The Tuatara, a Still-Evolving Original

http://www.nytimes.com/2010/11/23/science/23angier.html?pagewanted=1&_r=1&src=fbmain

By NATALIE ANGIER
Published: November 22, 2010

As a femur-shaped island paradise that snapped away from the Gondwana supercontinent some 80 million years ago, New Zealand is famously home to eccentric forms of wildlife that look like pets for a Hobbit.

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the animal that may well be New Zealand’s most bizarrely instructive species at first glance looks surprisingly humdrum: the tuatara. A reptile about 16 inches long with bumpy, khaki-colored skin and a lizardly profile, the tuatara could easily be mistaken for an iguana. Appearances in this case are wildly deceptive. The tuatara — whose name comes from the Maori language and means “peaks on the back” — is not an iguana, is not a lizard, is not like any other reptile alive today.

In fact, as a series of recent studies suggest, it is not like any other vertebrate alive today. The tuatara, scientists have learned, is in some ways a so-called living fossil, its basic skeletal layout and skull shape almost identical to that of tuatara fossils dating back hundreds of millions of years, to before the rise of the dinosaurs. Certain tuatara organs and traits also display the hallmarks of being, if not quite primitive, at least closer to evolutionary baseline than comparable structures in other animals.

For example, the tuatara has a third eye at the top of its skull, the legendary if poorly understood pineal eye, which is found in only a sprinkling of reptile species and which vision researchers suspect harks back to nature’s original eye — pretty much a few light-sensitive cells on a stalk. A tuatara’s teeth likewise follow the no-nonsense design seen in dinosaur dentition, erupting directly from the jawbone and without the niceties of tooth sockets and periodontal ligaments that characterize the teeth of all mammals and many reptiles. Some researchers are looking at tuataras for clues to how dental implants, which are inserted directly into the jaw, might be improved.

Yet in a startling counterpoint to the notion of the tuatara as a holdover from Triassic Park, researchers lately have discovered that a few regions of tuatara DNA appear to be evolving at hyperspeed, possibly the fastest mutation rate yet clocked in a vertebrate genome. The quick-changing sequences are limited to so-called neutral regions of the tuatara’s DNA, affecting filler codes, rather than the molecular blueprints for how to build a tuatara. The researchers have yet to determine what the observed hypermutability is all about, but obviously, said David M. Lambert of Griffith University, in Brisbane, Australia, an author of the study, “the processes that govern skeletal morphology are decoupled from the biological processes that govern changes in DNA.”

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