Skip to main page content
U.S. flag

An official website of the United States government

Dot gov

The .gov means it’s official.
Federal government websites often end in .gov or .mil. Before sharing sensitive information, make sure you’re on a federal government site.

Https

The site is secure.
The https:// ensures that you are connecting to the official website and that any information you provide is encrypted and transmitted securely.

Access keys NCBI Homepage MyNCBI Homepage Main Content Main Navigation
. 2011 Aug 30;108(35):14545-8.
doi: 10.1073/pnas.1108927108. Epub 2011 Aug 22.

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

Julia M Fahlke  1 , Philip D Gingerich, Robert C Welsh, Aaron R Wood
Affiliations

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

Julia M Fahlke et al. Proc Natl Acad Sci U S A. .

Abstract

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.

PubMed Disclaimer

Conflict of interest statement

The authors declare no conflict of interest.

Figures

Fig. 1.
Fig. 1.
Asymmetry in the evolution of whales. (A) Evolutionary relationships between terrestrial Artiodactyla, Eocene archaeocetes, and modern Mysticeti and Odontoceti. Cranial asymmetry is present in archaeocetes and Odontoceti, but absent in artiodactyls and Mysticeti. Archaeocetes are represented by Artiocetus, Mysticeti are represented by Balaenoptera, Odontoceti are represented by Tursiops, and terrestrial artiodactyls are represented by Elomeryx. (B) Skull of the protocetid archaeocete A. clavis (GSP-UM 3458) in dorsal view showing maxillary and frontal sinuses visible in a 3D micro-CT reconstruction. Note the rightward deviation of the midcranium. fc, caudal frontal sinus; fr, rostral frontal sinus; ml, lateral maxillary sinus; mm, medial maxillary sinus.
Fig. 2.
Fig. 2.
Mean relative deviation of the dorsal midline suture from RC (mean δx/RC) for archaeocete crania. Means are calculated for lateral deviations (mm) at nine evenly spaced points along RC (mm) for each specimen. Mean δx/RC for 24 artiodactyl specimens (included taxa are given in Table S1) serve as modern symmetrical comparison and show a normal distribution with zero mean and standard deviation s = 0.0045 (green curve; one and two standard deviation intervals highlighted in yellow). Note that archaeocete midline sutures deviate consistently to the right, and that the deviation is statistically significant in four of six cases.
Fig. 3.
Fig. 3.
Torsion of the cranium of the middle Eocene protocetid Artiocetus clavis (GSP-UM 3458). Absolute deviations of dorsal and ventral midline sutures (black lines) are shown in dorsal and ventral view, respectively. Torsion is documented by successive deflection angles, where each is the angle between a line connecting dorsal and ventral midline points, measured from the xz-plane, for 1,000 successive values of x. Note that torsion is clockwise and greatest in the rostrum.
Fig. 4.
Fig. 4.
Bone thickness of the lateral wall of the left and right (mirrored) dentaries of late Eocene B. isis (WH-74). Note that the thinnest area (pan bone, outlined in black) lies in front of the mandibular foramen (red line) in each dentary. Note also the asymmetrical positions of the pan bones.
See this image and copyright information in PMC

References

    1. Feldhamer GA. Mammalogy: Adaptation, Diversity, Ecology. Baltimore: Johns Hopkins University Press; 2007. p. 643.
    1. Miller GS. The telescoping of the cetacean skull. Smithsonian Miscellaneous Collections. 1923;76:1–70.
    1. Slijper EJ. Die Cetaceen, Vergleichend-Anatomisch und Systematisch (Cetacea, comparative anatomy and systematics) Capita Zoologica. 1936;6–7:1–590. (in German)
    1. Ness AR. A measure of asymmetry of the skulls of odontocete whales. J Zool. 1967;153:209–211.
    1. Mead JG. Anatomy of the external nasal passages and facial complex in the Delphinidae (Mammalia: Cetacea) Smithson Contrib Zool. 1975;207:1–72.

Publication types

LinkOut - more resources