In spite of our advances in understanding of the pattern of
cetacean origins, it remains unclear which process caused
this pattern: Why did cetaceans enter the oceans? The
availability of rich new food sources has been proposed as a
reason for the cetacean entry into the water, but this is
unlikely, given that cetacean ancestors already lived in very
shallow freshwater. The new find of aquatic behaviors in
raoellids suggests that these animals used the water as a
refuge against danger. Raoellid teeth are very different from
those of early cetaceans, suggesting that a dietary shift took
place after the habitat change and may have been critical in
the early diversification of cetaceans but not in their entry
into the water. On the other hand, it is not clear what
raoellids ate, and neither raoellid nor early cetacean
dentitions have good modern analogs. It has been suggested
that early cetaceans ate fish (O'Leary and Uhen 1999).