This reptile is born with a working third eye on top of its head

Tuatara hatchlings have a parietal eye complete with its own lens and retina, visible as a pale patch of skin on the skull. It can't form images, but its light-sensitive cells appear to track the day-night cycle, likely helping regulate hormones tied to circadian and seasonal rhythms. The patch is usually covered over by scales within about six months.

— Richard M. Eakin, The Third Eye — University of California Press, 1973
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