Tuatara are greenish brown and grey, and measure up to from head to tail-tip and weigh up to with a spiny crest along the back, especially pronounced in males. They have two rows of teeth in the upper jaw overlapping one row on the lower jaw, which is unique among living species. They are also unusual in having a pronounced photoreceptive eye, the third eye, which is thought to be involved in setting circadian and seasonal cycles. They are able to hear, although no external ear is present, and have unique features in their skeleton, some of them apparently evolutionarily retained from fish. Tuatara are sometimes referred to as "living fossils", which has generated significant scientific debate. This term is currently in disuse among paleontologists and evolutionary biologists. Although tuatara have preserved the morphological characteristics of their Mesozoic ancestors (240-230 million years ago), there is no evidence of a continuous fossil record to support this. Researchers mapping its genome have discovered that the species has between 5 and 6 billion base pairs of DNA sequence, nearly twice that of humans. The tuatara ("Sphenodon punctatus") has been protected by law since 1895. A second species, the Brothers Island tuatara ("S. guntheri", Buller, 1877), was recognised in 1989, but since 2009 it has been reclassified as a subspecies ("S. p. guntheri"). Tuatara, like many of New Zealand's native animals, are threatened by habitat loss and introduced predators, such as the Polynesian rat "(Rattus exulans)". Tuatara were extinct on the mainland, with the remaining populations confined to 32 offshore islands until the first North Island release into the heavily fenced and monitored Karori Wildlife Sanctuary (now named "Zealandia") in 2005.