Toothless, 33-Million-Year-Old Whale Could Be An Evolutionary 'Missing Link'

Top and bottom views of the remarkably well-preserved Maiabalaena nesbittae skull, an ancient species of toothless whale. (Image: C. M. Peredo et al., 2018/Current Biology)

A closer examination of a fossil found more than four decades ago has led to the identification of a new species of whale — a 33-million-year-old cetacean featuring neither teeth nor baleen. Its discovery could solve a longstanding mystery about the origin of filter-feeding whales, but some scientists say the new analysis isn't wholly convincing.

Introducing Maiabalaena nesbittae, an entirely new genus and species of ancient whale. Roughly the size of a modern beluga whale, this 4.57m-long cetacean didn't have teeth or baleen (rows of hair-like plates that whales use to filter tiny prey from the water), relying instead on suction feeding.

As such, Maiabalaena nesbittae, meaning "mother whale," represents an intermediate stage between ancient toothed whales and modern filter-feeders, according to new research published today in Current Biology.

Artist's impression of Maiabalaena nesbittae with calf. (Illustration: Alex Boersma)

Today, whales can be broadly lumped into two main groups: toothed whales, such as orcas and dolphins, and filter-feeding whales (or mysticeti), such as humpbacks, fin whales, blue whales, and minke whales. Baleen is the remarkable evolutionary invention that makes filter feeding possible, allowing large marine whales to consume several tonnes of food each day without ever having to chomp or chew.

Whales are the first and only mammals to evolve baleen, but the origin of this feeding strategy isn't entirely clear. Whales are descended from terrestrial mammals, who retained their teeth after adapting an aquatic lifestyle.

With their razor-sharp teeth, ancient whales continued to chew their food. But the environment changed, as did their prey, so these whales had to adopt new feeding strategies. Eventually, this resulted in the emergence of filter feeding whales.

As to how whales went from having teeth to having baleen — a substance made of keratin, which is what hair and fingernails and made of — is the subject of much controversy.

Some scientists have speculated that ancient whales used their teeth for sifting water, and that this feeding strategy led directly to baleen. This theory took a direct hit last year by Monash University palaeontologists who showed that the sharp teeth employed by ancient whales couldn't have possibly been used as filters, concluding that ancient whales never passed through a tooth-based filtration phase, and that some kind of intermediary, yet-to-be-found species must have existed.

Illustration of the proposed feeding transitions in ancient whales. (Graphic: C. M. Peredo et al., 2018, Current Biology)

Part of the problem is that keratin doesn't preserve well in the fossil record. For palaeontologists who study ancient whales, this mystery is akin to the study of flight in ancient animals, and the seemingly endless quest to discover the "missing link" between gliding birds and those capable of self-powered flight.

In the case of whales, palaeontologists have been searching for an intermediate species of whale positioned between toothed whales and filter-feeding whales. The discovery of the toothless, baleenless Maiabalaena nesbittae could very well be this missing link.

The partial skeleton of Maiabalaena nesbittae, which includes a nearly complete skull, was uncovered in Oregon back in the 1970s, and it has languished at the Smithsonian's national collection ever since. To this point, a detailed analysis of the fossil wasn't possible because it's inundated with rock and other materials.

The lead author of the new study, Carlos Mauricio Peredo of George Mason University and the National Museum of Natural History, took a look at this old fossil with new eyes using state-of-the-art CT scanning technology. By peering into the rock, the researchers were able to identify the tell-tale signs of a toothless and baleenless whale — including a thin and narrow upper jaw that had no proper surface from which to suspend baleen.

"A living baleen whale has a big, broad roof in its mouth, and it's also thickened to create attachment sites for the baleen," said Peredo in a statement. "Maiabalaena does not. 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."

Other evidence points to this animal as a filter feeder. Muscle attachments on the bones of its throat imply the presence of strong cheeks and a retractable tongue — characteristics that would have allowed this whale to suck water into its mouth, sopping up fish and small squid in the process.

Equipped with this ability, these whales no longer needed their chompers, so their teeth gradually faded away. The eventual loss of teeth and the origin of baleen, the researchers, argue, where therefore separate evolutionary events.

As to why toothed whales abandoned biting and chewing in favour of sucking, the researchers say it was a transition forced upon them by a changing environment. Maiabalaena lived during the transitional period that divided the Eocene from the Oligocene, which happened some 33 million years ago. This was a critical time for whales, as the continents shifted and separated, and as ocean currents from the Antarctic cooled the oceans.

As the planet's geology changed, so too did the ocean environment — and its animals. The prey of toothed whales changed or disappeared, forcing them to find new prey, which resulted in the transition from toothed to suction feeding, the researchers speculate. Eventually, some 5-7 million years later, around 26-28 million years ago, the toothless whales began to sprout baleen, facilitating yet another transition, this time from suction feeding to filter feeding.

"In general, I think this is a good study, and I agree with its general conclusions," Felix G. Marx, a palaeontologist at Monash University not affiliated with this new research, told Gizmodo. "Crucially, though, Maiabalaena seems to be right in the middle of this transition, with no teeth, and possibly no baleen."

Possibly no baleen.

That's the the key phrase, here. As noted, baleen, which is made from soft tissue, doesn't fossilise very well. Typically, scientists can detect the presence of baleen in a fossil by looking for traces of corresponding blood vessels on their bones. And in fact, traces of blood vessels were detected in the Maiabalaena fossil. The question, however, is whether these blood vessels always correlate with baleen.

"The new study says no, and argues that similar structures also existed in ancient toothed whales that clearly did not filter feed," said Marx. "I agree, but this is still an interpretation, and I suspect not everyone will buy into it. Luckily, there are more things we can do to tackle this question, for example by examining how baleen actually develops in the womb."

Monash University palaeontologist Alistair Evans, a co-author of the aforementioned 2017 study, agrees with Marx's assessment, saying the absence of teeth in this species is fairly evident, but the absence of baleen, not so much.

"Because baleen is so rarely fossilised, its presence can rarely be seen directly," Evans told Gizmodo. "As has been suggested before — and [as this new paper] gives more evidence — there are no silver bullets in the bones that can tell us for sure that baleen was present. So unfortunately there is no strong evidence of baleen being absent, but we also may never find such evidence."

Evans says the conclusions made in the new study are "fairly reasonable," but he would like to see other specimens of this species and related ones that are better preserved in the region where baleen would be if present.

"I was happy that they found a fossil that we predicted would occur, but the evidence is not a slam dunk that it really did fit in this slot," added Evans.

So is Maiabalaena nesbittae the missing link we've been looking for? Quite possibly yes — but we won't know for sure until more fossils are recovered.

[Current Biology]

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