TOEFL reading test 40 with answers
Mass Extinctions [■] Cases in which many species become extinct within a geologically short interval of time are called mass extinctions. [■] There was one such event at the end of the Cretaceous period (around 70 million years ago). [■] There was another, even larger, mass extinction at the end of the Permian period (around 250 million years ago). [■] The Permian event has attracted much less attention than other mass extinctions because mostly unfamiliar species perished at that time. The fossil record shows at least five mass extinctions in which many families of marine organisms died out. The rates of extinction happening today are as great as the rates during these mass extinctions. Many scientists have therefore concluded that a sixth great mass extinction is currently in progress. What could cause such high rates of extinction? There are several hypotheses, including warming or cooling of Earth, changes in seasonal fluctuations or ocean currents, and changing positions...