The origin of whales (order Cetacea) is one of the best-documented examples of macroevolutionary change in vertebrates1,2,3. As the earliest whales became obligately marine, all of their organ systems adapted to the new environment. The fossil record indicates that this evolutionary transition took less than 15 million years, and that different organ systems followed different evolutionary trajectories. Here we document the evolutionary changes that took place in the sound transmission mechanism of the outer and middle ear in early whales. Sound transmission mechanisms change early on in whale evolution and pass through a stage (in pakicetids) in which hearing in both air and water is unsophisticated. This intermediate stage is soon abandoned and is replaced (in remingtonocetids and protocetids) by a sound transmission mechanism similar to that in modern toothed whales. The mechanism of these fossil whales lacks sophistication, and still retains some of the key elements that land mammals use to hear airborne sound.
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We thank E. Blum, K. Grecco and F. Spoor for help with CT scans; W. Lancaster for information about basilosauroid ossicles; M. Filon for artwork; and S. Hemilä, S. Madar, T. Reuter and L. Sundström for commenting on the manuscript. J. Schiebout (Louisiana State University) and H. Baagøe and M. Andersen (Zoological Museum, University of Copenhagen) loaned specimens. The Geological Survey of Pakistan assisted in the collection and loan of some of the fossils. Financial support was provided to J.G.M.T by the National Science Foundation and to S.B. by the Department of Science of Technology of India.
Rostrolateral view of RUSB 2828, with tympanic.
Rostrolateral view of RUSB 2828, without tympanic.
Ventral view of RUSB 2828, with tympanic.
Ventral view of RUSB 2828, without tympanic.
List of specimens studied for this research..
Description of how the drawings in Fig. 1f-g were prepared, and the origin of data presented in Fig. 3.
Ossicular masses, tympanic membrane and/or tympanic plate areas for those data points of Fig. 3 for which the numerical values have not been published elsewhere.
Background information for the sound transmission mechanisms in odontocetes and mysticetes, in connection to Fig. 2. Additional information as for the systematic relationships in Fig. 4.
About this article
Journal of Mammalian Evolution (2012)