When Did Cetaceans Evolve Echolocation?

Modern whales include baleen whales (Mysticeti) and toothed whales (Odontoceti). Baleen whales are large, filter feeding whales that do not echolocate. Odontocete whales tend to be smaller in size, predatory, and are capable of producing high-frequency sounds used in echolocation. In addition, modern odonotcetes have an asymmetrical skull, where the bones of the skull roof extend posteriorly (telescoped) and are shifted to the left side of the skull (asymmetry) (Figure 1). This pronounced asymmetry is linked with the production of high frequency sounds and the reception of the returning echoes.

WhaleSkullDorsal
Figure 1. Dorsal view of the skull of a bottlenosed dolphin (Tursiops truncatus) showing the posterior position of the nasals, frontals, maxilla, and premaxilla bones (telescoping) and the asymmetry of the bones surrounding the blow hole. (copyright Jim Ryan)

Modern mysticete and odontocete whales both arose from Eocene
archaeocete whales. The skulls of archaeocete whales were presumed to by symmetrical. However, recent evidence casts doubt on that assumption. Julia Fahlke, working with Philip Gingerich, Robert Welsh, and Aaron Wood report in the Proceedings of the National Academy of Science that some archaeocete skulls show distinct asymmetry without telescoping (Figure 2).

whaleskull2
Figure 2. A graph of the mean deviation of the dorsal midline suture for 24 artiodactyl skulls (top row), which show a high degree of symmetry, compared with 6 archaeocete whale skulls (bottom, two protocetids and four basilosaurids). All six archaeocete species show asymmetry to the same side of the skull. (From Fahlke et al., 2012).

The researchers propose that Eocene archaeocetes evolved cranial asymmetry “as part of a complex of traits linked to directional hearing.” Some of the other traits associated with hearing high frequency sounds include the thinning of the pan-bone of the lower jaws (Figure 3), and isolation of the ear region from the rest of the skull.

whalejaw
Figure 3. Dentary bones of a late Eocene archaeocete whale (Basilosaurus isis) showing the relative bone thickness. The thinnest region represents the location of the pan bone. (From Fahlke et al., 2012)


The authors propose the following sequence of events: 1) Eocene archaeocete whales evolved modest cranial asymmetry along with a change to more directional hearing of high frequency sounds. They also evolved thinner pan bones in the jaw to aid the reception of water borne sounds. 2) Later on, Oligocene odontocete whales evolved more refined high frequency echolocation, which further shifted the cranial roofing bones posteriorly (telescoping) and to the left side of the skull (asymmetry). 3) At the same time, Oligocene mysticete whales lost the skull asymmetry of their archaeocete ancestors as their skulls became modified for a bulk filter-feeding mode with low-frequency hearing. Thus, cranial asymmetry is probably an ancestral condition in whales.


References

Fahlke, J., Gingerich, P., Welsh, R., & Wood, A. (2011). Cranial asymmetry in Eocene archaeocete whales and the evolution of directional hearing in water Proceedings of the National Academy of Sciences, 108 (35), 14545-14548 DOI: 10.1073/pnas.1108927108