August 18, 2011

Fossil Sheds Light On Evolution Of Whales’ Mouths

Scientists have identified a critical step in the evolution of filter-feeding whales' enormous mouths.

These whales, otherwise known as baleen whales or mysticetes, have feeding adaptations that are unique among mammals, in that they can filter small marine creatures from huge volumes of water.  The whales accomplish this by using their "loose" lower jaw joints, which enable them to produce a vast filter-feeding gape.

A new study of this ancient jawbone has overturned a long-held belief about how baleen whales evolved, finding that nature's largest mouths likely evolved to suck in large prey rather than to engulf plankton-filled water.

The researchers from Australia and the United States found that the fossilized prehistoric jaw differed greatly from the mouths of today's baleen whales.

In modern whales, the lower jaw does not unite at the "chin", but instead consists of a specialized jaw joint that allows each side to rotate.  By having two curved lower jawbones that rotate in this manner, modern baleen whales are able to create vast gapes to take in large quantities of water and prey.

The study provides "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," lead researcher Dr. Erich Fitzgerald from the Museum Victoria in Melbourne, Australia, told BBC News.

However, it is important to note that the fossilized whale, dubbed Janjucetus hunderi, did have a wide upper jaw, something Dr. Fitzgerald said was the earliest step in the evolution of modern whales' enormous mouths.

Dr. Fitzgerald charted the anatomical features of whales on an "evolutionary tree" - from Janjucetus hunderi to today's blue whale.

"I was able to discover the sequence of jaw evolution from the earliest whales to the modern giants of the sea," he told BBC News reporter Victoria Gill.

The chart showed 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 "ordinary" teeth, and had not yet evolved its comb-like baleen.

The fossilized jawbone analyzed in the study was discovered in the 1970s in a coastal town in Victoria, Australia.

"I first saw [it] while visiting a private collection in 2008," said Dr. Fitzgerald.

"I immediately recognized the characteristic shape of the lower jaws of a whale."

Researcher Jeremy Goldbogen from the Cascadia Research Collective in Washington, an expert in the feeding strategies of modern whales, described bulk filter feeding as "one of the most fascinating adaptations in the animal kingdom".

"An important point to note is that bulk filter feeding using [rotating jawbones] does not necessarily mean that suction is not used," he told BBC's Gill.

"A prime example of this are grey whales which are notorious suction filter feeders," he noted.

Dr. Fitzgerald described the whales' mouths as an elegant example of an exaptation, in which a feature evolved to serve a particular function but was later co-opted into a new role.

He 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.

"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," he said in a press release.

"This is the kind of fossil paleontologists dream of finding because it shows a transitional form."

"It's an exciting discovery, but actually not as surprising as you might think," he concluded.

"Evolution by natural selection implies that we should expect to find these kinds of fossils in the rocks."

The findings were published Wednesday in the journal Biology Letters.


Image 1: Illustration of the biggest mouth in history at work. The Blue Whale can expand its mouth to gulp huge volumes of krill-filled water. Credit: Carl Buell/Museum Victoria

Image 2: The fossilised jaws of Janjucetus, clearly showing the immobile symphysis at the tip. Credit: Jon Augier/Museum Victoria


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