A new study, published in the journal Nature Ecology & Evolution, has analyzed the conservation status of over 29,000 animal species and found that the existence of more than 500 species remains uncertain. This number includes both species that are known to exist but have not been seen in the wild for many years, as well as species that have never been formally described.

The study, which was led by researchers from the International Union for Conservation of Nature (IUCN), found that the most uncertain species are those that live in remote or inaccessible regions, or those that are nocturnal or cryptic in behavior. Among the species with uncertain status are the Yangtze giant softshell turtle, which has not been seen in the wild since 2005; the Amur leopard, of which only about 100 individuals remain; and the cryptic species complex of Amazonian marmosets, which is thought to contain several species that have yet to be described.

The researchers say that the high number of uncertain species highlights the need for more research on the conservation status of wildlife. They also call for greater efforts to protect these species from extinction, as many of them are likely to be at risk due to habitat loss, hunting, or other human activities.

"This study provides a starting point for identifying and prioritizing species that need further research and conservation efforts," said lead author Dr. Gernot Segelbacher. "By shedding light on the species that we don't know much about, we can help to ensure that they don't become lost forever."

The study is the most comprehensive assessment of the conservation status of animal species ever conducted. It involved the collaboration of over 100 scientists from around the world.