Fossil shows huge mouth evolution
An ancient whale fossil has shown a key step in the evolution of filter-feeding whales' enormous mouths.
Modern baleen whales, such as blue whales, can filter small marine creatures from huge volumes of water.
Their "loose" lower jaw joints enable them to produce a vast filter-feeding gape.
A study of this ancient jawbone showed that nature's largest mouths probably evolved to suck in large prey rather than to engulf plankton-filled water.
The researchers, from Australia and the US, reported their findings in the Royal Society journal, Biology Letters. The prehistoric jaw, they noted, was very different from modern baleen whales.
In modern whales, the lower jaw does not fuse at the "chin". Instead there is a specialised jaw joint that allows each side of the jaw to rotate.
By having two curved lower jaw bones that rotate in this way, baleen whales are able to produce huge gapes to take in massive quantities of water and prey.
- Baleen is a hair-like substance that grows from the roof of baleen whales' mouths
- Baleen whales, such as grey, humpback and blue whales are the oceans's gentle giants. Rather than hunt large prey, they engulf vast quantities of water and filter out small fish, or plankton, with their baleen
- The efficiency of this filter feeding has allowed baleen whales to evolve into some of the world's biggest animals
- A blue whale, the largest animal that has ever lived, has a mouth cavity so vast that it can engulf a volume of water equivalent to its own body mass
Lead researcher, Erich Fitzgerald from the Museum Victoria in Melbourne, said: "This is compelling evidence that these archaic baleen whales could not expand and rotate their lower jaws, which enables living baleen whales to engulf and expel huge volumes of seawater when filter feeding on krill and other tiny animals."
Crucially though, the fossilised whale, named Janjucetus hunderi, did have a very wide upper jaw. Dr Fitzgerald says that this widening was the earliest step in the evolution of today's whales' gigantic mouths.
He charted the anatomical features of whales on an "evolutionary tree" - from Janjucetus hunderi to the blue whale.
"I was able to discover the sequence of jaw evolution from the earliest whales to the modern giants of the sea," he said.
This chart indicated that "the first step towards the huge mouths of baleen whales may have been increasing the width of the upper jaw [to] suck fish and squid into the mouth one-at-a-time.
"The loose lower jaw joint that enables living baleen whales to greatly expand their mouths when filter feeding evolved later."
This particular whale was so primitive that it had not evolved its comb-like baleen; it had "ordinary" teeth.
Its fossilised jawbone was discovered in the 1970s by an amateur collector in a coastal town in Victoria, south-east Australia.
"I first saw [it] while visiting a private collection in 2008," recalled Dr Fitzgerland. "I immediately recognised the characteristic shape of the lower jaws of a whale."
Jeremy Goldbogen, a researcher from the Cascadia Research Collective in Washington, who studies the feeding strategies of modern whales, said that bulk filter feeding was "one of the most fascinating adaptions in the animal kingdom".
He told BBC Nature: "An important point to note is that bulk filter feeding using [rotating jawbones] does not necessarily mean that suction is not used.
"A prime example of this are grey whales which are notorious suction filter feeders."