How The Biggest Mouth On The Planet Evolved

A great blue whale opens its vast mouth

Whales have the largest mouths of any creature on the planet. They were originally large predators and exactly how they made the change from vicious carnivores to gentle giants in such a short time span is an evolutionary mystery. Today, most whales have a series of thick, stiff bristles in their mouths which they use to filter algae and plankton out from vast mouthfuls of sea water that weigh more than their entire body. However Dr Eric Fitzgerald from the Victoria Museum, Melbourne, Australia believes that he has solved the puzzle using a mixture of fossil and modern whale jaw characteristics.

Baleen whales - cetaceans that possess filter feeding bristles - have highly elastic lower jaws that allow them to open their mouths in a cavernous gape that is large enough to swallow a kombi van. Elastic jaws are thought to be a feature of all baleen whales both fossil and modern. Fitzgerald's theory states that instead of going from large stiff jawed cetaceans to baleen whales, there was an intermediate step that mixed the two characteristics of the groups. The final step was to find this missing link.

The fossilised skull of Janjucetus hunderi

Fitzgerald found this link on a beach near Torquay in Victoria in 2006. He found the fossilised jawbone of a 25 million year old whale which he named Janjucetus hunderi. The fossil jaw was incomplete until he discovered that an amateur fossil collector called Brian Crichton had discovered the missing pieces in the late 1970s. When he reconstructed the jaw, he found that it was a species of baleen whale that had a fused lower jaw which did not allow it to open its mouth wide enough to filter feed.

However a second feature is needed for successful filter feeding: a wide upper jaw which creates a large cavity inside the mouth. Janjucetus had the wide upper jaw possessed by cetaceans. Fitzgerald states that, in the time that modern baleen whales evolved, it would have been easier to go to whales that had a wide jaw, but used their teeth to catch prey and then onto a creature with an elastic jaw that could open wide enough to create suction. Such a creature would have been very similar to dolphins with their stiff lower jaw and teeth.