How the world's biggest mouth evolved

Blue whales are the largest creatures that have ever lived (Source: Carl Buell)

Anna Salleh
ABC

An Australian palaeontologist has figured out a missing step in the evolution of giant filter-feeding mouths characteristic of blue whales.

Dr Erich Fitzgerald from Museum Victoria in Melbourne reports his argument in today's issue of Biology Letters.

"You could fit an average garden-variety kombi van in the mouth of a blue whale," says Fitzgerald, adding that blue whales are the largest animal ever known to inhabit the earth.
They have no teeth but, like other such whales, live on a diet of krill and other marine organisms that they filter out from seawater, using bristles on the roof of their mouths, called baleen.
Central to this baleen whale filter-feeding system is a cavernous mouth with a wide upper jaw and an elastic lower jaw that can open up wide to allow more than the whale's own bodyweight in sea water to enter in one gulp.

"[Modern baleen whales] have extremely mobile lower jaws, which is quite frankly bizarre because no other mammals have that sort of specialisation," says Fitzgerald.

This elastic lower jaw, in which the left and right hand sides are able to stretch apart, was until now believed to be a feature of all baleen whales, even fossil ones.

Scientists have long wondered how ancestral baleen whales, which used their teeth to catch large prey (like killer whales do) evolved into toothless filter feeders.

"This is a huge evolutionary jump," says Fitzgerald.

He now believes he has found the evolutionary missing link in the story.

Missing link

Fitzgerald has found the first fossil evidence of a toothed baleen whale that has no elastic lower jaw.
The newly-described jaw belonged to a tiny 25 million-year-old primitive baleen whale called Janjucetus hunderi, which was at most just three metres long, the size of a bottlenose dolphin.

"This is the clearest evidence yet that the earliest baleen whales could not filter feed and that's interesting because it had previously been thought that all baleen whales were filter feeders," says Fitzgerald.

He first analysed and named this creature in 2006, but at that stage he only had an incomplete lower jaw.

Fitzgerald then came across missing lower jaw bones in the collection of an amateur fossil hunter, by the name of Brian Crichton, who originally found them in the 1970s on a beach near Torquay in Victoria.

These new bones showed that the two halves of the lower jaw bone in Janjucetus hunderi were fused, and unable to open up to allow filter feeding.

Yet, Fitzgerald had previously found the animal had evolved another feature thought to be essential for the filter feeding - a wide upper jaw that creates a large space inside the mouth.

So why did this toothed whale evolve a wide upper jaw?

Suction feeding

Fitzgerald finds a clue in the mouths of modern dolphins, which also lack an elastic lower jaw. Those with really wide upper jaws feed by sucking in large individual prey, he says.

"They generate a vacuum [helped by the wide upper jaw] and hoover up fish and squid, sucking them in through a relatively small opening at the front of their mouths," says Fitzgerald.

"I argue that the big mouth of baleen whales possibly originally evolved to enhance the ability to generate suction."

He says it would be less of an evolutionary leap to go from baleen whales that catch large prey with their teeth to those that suction feed, than directly to those that filter feed of lots on tiny organisms.
After being decimated by past whaling the numbers of blue whales remain low with only about 10,000 individuals left, mainly in the Southern Ocean, says Fitzgerald.

Although they are now protected, he says they remain under threat due to changes in the ocean ecosystem that may affect levels of krill.