Hunting by ear. Cotylocara macei's fossilized skull shows that this 28-million-year-old whale might have used echolocation to find its prey.

James Carew and Mitchell Colgan; (Inset) Carl Buell

Hunting by ear. Cotylocara macei's fossilized skull shows that this 28-million-year-old whale might have used echolocation to find its prey.

Fossil Whale Offers Clues to Origins of 'Seeing With Sound'

When you’re trying to track a fish in the murky ocean, forget about using your eyes—use your ears. Dolphins, orcas, and other toothed whales—known as odontocetes—pinpoint their prey by producing high-frequency sounds that bounce around their marine environment and reveal exactly where tricky fish are trying to hide. But when did whales evolve this sonarlike ability, known as echolocation? A newly named, 28-million-year-old whale may hold the answer.

Found in South Carolina among rocks dating back to the Oligocene epoch and christened Cotylocara macei, the fossil whale is named after Mace Brown, a curator at the College of Charleston’s Mace Brown Natural History Museum in South Carolina who acquired the specimen for his private collection about a decade ago. It was in that private accumulation of fossils that Jonathan Geisler, a paleontologist at the New York Institute of Technology College of Osteopathic Medicine in Old Westbury, first saw the skull. “I knew it was special then,” he says.

The only known specimen of the early odontocete includes a nearly complete skull and jaw, three neck vertebrae, and fragments of seven ribs. It’s the skull that makes Cotylocara so remarkable. While the whale’s soft tissue rotted away long ago, the skull bones show several features—such as a downturned snout and a slight asymmetry of the skull—that suggest Cotylocara was one of the earliest whales to use echolocation, Geisler’s team reports online today in Nature.

The strongest pieces of evidence for this hypothesis, Geisler explains, are cavities at the base of the snout and on top of the skull that probably held air sinuses. “These air sinuses are thought to have important roles in the production of high-frequency vocalizations that living odontocetes use for echolocation,” Geisler says, possibly helping direct returning sound waves or store air that can be used to make continuous sound.

“I think the authors have a good case for inferring that Cotylocara had some ability to produce some sound from its forehead, just as living toothed whales do today,” says Nicholas Pyenson, a marine mammal paleontologist at the Smithsonian Institution’s National Museum of Natural History in Washington, D.C. But even if Cotylocara made those sounds, could it have heard them? Living whales have specialized ear bones that let them hear the high-frequency sounds bouncing off their prey. The only known skull of Cotylocara doesn’t have well-preserved ear bones, and, therefore, knowing whether the whale could have actually used echolocation for hunting is unclear. “Overall, the description of Cotylocara underscores the need to investigate the inner ear of fossil Oligocene cetaceans in much more detail, because that’s where the answer will be,” Pyenson says. 

Nevertheless, the whale’s probable sound-producing abilities give Cotylocara an important place in whale evolution. Whale’s biological sonar is thought to have evolved only once along the ancestral line leading to today’s toothed whales, Geisler notes. Cotylocara lies along that evolutionary stem, as do other Oligocene fossil whales that have already been found. The skull features that allowed Cotylocara to create sound, Geisler says, “can now be investigated in other fossil whales to more fully understand the evolution of echolocation.” For now, the evolutionary epic of whale echolocation is only just beginning to be heard.

Posted in Paleontology, Plants & Animals Whales