its historic range and failed to find evidence of a surviving individual. Possibly extinct Extinction of taxa is difficult to detect, as a long gap without a sighting is not definitive. Some mammals declared as extinct may very well reappear. 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. As of December 2015, the IUCN listed 30 mammalian species as "critically endangered (possibly extinct)". See also Holocene extinction List of extinct animals List of extinct bird species since 1500 Lists of mammals by population Notes References Mammals M recently extinct