The Devonian Period
The Devonian period is a slice of geologic time beginning at 416ma and lasting until 359ma. It occurs as a relative time period after the Silurian period and before the Mississippian epoch. Much of the Devonian period was characterized by epeiric seas which covered much of the North American craton, as well as other continental land masses worldwide. Evidence of this is seen in the widespread marine rocks deposited during this time. A multitude of marine organisms lived in the warm Devonian seas. Especially noteworthy is the high number of brachiopod and reef communities compared to the other geologic time periods. This was also a time when vertebrates began to diversify. Fish began to develop amphibian qualities which sparked a migration from sea to land.
The end of the Devonian period is infamous as it records one of five major biotic catastrophes in Earth's history, commonly known as the Frasnian — Famennian mass extinction. It turns out that the term "mass extinction" is probably incorrect because most paleontologists would prefer to call the Frasnian — Famennian event a "biodiversity crisis". A biodiversity crisis is a result of fewer species being formed through adaptation rather than an increase in species that become extinct.