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。地球历史上第二大灭绝事件发生在奥陶纪末期，导致当时约85%的物种消失。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，一群灭绝的半索动物。它们数量丰富，地理分布广泛，进化相对迅速，这意味着它们是识别地质时间短间隔和地球仪大面积区域这些间隔的优秀化石。深入了解这些生物的分类，进化和古生物学，以及它们在沉积地层中的分布，最大限度地发挥我们利用它们来解决地质问题的潜力，如构造和沉积盆地历史，以及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.