2018-10-15 19:16dgukedu 互联网







  内容回忆:文章主要内容是追溯鸟的起源。第一段介绍了一些名词,说科学家们通常认为 KT extinction 是旧鸟类和新鸟类的分界,但具体是什么时候,却仍不清楚(第一段末尾有插入题)。一些科学家用了新技术,通过追溯鸟基因里的某种物质进行追溯,并成功追溯到 10 million years 以前,比普遍认识的都要早。其他人也用这种方法做了实验,虽然结果有所不同,但实质都证明了鸟起源、扩散比预想要早(考点)。有另一拨人找到化石进行化验,得出结论认为可追溯到 68-70 million 年前。有两种化石(给出了学名),他们都跟 modern birds 有关。后一种鸟化石中的鸟已经灭绝了,它属于跟 duck 一样的种族(考点)。虽然化石结果和上述实验结果有不同,但不能断定孰优孰劣。



  The discovery that birds evolved from small carnivorous dinosaurs of the Late Jurassic was made possible by recently discovered fossils from China, South America, and other countries, as well as by looking at old museum specimens from new perspectives and with new methods. The hunt for the ancestors of living birds began with a specimen of Archaeopteryx, the first known bird, discovered in the early 1860s. Like birds, it had feathers along its arms and tail, but unlike living birds, it also had teeth and a long bony tail. Furthermore, many of the bones in Archaeopteryx's hands, shoulder girdles, pelvis, and feet were distinct, not fused and reduced as they are in living birds. Based on these characteristics, Archaeopteryx was recognized as an intermediate between birds and reptiles; but which reptiles?

  In the 1970s, paleontologists noticed that Archaeopteryx shared unique features with small carnivorous dinosaurs called theropods. All the dinosaur groups on this evogram, except the ornithischian dinosaurs, are theropods. Based on their shared features, scientists reasoned that perhaps the theropods were the ancestors of birds. When paleontologists built evolutionary trees to study the question, they were even more convinced. The birds are simply a twig on the dinosaurs' branch of the tree of life.

  As birds evolved from these theropod dinosaurs, many of their features were modified. However, it's important to remember that the animals were not "trying" to be birds in any sense. In fact, the more closely we look, the more obvious it is that the suite of features that characterize birds evolved through a complex series of steps and served different functions along the way.

  In theropods even more closely related to birds, like the oviraptorosaurs, we find several new types of feathers. One is branched and downy, as pictured below. Others have evolved a central stalk, with unstructured branches coming off it and its base. Still others (like the dromaeosaurids and Archaeopteryx) have a vane-like structure in which the barbs are well-organized and locked together by barbules. This is identical to the feather structure of living birds.




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