Evolution

The Evolution of Whales



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"The Evolution of Whales"
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Modern whales are amazing animals, perfectly evolved for life in the sea. Whales have blow-holes rather than nostrils, flippers and flukes in place of legs, ears and voices that work underwater, and large brains. They live in complex social groups, and communicate with an array of songs, clicks, and burbles that may constitute language. Yet their ancestors were land animals that looked like long-faced wolves with hooves. How did whales' bodies and behaviors change so completely? The story begins 52 million years ago, when these creatures started to move back to the sea from which all life emerged.

BACK TO THE WATER

"Pakicetus," the earliest known cetacean, was probably a member of the artiodactyl order that includes modern antelope, cows, hippos, camels, and giraffes. Unlike its modern hoofed cousins, Pakicetus was a carnivore. A freshwater creature, it probably hunted in shallow water along rivers and lakes. It had a relatively small brain, and its nostrils were situated at the end of its snout. The creature also had a pelvis that was fused to its spine, like other land mammals (and unlike today's whales). Paleontologists have been able to establish its relationship to modern whales and dolphins because of the unique S-shape of its inner ear bones- a shape half-way between that of a land mammal, and that of modern whales.

LEARNING TO SWIM

Proto-whale "Ambulocetus" (the "walking whale"), from 49 million years ago, had a lifestyle similar to a crocodile's. Its legs would have worked well either for swimming or walking on dry land. This animal probably hunted like a croc, as well, lurking in the water of marine estuaries and bays, and ambushing prey. "Ambulocetus" still needed to drink fresh water, according to analysis of oxygen isotopes found in its fossil teeth. This amphibious mammal was about 10 feet long and may have weighed about 600 pounds. It swam by moving its spine vertically, like otters, seals, and whales.

ADAPTING TO SALT WATER

The first group of early whales to spread throughout tropical seas was the Protocitedae, including such species as "Rodhocetus," which lived about 45 million years ago. Fossils of these animals have been found in Indo-Pakistan, Africa, Europe, and North America. The Protocitedae were the first whales known to have tail flukes; they still had four short legs which may have been capable of supporting them on land, but the pelvis was no longer fused to the spine. This allowed animals like "Rodhocetus" to move with speed and agility in the water. It isn't known whether the creatures returned to land to mate and raise young, like modern sea lions. These were the first creatures to look more like a modern whale than a land mammal. Like modern whales, they did not need to drink fresh water.

AT HOME IN THE SEA

By the late Eocene era, 41 to 35 million years ago, the ancestors of whales and dolphins had split into at least two distinct forms. The larger group is called Basilosaurids after the best-known species, the "Basilosaurus." This creature was 60-65 feet long, and had a small head with a mouth full of sharp teeth, and a long, sinuous body. "Basilosaurus" had a tail fluke and tiny but complete back limbs, which were probably useless for propulsion. The smaller lineage that existed at this time was the Dorudontids, which resemble dolphins about 20 feet long. The "Dorudon" had small, flattened flippers and a long, toothy snout. Both Basilosaurids and Dorudontids had small brains, indicating that they may have been solitary creatures. In addition, they lacked the "melon organ" that characterizes modern cetaceans, allowing today's whales and dolphins to hunt by ultrasound, and to sing to other members of their pods. These animals mated and gave birth at sea.

LOSING LEGS, GAINING A BLOWHOLE

Around 30 million years ago, the whale family tree split into its two-part modern division. The major groups are: toothed whales, or Odontocetes, which includes belugas, sperm whales, narwhals, orcas, and all of the dolphins and porpoises; and baleen whales, Mysticetes, such as blue whales, fin whales, humpbacks, etc. Rather than using echolocation to hunt fish or smaller marine mammals like their toothed cousins, baleen whales employ plates of comb-like fibers to strain plankton from the water.

The earliest-known baleen whale was called "Aetiocetus," which had a loose jaw hinge like modern mysticetes, but still had teeth. This animal was only about 10 feet long, 1/10th the size of its blue whale descendents. By this time, 30 million years ago, the whale's nostrils had migrated from the tip of its snout to a place in the middle of the skull, between the eyes.

MODERN WHALES EMERGE, BRAINY AND SOCIAL

In the Miocene era, 24 to 5 million years ago, whales and dolphins radiated into a wide array of different species like that seen today. Modern bowheads and right whales, which move slowly and use their baleen plates to skim food from the water, appeared as early as 22 million years ago. The rorqual group, massive, fast-moving giants such as blue and sei whales, show up in the fossil record around 15 million years ago. Rorquals use accordion-like pleated skin on their necks to gulp down huge quantities of food-rich sea water and then force the water back out over their baleen. Before the end of the Miocene, all of the important families of toothed whales and dolphins had also put in an appearance: sperm whales, orcas, belugas and narwhals, dolphins, porpoises, and beaked whales. Modern innovations such as echolocation, singing for communication, and elaborate social structures probably all developed during this era. The history of such behaviors is hard to trace, since they do not leave direct fossil evidence. Nonetheless, physical clues such as large brains and the "melon organ" present in these early modern cetaceans' heads indicate that the animals would have been capable of these advanced social interactions.

THE FUTURE?

"Pakicetus" and "Ambulocetus" began a move back into the water that gave birth to us all. Fifty million years later, the descendents of these marine pioneers patrol all of the world's oceans, humbling us with their intelligence, speed, grace and majesty. May they continue to roam for 50 million more!

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