MV Blog

DISPLAYING POSTS TAGGED: janjucetus (1)

Evolving the biggest mouth in history

Author
by Kate C
Publish date
17 August 2011
Comments
Comments (1)

Imagine that your face was articulated so that your jaw could split down the middle and expand sideways until the tips were out as wide as your ears. Imagine that you could move all the bones of your face... not just the soft tissue, but the bones themselves.

Sound bizarre? Alien, even? Yet this is exactly what happens every time a Blue Whale takes a gulp of water. The filter-feeding whales, otherwise known as baleen whales or mysticetes, have feeding adaptations that are unique among mammals. Their intriguing evolutionary history is the subject of Dr Erich Fitzgerald's research, and today he's published a paper that overturns a long-held belief about how the baleen whales evolved.

Blue Whale Illustration of the biggest mouth in history at work. The Blue Whale can expand its mouth to gulp huge volumes of krill-filled water.
Image: Carl Buell
Source: Museum Victoria
 

For several years, he has worked on an extraordinary 25 million-year-old species known from fossils that were found in the 1990s near Jan Juc on Victoria's west coast. Called Janjucetus, this early baleen whale predated the evolution of baleen – the hairy structure used by modern baleen whales to filter tiny crustaceans from the sea. Instead, Janjucetus had the large eyes and ferocious teeth of a hunter.

Erich Fitzgerald with Janjucetus Dr Erich Fitzgerald holding the jaws of Janjucetus with Melbourne Museum's massive Blue Whale skeleton in the background.
Image: Jon Augier
Source: Museum Victoria
 

There are two key changes in the skull that permit the filter feeding of modern whales. The first is a lower jaw that can split down the middle. In humans, the seam (or symphysis) where the two halves of the jawbone meet at our chin is fused, thus our jaws are rigid. In contrast, baleen whales have greatly elongated jawbones that do not meet in the middle. The second change is in the width of the upper jaw; baleen whales have evolved a wide mouth, allowing them to engulf massive volumes of water.

"Previously it was thought that the origins of both features were intimately linked to filter-feeding and that's what differentiated baleen whales from toothed whales and dolphins," explains Erich. His research has just overturned this theory since Janjucetus had a wide upper jaw yet its lower jaw had a tightly connected, immobile symphysis. "So, the loose symphysis is not typical of all baleen whales, it's a later innovation. The earliest baleen whales could not expand and contract their lower jaws so were anatomically incapable of filter-feeding, yet they had these wide upper jaws."

Jaws of Janjucetus The fossilised jaws of Janjucetus, clearly showing the immobile symphysis at the tip.
Image: Jon Augier
Source: Museum Victoria
 

What Erich describes is an elegant example of an exaptation, where a feature evolved to serve a particular function but was later co-opted into a new role. Erich believes that its wide jaw helped Janjucetus to suck in large singe prey items, such as squid or fish, and didn't evolve for filter-feeding at all.

Says Erich, "Charles Darwin reflected upon this in The Origin of Species. He wondered how you could go from a whale that has big teeth like Janjucetus does and catching fish and squid one at a time, to something like a modern Blue Whale that feeds en masse. This is the kind of fossil palaeontologists dream of finding because it shows a transitional form."

"It's an exciting discovery, but actually not as surprising as you might think," concludes Erich. "Evolution by natural selection implies that we should expect to find these kinds of fossils in the rocks." The next question he looks forward to answering is how whales shifted from suction feeding to filter-feeding. "I think we're really close to finding a transitional series of fossils that illuminate this."

Erich's paper about this discovery, 'Archaeocete-like jaws in a baleen whale', is published today in Biology Letters.

Links:

Video: Erich discusses whale evolution

MV News: Ferocious fossil

Dr Erich Fitzgerald

Baleen and toothed whales

About this blog

Updates on what's happening at Melbourne Museum, the Immigration Museum, Scienceworks, the Royal Exhibition Building, and beyond.

Categories