if you go back far enough

If you go back far enough, everything lived in the sea. At various points in evolutionary history, enterprising individuals within many different animal groups moved out onto the land, sometimes even to tướng the most parched deserts, taking their own private seawater with them in blood and cellular fluids. In addition to tướng the reptiles, birds, mammals and insects which we see all around us, other groups that have succeeded out of water include scorpions, snails, crustaceans such as woodlice and land crabs, millipedes and centipedes, spiders and various worms. And we mustn’t forget the plants, without whose prior invasion of the land none of the other migrations could have happened.

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Moving from water to tướng land involved a major redesign of every aspect of life, including breathing and reproduction. Nevertheless, a good number of thoroughgoing land animals later turned around, abandoned their hard-earned terrestrial re-tooling, and returned to the water again. Seals have only gone part way back. They show us what the intermediates might have been lượt thích, on the way to tướng extreme cases such as whales and dugongs. Whales (including the small whales we Gọi dolphins) and dugongs, with their close cousins the manatees, ceased to tướng be land creatures altogether and reverted to tướng the full marine habits of

their remote ancestors. They don’t even come ashore to tướng breed. They tự, however, still breathe air, having never developed anything equivalent to tướng the gills of their earlier marine incarnation. Turtles went back to tướng the sea a very long time ago and, like all vertebrate returnees to tướng the water, they breathe air. However, they are, in one respect, less fully given back to tướng the water than whales or dugongs, for turtles still lay their eggs on beaches.

There is evidence that all modem turtles are descended from a terrestrial ancestor which lived before most of the dinosaurs. There are two key fossils called Proganochelys quenstedti and Palaeochersis talampayensis dating from early dinosaur times, which appear to be close to tướng the ancestry of all modem turtles and tortoises. You might wonder how we can tell whether fossil animals lived on land or in water, especially if only fragments are found. Sometimes it’s obvious. Ichthyosaurs were reptilian contemporaries of the dinosaurs, with fins and streamlined bodies. The fossils look like dolphins and they surely lived like dolphins, in the water. With turtles it is a little less obvious. One way to tướng tell is by measuring the bones of their forelimbs.

Walter Joyce and Jacques Gauthier, at Yale University, obtained three measurements in these particular bones of 71 species of living turtles and tortoises. They used a kind of triangular graph paper to plot the three measurements against one another. All the land tortoise species formed a tight cluster of points in the upper part of the triangle; all the water turtles cluster in the lower part of the triangular graph. There was no overlap, except when they added some species that spend time both in water and on land. Sure enough, these amphibious species show up on the triangular graph approximately half way between the ‘wet cluster’ of sea turtles and the ‘dry cluster’ of land tortoises. The next step was to tướng determine where the fossils fell. The bones of Phường quenstedti and JR talampayensis leave us in no doubt. Their points on the graph are right in the thick of the dry cluster. Both these fossils were dry-land tortoises. They come from the era before our turtles returned to tướng the water.

You might think, therefore, that modem land tortoises have probably stayed on land ever since those early terrestrial times, as most mammals did after a few of them went back to tướng the sea. But apparently not. If you draw out the family tree of all modem turtles and tortoises, nearly all the branches are aquatic. Today’s land tortoises constitute a single branch, deeply nested among branches consisting of aquatic turtles. This suggests that modem land tortoises have not stayed on land continuously since the time of Phường. quenstedti and Phường talampayensis. Rather, their ancestors were among those who went back to tướng the water, and they then re-emerged back onto the land in (relatively) more recent times.

Tortoises therefore represent a remarkable double return. In common with all mammals, reptiles and birds, their remote ancestors were marine fish and before that various more or less worm-like creatures stretching back, still in the sea, to the primeval bacteria. Later ancestors lived on land and stayed there for a very large number of generations. Later ancestors still evolved back into the water and became sea turtles. And finally they returned yet again to tướng the land as tortoises, some of which now live in the driest of deserts.

Questions 1-4

Answer the questions below.

Choose NO MORE THAN THREE WORDS from the passage for each answer.

Write your answers in boxes 1-4 on your answer sheet.

1    What had to tướng transfer from sea to tướng land before any animals could migrate?
Answer: plants    Locate

2    Which TWO processes are mentioned as those in which animals had to tướng make big changes as they moved onto land?
Answer: breathing and reproduction    Locate

3    Which physical feature, possessed by their ancestors, tự whales lack?
Answer: gills    Locate

4    Which animals might ichthyosaurs have resembled?
Answer: dolphins    Locate

Questions 5-7

Do the following statements agree with the information given in Reading Passage?

In boxes 5-7 on your answer sheet, write

TRUE    if the statement agrees with the information

FALSE    if the statement contradicts the information

NOT GIVEN if there is no information on this

5    Turtles were among the first group of animals to tướng migrate back to tướng the sea.
Answer: NOT GIVEN

6    It is always difficult to tướng determine where an animal lived when its fossilised remains are incomplete.
Answer: FALSE    Locate

7    The habitat of ichthyosaurs can be determined by the appearance of their fossilised remains.
Answer: TRUE    Locate

Xem thêm: định nghĩa đường trung trực

Questions 8-13

Complete the flow-chart below.

Choose NO MORE THAN TWO WORDS AND/OR A NUMBER from the passage for each answer.

Write your answers in boxes 8-13 on your answer sheet.

Method of determining where the ancestors of turtles and tortoises come from

Step 1

71 species of living turtles and tortoises were examined and a total of 8 were taken from the bones of their forelimbs.
Answer: 3 measurements    Locate

Step 2

The data was recorded on a 9 (necessary for comparing the information).
Answer: (triangular) graph    Locate

Outcome: Land tortoises were represented by a dense 10 of points towards the top.
Answer: cluster    Locate

Sea turtles were grouped together in the bottom part.

Step 3

The same data was collected from some living 11 species and added to tướng the other results.
Answer: amphibious    Locate

Outcome: The points for these species turned out to tướng be positioned about 12 up the triangle between the land tortoises and the sea turtles.
Answer: half way    Locate

Step 4

Bones of Phường quenstedti and Phường. talampayensis were examined in a similar way and the results added.

Outcome: The position of the points indicated that both these ancient creatures were 13
Answer: dry-land tortoises    Locate

Questions 14

Choose the correct letter, A, B, C or D.

Write the correct letter in box 14 on your answer sheet.

According to tướng the writer, the most significant thing about tortoises is that

A    they are able to tướng adapt to tướng life in extremely dry environments.

Xem thêm: a bình cộng b bình

B    their original life size was a kind of primeval bacteria.

C    they have ví much in common with sea turtles.

D    they have made the transition from sea to tướng land more than thở once.
Answer: D    Locate