Whales: From the "ear of the earth" to the "water ear"

Development of the hearing system on fossils reconstructed

Middle ear of fossil whale Remingtonocetus Lauren Stevens
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Whales and dolphins come from originally land-dwelling mammals. Now scientists have used fossils for the first time to understand how the ear has adapted to this step from land to water. The study published in Nature is based on skull analyzes of four primeval whale species.

"This study of the early evolution of whales demonstrates the changes that took place in the outer and middle ear, which were necessary for the transition from land to water life, " said Rich Lane, head of the National Science Foundation's Geology and Paleontology Program. which promoted the international project.

The ear is one of the most important sense organs for modern toothed whales, as these whales locate their prey via reflected sound waves. Without your ability to pinpoint the direction of sound, these whales would starve to death.

Based on the study of four different fossil whale groups, the new study shows how the hearing has developed towards this capability. The earliest whale species, the Pakicetids, populated the primeval oceans some 50 million years ago. They still used the same sound transmission system as the land mammals and therefore had only a very limited hearing under water. Their successors, the Remingtonocetiden and Protocetiden, who lived about 43 - 46 million years ago, still retained the land mammal system, but developed parallel to a new hearing system.

"The fossils document how the whales' hearing has changed, starting with the ear fossils of their ancestral ancestors and ending with the hearing of the near-modern whales, " explains Hans Thewissen, an anatomist at Northeastern Ohio Universities College of Medicine (NEOUCOM) and one the two study leaders. display

The later whales could already hear much better under water than the Pakicetiden, at the same time but also record sound over the air. The retention of both systems, however, brought significant performance losses.

Only with the development of the Basilosauroids around 40 million years ago, the old land mammal ear disappeared and the modern sound detection system of today's Cetacaea began its development. Although the basilosauroids still had no real sound localization, their ear was already a big step towards a refined hearing under water.

(National Science Foundation, 17.08.2004 - NPO)