August 23, 2011
Ancient Whales Had Directional Hearing
Scientists affiliated with the University of Michigan found that skewed skulls may have helped early whales find the direction of sounds in water.
Asymmetric skulls are a well-known characteristic of the modern whale group known as "odontocetes" or toothed whales.
These whales have modified nasal structures that help produce high-frequency sounds for echolocation.
The other modern whale group known as "masticates," or baleen whales, has symmetrical skulls and does not echolocate.
The scientists say that an extinct ancient whale species, archaeocetes, had symmetrical skulls that helped developed asymmetry later on in toothed whales.
A new analysis of archaeocete skulls by University of Michigan paleontologist Julia Fahlke and researchers shows that asymmetry evolved 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," Fahlke, who is working with scientist Philip Gingerich at the University of Michigan Museum of Paleontology, said in a press release.
"Modern whales don't chew their food," Fahlke said. "Toothed whales just bite it and swallow it, and baleen whales filter-feed. But archaeocetes have characteristic wear patterns on their teeth that show that they've been chewing their food."
She started studying the skull of Basilosaurus isis in hopes of piecing together how and what whales ate and how their eating habits changed over time.
Fahlke studied the skull of the 37-million-year-old whale by using a three-dimensional digital model generated from CT scans of the fossil that were acquired at the University of Michigan Medical School Department of Radiology.
Once Fahlke began working with the model, the jaws did not seem to fit together.
"Finally it dawned on me: maybe archaeocete skulls really were asymmetrical," she said.
She said she started examining archaeocete skulls and found "they all showed the same kind of asymmetry--a leftward bend when you look at them from the top down."
The researchers selected six well-preserved skulls that showed no signs of artificial deformation and measured their deviation from a straight line drawn from snout to back of skull.
They made similar measurements of the symmetrical skulls of artiodactyls, the group of terrestrial mammals from which whales evolved.
"Taken together, the six skulls deviate significantly from symmetry," Fahlke said in a press release. "Taken individually, four of them deviate significantly."
The other two appear asymmetrical, but their measurements are within the range of the symmetrical comparative sample.
"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."
The link between asymmetry and directional hearing is not unique to just whales.
"Owls have asymmetrical ear openings, which help them discriminate the rustling of leaves from the rustling of a mouse," Fahlke said in a press release.
"Such ability would also be helpful when you're trying to detect prey in the water, so we interpret that the same kind of mechanism was operating for archaeocetes."
The research was published in the journal Proceedings of the National Academy of Sciences (PNAS). It was funded by the National Science Foundation (NSF).
Image Caption: 3-D model of asymmetrical skull of archaeocete whale Basilosaurus isis, from CT scans. Credit: Julia Fahlke
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