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论文标题:Rapid recovery of life at ground zero of the end-Cretaceous mass extinction
作者:Christopher M. Lowery, et al.
期刊:Nature
发表时间:2018/05/30
数字识别码:10.1038/s41586-018-0163-6
摘要:
The Cretaceous/Palaeogene mass extinction eradicated 76% of species on Earth. It was caused by the impact of an asteroid on the Yucatán carbonate platform in the southern Gulf of Mexico 66 million years ago, forming the Chicxulub impact crater. After the mass extinction, the recovery of the global marine ecosystem—measured as primary productivity—was geographically heterogeneous; export production in the Gulf of Mexico and North Atlantic–western Tethys was slower than in most other regions, taking 300 thousand years (kyr) to return to levels similar to those of the Late Cretaceous period. Delayed recovery of marine productivity closer to the crater implies an impact-related environmental control, such as toxic metal poisoning, on recovery times. If no such geographic pattern exists, the best explanation for the observed heterogeneity is a combination of ecological factors—trophic interactions, species incumbency and competitive exclusion by opportunists—and ‘chance’. The question of whether the post-impact recovery of marine productivity was delayed closer to the crater has a bearing on the predictability of future patterns of recovery in anthropogenically perturbed ecosystems. If there is a relationship between the distance from the impact and the recovery of marine productivity, we would expect recovery rates to be slowest in the crater itself. Here we present a record of foraminifera, calcareous nannoplankton, trace fossils and elemental abundance data from within the Chicxulub crater, dated to approximately the first 200 kyr of the Palaeocene. We show that life reappeared in the basin just years after the impact and a high-productivity ecosystem was established within 30 kyr, which indicates that proximity to the impact did not delay recovery and that there was therefore no impact-related environmental control on recovery. Ecological processes probably controlled the recovery of productivity after the Cretaceous/Palaeogene mass extinction and are therefore likely to be important for the response of the ocean ecosystem to other rapid extinction events.
2005年,美国芝加哥大学的戴维·艾拉·雅布隆斯基(David Ira Jablonski)曾提出,大灭绝还遵循着“非建设性选择”(non-constructive selectivity)的原则。这一原则认为,一个演化分支能否从灾难中幸存,既不完全取决于其个体性状,也不完全随机。比如,一些物种能够幸存,最重要的原因在于它们分布得足够广泛。那些仅生活在一小片区域,或是只生活在一座岛上的物种就可能面临更大的灭绝风险。不过也有一些例外的情况,比如在三叠纪末期大灭绝中陆生动物的灭绝就不符合这一原则。
不过,通常情况下,内因和外因会共同发挥作用:红色皇后要求物种不停地跑,但方向却是小丑的音乐,受到外界的控制。美国丹佛市自然科学博物馆(Denver Museum of Natural Sciences)的泰勒·利松(Tyler Lyson)和同事曾试图复原了白垩纪末期大灭绝后复苏期的详细情景。这还要多亏了在美国科罗拉多州科拉尔断崖发现的化石群,这里保存了白垩纪最后10万年和古新世前100万年间的精美化石。
灭绝后的复苏并不一定总是这么容易。伯明翰大学的理查德·J·巴特勒(Richard J. Butler)告诉我们,在二叠纪末期大灭绝发生后100万年的时间里,陆地动物群的多样性仍然很低,直到500万年后才发生了真正广泛的演化,出现了食草动物和大型捕食者。直到中三叠世,生物多样性才恢复至正常水平。为什么这次复苏需要这么久呢?一方面是因为这场大灭绝太严重了,生态系统的损失越大,回归原本状态所需的时间就越长。而另一个原因,就是当时的地球发生了地质环境和气候条件的巨变,而地球很难摆脱这一事件的影响。
在每次大灭绝发生后,总有一些类群呈“爆发式”演化,它们会以令人震惊的速度形成新的物种。剑桥大学的丹尼尔·J·菲尔德(Daniel J. Field)指出,甚至在希克苏鲁伯撞击事件发生后不到100万年,不只是哺乳动物,鸟类也演化出了大部分现存的目。鸟类在这一阶段的演化速度太快了,以至于我们几乎不可能建立鸟类的演化树:看起来鸟类的不同演化分支似乎都是在同一时间点开始的。不过,也有一些生物并不符合这一规律。
珊瑚:即使不考虑高排放情景,到2070年,也预计有75%的珊瑚礁面临灭绝危险。然而,理查德·J·特威切特(Richard J. Twitchett)发现,现生珊瑚(属于Scleractinia,石珊瑚目)已经经历过几次严重的全球变暖事件,但仍幸存至今,这其中就包括约2亿年前三叠纪末期的那次全球变暖事件。
理查德·J·特威切特(Richard J. Twitchett)是伦敦自然历史博物馆的古生物学家,他相信通过研究约2亿年前的三叠纪末期大灭绝,能够为生命未来的发展方向提供线索。另外,3亿多年前还发生了一次大灭绝事件,它可能不如其他大灭绝知名,但这是唯一一次由于地球迅速变暖导致的大灭绝。期间,一些分布广泛且有重要生态价值的生物,比如珊瑚,幸存了下来。同样地,这一事件也有可能帮助我们预测人类世未来的生态系统,因为它展示了生命是如何应对缺氧与高温导致的更高新陈代谢需求的。