Outside References
The sudden extinction of the dinosaurs 65.5 million years ago is known as the Cretaceous–Paleogene extinction event. A similar massive reduction in Earth's biodiversity took place 251 million years ago; this is the Permian-Triassic extinction event. One of the Big Finish Productions audio plays, The Land of the Dead (also featuring the Fifth Doctor and Nyssa), provides a possible (fictional) explanation for that event as well. As with all the spin-off media, the canonicity of the audio plays is unclear.
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