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We know that there are many exceptions to this, because of geological processes such as tectonics, metamorphic folding, subduction, etc., but these processes are added to the basic principal of deeper=older.
By the end of the 18th century, fossils were accepted as remains of past life, and in the early 1800s William Smith (England), and George Cuvier and Alexandre Brongniart (France) documented that different layers contained distinctive fossils that characterized their chronological periods, opening the doors to the use of fossils to establish a sequence of rock layers through time, and across global geographies. It is defined by the First Appearence Datum (FAD) of a specific short-range index species, and ends at the FAD of the next chosen index species.
This has entered popular knowledge: the Mesozoic is the "Age of Dinosaurs", and the Paleozoic is the era marked by trilobites.
The chart at right (adapted from Tormo 2002) shows geological strata of the middle Middle Cambrian in the Montagne Noire area, southern France, SW Europe, and the trilobites found within them (various species of Solenopleuridae), recording their presence as dots on the right half of the chart.
The As mentioned earlier, trilobites play a large role in the biostratigraphy of the Paleozoic, but their major utility is in the Cambrian, when trilobite diversity was at its greatest.The emerging standards for the Ordovician, in contrast, rely much more on graptolites (colonial marine invertebrates with distinctive skeletons).Other major fossil groups used along with trilobites for biostratigraphy include foraminifera (tiny planktonic single-celled eukaryotes rather like amoebas with shells), conodonts (now known to be the hard mouthparts of jawless, swimming marine vertebrates), and archaeocyathans (early Cambrian sponge-like creatures that formed the first reefs, living in cup-shaped calcium carbonate structures). Using the relative patterns of stratigraphy that emerged from the study of these different fossils, scientists (separately at first, but now as international committees) continue to refine the geological time scale, combining geology, paleontology, and radioisotope chronology to set benchmarks in time across the globe.The first index species can also occur in the second biozone.
By the 1890s, several of the main divisions of the Paleozoic era, such as the Cambrian and Carboniferous periods were internationally recognized.
These mark the boundary between the Cambrian Terreneuvian Series (lacking trilobites), and Cambrian Series 2.