Recently extinct mammals are any mammal that went extinct since the year 1500 C. E., as defined by the International Union for Conservation of Nature (IUCN). Extinction of taxa is difficult to detect as long gaps without a sighting is not definitive, but before 1995 a threshold of 50 years without a sighting was used to declare extinction. 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. 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 lists 30 mammalian species as "cr