Warty tree frog (Scinax boulengeri), Costa Rica.
© Piotr Naskrecki
Scientists believe that ancient amphibians were the first vertebrates to leave the water and colonize our planet's shores millions of years ago.
Modern-day amphibians – frogs, toads, newts, salamanders and caecilians – also constitute an important link between terrestrial and aquatic ecosystems, perpetuating nutrient cycling in their environments. They may be a food source for larger animals, but amphibians themselves also control pests, including insects known to be vectors of human diseases like malaria.
Their moist permeable skin makes amphibians particularly susceptible to environmental changes; they are consequently considered to be good indicators of ecosystem health. In recent years, habitat loss, widespread disease and climate change have caused drastic population crashes in amphibian populations across the globe.
IN DEPTH: Global Search for 'lost' frogs yields few findings, important warnings
Here are just a few of the many fascinating amphibian species who have not been seen for over a decade. Some may be lost forever, while others may still exist, hidden under rocks in a remote stream, waiting to be rediscovered.