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After the Permo-Triassic mass extinction (the largest extinction of all), there was a radiation of life from the Triassic on till the end of the Cretaceous that became to be known as Mesozoic life. These newly evolved life forms included: the dinosaurs, mammals, pterosaurs (flying reptiles), amphibians (frogs, turtles) and the first known birds (appeared in the Jurassic). The Permian-Triassic extinction left many niches open; consequently allowing major radiations in some of the surviving groups, especially the marine organisms: ammonoids, belemnoids, corals, marine reptiles, bivalves, brachiopods, etc. The end of the Cretaceous at 65 Ma marks the last major extinction in geologic past, wiping out close to 60-70% of the species present and allowing the radiation of mammals from the Tertiary onwards. The Cretaceous-Tertiary ranks as one of the four major extinction events in geologic past - arguably ranks as the second severest extinction after the Permo-Triassic extinction.

Figure: K/T ranks as one of the four major extinctions.

The K/T extinction is the most highly publicized paleontological event. Scientists and the media alike have always gravitated towards this event with one trying to discover its secrets, the other trying to report them. The allure of the dinosaurs has always been an attraction to both groups. Early theories dealing with the K/T extinction event naturally focused only on the dinosaurs. This has led to many of the earlier theories being highly biased, explaining the extinction of only the dinosaurs - which, in all regards, were not the most abundant organisms in the Cretaceous. Subsequent to the unanimous accepting of the theories that explained only the extinction of the dinosaurs, more research was conducted in the area. This research has led to more credible theories being formulated, shifting the focus of the dinosaurs and moving it to more towards the marine species that were exterminated in much larger numbers than the dinosaurs.