33 million-year-old toothless whale weighs in on the evolution of oral structures

FRIDAY, 24 AUGUST 2018

The blue whale is the largest animal to ever have lived on our planet, with the heaviest recorded weighing in at an impressive 173 tonnes. Blue whales have to consume enormous amounts to sustain their massive size, and can eat up to 3,600 kilograms of food a day. Their dish of choice is krill - tiny crustaceans that are an important part of the ocean food chain. How certain mouth structures of whales evolved has long been a mystery to whale scientists, but a new study recently published in Current Biology may have found the answer.

The whales eat krill through a process called ‘filter-feeding’, where they gather large amounts of krill and seawater into their mouths. They then filter out the seawater through structures called baleen - bristles inside their mouths which prevent the krill from being pushed out with the seawater.

Scientists previously assumed that baleen evolved directly from teeth via gradual evolutionary change, giving rise eventually to the filter-feeder whales we see today. However Dr Carlos Mauricio Peredo, of George Mason University and the Smithsonian National Museum of Natural History in Washington, DC, recently described a new fossilised whale which challenges that understanding of the evolutionary process.

“Our study reports a 33 million year old fossil whale that we named Maiabalaena” said Dr Carlos, “which provides key evidence for the evolutionary problem of tooth loss and the origin of baleen in whales”. Maiabalaena, whose name means ‘mother whale’, is an ancient ancestor of the blue whale and other filter-feeding whales and was found in Oregon, USA. Maiabalaena does not have teeth, which lead to researchers looking for baleen structures. However, Dr Peredo reports that they also could not find any baleen structures either - so not only is Maiabalaena the oldest recorded toothless whale, but when compared to other fossilised whales it suggests that whales lost their teeth and then, later, evolved to have baleen. Dr Nicholas Pyenson, another author on the paper, summarised “this fossil demonstrates that the loss of teeth and the origin of baleen are separate evolutionary changes, and that the two changes did not overlap”.

Based on analysis of the bone structure, Dr Peredo suggests that the lack of teeth and baleen means that Maiabalaena was a suction-feeder that got by just fine without oral structures to filter or chew food. Future work will involve continuing to try and catalogue different feeding mechanisms of ancient whales to understand what caused whales to lose their teeth, and then what prompted the evolution of the unusual baleen structures. Maiabalaena represents an important step forward in understanding this process, suggesting that the name ‘mother whale’ is well-deserved.

Eva Higginbotham is a PhD student at the Department of Zoology