THE ORIGINS OF CETACEANS
It should be obvious that cetaceans—whales, porpoises, and dolphins—are mammals.
They breathe through lungs, not through gills, and give birth to live young. Their streamlined bodies, the absence of hind legs, and the presence of a fluke1and blow-hole2cannot disguise their affinities with land-dwelling mammals. However, unlike the cases of sea otters and pinnipeds (seals, sea lions, and walruses, whose limbs are functional both on land and at sea), it is not easy to envision what the first whales looked like. Extinct but already fully marine cetaceans are known from the fossil record. How was the gap between a walking mammal and a swimming whale bridged? Missing until recently were fossils clearly intermediate, or transitional, be-tween land mammals and cetaceans.
Very exciting discoveries have finally allowed scientists to reconstruct the most likely origins of cetaceans. In 1979, a team looking for fossils in northern Pakistan found what proved to be the oldest fossil whale. The fossil was officially named Paki-cetus in honor of the country where the discovery was made. Pakicetuswas found embedded in rocks formed from river deposits that were 52 million years old. The river that formed these deposits was actually not far from an ancient ocean known as the Tethys Sea.
The fossil consists of a complete skull of an archaeocyte, an extinct group of an-cestors of modern cetaceans. Although limited to a skull, the Pakicetusfossil provides precious details on the origins of cetaceans. The skull is cetacean-like but its jawbones lack the enlarged space that is filled with fat or oil and used for receiving underwater sound in modern whales. Pakicetusprobably detected sound through the ear opening as in land mammals. The skull also lacks a blowhole, another cetacean adaptation for diving. Other features, however, show experts that Pakicetusis a transitional form between a group of extinct flesh-eating mammals, the mesonychids, and cetaceans. It has been suggested that Pakicetus fed on fish in shallow water and was not yet adapted for life in the open ocean. It probably bred and gave birth on land.
Another major discovery was made in Egypt in 1989. Several skeletons of another early whale, Basilosaurus, were found in sediments left by the Tethys Sea and now ex-posed in the Sahara desert. This whale lived around 40 million years ago, 12 million years after Pakicetus. Many incomplete skeletons were found but they included, for the first time in an archaeocyte, a complete hind leg that features a foot with three tiny toes. Such legs would have been far too small to have supported the 50-foot-long Basilosauruson land. Basilosauruswas undoubtedly a fully marine whale with possi-bly nonfunctional, or vestigial, hind legs.
An even more exciting find was reported in 1994, also from Pakistan. The now ex-tinct whale Ambulocetus natans(“the walking whale that swam”) lived in the Tethys Sea 49 million years ago. It lived around 3 million years after Pakicetusbut 9 million
beforeBasilosaurus. The fossil luckily includes a good portion of the hind legs. The legs were strong and ended in long feet very much like those of a modern pinniped.
The legs were certainly functional both on land and at sea. The whale retained a tail and lacked a fluke, the major means of locomotion in modern cetaceans. The structure of the backbone shows, however, that Ambulocetus swam like modern whales by moving the rear portion of its body up and down, even though a fluke was missing.
The large hind legs were used for propulsion in water. On land, where it probably bred and gave birth, Ambulocetusmay have moved around very much like a modern sea lion. It was undoubtedly a whale that linked life on land with life at sea.
1. Fluke: the two parts that constitute the large triangular tail of a whale 2. Blowhole: a hole in the top of the head used for breathing
It should be obvious that cetaceans—whales, porpoises, and dolphins—are mammals.
They breathe through lungs, not through gills, and give birth to live young. Their streamlined bodies, the absence of hind legs, and the presence of a fluke1and blow-hole2cannot disguise their affinities with land-dwelling mammals. However, unlike the cases of sea otters and pinnipeds (seals, sea lions, and walruses, whose limbs are functional both on land and at sea), it is not easy to envision what the first whales looked like. Extinct but already fully marine cetaceans are known from the fossil record. How was the gap between a walking mammal and a swimming whale bridged? Missing until recently were fossils clearly intermediate, or transitional, be-tween land mammals and cetaceans.
