A common theme in evolution is that large animals descend from much smaller ancestors--and nowhere is this more true than in the case of multi-ton sperm and gray whales, whose ultimate forebears were small, dog-sized mammals that prowled the riverbeds of Asia 50 million years ago. Even more intriguingly, whales are a case study in the gradual evolution of mammals from fully terrestrial to fully aquatic lifestyles, with corresponding adaptations (elongated bodies, webbed feet, etc.) at various key intervals along the way. (See a gallery of prehistoric whale pictures.)
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Until the turn of the 21st century, the origin of whales was shrouded in mystery, with few fossil remains of early species. That all changed with the discovery of a huge trove of fossils in Pakistan, some of which are still being studied and described. These bones, which date from only 15 to 20 million years after the demise of the dinosaurs, prove that the ancestors of whales were closely related to artiodactyls (the even-toed, hoofed mammals typified by modern pigs and sheep).
The First Whales - Pakicetus, Ambulocetus and Rodhocetus
In most ways, Pakicetus (Greek for "Pakistan whale") was indistinguishable from the other medium-sized mammals of the early Eocene epoch: about 50 pounds or so, with long, dog-like legs, a long tail, and a narrow snout. Crucially, though, the anatomy of this creature's inner ears closely matches that of modern whales, the main "diagnostic" feature that places Pakicetus at the origins of cetacean evolution. One of Pakicetus' close relatives was Indohyus, an ancient artiodactyl with some aquatic adaptations, such as a thick, hippopotamus-like hide.
Ambulocetus ("walking whale") flourished a few million years after Pakicetus, and already showed some distinctly whale-like adaptations. Whereas Pakicetus was mostly terrestrial, occasionally dipping into lakes or rivers to find food, Ambulocetus had a long, slender, otter-like body, with webbed, padded feet and a narrow, crocodile-like snout. It was much bigger than Pakicetus--about 10 feet long and 500 pounds, much closer to a blue whale than a guppy--and probably spent a significant amount of time in the water.
Rodhocetus, named after the region of Pakistan where it was found, shows even further adaptations to an aquatic lifestyle. This early whale also led an amphibious lifestyle, crawling up on land to forage for food and (possibly) give birth. In evolutionary terms, though, its most telling feature is the structure of its hip bones, which weren't fused to its backbone and thus lent it greater flexibility when swimming.
The Next Whales - Protocetus, Maiacetus and Zygorhiza
The fossils of Rodhocetus and its predecessors have been found mostly in central Asia, but the larger prehistoric whales of the late Eocene epoch (which were able to swim faster and further) have been dug up in more diverse habitats. The deceptively named Protocetus (it wasn't really the "first whale") had a long, seal-like body, powerful legs for propelling itself through the water, and nostrils that had already begun to migrate halfway up it forehead--foreshadowing the blowholes of modern whales.
Protocetus appears to have shared one key characteristic with two roughly contemporary prehistoric whales, Maiacetus and Zygorhiza. The front limbs of Zygorhiza were hinged at the elbow, indicating that it crawled onto land to give birth, and a specimen of Maiacetus ("good mother whale") has been found with a fossilized embryo inside, positioned in the birth canal for terrestrial delivery. Clearly, the prehistoric whales of this period had a lot in common with modern giant tortoises!
The Giant Prehistoric Whales: Basilosaurus and Relatives
By about 35 million years ago, some prehistoric whales had reached gigantic sizes, bigger even than modern blue or sperm whales. The largest genus yet known is Basilosaurus, the bones of which (discovered in the mid-19th century) were thought to belong to a dinosaur--hence its deceptive name, meaning "king lizard." Despite its 100-ton size, Basilosaurus had a relatively small brain, and didn't use echolocation when swimming. Even more important from an evolutionary viewpoint, Basilosaurus was fully aquatic, birthing as well as swimming and feeding in the ocean.
Contemporaries of Basilosaurus were much less fearsome, perhaps because there was only room for one giant mammalian predator in the prehistoric food chain. Dorudon was once thought to be a baby Basilosaurus; only later was it realized that this small whale (only about 16 feet long and half a ton) merited its own genus. And the much later Aetiocetus (which lived about 25 million years ago), while weighing only a few tons, shows the first primitive progress to plankton filtering--sporting small plates of baleen alongside its ordinary teeth.
Here's a list of the 12 most notable prehistoric whale genuses; just click on the links for more information.
Aetiocetus This primitive whale used both teeth and plankton-filtering baleen.
Ambulocetus This "walking whale" led an otter-like lifestyle.
Basilosaurus The biggest prehistoric whale that ever lived.
Cetotherium A sleek ancestor of the modern gray whale.
Dorudon This small whale was once mistaken for a baby Basilosaurus.
Indohyus This ancient hoofed creature may (or may not) have been ancestral to whales.
Kutchicetus A long-tailed relative of the early whale Ambulocetus.
Maiacetus This prehistoric whale gave birth on land.
Pakicetus This dog-like mammal was ancestral to modern whales.
Protocetus This "first whale" looked more like a seal.
Rodhocetus Yet another early step in whale evolution.
Zygorhiza This early whale may have crawled onto land to give birth.
