What Does the Fossil Record Tell Us About the Evolution of Gigantism in Whales?

Cetacean evolution represents one of the best-documented transitions from land to sea, and paleontological evidence in particular has made this evolutionary transformation a textbook case of macroevolution. However, the transition from land to sea represents only the first third of cetacean evolution, during the Eocene; over 30 million years of their evolutionary history occurred in the oceans, culminating in the evolution of the largest vertebrates ever. How did this occur? And what does the fossil record tell us that we would not know otherwise? Three examples provide an outline of these answers. First, baleen whales include the largest (longest and heaviest) exemplars of gigantism in cetaceans, with rorquals and right and bowhead whales. The fossil record of these filter-feeding groups shows the maximum size of extinct relatives at a fraction of their largest living relatives; the sudden rise to today's gigantic sizes seems to coincide with the onset of glaciation in the Northern Hemisphere during the Pliocene, perhaps causing changes in patchiness and intensity of prey resources. Second, sperm whales are renowned as the deepest diving cetaceans; they are also the largest toothed whale, or odontocete, with complex social organization. The entirety of their fossil record is also composed of much smaller members. Given the benefits of gigantism related to extreme diving depths and durations, it is likely that gigantism in sperm whales conferred benefits that also mirrored an escalation of predation on cephalopod resources, culminating in their iconic predator-prey relationship with the largest cephalopods known, giant squid, and other Architeuthidae. Lastly, killer whales are the largest oceanic dolphin species, occupying a dominant role in many marine food webs. While their fossil record is poor, their diverse feeding modes -- including some populations that feed on other cetaceans -- suggests that gigantism can evolve rapidly in cetaceans. Moreover, it is likely that the origin of killer whales precipitated evolutionary arms races with other cetaceans, resulting the flight or fight behavioral modes observed among baleen whales. Such comparative work, in the aforementioned examples, highlights the value of examining ecological hypotheses in an evolutionary framework. The fossil record of cetaceans is especially relevant to understanding the evolution of gigantism because it offers a vantage over geologic timescales, and past conditions and adaptations that may not be present among today’s species.