Science, Tech, Math › Animals & Nature Cretaceous-Tertiary Mass Extinction The Event That Killed off the Dinosaurs Share Flipboard Email Print KARSTEN SCHNEIDER / Getty Images Animals & Nature Evolution History Of Life On Earth Human Evolution Natural Selection Evolution Scientists The Evidence For Evolution Resources Amphibians Birds Habitat Profiles Mammals Reptiles Wildlife Conservation Insects Marine Life Forestry Dinosaurs View More By Heather Scoville Science Expert M.A., Technological Teaching and Learning, Ashford University B.A., Biochemistry and Molecular Biology, Cornell University Heather Scoville is a former medical researcher and current high school science teacher who writes science curriculum for online science courses. our editorial process Heather Scoville Updated August 11, 2019 Scientists across several disciplines, including geology, biology, and evolutionary biology, have determined that there have been five major mass extinction events throughout the history of life on Earth. For an event to be considered a major mass extinction, more than half of all known life forms in that time period must have been wiped out. Cretaceous-Tertiary Mass Extinction Probably the best-known mass extinction event took out all the dinosaurs on Earth. This was the fifth mass extinction event, called the Cretaceous-Tertiary Mass Extinction, or K-T Extinction for short. Although the Permian Mass Extinction, also known as the "Great Dying," was much larger in the number of species that went extinct, the K-T Extinction is the one most people remember because of public fascination with dinosaurs. The K-T Extinction divides the Cretaceous Period, which ended the Mesozoic Era, and the Tertiary Period at the start of the Cenozoic Era, which we currently live in. The K-T Extinction happened around 65 million years ago, taking out an estimated 75% of all living species on Earth at the time. Many people know that land dinosaurs were casualties of this major mass extinction event, but numerous other species of birds, mammals, fish, mollusks, pterosaurs, and plesiosaurs, among other groups of animals, also went extinct. Asteroid Impacts The main cause of the K-T Extinction is well documented: an unusually high number of extremely large asteroid impacts. Evidence can be seen in various parts of the world in layers of rock that can be dated to this time period. These rock layers have unusually high levels of iridium, an element not found in large amounts in the Earth's crust but is very common in space debris such as asteroids, comets, and meteors. This universal layer of rock has come to be known as the K-T boundary. By the Cretaceous Period, the continents had drifted apart from when they were one supercontinent called Pangaea in the early Mesozoic Era. The fact that the K-T boundary can be found on different continents indicates the K-T Mass Extinction was global and happened quickly. 'Impact Winter' The impacts weren't directly responsible for the extinction of three-quarters of the Earth's species, but their residual effects were devastating. Perhaps the biggest issue caused by the asteroids hitting Earth is termed "impact winter." The extreme size of the space debris vaulted ash, dust, and other matter into the atmosphere, essentially blocking out the Sun for long periods of time. Plants, no longer able to undergo photosynthesis, began to die off, leaving animals with no food, so they starved to death. It's also thought that oxygen levels declined due to the lack of photosynthesis. The disappearance of food and oxygen affected the largest animals, including land dinosaurs, the most. Smaller animals could store food and needed less oxygen; they survived and thrived once the danger passed. Other major catastrophes caused by the impacts included tsunamis, earthquakes, and possibly increased volcanic activity, yielding the devastating results of the Cretaceous-Tertiary Mass Extinction event. Silver Lining? As horrific as they must have been, mass extinction events were not all bad news for those that survived. The extinction of the large, dominant land dinosaurs allowed smaller animals to survive and thrive. New species emerged and took on new niches, driving the evolution of life on Earth and shaping the future of natural selection on various populations. The end of the dinosaurs particularly benefited mammals, whose ascendance led to the rise of humans and other species on Earth today. Some scientists believe that in the early 21st century, we are in the middle of the sixth major mass extinction event. Because these events often span millions of years, it's possible that the climate changes and Earth changes—physical changes to the planet—that we are experiencing will trigger the extinction of several species and in the future will be seen as a mass extinction event. Sources "K-T Extinction: Mass Extinction." Encyclopaedia Britannica."Cretaceous-Tertiary Extinction Event." ScienceDaily.com."Why Did the Dinosaurs Go Extinct?" National Geographic.