K-Pg extinction event

An artist's conception of the asteroid collision which probably caused the Cretaceous-Tertiary Extinction Event

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The Cretaceous-Paleogene extinction event (aka the Cretaceous-Tertiary extinction event, abbreviated as either the K-T extinction event or the K-Pg extinction event) was a major mass extinction event that took place around 66 million years ago, and led to the deaths of a large number of organisms around the globe, including all of the dinosaurs (except birds). About 50% of all the plants and animals that existed at the time died out.

Causes of the extinction

There may have ultimately been multiple different causes for the K-Pg extinction event, but what is widely agreed upon is that the major catalyst for the extinction was a 10–15 km asteroid or comet that hit what is now the Yucatán peninsula of Mexico, and formed the Chicxulub crater. Other causes may have included the Deccan Traps Strangely enough, the extinction ratios of clades affected by the extinction suggests that the extinction was ultimately due to the re-entry of ejecta into the atmosphere after the impact occurred.

Alternate extinction hypotheses

There is some residual debating regarding the exact causes of the K-Pg extinction event, with some people believing that the event was actually caused by something other than the asteroid impact. In general the evidence favors a combination of factors including the Deccan Traps and the impact, but the impact was definitely the main factor. While it has been debated whether or not dinosaurs experienced a decline prior to the terminal Cretaceous extinction event, a study published in the journal The Proceedings of the National Academy of Sciences conducted a thorough analysis for the first time and found that the data supports a long term decline among non-avian dinosaurs prior to the aforementioned end-Cretaceous extinction event.

Extinction ratios

Freshwater biomes were in general less affected by the K-Pg extinction event than their terrestrial or marine counterparts, with amphibians in particular not being generally affected by the extinction event, with only a few extinctions among them. Terrestrial organisms were hit hard, however, with the extinction of all birds outside of Neornithes, all pterosaurs, all non-avian dinosaurs, various groups of marine fish, and entire clades of mammals (marsupials used to be the dominant mammals in North America, but after the K-Pg extinction event only a few marsupials remained, among them Peradectes, with the mammals above the K-Pg boundary layer that dominated North America being placentals). The oceans were hit hard as well, with almost every marine reptile outside of sea turtles going extinct, not to mention the extinction of the ammonites and the belemnites.

The nature of the victims of this extinction event notably make pseudo-scientific cryptozoological claims all the more ridiculous. If sparrow-sized birds didn't make it, do you seriously think a sauropod would?

See also

• History of the Earth
• Palaeocene — the first epoch of the Paleogene period of the modern Cenozoic era
• Dinosaurs - The most notable taxon to be rendered extinct by this extinction event, excluding neornithine birds.