The ancestors of today's largest whales were gruesome carnivores that sucked giant fish and squid into tooth-filled jaws. Now, thanks to the analysis of the ancient jawbone of a sharp-toothed, dolphinlike creature, researchers think they know how such a terrifying animal gave rise to its docile, filter-feeding kin.
Modern-day baleen whales are among the largest creatures that have ever lived. At 30 meters in length, an adult blue whale weighs about 180 metric tons—almost twice the heft of the largest dinosaur known—and consumes about 3.6 metric tons of food each day. Baleen whales get their name from the frayed slabs of fingernail-like material they use to strain tiny crustaceans and small fish from mouthfuls of seawater.
To nourish their great bulk, the whales have evolved several anatomical features that allow them to slurp in great volumes of prey-laden water when they feed. Most species have a broad skull, which is key to a voluminous mouth. Moreover, the bones in the front part of a baleen whale's lower jaw, or mandible, aren't fused as they are in other mammals. That feature allows the bones to rotate, broadening the lower jaw and thereby increasing the volume of seawater that can be taken in a single gulp. Scientists weren't clear about the order in which these features evolved, however.
Now, newly described fossils of Janjucetus, a 25-million-year-old, dolphin-sized member of the baleen whale clan, suggest that the broad skull evolved first, Fitzgerald reports online today in Biology Letters. Although the arrangement of bones in its face and skull indicate that Janjucetus is most closely related to modern-day baleen whales, the species had teeth and not baleen.
Erich Fitzgerald, a paleobiologist at Museum Victoria in Melbourne, Australia, first described Janjucetus  in 2006, based on only a fossil skull whose upper jaw sported large, conical teeth. The new fossils include a mandible. Although this jaw's bones are not fused, they are tightly connected and would not have allowed the lower jaw to expand. Thus Janjucetus didn't feed as modern baleen whales do. Instead, Fitzgerald contends, the whale depended on suction feeding: By dropping its tongue and lower jaw, it pulled in water and slurped prey such as large fish or squid into its toothy maw . The whale's wide skull would have boosted the volume of its mouth and, in turn, the efficiency of suction feeding, he suggests.
Over time, the evolution of baleen and the flexible jaw allowed the whales to capture food more efficiently than toothy jaws did—a combination that enabled baleen whales to grow to immense size by eating vast amounts of small but plentiful prey.
The newly described mandible is an important find, says Nicholas Pyenson, a vertebrate paleontologist at the Smithsonian Institution's National Museum of Natural History in Washington, D.C. Although today's whales, dolphins, and porpoises have a variety of feeding styles, it's clear that the ancestors of all whales were suction feeders, he notes.
Nevertheless, he says, at this point the evidence that the dolphin-sized Janjucetus was a suction feeder is only circumstantial. "If we want to understand the factors that led to gigantism in baleen whales, we'll need to look at fossils yet to be discovered or described."