Modern whales have many adaptations that allow them to overcome these obstacles. How these features developed, however, remained a mystery until the new fossil discoveries.
The finds enabled Thewissentogether with his graduate student, Sirpa Nummela, and a team of biologists and paleontologiststo document the evolutionary transition of whale ears.
From Air to Water
The team found fossil specimens of four groups of early whales from Pakistan, which allowed them to interpret how the ear evolved as whales took to the water. (These detailed fossils include ear bones as well as marks on the skull that hint at how those ears worked).
The earliest ancestors of whales, found in 50 million-year-old sediments, bore little resemblance to modern whales. These animals, known as pakicetids, were wolf-size terrestrial carnivores. They were most closely related to cows, pigs, antelopes, and other hoofed animals.
Skull features give strong evidence that pakicetids were the ancestors of whales. New fossils reveal that pakicetids had ears similar to those of terrestrial mammals, however, and would have had very poor hearing underwater, according to Thewissen.
Researchers have also recently found new fossils for later intermediate species of whales associated with 43- to 46-million-year-old deposits. Fossils from these species, known as remingtonocetids and protocetids, revealed that they had developed some of the features of the modern whale ear.
Remingtonocetids had short legs, long tails, and long snouts. Some were the size of river otters. Protocetids had powerful hind limbs for swimming and were the first whales to disperse to tropical oceans across the world.
Both of these ancient whale ancestors had developed a system whereby sound vibrations were detected from the water and transmitted to the ear via a large fat pad in the lower jaw. (The animals still retained the air-filled outer ear canal for hearing on land, however.)
Analysis of specimens of one more group of whalesbasilosauroids, which lived around 40 million years agoreavealed that they had developed one of the final defining features of modern whale ears.
Unlike their semi-aquatic predecessors, the massive, snakelike basilosauroids had tiny residual hind limbs and would not have been able to leave the water.
The mammals, which grew up to 60 feet (18 meters) in length, possessed air-filled sacs that insulated each ear from sound on all sidesexcept the left or right side of the head. This adaptation allowed high-definition, directional hearing underwater and is used by whales today, Thewissen said.
Annalisa Berta, an expert on the evolution of marine mammals at San Diego State University in California, said the new study on whale-ear evolution "elucidates how a land-mammal ear capable of hearing airborne sounds was transformed into the precise, directional underwater ear exemplified by modern toothed whales."
"The evolution of the ear and hearing in whales adds to an impressive suite of [physical] transformations, such as teeth and limbs, that document the remarkable transition of whales from land to sea," she added.
Zhe-Xi Luo, a vertebrate paleontologist at the Carnegie Museum of Natural History in Pittsburgh, Pennsylvania, described the new study as "elegant" and said it "demonstrates for the first time that [intermediate species such as remingtonocetids] had a fascinating mosaic of primitive characters for hearing sound in air, and advanced characters for hearing sound underwater."
Thewissen, meanwhile, noted that his study of how the whale ear developed only recently become possible as "fossils have slowly accumulated"and because the mechanism of hearing in modern whales has been explained only in the last decade.
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