The astounding transition came shortly after the rise of modern mammal groups, around 55 million years ago, during a hot period in the Earth’s history. Dinosaurs, other than birds, and large marine reptiles had disappeared some millions of years previously.
One group of hoofed mammals spent more and more time in the water, living on the abundant food there. Eventually they left land altogether—evolving into the fully aquatic whales. Take a look at a few of these early extinct whales below.
Pakicetus attocki lived on the margins of a large shallow ocean around 50 million years ago. Chemical information from some of these wolf-sized meat-eaters show that they ate fish.
Ear bones from Pakicetus show a feature that is unique to whales, placing it as the earliest known member of the modern whale lineage.
Ambulocetus natans means ‘walking whale that swims,” referring to its lifestyle both in water and on land. It probably swam by paddling with its legs and dived by tucking in its forelimbs and giving powerful kicks with its hind limbs, a distinctive way of moving in the water.
It seems Ambulocetus heard sound through its lower jaw bone. Sound passed from the jaw through soft tissues leading to the ear. This small adaptation foreshadows the remarkable sound-receiving system used by modern toothed whales.
Kutchicetus minimus, with its small, otterlike skeleton, lived between 43 and 46 million years ago. Like other early whales, Kutchicetus lived in tropical seas. Its fossils are found in sediment that formed in shallow seas sheltered by barrier islands.
How did Kutchicetus swim? Its hind legs are smaller than those of earlier whales and probably had little to do with propulsion. Perhaps that long tail helped, although there is no evidence of tail flukes as seen in living whales.
Kutchinecetus probably spent more time diving than Pakicetus. Hair was no longer needed in its aquatic environment. Blubber provided insulation and helped with body streamlining, which in turn aided swimming.
From the Museum’s collection: Andrewsarchus mongoliensis
This close whale relative, a member of the hoofed mammal group that includes hippos and whales, is known from a single skull discovered in 1923 during a Museum expedition to Mongolia and China. The rarely displayed specimen is now on exhibit in Whales: Giants of the Deep.
The exhibition features a cast of a Dorudon atrox skeleton and skull that represents a group of early fossil whales called basilosaurids. These whales were fully aquatic and lived between 34 and 40 million years ago. From a distance, living basilosaurids probably looked very much like modern species.
The nostrils, or blowhole, had moved toward the top of the head. The structure of the ear bones suggests that basilosaurids could hear well under water. Forelimbs became paddlelike flippers, while the hindlimbs were rudimentary. The pelvis had detached from the spinal column, freeing up the lower spine to power greater tail movement. Squared-off vertebrae at the tip of the tail would have supported flukes.
This group of whales came to inhabit all the oceans of the world.