Two very old whales are telling us a fascinating story. They are fossils—a male and a pregnant female—that were discovered in Pakistan in 2000 and 2004. Researchers say that these 47.5-million-year-old cetaceans are beginning to reveal that, without a doubt, our modern whales originated from terrestrial ancestors.
The skeleton of the fetus is the first to be found of an extinct whale in the group known as Archaeocetes. It represents a new species that has been named Maiacetus inuus (Maiacetus means “mother whale,” and Inuus was a Roman fertility god). What makes this discovery so important is that the fetus is positioned for a headfirst delivery, similar to that in land mammals but unlike that in modern whales. This indicates that these whales gave birth on land.
The fossil whales have large teeth, suited for catching and eating fish. The animals probably made their living in the sea, coming onto land only to rest, mate and give birth. Like other whales in the group Archaeocetes, Maiacetus had four legs modified for foot-powered swimming. Although they were capable of supporting their weight on their flipper-like limbs, they probably couldn’t travel very far on land. But they clearly were living at the edge, where shore meets sea.
Liking the land
Until fairly recently, the history of the earliest whales was almost unknown. Then in 1979, University of Michigan’s paleontologist Dr. Philip Gingerich and his team found a whale skull embedded in rock in Pakistan. Dr. Gingerich named the 48.5-million-year-old whale ancestor “Pakicetus,” and some scientists speculate it looked like a modern seal or sea lion.
Unlike current whales, however, Pakicetus lacked today’s typical whale ear, backbone and limbs. In living whales, the ears contain large sinuses that can be filled with blood, allowing them to maintain pressure while diving. Modern whales also transmit sound vibrations to the inner ear by the use of a pad of fat, which allows them to hear directionally underwater. Pakicetus lacked both of these features, indicating that it was unable to dive deeply and that it could not hear well when submerged. Since the Pakicetus bones were found in deposits laid down at the mouth of a river on the shore of a shallow sea—where the opportunities for deep diving would be limited—these anatomical features make sense.
In 1990, Gingerich found another fossil in Egypt, a specimen of Basilosaurus that lived 40 to 34 million years ago, again corroborating the whale-land connection. Like his Pakicetus find, Gingerich’s new specimen included a four-foot long skull and a number of ribs, all of which bore the unmistakable characteristics of whales. But more importantly, Gingerich also found a femur, a kneecap, both lower leg bones, a number of anklebones and three toes, demonstrating that Basilosaurus had complete rear limbs. Although the limbs were fully formed and functional, however, they did not connect to the whale’s pelvic bones. Thus, they couldn’t support the animal’s weight out of water. Basilosaurus, therefore, was a whale with a prominent rear leg, but one that was adapted to a marine existence.
But loving the sea
Between Pakicetus and Basilosaurus, then, the new 2000 and 2004 discoveries of Maiacetus inuus occupy an intermediate position on the evolutionary path that whales traversed as they made the transition from full-time land dwellers to devotees of the deep.
Going back even further, whales, we believe, developed from a land mammal that was a wolf-like creature, called Sinonyx. Over millions of years, front legs became fins, rear legs disappeared and bodies lost fur. It seems incongruous today to think of whales living on the land, but this mental picture of species flowing back and forth across Earth’s mediums intrigues me.
I have to wonder if this particular return to the water started out long ago as one wolf’s great sea adventure. What was he thinking? Was the land becoming too crowded for him or too dangerous? Or did he prefer the grace of a swimming life—or maybe even just have a craving for lobster?
Perhaps the only major difference between us and the planet’s other creatures is in the decisions we make: Whales happened to choose to leave us and terra firma behind.
Here’s to your adventures, in whatever corner of the world you find them,