Index fossils and relative dating
Although the recognition of fossils goes back hundreds of years, the systematic cataloguing and assignment of relative ages to different organisms from the distant past — paleontology — only dates back to the earliest part of the 19th century.The oldest undisputed fossils are from rocks dated around 3.5 Ga, and although fossils this old are typically poorly preserved and are not useful for dating rocks, they can still provide important information about conditions at the time.The oldest well-understood fossils are from rocks dating back to around 600 Ma, and the sedimentary record from that time forward is rich in fossil remains that provide a detailed record of the history of life.However, as anyone who has gone hunting for fossils knows, that does not mean that all sedimentary rocks have visible fossils or that they are easy to find.[SE] Some organisms survived for a very long time, and are not particularly useful for dating rocks.Sharks, for example, have been around for over 400 million years, and the great white shark has survived for 16 million years, so far.Insects, which evolved from marine arthropods, came onto land during the Devonian (400 Ma), and amphibians (i.e., vertebrates) came onto land about 50 million years later.By the late Carboniferous, trees had evolved from earlier plants, and reptiles had evolved from amphibians.
The second most significant extinction was at the Cretaceous-Paleogene boundary (K-Pg, a.k.a. At that time, about 75% of marine species disappeared.The major groups of organisms that we are familiar with evolved between the late Proterozoic and the Cambrian (~600 Ma to ~520 Ma).Plants, which evolved in the oceans as green algae, came onto land during the Ordovician (~450 Ma).Some well-studied groups of organisms qualify as biozone fossils because, although the genera and families lived over a long time, each species lived for a relatively short time and can be easily distinguished from others on the basis of specific features.For example, ammonites have a distinctive feature known as the suture line — where the internal shell layers that separate the individual chambers (septae) meet the outer shell wall, as shown in Figure 8.3.3.