There are about 500 species of lost animals on Earth that have not been declared extinct, the article published in Animal Conservation reports.

Researchers reviewed information on 32,820 species from the International Union for Conservation of Nature's Red List. According to Union rules, a species is considered extinct "when there is no reasonable doubt that the last species died." This is rather difficult to verify. A total of 311 terrestrial vertebrate species have been declared extinct since 1500.

Arne Mooers of Canada's Simon Fraser University has analyzed how many animal species can be considered lost, that is, their representatives have not been seen for more than 50 years. In total, scientists counted 562 species: Reptiles led the way with 257 species considered lost, followed by 137 species of amphibians, 130 species of mammals and 38 species of birds. Most of these lost animals were last seen in megadiverse countries such as Indonesia (69 species), Mexico (33 species) and Brazil (29 species).

The scientists hope that future research efforts will focus on identified endangered regions where the existence of many species is in question. Their extinction needs to be either confirmed or disproved.