Graptolite biodiversity history, paleoenvironmental cycles, chronostratigraphy of the silurian period, and phylogeny of paleozoic hemichordates

寒武纪、奥陶纪和志留纪时期（约5.42 - 4.16亿年前）是气候、海平面和生物多样性急剧变化的时期。地球历史上第二大物种大灭绝事件发生在奥陶纪末期，当时大约85%的物种灭绝。在这些时期的海洋浮游生物化石记录中，最重要的动物是笔石，这是一组已灭绝的半足动物。它们数量丰富，地理分布广泛，进化速度相对较快，这意味着它们是识别短地质时间间隔和在全球大片地区进行这些间隔的相关性的极好化石。透彻地了解这些生物的分类、演化和古生物学，以及它们在沉积地层中的分布，可以最大限度地利用它们来解决构造学和沉积盆地历史等领域的地质问题，并帮助重建生命的历史。

The Cambrian, Ordovician and Silurian periods (ca. 542-416 million years ago) were times of dramatically changing climate, sea level, and biodiversity. The second biggest mass extinction event in Earth history occurred near the end of the Ordovician Period and was responsible for the disappearance of approximately 85% of the species living at the time. The most important animals in the fossil record of marine plankton during these time periods was the graptolites, a group of extinct hemichordates. They were abundant, geographically widespread and evolved relatively rapidly, which means that they are excellent fossils for identifying short intervals of geologic time and correlation of those intervals over large areas of the globe. A thorough understanding of the classification, evolution, and paleobiology of these organisms, and their distribution within sedimentary strata maximizes our potential to use them to solve geological problems in areas such as tectonics and sedimentary basin history, as well as to help reconstruct the history of life.