed fish were still several meters long. Their extinction at the Cretaceous-Paleogene boundary 65.5 million years ago may have cleared the seas for the evolution of whales.