1. Fluke: the two parts that constitute the large triangular tail of a whale 2. Blowhole: a hole in the top of the head used for breathing
Directions:Mark your answer by filling in the oval next to your choice.
1. In paragraph 1, what does the author say about the presence of a blow-hole in cetaceans?
It clearly indicates that cetaceans are mammals.
It cannot conceal the fact that ceta-ceans are mammals.
It is the main difference between cetaceans and land-dwelling mam-mals.
It cannot yield clues about the ori-gins of cetaceans.
2. Which of the following can be in-ferred from paragraph 1 about early sea otters?
It is not difficult to imagine what they looked like.
There were great numbers of them.
They lived in the sea only.
They did not leave many fossil remains.
Another major discovery was made in Egypt in 1989. Several skeletons of another early whale, Basilosaurus, were found in sediments left by the Tethys Sea and now exposed in the Sahara desert. This whale lived around 40 million years ago, 12 million years after Pakicetus. Many incomplete skeletons were found but they included, for the first time in an archaeocyte, a complete hind leg that features a foot with three tiny toes. Such legs would have been far too small to have supported the 50-foot-long Basilosauruson land. Basilosauruswas undoubtedly a fully marine whale with possi-bly nonfunctional, or vestigial, hind legs.
The fossil consists of a complete skull of an archaeocyte, an extinct group of ancestors of modern cetaceans. Although limited to a skull, the Pakicetus fossil provides precious details on the origins of cetaceans. The skull is cetacean-like but its jawbones lack the enlarged space that is filled with fat or oil and used for receiving underwater sound in modern whales. Pakicetusprobably detected sound through the ear opening as in land mammals. The skull also lacks a blowhole, another cetacean adaptation for diving. Other features, however, show experts that Pakicetusis a transitional form be-tween a group of extinct flesh-eating mammals, the mesonychids, and cetaceans. It has been suggested that Pakicetus fed on fish in shallow water and was not yet adapted for life in the open ocean. It probably bred and gave birth on land.
P
3. The word precious in the passage is closest in meaning to
exact
scarce
valuable
initial
4. Pakicetusand modern cetaceans have similar
hearing structures
adaptations for diving
skull shapes
6. The word exposed in the passage is closest in meaning to
explained
visible
identified
located
7. The hind leg of Basilosauruswas a significant find because it showed thatBasilosaurus
lived later than Ambulocetus natans
lived at the same time as Pakicetus
was able to swim well
could not have walked on land
8. It can be inferred that Basilosaurus bred and gave birth in which of the following locations?
On land
Both on land and at sea
In shallow water
In a marine environment
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An even more exciting find was reported in 1994, also from Pakistan. The now extinct whaleAmbulocetus natans(“the walking whale that swam”) lived in the Tethys Sea 49 million years ago. It lived around 3 million years after Pakicetusbut 9 million before Basilosaurus. The fossil luckily includes a good portion of the hind legs. The legs were strong and ended in long feet very much like those of a modern pinniped. The legs were certainly functional both on land and at sea. The whale retained a tail and lacked a fluke, the major means of locomotion in modern cetaceans. The structure of the backbone shows, however, thatAmbulocetusswam like modern whales by moving the rear portion of its body up and down, even though a fluke was missing. The large hind legs were used for propulsion in water. On land, where it probably bred and gave birth,Ambulocetusmay have moved around very much like a modern sea lion. It was undoubtedly a whale that linked life on land with life at sea.
9. Why does the author use the word luckily in mentioning that the Ambu-locetus natansfossil included hind legs?
Fossil legs of early whales are a rare find.
The legs provided important infor-mation about the evolution of cetaceans.
The discovery allowed scientists to reconstruct a complete skeleton of the whale.
Until that time, only the front legs of early whales had been discovered.
