When Carnivores Took to the Water

When Carnivores Took to the Water - Transcript
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Carnivores are a highly diverse group of mammals, adapted to living on land and in water. Yet all modern carnivores share a common land-dwelling ancestor that lived 43 million years ago. How & when did the ancestors of otters, seals and sea lions take to the water?
Dr. Camille Grohé (Post-Doctoral Researcher, American Museum of Natural History):
My name is Camille Grohé. I'm a post-doctoral researcher at the American Museum of Natural History.
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Grohé studies mammalian carnivore evolution. She's tracing the evolutionary history of modern carnivores by looking at the inner ear.
Grohé:
The inner ear is important because it relates to hearing and also to detecting movement in space, and maintaining balance.
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The inner ear is a multi-purpose sensory organ.

Hearing

Detecting head motion and gravity

Balancing

Stabilizing gaze

Grohé:
Understanding the evolution of the inner ear through time and through transition, for example, from terrestrial mammals to aquatic mammals—like the seals and the sea lions—permits us to understand what are the morphological characters related to the adaptation to an aquatic environment.
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Grohé and her colleagues studied 31 modern carnivore species. The researchers found that modern terrestrial and semi-aquatic carnivores have distinctly shaped inner ears. Why the different shapes? Small morphological changes can indicate big adaptations. Each environment requires animals to move and balance in unique ways. As their ancestors adapted to these environments, their inner ears evolved different shapes.
Grohé:
Before the use of the CT scan we didn't have much information about the morphology of the inner ear.
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The soft inner ear decomposes quickly when an animal dies. But the bone around in inner ear, called the bony labyrinth, persists.
Enter technology. Using a CT scanner, Grohé isolates the bony labyrinth in a skull.
Grohé:
Some aquatic mammals, like seals, sea lions or otters, have a terrestrial ancestor. So with the fossil record, we're able to see this transition from a terrestrial to an aquatic environment.
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Grohé can compare the inner ear shape of an extinct carnivore to that of modern species. 

Aquatic carnivore

Terrestrial carnivore

Transitional species

By comparing extinct and modern species, scientists can locate evolutionary transitions, the source of diversity in the tree of life.