第三次生物大灭绝，又称二叠纪大灭绝。

　　二叠纪(Permian period)是古生代的最后一个纪，也是重要的成煤期。二叠纪分为早二叠世， 中二叠世和晚二叠世。二叠纪开始于距今约2.95亿年，延至2.5亿年，共经历了4500万年。二叠纪的地壳运动比较活跃，古板块间的相对运动加剧，世界范围内的许多地槽封闭并陆续地形成褶皱山系，古板块间逐渐拼接形成联合古大陆(泛大陆)。陆地面积的进一步扩大，海洋范围缩小，自然地理环境的变化，促进了生物界的重要演化，预示着生物发展史上一个新时期的到来。

　　距今2.5亿年前的二叠纪末期，发生了有史以来最严重的大灭绝事件，估计地球上有96%的物种灭绝，其中90%的海洋生物和70%的陆地脊椎动物灭绝。三叶虫、海蝎以及重要珊瑚类群全部消失。陆栖的单弓类群动物和许多爬行类群也灭绝了。这次大灭绝使得占领海洋近3亿年的主要生物从此衰败并消失，让位于新生物种类，生态系统也获得了一次最彻底的更新，为恐龙类等爬行类动物的进化铺平了道路。 科学界普遍认为，这一大灭绝是地球历史从古生代向中生代转折的里程碑。其他各次大灭绝所引起的海洋生物种类的下降幅度都不及其1/6，没有使生物演化进程产生如此重大的转折。

　　科学家认为，在二叠纪曾经发生海平面下降和大陆漂移，造成了最严重的物种大灭绝。所有的大陆聚集成了一个联合的古陆，富饶的海岸线急剧减少，大陆架也缩小了，生态系统受到了严重的破坏，很多物种的灭绝是因为失去了生存空间。更严重的是，当浅层的大陆架暴露出来后，原先埋藏在海底的有机质被氧化，这个过程消耗了氧气，释放出二氧化碳。大气中氧的含量减少，对生活在陆地上的动物非常不利。随着气温升高，海平面上升，又使许多陆地生物遭到灭顶之灾， 海洋里也成了缺氧地带。地层中大量沉积的富含有机质的页岩是这场灾难的证明。

　　这次大灭绝是由气候突变、沙漠范围扩大、火山爆发等一系列原因造成。

Permian-Triassic extinction

　　The Permian-Triassic (P-T or PT) extinction event, sometimes informally called the Great Dying, was an extinction event that occurred approximately 251 million years ago, defining the boundary between the Permian and Triassic periods. It was the Earth's most severe extinction event, with about 90 percent of all marine species and 70 percent of terrestrial vertebrate species going extinct.

　　For some time after the event, fungal species were the dominant form of terrestrial life. Though they only made up approximately 10 percent of remains found before and just after the extinction horizon, fungal species subsequently grew rapidly to make up nearly 100 percent of the available fossil record (Eshet et al. 1995). However, some researchers argue that fungal species did not dominate terrestrial life, as their remains have only been found in shallow marine deposits (Wignall 1996). Alternatively, others argue that fungal hypha (long, branching filament) are simply better suited for preservation and survival in the environment, creating an inaccurate representation of certain species in the fossil record (Erwin 1993).

　　At one time, this die-off was assumed to have been a gradual reduction over several million years. Now, however, it is commonly accepted that the event lasted less than a million years, from 252.3 to 251.4 million years ago (both numbers ±300,000 years), a very brief period of time in geological terms. Organisms throughout the world, regardless of habitat, suffered similar rates of extinction, suggesting that the cause of the event was a global, not local, occurrence, and that it was a sudden event, not a gradual change. New evidence from strata in Greenland shows evidence of a double extinction, with a separate, less dramatic extinction occurring 9 million years before the Permian-Triassic (P-T) boundary, at the end of the Guadalupian epoch. Confusion of these two events is likely to have influenced the early view that the extinction was extended.

若想获取更多详尽出国留学攻略以及托福培训资讯，可以打开我们【上海新东方托福网】，涵盖托福培训，托福写作、口语、听力、阅读以及留学名校介绍等。上海新东方托福网在这里预祝各位考生学习顺利，都能考取自己满意的学校。