The tuatara is not a lizard, but the last survivor of an ancient lineage.
In short: Often mislabeled as a lizard, the tuatara is actually the sole remaining member of Rhynchocephalia, a once-diverse order of reptiles that dominated the planet during the Mesozoic era. This living fossil, endemic to New Zealand, possesses unique anatomical features—including a third eye—that set it apart from all other modern reptiles.
The order Rhynchocephalia first emerged in the Middle Triassic, roughly 244 million years ago. While they are now restricted to a single genus, Sphenodon, they were once a widespread and ecologically varied group. During the Triassic and Jurassic periods, they occupied diverse niches, including herbivorous, carnivorous, and even aquatic forms, before suffering a massive decline during the Cretaceous. By the beginning of the Cenozoic, the group had almost entirely vanished.
Physically, the tuatara is distinct from the order Squamata—which includes lizards and snakes—despite their shared ancestry within the superorder Lepidosauria. Key differences include the tuatara's retention of belly bones called gastralia and a lack of a penis, a secondary loss that contrasts with the hemipenes found in squamates. Furthermore, the tuatara features a parietal eye on the top of its head. While this organ possesses a rudimentary lens, it does not form images; instead, it serves an endocrine function, helping the animal regulate hormone production and circadian rhythms by sensing light cycles.
The tuatara's biology is marked by extreme longevity and slow development. Individuals commonly live 60 years or more, with some capable of reproduction after age 60. Their reproductive cycle is equally deliberate, with females laying eggs only every four years, and those eggs requiring 11 to 16 months to incubate. Today, these reptiles are threatened by introduced mammals like the Polynesian rat, which prey on their eggs and young. Their survival remains a focal point for scientists studying the evolutionary history of reptiles, as they provide a rare window into a lineage that diverged from other reptiles some 250 million years ago.
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Sources
- Summary by Intellect · source: Rhynchocephalia | reptile order | Britannica
- Also: Rhynchocephalia
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