剑桥雅思9Test1阅读Passage3原文翻译 The history of the tortoise 乌龟的历史
剑桥雅思9 Test1 Passage3阅读原文翻译
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 the most parched deserts, taking their own private seawater with them in blood and cellular fluids. In addition to 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, spider 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.
Moving from water to 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 like, on the way to extreme cases such as whales and dugongs. Whales (including the small whales we call dolphins) and dugongs, with their close cousins the manatees, ceased to be land creatures altogether and reverted to the full marine habits of their remote ancestors. They don’t even come ashore to breed. They do, however, still breathe air, having never developed anything equivalent to the gills of their earlier marine incarnation. Turtles went back to the sea a very long time ago and, like all vertebrate returnees to the water, they breathe air. However, they are, in one respect, less fully given back to the water than whales or dugongs, for turtles still lay their eggs on beaches.
There is evidence that all modern turtles are descended from a terrestrial ancestor which lived before most of the dinosaurs. There are two key fossils called Proganochleys quenstedti and Palaeochersis talampayensis dating from early dinosaur times, which appear to be close to the ancestry of all modern turtles and tortoises. You might wonder how we can tell whether fossil animal 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 dolphines, in the water. With turtles it is a little less obvious. One way to tell is by measuring the bones of their forelimbs.
有证据表明，所有现代海龟均来自同一陆地祖先。它生活在比大多数恐龙还要遥远的时代。两个关键的化石，Proganochleys quenstedti 和Palaeochersis talampayensis，可以追溯到恐龙时代早期。它们似乎与所有现代乌龟和海龟的祖先十分接近。您可能想知道我们如何分辨化石中的动物究竟是生活在陆地上还是在水里，特别是如果仅发现碎片的时候。有时这很明显。带有鳍和流线型身体的鱼龙是生活在恐龙时代的爬行动物。其化石看起来像海豚，它们也肯定像海豚一样生活在水中。对于海龟来说，这一点并不那么明显。一种判断方法是测量前肢的骨头。
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 halfway between the ‘wet cluster’ of sea turtles and the ‘dry cluster’ of land tortoises. The next step was to determine where the fossils fell. The bones of P. quenstedti and P. talampayensis leave us in no doubt