Extinction of taxa is difficult to confirm, as a long gap without a sighting is not definitive, but before 1995 a threshold of 50 years without a sighting was used to declare extinction.[1]
One study found that extinction from habitat loss is the hardest to detect, as this might only fragment populations to the point of concealment from humans. Some mammals declared as extinct may very well reappear.[1] For example, a study found that 36% of purported mammalian extinction had been resolved, while the rest either had validity issues (insufficient evidence) or had been rediscovered.[3]
As of June 2023, the IUCN listed 233 mammalian species as critically endangered, while 27% of all mammalian species were threatened with extinction.[4]
^Ceballos, G.; Ehrlich, A. H.; Ehrlich, P. R. (2015). The Annihilation of Nature: Human Extinction of Birds and Mammals. Baltimore, Maryland: Johns Hopkins University Press. ISBN978-1421417189. "69"
^"IUCN Red List version 2022.2". The IUCN Red List of Threatened Species. International Union for Conservation of Nature and Natural Resources (IUCN). Retrieved 21 June 2023.