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

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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.