one of the greatest changes in Earth history is marked by the Precambrian-Phanerozoic transition. At this time the Earth witnessed the first widespread appearance of organisms with hard shells, and there was a huge adaptive radiation unparalleled in the rest of Earth history. By this time most of the cratons on the planet had formed and large continents existed, and plate tectonics had already been through several supercontinent cycles.
The history of the early Paleozoic can be interpreted from shallow seas that repeatedly flooded the cratons and from orogenic belts on the margins where island arcs and other terranes collided with and were sutured to the cratons. The principles of sequence stratigraphy prove useful for interpreting the history of the Paleozoic, as this era is marked by many transgressions and regressions caused by global rise and fall of sea level, and these formed several globally recognized unconformity-bounded sequences.
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