Discovery: Skewed skulls helped early whales tell where sounds were coming from U-M’s
Julia Fahlke explains how whales hear and how she made the surprising skull discovery.
Julia Fahlke: So this is the skull of a Toothed whale of a spotted dolphin. Toothed whales
hear using a fat pad that is located in their lower jaws and they can receive sound waves
through this opening here and via vibration of a very thin bone on the outside of the
lower jaws and sound waves get then transmitted via the fat pad to the ear region.
You have two groups of modern whales; the mysticetes that are the baleen whales and
the odontocetes that are the toothed whales. And the baleen whales have symmetrical skulls
and the toothed whales have asymmetrical skulls. The toothed whales use echo location to navigate
and to find prey, and they have certain structures in the nasal complex that allow them to do
that.
So it was commonly assumed that asymmetry evolved with echo location in odontocete whales.
But now we found out that the archaeocetes -- this is the group of whales that gave rise
to both baleen and toothed whales -- have asymmetrical skulls. We think that asymmetry
helped the archaeocete whales to figure out where underwater sounds came from that they
heard.
When I first came here, I wanted to study something completely different. I wanted to
study tooth forming function in archaeocete whales and how that changes over time during
the evolution from a land mammal to a mammal that lives in the sea.
When you look at modern whales today, the baleen whales, they filter-feed and the toothed
whales catch their prey and eat it whole or in big pieces. But archaeocete whales have
certain tooth wear features on their teeth that showed that they did chew their food,
and I wanted to find out how they did it and what they were eating.
As we created three-dimensional digital model of the skull of the basilosaurus from CT scans
and I wanted to measure the orientation of those wear features with regard to the skull,
but we saw that the skull of the basilosaurus was asymmetrical.
But like everybody else before us we thought it was due to deformation that had happened
during fossilization. Animal remains get buried, they get compressed and they can get deformed.
We created a three-dimensional digital model of the skull of the basilosaurus and we saw
that deformation or what we thought was deformation and we digitally straightened that model,
and I wanted to align models of the lower jaws to the model of the cranium. But I saw
that the lower jaws will not occlude and the measurements that I had taken on the straightened
model would just not make sense. And I got very frustrated with it and I stood right
here in my office in front of this skull cast of the sorceress and I was pouting and I was
looking at it, staring at it, and I was actually talking to it, why on earth are you not symmetrical.
And it took me, probably a quarter of an hour of staring until it finally dawned on me that
it’s maybe because you’re not symmetrical. Archaeocetes do not show any evidence of structures
that are related to echolocation, so we are very sure that they did not echolocate.
So taking together all of our findings mean that asymmetry evolved much earlier in whale
evolution than previously thought and that the initial asymmetry is not connected to
echolocation but to directional hearing in water.
And what I would like to study next is when asymmetry got lost in the baleen whales and
when they started to hear lower frequencies and how this is connected.