A study carried out by Professor Jonathan Geisler at the New York Institute of Technology College of Osteopathic Medicine has provided evidence for a primitive form of the biological sonar, known as echolocation, which existed in a relative of present day odontocetes (toothed whales and dolphins) that lived around 28 million years ago. The study was published yesterday in Nature.
Echolocation involves the emission of calls that produce sound waves, which travel into the environment and eventually bounce off objects. The animal then listens to echoes that return. This allows the animal to detect how far away a particular object is by the length of time it takes for the echo to return. The animal can also determine the location, size and movement of objects. This allows the animal to navigate through dark environments where vision is poor, like muddy waters or dark caves. Echolocation is used by numerous animals other than odontocetes, including bats, birds and shrews. Echolocation is usually at such a high pitch that it cannot be detected by humans.
Discoveries of complex adaptations are always exciting, and it is often found that they evolve in a step-wise, gradual fashion. Through the use of fossil records, we can observe small changes over time which lead to particular features that adapted the organism to its changing environment. Odontocetes are unique in their mechanism of echolocation; they produce high-frequency vocalisations in a constricted area of nasal passage just beneath the blowhole called the phonic lips. This vocalization is then modulated by a large fatty organ named the melon. Until now, the origins of echolocation remained a mystery.
The researchers discovered an odontocete fossil in South Carolina, which was from the Oligocene, part of the Paleogene period that extends from around 40 to 23 million years ago. The fossil, Cotylocara macei, presented several features which suggested it was capable of echolocation. This included a thick and downturned rostrum, and a cavity at the top of its head which could potentially have served to store air and reflect sound. They also discovered a bowl-like area of bone near the nasal openings which could have served to reflect sound and thus improve echolocation in this animal.
Their analysis provided evidence that a primitive form of echolocation in odontocetes evolved in the early Oligocene, not long after they split off from the ancestors of mysticetes (filter-feeding whales). Following this, the facial muscles known to modulate echolocation calls started to increase in size, consequently causing changes in the shape of the skull. The skull of Cotylocara macei is a unique discovery; nothing similar has been previously described.