Filter-feeding, or baleen, whales have long been known to have evolved from earlier toothed species, but the transition is poorly understood.

Some scientists cite evidence to suggest an intermediary phase involving species that had both teeth and baleen, and thus “that combined raptorial feeding with incipient bulk filtering”.

Others point to fossil evidence of long-extinct whale species that were clearly members of the baleen-feeding group – known as Mysticeti – but lacked baleen itself and instead used teeth to hunt large prey in a manner much like the modern leopard seal (Hydrurga leptonyx).

Now, a third possibility has been uncovered.

US researchers have discovered that Maiabalaena nesbittae, a 4.5-metre whale that lived around 33 million years ago, had neither teeth nor filters. Instead it appears to have used its strong cheeks and retractable tongue to suck water into its mouth, taking up fish and small squid in the process. This rendered teeth, which require a lot of energy to grow, unnecessary.

The loss of teeth appears to have set the evolutionary stage for the baleen, which the researchers estimate did not arise until five to seven million years later.

“A living baleen whale has a big, broad roof in its mouth and it’s also thickened to create attachment sites for the baleen; Maiabalaena does not,” says Carlos Mauricio Peredo from the Smithsonian National Museum of Natural History in Washington, who led the research.

“We can pretty conclusively tell you this fossil species didn’t have teeth, and it is more likely than not that it didn’t have baleen, either.”

The findings are reported in a paper published in the journal Current Biology.

M. nesbittae was discovered in Oregon in the 1970s and has been part of the museum’s collection for a number of years. However, the material which surrounded the fossil obscured many of its key features until recently, when Peredo brought in state-of-the-art computed tomography (CT) scanning technology.{%recommended 6998%}

The lack of teeth is readily apparent from the preserved bone, but the CT scans, which revealed the fossil’s internal anatomy, told the scientists something new: its upper jaw was thin and narrow, making it an inadequate surface from which to suspend baleen.

Whales were the only mammals to evolve baleen – rows of flexible hair-like plates. No other mammal uses any anatomical structure even remotely similar to it to consume its prey, Peredo says.

However, it has been difficult to study baleen because it does not preserve well; its chemical composition is more like that of hair or fingernails than bone.

As palaeontologists have had little direct evidence of its origins, they have had to rely on inferences from fossils and studies of foetal-whale development in the womb to try to trace its evolution.

An initial assumption was that ocean-dwelling mammals must have needed either teeth or baleen to eat, but several living whales contradict that idea.

Sperm whales, for example, have teeth in their bottom jaw but none on the top, so they cannot bite or chew. Narwhals’ only teeth are their long tusks, which they do not use for feeding, and some species of beaked whales, despite being classified as toothed, have no teeth at all.

M. nesbittae adds further weight to the argument.

“When we talk about whale evolution, textbooks tend to focus on the early stages, when whales went from land to sea,” says Nick Pyenson, the Smithsonian’s curator of marine mammals.

“Maiabalaena shows that the second phase of whale evolution is just as important for evolution over big scales. For the first time, we can now pin down the origin of filter-feeding, which is one of the major innovations in whale history.”

Peredo and Pyenson see studying whale evolution as key to understanding their survival. Like the emergence of baleen, tooth loss is evidence of adaptability, suggesting whales might be able to adapt to challenges posed in the ocean today.

Peredo cautions, however, that evolutionary change may be slow for the largest whales, which have long life spans and take many years to reach reproductive age.

“Given the scale and rate of changes in the ocean today, we don’t exactly know what that will mean for all of the different species of filter-feeding whales,” he says.

“We know that they’ve changed in the past. It’s just a matter of whether they can keep up with whatever the oceans are doing–and we’re changing the oceans pretty quickly right now.”