The (Fossil) Whale Tail That Tells Tales

Evolutionary biologists use three families of mammals to demonstrate that we do, in fact, have the “transitional fossils” necessary to provide support for natural selection: horses, humans, and whales. While human evolution is my specialty, several recent stories about fossil whales have piqued my interest and illustrate how much information can be gleaned from a single fossil.

First, in February, a team from the University of Michigan published a remarkable find—a female whale that died 48 million years ago with a term fetus still in utero. The article discusses what can be learned about the species’ behavior based on body and tooth size differences between the female and an adult male found nearby, but just briefly mentions that the birth position of the fetus is also important. (In the image to the right, the maternal bones are colored pink, and the fetal bones are colored blue, illustrating the head-first position of birth, unusual for aquatic whales since they are born tail-first.) Since whales are marine mammals, infants are born tail-first, so they can end up swimming in the same direction as their mother immediately, a clear evolutionary advantage for both mother and offspring. Terrestrial mammals tend to be born head first, since our easily-splayed limbs can make any other position exceptionally dangerous. This fossil whale was giving birth like a terrestrial mammal, indicating that it was one of the first whale species to emerge from the ocean—a “missing link” between aquatic and terrestrial mammals if ever there was one (and there wasn’t!).

A second amazing whale find was published just last week. Paleontologists reported finding a partially healed broken jaw in a 14-million-year old fossil whale. The injury mimicked those seen in modern whales hit by large fishing boats, but Miocene apes weren’t driving boats around modern-day Virginia. The jaw was broken on the left side, indicating that perhaps the animal collided with something while turning left. Bottom-feeding whales (less common than the filter feeding types) could have come into with large objects on the sea floor with enough force to break bones. Interestingly, 80% of bottom-feeding marine mammals alive today consistently turn to the right when trolling sediment from the ocean floor for food. This individual must have been turning to the left if the colliding-with-something-big-while-feeding hypothesis holds. This may seem like a bit of a stretch (and not just to you—other paleontologists aren’t embracing the idea fully), but it’s a fascinating example of the possible conclusions that can be drawn from a single partially healed fossil.

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