By examining the Echo Hunter’s inner ear, the researchers found evidence of its ability to receive ultrasonic frequencies. A soft tissue structure called the basilar membrane, while not present in the fossil itself, was indicated by other parts of the ear to be of a size and thickness consistent with high-frequency hearing. Another part of the inner ear, a thin, bony structure within the cochlea, provided further evidence of the ability.

Earlier research had shown that odontocetes had developed the ability to produce high-pitched sounds relatively early in their evolution as a group.

“We had suspicions that they were echolocating but to really get down to a rough estimate of frequency, you really had to look in the inner ear in more detail and that’s where this project comes in,” said Jonathan Geisler, a professor at N.Y.I.T. and one of Dr. Churchill’s co-authors.

Echo Hunter would have been about the size of a modern day harbor porpoise, and would have resembled a small dolphin, Dr. Churchill said. Its remains indicate that it lived in warm coastal waters and preyed upon small fish, using echolocation to help it forage in murky, sediment-filled waters.

Mark D. Uhen, a professor at George Mason University who in 2008 named the whale family to which the new fossil belongs, said that previous research had suggested that the earliest toothed whales could echolocate, but that the new paper provided a clearer picture.

“They’re sort of triangulating when and how the high-frequency part of echolocation in odontocetes first arose,” he said.