10. Which of the sentences below best expresses the essential information in the highlighted sentence in the passage?Incorrectchoices change the meaning in important ways or leave out essential information.
Even though Ambulocetusswam by moving its body up and down, it did not have a backbone.
The backbone of Ambulocetus, which allowed it to swim, provides evidence of its missing fluke.
AlthoughAmbulocetushad no fluke, its backbone structure shows that it swam like modern whales.
By moving the rear parts of their bodies up and down, modern whales swim in a different way from the wayAmbulocetusswam.
11. The word propulsion in the passage is closest in meaning to
staying afloat
Extinct but already fully marine cetaceans are known from the fossil record. 7How was the gap between a walking mammal and a swimming whale bridged? 7Missing until recently were fossils clearly intermediate, or transitional, between land mammals and cetaceans.
7 Very exciting discoveries have finally allowed scientists to reconstruct the most likely origins of cetaceans. 7In 1979, a team looking for fossils in northern Pakistan found what proved to be the oldest fossil whale.
12. Look at the four squares [7] that indicate where the following sentence can be added to the passage.
This is a question that has puzzled scientists for ages.
Where would the sentence best fit?
Extinct but already fully marine cetaceans are known from the fossil record. This is a question that has puzzled scientists for ages. How was the gap between a walking mammal and a swimming whale bridged? 7Missing until recently were fossils clearly intermediate, or transitional, between land mammals and
cetaceans.
7Very exciting discoveries have finally allowed scientists to reconstruct the most likely origins of cetaceans. 7In 1979, a team looking for fossils in northern Pakistan found what proved to be the oldest fossil whale.
Extinct but already fully marine cetaceans are known from the fossil record. 7 How was the gap between a walking mammal and a swimming whale bridged?
This is a question that has puzzled scientists for ages. Missing until recently were fossils clearly intermediate, or transitional, between land mammals and cetaceans.
7Very exciting discoveries have finally allowed scientists to reconstruct the most likely origins of cetaceans. 7In 1979, a team looking for fossils in northern Pakistan found what proved to be the oldest fossil whale.
Extinct but already fully marine cetaceans are known from the fossil record. 7 How was the gap between a walking mammal and a swimming whale bridged?
7Missing until recently were fossils clearly intermediate, or transitional, be-tween land mammals and cetaceans.
This is a question that has puzzled scientists for ages. Very exciting discov-eries have finally allowed scientists to reconstruct the most likely origins of cetaceans.7In 1979, a team looking for fossils in northern Pakistan found what proved to be the oldest fossil whale.
Extinct but already fully marine cetaceans are known from the fossil record. 7 How was the gap between a walking mammal and a swimming whale bridged?
7Missing until recently were fossils clearly intermediate, or transitional, be-tween land mammals and cetaceans.
7Very exciting discoveries have finally allowed scientists to reconstruct the most likely origins of cetaceans. This is a question that has puzzled scientists for ages. In 1979, a team looking for fossils in northern Pakistan found what proved to be the oldest fossil whale.
P
13–14. Directions:An introductory sentence for a brief summary of the passage is provided below. Complete the summary by selecting the THREE answer choices that express the most important ideas in the passage. Some answer choices do not belong in the summary because they express ideas that are not presented in the passage or are minor ideas in the passage. This question is worth 2 points.
This passage discusses fossils that help to explain the likely origins of cetaceans— whales, porpoises, and dolphins.
b
b
b
Answer Choices 1. Recent discoveries of fossils have
helped to show the link between land mammals and cetaceans.
2. The discovery of Ambulocetus natansprovided evidence for a whale that lived both on land and at sea.
3. The skeleton of Basilosauruswas found in what had been the Tethys Sea, an area rich in fossil evidence.
4. Pakicetusis the oldest fossil whale yet to be found.
5. Fossils thought to be transitional forms between walking mammals and swimming whales were found.
6. Ambulocetus’ hind legs were used for propulsion in the water.