A whale of a find: Fossil sheds light on cetacean sonar's origin

2014-03-13 12:49
Washington - The deadly threat posed by German submarines during World War One helped spur scientists to develop sonar, using underwater sound signals to locate objects like subs that might be taking aim with a torpedo.

In the 20th century, it was an important technological breakthrough.

But it was old technology as far as whales go. These marine mammals have been using echolocation, bouncing high-frequency sounds off underwater objects to find prey for tens of millions of years.

US scientists on Wednesday announced the discovery of the most ancient whale known to have used echolocation, a creature called Cotylocara macei, a bit larger than a bottlenose dolphin, which lived about 28 million years ago.

The discovery suggests that echolocation evolved in toothed whales, the group that includes modern day varieties like sperm whales, killer whales, dolphins and porpoises, perhaps 32 million to 34 million years ago, the scientists said.

That was relatively soon after whales, around 35 million years ago, split into two major cetacean groups, toothed whales that were active hunters and toothless baleen whales that were filter feeders, straining food like krill from the ocean.

Jonathan Geisler, an anatomy professor at New York Institute of Technology who led the research published in the journal Nature, called echolocation "an amazing trait".

"It's a sonar-like system which allows them basically to navigate and find food, particularly in waters where there's little light, either at great depth or in very turbulent waters with a lot of mud, like estuaries or around marshes", he added.

Cotylocara, whose fossilised remains include a 56cm skull, neck vertebrae and ribs, was about 3 to 3.4m long and probably swam in a shallow ocean environment, feeding on fish and squid, Geisler said.

The fossils were unearthed near Summerville, South Carolina, outside Charleston, said College of Charleston geology professor James Carew, another of the researchers.

'An extinct family'

While Cotylocara looked superficially like some smaller modern-day toothed whales, it was not closely related to them.

"This is a member of an extinct family that split off very early from other echolocating whales, dolphins and porpoises. They went extinct 25 million or 26 million years ago and they don't have any living relatives", Geisler said in a telephone interview.

Whales that use echolocation produce very high-frequency vocalisations through a soft-tissue nasal passage located between the blowhole and skull. Other mammals, including people, produce sounds using the voice box, or larynx, inside the neck.

When air is pushed through the whale's nasal passage, it produces extremely high frequency clicks, squeaks and squeals that then echo off objects in the water, enabling the whale to get a high-resolution audio image of its surroundings.

"They can 'see' the fish and then they know to swim in that direction to catch it", Geisler said.

The sound-producing mechanism is complex, with big muscles, air pockets and bodies of fat, all in a small facial area.

The sound is too high frequency for human ears to hear.

Modern-day whales that use echolocation possess a melon, or a fat-filled organ in the head, that focuses the sound wave. Geisler said he suspects that Cotylocara already had this organ.

The whale's genus name, Cotylocara, means "cavity head" in recognition of a very deep pocket atop its skull thought to be associated with an air sinus used in echolocation.

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