Cambrian period do the Appalachian mountains first start to appear. The Paleozoic Era and the Phanerozoic Eon were both preceded by the Cambrian Period.
An crucial turning point in the evolution of life on Earth occurred during the Cambrian Period, when numerous invertebrate species and the first fish-like vertebrates first appeared in the fossil record. The greatest record of Cambrian animal fossils, including soft-bodied creatures, may be found in the Burgess Shale. The Paleozoic era's Cambrian phase saw the most rapid explosion of evolution ever recorded. A staggering diversity of life, including many of the main animal groupings still extant today, emerged during the Cambrian Explosion. The chordates, which include vertebrates (animals with backbones) like humans, were among them. According to some experts, the appearance of predators was made possible by a slight, possibly transient increase in oxygen that abruptly crossed an ecological threshold. Carnivory's emergence would have sparked an evolutionary arms race that produced the explosion of various body kinds and behaviours that now populate the oceans. The team comes to the conclusion that a scarcity of oxygen and toxic hydrogen sulphide killed off a large number of Cambrian species. It is believed that similar circumstances led to the devastating Permian extinction 251 million years ago. According to Gill, there were multiple instances of fast extinction in the second half of the Cambrian.
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