Fossil reveals ancient sonar system
The development of sonar technology in World War I to locate enemy submarines was considered a technological breakthrough. Yet in the marine world the revelation was already yesterday's news, a study reveals.
A team of US scientists today 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, published in the journal Nature, suggests 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.
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.
Lead researcher Jonathan Geisler, an anatomy professor at New York Institute of Technology, says echolocation is "an amazing trait".
"It's a sonar-like system that 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 says.
Cotylocara, whose fossilised remains include a 56 centimetre skull, neck vertebrae and ribs, was about three to 3.4 metres long and probably swam in a shallow ocean environment, feeding on fish and squid, Geisler says.
The fossils were unearthed near Summerville, South Carolina, outside Charleston, says College of Charleston geology professor James Carew, another of the researchers.
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 says.
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 says.
The sound-producing mechanism is complex, with big muscles, air pockets and bodies of fat - all in a small facial area.
The sound frequency is too high 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 says he suspects 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 17 metres long. Echolocation was a later adaptation