University of Michigan researchers have suggested that skewed skulls may have helped early whales discriminate the direction of sounds in water and are not solely, as previously thought, a later adaptation related to echolocation.
Asymmetric skulls are a well-known characteristic of the modern whale group known as odontocetes (toothed whales).
These whales also have highly modified nasal structures with which they produce high-frequency sounds for echolocation, a sort of biological sonar used to navigate and find food.
The other modern whale group, mysticetes (baleen whales), has symmetrical skulls and does not echolocate.
These observations led scientists to believe that archaeocetes, the extinct ancient whales that gave rise to all modern whales, had symmetrical skulls, and that asymmetry later developed in toothed whales in concert with echolocation.
But a new analysis of archaeocete skulls by U-M postdoctoral fellow Julia Fahlke and co-authors shows that asymmetry evolved much earlier, as part of a suite of traits linked to directional hearing in water.
“This means that the initial asymmetry in whales is not related to echolocation,” said Fahlke, who is working with Philip Gingerich, an internationally recognized authority on whale evolution, at the U-M Museum of Paleontology.
Fahlke began examining archaeocete skulls, and to her astonishment, “they all showed the same kind of asymmetry, a leftward bend when you look at them from the top down,” she said.
“This shows that asymmetry existed much earlier than previously thought, before the baleen whales and toothed whales split,” Fahlke said.
“This means that the earliest baleen whales must have had asymmetrical skulls, which later became symmetrical,” she concluded.
The finding has been published online in the Proceedings of the National Academy of Sciences during the week of August 22.