How the Whale Lost Its Legs and Returned to the Sea

How the Whale Lost Its Legs And weighing 600 to 700 pounds and evolutionary change, along with the Returned To the Sea measuring about 10 feet from snout first fishlike animals establishing their By JOHN NOBLE WILFORD to tail. Judging by fossil remains, the beachhead 370 million years ago, May 3, 1994 animal was an amphibious species birds becoming flying animals and clearly intermediate between a early human ancestors developing AGES after some adventurous (or terrestrial ancestor of whales and upright walking. But why would misadventurous) fish left the sea and aquatic modern whales. animals like whales, with all the planted the flag of vertebrate animal apparatus for successful living on the This fossil animal, named life on land, their descendants had it land and ages of experience doing it, Ambulocetus natans, which means both ways as amphibians and then seek such a different habitat? swimming walking-whale, was completed the epic transition, excavated from sediments of an "The motive for any such transition is evolving into terrestrial reptiles, ancient seabed in Pakistan. always opportunity," Dr. Gingerich mammals and birds. But something Ambulocetus was just what scientists said. "Look at what changed first, the about the water must have kept would have expected to find. The teeth. What was initially important in beckoning, until a few irredentists animal still had four limbs for walking the transition was food. These land among the mammals did eventually on land, though probably with animals were exploiting an reclaim a place in the sea. diminished agility. It could also hunt opportunity to feed on fish in the Most prominent of these mammals in the sea, probably swimming by sea." are the whales. Although they may kicking its big feet. Other change over time involved swim the oceans with power and Another discovery in Pakistan has improvements in the animals' grace, these leviathans are more now advanced the transition story maneuverability in their new closely related to the camel and cow three or four million years and out environment. These included hearing than any fish in their wake. Their into deeper waters. Scientists systems evolved for underwater anatomies retain vestiges of the four- reported last week that they had communications and navigation and legged land animals in their ancestry, found fossils of an animal that had completely new ways of locomotion, the ones that began the bold return obviously made a more resolute first as amphibians and then as fully to the sea more than 50 million years commitment to a life at sea. developed marine animals. ago. Writing in the current issue of the For a long time, scientist had almost As early as Aristotle, people journal Nature, Dr. Philip D. Gingerich, no direct evidence about the nature recognized that more than size set a paleontologist at the University of of the whale transition. The best they whales apart from other marine life. Michigan at Ann Arbor, said the new could do was make inferences based Aristotle noted that whales as well as species, Rodhocetus kasrani, was the on a comparison of modern whales dolphins, the other members of the earliest known transitional whale with and fossils of archaic whales with the cetacean group, bear their young live, an anatomy adapted for swimming remains of their putative terrestrial unlike fish. This made them, by like a whale. It had a more relatives. definition, mammals. (It is now known streamlined body and a fully flexible that some fish do bear live young.) From this scientists concluded that rear spinal column, which could have whales were distantly related to New fossil discoveries have now produced the motions for the ungulate mammals, a group whose revealed several of the critical powerful beat of a horizontal tail modern members include camels, evolutionary steps in the earliest fluke that propels modern whales. cows, pigs and deer. The link between history of whales. Scientists have Whether Rodhocetus had indeed other ungulates and whales is thought identified some intermediate species made this important advance cannot to be mesonychids, extinct four- as land mammals steadily changed be determined until more complete legged mammals that sometimes physical form while adapting to tail fossils are uncovered. But the feasted on fish at river edges. swimming, diving, feeding and evidence, Dr. Gingerich and otherwise thriving in their new Beginning in 1983, paleontologists colleagues concluded, "shows that tail habitat. One surprise is that the have collected more revealing swimming evolved early in the history transformation of four-legged land evidence of the transition. In that of cetaceans." mammals into an animal completely year, Dr. Gingerich reported the adapted to marine life took only 10 Besides whales and dolphins, two discovery of the oldest whale, which million years -- hardly any time at all other major types of mammals have lived about 52 million years ago. A in evolutionary terms. made this dramatic transition: Sirenia, fossil skull was found in Pakistan in which includes dugongs and river sediments near an ancient sea. One discovery, which Dr. Gould called manatees, and the group composed The animal was given the name "a remarkable smoking gun," of seals, sea lions and walruses. Pakicetus. introduced scientists to an animal that lived 50 million years ago. It was The migration of whales to the sea is Although they had nothing to ponder about the size of a modern sea lion, a classic example of transcendental but this skull, scientists could see that Pakicetus had teeth resembling those intermediacy, and so the search In this intermediate phase, the up- of mesonychids, but it was well continued. and-down undulations probably adapted to feeding on fish in surface operated in concert with the vigorous If the matter of locomotion is the waters of shallow seas. Other parts of paddling of Ambulocetus's large feet. functional test of intermediacy, as Dr. the skull were becoming whalelike, In modern whales, they contribute Gould wrote, Dr. Thewissen's but it lacked the auditory equipment mightily to the propulsive beat of the Ambulocetus came closest to the for a fully marine existence. tail fluke. long-sought breakthrough in early As Dr. Gould noted in his review of whale evolution. Its toes are even Whales in their present form began recent whale research, the next terminated by hooves as in appearing about 30 million years ago. important discovery was the first mesonychids and other ungulates, Their flippers are what remains of the complete hind limbs of a fossil whale, those earlier ancestors. The skeleton forelimbs of their terrestrial past. The Basilosaurus isis, which lived 5 million also revealed that the animal had only hints of the former hind limbs to 10 million years after Pakicetus. stubby forelimbs with hands that are the vestiges of a pelvis and femur, splayed outward like the flippers on a the upper leg bone, embedded in the Since the hind limbs were a mere 2 sea lion, while the rear legs were still body wall. feet long and the whole body was 50 large and powerful, with possibly feet long, the discoverers concluded Reflecting on the recent succession of webbed feet. It had not yet evolved a that the legs could not have discoveries, Dr. Novacek said, "This tail fluke, but its spine seemed to be supported the body on land or expanding fossil casebook on the flexible enough to allow the assisted in swimming. This fossil origins of whales is one of the undulations associated with whale whale had passed the point of triumphs of modern vertebrate propulsion. paleontology." Mythbusters--- How the Whale Lost Its Legs and Returned to the Sea

Before reading, check the column labeled. “You” for those statements you could agree with. Think about why and be prepared to support your positions with examples.

After reading, check the column labeled “author” for those statements with which you feel he would agree. Be prepared to cite specific evidence from the text to support your interpretation. (Put the number next to text associated with the specific statement).

You Author

______1. Whales once walked on land.

______2. Whales are more like a dog than a fish.

______3. A change in the environment caused the transition to marine life.

______4. It wasn’t until intermediate transitional species bones were found that scientists began to think

whale’s ancestors went from land back to water.

______5. Loss of legs was the reason whale’s ancestors headed back to water.

______6. Change of whales over time is evidence of evolution.

Mythbusters--- How the Whale Lost Its Legs and Returned to the Sea