Tuataras, the "Living Fossil" Reptiles

Tuataras are a rare family of reptiles restricted to the rocky islands off the coast of New Zealand. Today, tuatara are the least diverse reptile group, with only one living species, Sphenodon punctatus; however, they were once more widespread and diverse than they are today, spanning  Europe, Africa, South America and Madagascar. There were once as many as 24 different genera of tuataras, but most of those disappeared by about 100 million years ago, during the middle Cretaceous period, doubtless succumbing to competition by better-adapted dinosaurs, crocodiles and lizards.

Tuatara are nocturnal burrowing reptiles of coastal forests, where they forage over a restricted home range and feed on bird eggs, chicks, invertebrates, amphibians, and small reptiles. Since these reptiles are cold-blooded and live in a cool climate, tuataras have extremely low metabolic rates, growing slowly and achieving some impressive life spans. Amazingly, female tuataras have been known to reproduce until they reach the age of 60, and some experts speculate that healthy adults can live for as long as 200 years (about in the neighborhood of some large species of turtles). As with some other reptiles, the sex of tuatara hatchlings depends on the ambient temperature; an unusually warm climate results in more males, while an unusually cool climate results in more females.

The oddest feature of tuataras is their "third eye": a light-sensitive spot, located on the top of this reptile's head, which is thought to play a role in regulating circadian rhythms (that is, the tuatara's metabolic response to the day-night cycle). Not simply a patch of skin sensitive to sunlight—as some people mistakenly believe—this structure actually contains a lens, cornea, and primitive retina, albeit one that is only loosely connected to the brain. One possible scenario is that the ultimate ancestors of the tuatara, dating to the late Triassic period, actually had three functioning eyes, and the third eye gradually degraded over the eons into the modern tuatara's parietal appendage.

Where does the tuatara fit in on the reptile evolutionary tree? Paleontologists believe that this vertebrate dates to the ancient split between lepidosaurs (that is, reptiles with overlapping scales) and archosaurs, the family of reptiles that evolved during the Triassic period into crocodiles, pterosaurs, and dinosaurs. The reason the tuatara deserves its epithet of "living fossil" is that it's the simplest identified amniote (vertebrates that lay their eggs on land or incubate them within the female's body); this reptile's heart is extremely primitive compared to those of turtles, snakes and lizards, and its brain structure and posture harks back to the ultimate ancestors of all reptiles, the amphibians.

Key Characteristics of Tuataras

• extremely slow growth and low reproductive rates
• reach sexual maturity at 10 to 20 years of age
• diapsid skull with two temporal openings
• prominent parietal "eye" on top of head

Classification of Tuataras

Turtles are classified within the following taxonomic hierarchy:

Animals > Chordates > Vertebrates > Tetrapods > Reptiles > Tuatara