对叶子化石的一项新研究表明，5000多万年前，全球突然变暖期间，昆虫蜂拥而至，进犯了今日美国北部的一些地区。该研究成果发表在不久前出版的美国《国家科学院院刊》（PNAS）上。研究指出，在我们现阶段的气候变暖过程中也可能会发生同样的情况。
一个地区的昆虫是否具有多样性，可以通过当地植物叶子遭受破坏的程度和类型推断出来。据该研究的发起人、宾夕法尼亚州立大学的研究生艾伦·柯拉诺介绍，大口咀嚼的昆虫在叶子上留下的疤痕不同，这可以在化石中看到。
研究人员发现，热带地区的昆虫种类最丰富，对植物叶子所造成的破坏也最多。这表明，较高的气温跟昆虫的多样性之间是相互关联的。
研究人员在来自怀俄明州毕葛红盆地（BighornBasin）的5000多块叶子化石中寻找疤痕证据。这些化石可以追溯到古新世-始新世极热时期(PETM)及其前后。古新世-始新世极热是大约5600万年前发生的一次全球突然变暖事件，跟目前的气候变化状况具有可比性。食叶昆虫留下痕迹的多少、类型以及吞掉叶子的比例，能够说明这个地区在不同的气候条件下爬行昆虫的多少。
研究人员发现，古新世末期较为凉爽，15%至38%的叶子遭受了昆虫的毁坏；在始新世初始阶段、极热时期过后气温有所下降时，33%的叶子遭到破坏；而在极热时期，有57%的叶子遭到破坏。可以看到，极热时期叶子遭到各种破坏的比例有了明显的上升。
柯拉诺及同事认为，这些昆虫的胃口变大是由于古新世-始新世极热时期二氧化碳水平上升造成的，这使得植物叶子的营养成分减少。柯拉诺解释说：“如果植物可以利用的二氧化碳较多，光合作用就会变得更加容易，植物无需在叶子中储存大量的蛋白质而同样能够提供自身所需的营养。因此，跟以前相比，昆虫必须吃掉更多的叶子才能得到同样多的营养。”
研究小组研究的三个不同时段的植物各不相同。在古新世-始新世极热时期，植物的多样性有所增加，这表明随着气温升高，植物品种逐渐地从墨西哥湾沿岸地区迁移出去。柯拉诺认为昆虫的迁移可能沿着相同的路线进行——“昆虫跟随着它们在遥远的南方食用的植物迁移过来的。”
在范围大小和变暖速度上，古新世-始新世极热时期跟目前的气候变化都具有可比性。因此，柯拉诺及同事认为，饥饿昆虫进犯的事件很可能发生于当今时代。
生物谷推荐原始出处：
PNAS 2008 105: 3815-3818; published online on March 3, 2008, 10.1073/pnas.0710180105
The oldest North American primate and mammalian biogeography during the Paleocene–Eocene Thermal Maximum
K. Christopher Beard
Section of Vertebrate Paleontology, Carnegie Museum of Natural History, 4400 Forbes Avenue, Pittsburgh, PA 15213
Edited by Alan Walker, Pennsylvania State University, University Park, PA, and approved January 10, 2008 (received for review October 25, 2007)
Undoubted primates first appear almost synchronously in the fossil records of Asia, Europe, and North America. This temporal pattern has complicated efforts to reconstruct the early dispersal history of primates in relation to global climate change and eustatic fluctuations in sea level. Here, I describe fossils from the Tuscahoma Formation on the Gulf Coastal Plain of Mississippi documenting an anatomically primitive species of Teilhardina that is older than other North American and European primates. Consistent with its antiquity, a phylogenetic analysis of dental characters recognizes Teilhardina magnoliana, sp. nov., as the most basal member of this genus currently known from either North America or Europe. Its stratigraphic provenance demonstrates that primates originally colonized North America near the base of the Paleocene–Eocene Thermal Maximum (PETM), but before an important fall in eustatic sea level. Correlation based on carbon isotope stratigraphy and sequence stratigraphy indicates that the earliest North American primates inhabited coastal regions of the continent for thousands of years before they were able to colonize the Rocky Mountain Interior. The transient provincialism displayed by early North American primates corresponds to similar biogeographic patterns noted among fossil plants. Decreased precipitation in the Rocky Mountain Interior during the early part of the PETM may have been an important factor in maintaining biotic provincialism within North America at this time. These results underscore the need to obtain multiple, geographically dispersed records bearing on significant macroevolutionary events such as the PETM.
