Cranial asymmetry in Eocene archaeocete whales and the evolution of directional hearing in water

Eocene archaeocete whales gave rise to all modern toothed and baleen whales (Odontoceti and Mysticeti) during or near the Eocene-Oligocene transition. Odontocetes have asymmetrical skulls, with asymmetry linked to high-frequency sound production and echolocation. Mysticetes are generally assumed to...

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Published inProceedings of the National Academy of Sciences - PNAS Vol. 108; no. 35; pp. 14545 - 14548
Main Authors Fahlke, Julia M, Gingerich, Philip D, Welsh, Robert C, Wood, Aaron R
Format Journal Article
LanguageEnglish
Published United States National Academy of Sciences 30.08.2011
National Acad Sciences
Subjects
Acoustic echoes
Animal anatomy
Animal sound production
Animals
Archaeocetes
Auditory perception
Biological Evolution
Biological Sciences
Bones
Cetacea
Cranium
ears
Evolution
fish
Frequencies
hearing
Hearing - physiology
jaws
Marine
Mysticetes
Mysticeti
Odontoceti
Predation
Skull
Skull - anatomy & histology
Sound
Ultrasonic technology
Whales
Whales & whaling
Whales - anatomy & histology
Whales - physiology
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Summary:Eocene archaeocete whales gave rise to all modern toothed and baleen whales (Odontoceti and Mysticeti) during or near the Eocene-Oligocene transition. Odontocetes have asymmetrical skulls, with asymmetry linked to high-frequency sound production and echolocation. Mysticetes are generally assumed to have symmetrical skulls and lack high-frequency hearing. Here we show that protocetid and basilosaurid archaeocete skulls are distinctly and directionally asymmetrical. Archaeocete asymmetry involves curvature and axial torsion of the cranium, but no telescoping. Cranial asymmetry evolved in Eocene archaeocetes as part of a complex of traits linked to directional hearing (such as pan-bone thinning of the lower jaws, mandibular fat pads, and isolation of the ear region), probably enabling them to hear the higher sonic frequencies of sound-producing fish on which they preyed. Ultrasonic echolocation evolved in Oligocene odontocetes, enabling them to find silent prey. Asymmetry and much of the sonic-frequency range of directional hearing were lost in Oligocene mysticetes during the shift to low-frequency hearing and bulk-straining predation.
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Author contributions: J.M.F., P.D.G., and A.R.W. designed research; J.M.F., P.D.G., and R.C.W. performed research; A.R.W. contributed new analytic tools; J.M.F., P.D.G., R.C.W., and A.R.W. analyzed data; and J.M.F. and P.D.G. wrote the paper.
Edited by David B. Wake, University of California, Berkeley, CA, and approved July 28, 2011 (received for review June 2, 2011)
ISSN:0027-8424
1091-6490
DOI:10.1073/pnas.1108927108