Extinctions

Picture credit: left-thinkquest.org (Trilobite), right- northallegheny.org (Dinosaur)

Two major extinctions occurred in the Earth's history. The extinction at the end of the Paleozoic era wiped out 50% of all marine invertebrate families and 90% of all marine invertebrate species became extinct. There are several theories what caused these extinctions but climatic changes (cooling) is one of the leading explanations.

The Mesozoic extinction which killed off the dinosaurs has been blamed on the impact of a meteorite. This impact put so much dust in the air it blocked out sunlight which halted photosynthesis and global temperatures also diminished. Proof of this meteorite collision lies in the discovery of the Cretaceous-Tertiary boundary. This is a worldwide layer of iridium. Iridium is found in high concentrations of some meteorites.