An extinction is when a living organism is completely wiped out. Extinctions have occurred due to climate change, natural disasters, new species introduction, etc. Major extinctions on Earth were the Oxygen Catastrophe, the Ordovician-Silurian extinction, the late Devonian extinction, the Permian-Triassic extinction, the Triassic-Jurassic extinction, and the Cretaceous-Paleocene extinction. The Oxygen Catastrophe happened when dioxygen entered the air in large amounts and killed off many of the obligate anaerobic monerans. The Ordovician-Silurian occurred when a massive ice age killed off about two thirds of all brachiopods, bivalves, echinoderms, bryozoans, and corals. The late Devonian extinction was caused by oceanic cooling and only affected the sea organisms. The Permian-Triassic extinction occurred when carbon dioxide that was trapped underground was released and caused massive global heating, wiping out over 90% of all life. The Triassic-Jurassic extinction occurred as a result of gradual climate change which wiped out all conodonts, 34% of sea life, all crurotarsans or non-dinosaurian archosaurs, the remaining Therapsids, and many of the large amphibians. Probably the most famous extinction event is the Cretaceous-Paleogene. This extinction event was caused a large meteorite that crashed into the Earth, wiping out lots of ammonites, almost all reptiles, all the dinosaurs, pterosaurs, and sea diapsids. No animals on land that were bigger than a dog survived. This extinction event paved the way for the mammals.