Is a Tuatara a Lizard? The Last of an Ancient Reptile Order

Why the tuatara looks like a lizard

The tuatara of New Zealand has a scaly skin, four sprawling legs, a long tail and a lizard-like head, so the mistake is easy to make. The resemblance is not a coincidence. Tuatara, lizards and snakes all belong to the same larger group of reptiles, the Lepidosauria, and they share a body plan inherited from a common ancestor.

They also share a few telltale features. In lepidosaurs the cloaca, the single opening for waste and reproduction, is a slit that runs across the body, and many of them can drop part of the tail when attacked and grow it back. Tuatara do both, which is why they are often grouped with lizards in popular books. For more on tail shedding, see Why do lizards drop their tails?

How tuataras differ from lizards

Under the skin, the tuatara is a very different animal. The table sums up the main differences; the sections below explain them.

Teeth and jaws: a natural saw

A tuatara’s teeth are not set in sockets or along the inner side of the jaw, as in most lizards. They are fused to the crest of the jawbone, so firmly that the boundary between tooth and bone is hard to see. The upper jaw carries two parallel rows, and the single row of the lower jaw closes between them. The lower jaw then slides forward, so the teeth work like a saw that cuts through beetle shells and small bones.

These teeth are never replaced. As they wear down over decades, old tuatara switch to soft food such as worms and slugs and eventually chew with the bare edges of the jaw. A few lizard groups, such as agamids and chameleons, also have teeth fused to the jaw crest, but none has the tuatara’s double row and sliding bite. The front of the upper jaw ends in a chisel-like point, which gave the order its name, Rhynchocephalia, meaning “beak heads”.

Skull, ears and the third eye

Behind each eye a tuatara has a complete bar of bone closing off an opening in the skull. Lizards have lost this lower bar, which lets their skulls flex as they bite. Oddly, the earliest rhynchocephalians also lacked it, so the tuatara’s closed bar is thought to have evolved again later, probably to strengthen the skull for its powerful bite.

The tuatara has no ear opening and no eardrum. Its middle ear is filled mostly with fatty tissue, and it hears only low sounds, roughly 100–800 Hz. Most lizards, by contrast, have an obvious eardrum on the side of the head.

Like many lizards, the tuatara has a parietal or “third” eye on top of the head, but in the tuatara it is especially well developed, with a lens and a retina. It is visible only in hatchlings, as a pale spot, before scales cover it. Read more in Do iguanas have a third eye?

Ribs, backbone and other hidden differences

The skeleton holds several features that lizards lost long ago. Tuatara have well-developed gastralia, rib-like bones in the belly wall that are not attached to the backbone. Their true ribs carry small hooked projections, called uncinate processes, a feature also seen in birds. They also keep a small bone, the proatlas, between the skull and the first neck vertebra, and their vertebrae are hourglass-shaped, concave at both ends, an ancient arrangement also found in fish.

Mating is different too. Male lizards and snakes have a pair of copulatory organs called hemipenes. Male tuatara have none, and sperm passes when the male presses his cloaca against the female’s, as in most birds. Finally, tuatara are far more cold-tolerant than lizards, staying active at about 5 °C (41 °F), a temperature at which most lizards are sluggish or inactive.

Where the tuatara fits in the reptile family tree

Rhynchocephalians first appear in the fossil record in the Middle Triassic, about 244–241.5 million years ago. During the Jurassic they were the dominant small reptiles in many parts of the world, and the group included plant-eaters and even long-bodied swimming forms. They declined during the Cretaceous, possibly in competition with lizards and mammals. Their last known record outside New Zealand comes from Patagonia in South America, around 66–60 million years ago.

New Zealand’s isolation saved the tuatara. Fossils of close relatives about 19–16 million years old have been found in Otago, which suggests the lineage was already there when New Zealand drifted away from the rest of the ancient southern continent of Gondwana.

Is the tuatara a living fossil?

The tuatara is often called a living fossil, and its body plan does resemble that of some Jurassic relatives. Many biologists now avoid the term, because the tuatara has kept evolving: its double tooth row and closed skull bar, for example, are later specializations, not leftovers from the earliest reptiles. Its genome, at about 5 billion base pairs, is also one of the largest of any reptile.

The label is debated for other ancient-looking animals too, such as the coelacanth. What is not in doubt is that the tuatara is the last survivor of a branch of the reptile family that has been separate from lizards and snakes for roughly a quarter of a billion years.