February 2019

The 380,000-pound blue whale is not only the largest animal on Earth, but the largest to have ever lived on Earth. A blue whale would even have tipped the scale on a titanosaur, the heaviest dinosaur, which weighed less than half of what a blue whale does. But these whales do not leverage their size to become the top predator of the seas. In fact, they are toothless and feed relatively low on the so-called food chain, feasting on tiny crustaceans called krill, which measure just a centimeter or two long. This is only possible because of a specialized filter-feeding structure called baleen, which hangs in sheets from the top of the whale’s mouth. Blue whales gulp up a mouthful of krill-rich water and then push it back out through their baleen, sieving out all the krill in the process. Without baleen, blue whales (and their behemoth brethren, all members of the group Mysticeti) could not achieve their enormous sizes. How did the evolutionary innovation of baleen arise? New research sheds light on this fascinating transition.

Where's the evolution?

Living whales fall into two groups: toothless baleen-bearing whales and Odontoceti (most of which have teeth). However, the ancestors of baleen whales had teeth. We know this from two lines of evidence. First, close living and extinct relatives of baleen whales have teeth. It’s much more likely that baleen whales simply lost teeth than that all of those different groups independently evolved teeth. Second, scientists studying whale development have discovered that, in the womb, baleen whales develop teeth and then reabsorb them. This strongly suggests that the ancestors of modern baleen whales had teeth and lost them over the course of evolution.

Teeth and baleen are not homologous — that is, baleen is not just an evolutionarily modified tooth — so the transition from tooth to baleen could not have occurred as a smooth transformation of one structure into another. Instead, one structure was lost and the other arose — but in what order? Some biologists hypothesize that baleen must have started to evolve before teeth were lost, and animals with both structures could perhaps switch between two modes of feeding. However, a new look at an old fossil supports a different idea.

Maiabalaena nesbittae is a 33-million year old fossil whale discovered in Oregon in the 1970s. Scientists had paid little attention to it until the team from George Mason University, Texas A & M, and the Smithsonian decided it warranted more study. Based on the shape of the available fossils (skull, jaw, ear bones, and front limbs), the researchers think that it is a close relative of modern baleen whales, putting it in a perfect position to tell us more about the origins of baleen.

So did Maiabalaena have baleen? Fossilized baleen was not a part of the Maiabalaena fossils. However, baleen is made of keratin (the same material that comprises hair and fingernails) and generally rots away before it can fossilize — so lack of fossil baleen doesn’t mean that the living animal didn’t have a mouth full of it. A more reliable indicator is the presence of small canals in the upper jaw that would have supplied the baleen-bearing gum tissue with blood. Maiabalaena’s skull does not seem to have these holes, so the researchers could find no evidence that the whale had baleen. Neither does it seem to have had teeth. No teeth were found among the fossil bones, and the jaws did not have tooth sockets.

If Maiabalaena had neither teeth nor baleen, how did it make a living? Based on the outsized proportions of a bone in its neck (the hyoid) and the habits of other living whales that lack both teeth and baleen, the biologists propose that Maiabalaena was a suction feeder. Modern beaked whales, for example, slurp up squid using muscles attached to their large hyoid bone. Maiabalaena may have taken the same approach.

The evolutionary transition this entails — loss of teeth accompanied by the evolution of suction feeding — has occurred several times in toothed whales. The researchers propose that this also occurred in the ancestor of all Mysticeti and that Maiabalaena is an offshoot from this time period. The ancestor of modern baleen whales then went on to experience natural selection that modified parts of the upper jaw into baleen and suction feeding into filter feeding … or so the hypothesis goes. The current research supports the idea that baleen and teeth did not overlap and that teeth were lost before baleen arose, but more work remains to be done. Paleontologists have difficulty identifying which animals had baleen and which lacked it based on skeletal remains, so answering this question with more certainty is likely to require more fossils and more studies of the development of baleen in utero.