Fossil Whale Shows Early Evolution of Sonar

Food-finding by sound evolved in toothed whales after they'd split from their baleen brethren.

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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 enemy submarines. In the early 20th century, it was a crucial technological breakthrough – but it was moldy old technology as far as whales are concerned.

These marine mammals have been using echolocation - bouncing high-frequency sounds off underwater objects - to find prey for tens of millions of years, which can now be proved. U.S. 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, that lived about 28 million years ago.

The discovery suggests that echolocation evolved in toothed whales 32 million to 34 million years ago, the scientists said.

The evolution of sonar came after whales split into two major cetacean groups around 35 million years ago: toothed whales, including the likes of the sperm whale, killer whale, dolphin and porpoise; and toothless baleen whales that are filter feeders, straining food like krill from the ocean.

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

The charming Cotylocara, whose fossilized remains include a 22-inch skull, neck vertebrae and ribs, was about 10 to 11 feet long. It probably swam in a shallow ocean environment, feeding on fish and squid, Geisler said.

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

'An extinct family'

While Cotylocara looked a bit like a midget modern-day toothed whale, it was not related.

"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 vocalizations 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.

Whales are not the only animals that use echolocation. Bats, which also first appeared more than 50 million years ago, use it while flying to pinpoint insects and other prey.

The first whales appeared more than 50 million years ago, arising from wolf-size land dwellers. Whales gradually became better suited to sea life and grew larger - one called Basilosaurus that lived about 40 million years ago was at least 56 feet (17 meters) long. Echolocation was a later adaptation.

The fossilized skull of the 28-million-year-old Cotylocara macei, the earliest known whale to use echolocation.Credit: Reuters

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