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Why do tuatara have a third eye on the top of their heads?
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tuatara
The species is the sole living member of the reptile order Rhynchocephalia.
The tuatara possesses a third eye, known as the parietal eye, located on the top of its head. While this structure contains a rudimentary lens, it is not used for vision. Instead, it is believed to serve an endocrine function by registering the dark-light cycle, which helps the animal regulate its hormones.1