New fossil suggests echolocation evolved early in whales

A roughly 27-million-year-old fossilized skull echoes growing evidence that ancient whales could navigate using high-frequency sound.

Discovered over a decade ago in a drainage ditch by an amateur fossil hunter on the South Carolina coast, the skull belongs to an early toothed whale. The fossil is so well-preserved that it includes rare inner ear bones similar to those found in modern whales and dolphins. Inspired by the Latin for “echo hunter,” scientists have now named the ancient whale Echovenator sandersi.
“It suggests that the earliest toothed whales could hear high-frequency sounds,” which is essential for echolocation, says Morgan Churchill, an anatomist at the New York Institute of Technology in Old Westbury. Churchill and his colleagues describe the specimen online August 4 in Current Biology.

Modern whales are divided on the sound spectrum. Toothed whales, such as orcas and porpoises, use high-frequency clicking sounds to sense predators and prey.

Filter-feeding baleen whales, on the other hand, use low-frequency sound for long-distance communication. Around 35 million years ago, the two groups split, and E. sandersi emerged soon after.

CT scans show that E. sandersi had a few features indicative of ultrasonic hearing in modern whales and dolphins. Most importantly, it had a spiraling inner ear bone with wide curves and a long bony support structure, both of which allow a greater sensitivity to higher-frequency sound. A small nerve canal probably transmitted sound signals to the brain.
“Scientists have long suspected that early toothed whales could produce the high-frequency sounds needed for echolocation based on features on their skulls,” says Travis Park of Monash University in Melbourne, Australia. Previous work points to early toothed whales sensing those high frequencies. Park and his colleagues reported in April in Biology Letters the discovery of a 26-million-year-old lone ear bone showing signs of high-frequency hearing. But it wasn’t connected to a skull and, thus, couldn’t be tied to a specific whale species.

Tracing inner ear features in CT scans of 24 ancient and modern whales, including E. sandersi, plus two hippos, whales’ closest living relatives, Churchill’s team ups the ante. Because primitive versions of the bony spiral and nerve canal appeared before the first known toothed whale, the researchers suggest that rudimentary high-frequency hearing might have emerged in the common ancestor of toothed and baleen whales at least 43 million years ago. If so, baleen whales lost their high-frequency hearing at some point. Determining whether that’s truly the case requires more analysis and a wider array of fossil data, says Park, who’s unconvinced.

But there is growing consensus that the first toothed whales could hear and produce sounds at high-frequency ranges. The skull of a 28-million-year-old toothed whale also suggests that such animals could make high-frequency calls (SN: 4/19/14, p. 6). “The next step is to look at when their brains got big enough to process echolocation signals,” says Nicholas Pyenson, a paleobiologist at the Smithsonian’s National Museum of Natural History who was not affiliated with the study. “This is great, but there’s more to be done.”