Part 1 Printing the Circuit Board. 1. Choose a method to use for creating the PCB. Your choice will usually be based on the availability of materials needed by the method, the technical difficulty level of the method or the quality of PCB you desire to obtain.
Permian-Triassic Extinction. In this video segment from Evolution: Extinction!, geologist Peter Ward shows rock layers laid down during the Permian and Triassic periods. The Permian layers contain abundant animal fossils and fossilized traces of animals, while the Triassic layers are almost devoid of fossils, suggesting a mass extinction event occurred 250 million years ago, at the end of the Permian.
A Brief History of Earth. Mass extinctions--when at least half of all species die out in a relatively short time--have happened only a handful of times over the course of our planet's history. The largest mass extinction event occurred around 250 million years ago, when perhaps 95 percent of all species went extinct. Top Five Extinctions