Other indigenous animals are represented by reptiles (tuatara, skinks and geckos), frogs, spiders, insects (), and snails. Some, such as the tuatara, are so unique that they have been called living fossils. Three species of bats (one since extinct) were the only sign of native land mammals in New Zealand until the 2006 discovery of bones from a unique, mouse-sized land mammal at least 16 million years old. Marine mammals, however, are abundant, with almost half the world's cetaceans (whales, dolphins, and porpoises) and large numbers of fur seals reported in New Zealand waters. Many seabirds breed in New Zealand, a third of them unique to the country. More penguin species are found in New Zealand than in any other country, with 13 of the world's 18 penguin species.