A unique feature of the tuatara is a "third eye" on the top of the head.
The "eye" has a retina, lens, and nerve endings, but is not used for seeing.
It is visible under juvenile tuatara skin but after a few months becomes covered with scales and pigment.
The unique eye is sensitive to light and is believed to help the tuatara judge the time of day or season.
Additionally, it has two parallel rows of teeth in its upper jaw, and the gap between these rows are where the teeth from the lower jaw fit to perform special grinding/sawing motion to crush prey.
Furthermore, the tuatara has a diapsid skull, but lacks a complete lower temporal bar, which separates it from other species as well as its acrodont dentition and overhanging pair of incisor-like teeth.
Tooth shape was originally designed for a strictly insectivore diet with piercing teeth.
Later on, the teeth became more diversified for various ancestors of the tuatara, which included herbivores, carnivores, and omnivores.
The teeth were complex enough to crush crab shells, while others maintained continuous growth in the lower jaw for the breakdown of plant material.
Their current dental structure is specialized for grinding up prey after capture.
The fossil record shows the tuatara lineage separating from squamates approximately 240 million years ago.

Tuatara were originally classified as lizards in 1831, when the species was discovered by John Edward Gray and the British Museum received a skull.
The fossil records show that they have been around since the Middle Triassic, approximately 240 million years ago.
The tuatara is traditionally called a "living fossil".
This statement has been challenged, as some studies have considered it a model of evolutionary adaptation well adapted to its current conditions, rather than an unchanging remnant of the rest of Sphenodontia.
A further major difference from lizards is the row of spine-like structures running part way down the back at least in the New Zealand species of Tuatara.

The genus remained misclassified until 1867, when Albert Günther of the British Museum noted features similar to birds, turtles, and crocodiles.
He proposed the order Rhynchocephalia (meaning "beak head") for the tuatara and its fossil relatives.
 Many disparately related species were subsequently added to the Rhynchocephalia, resulting in what taxonomists call a "wastebasket taxon".
These include the superficially similar (both in shape and name) but unrelated rhynchosaurs, which lived in the Triassic.
Williston proposed the Sphenodontia to include only tuatara and their closest fossil relatives in 1925.
Sphenodon is derived from the Greek for "wedge" (σφήν/sphen) and "tooth" (ὀδούς/odous).
However, today Rhynchocephalia is used to include Gephyrosaurus and Sphenodontia, while Sphenodontia excludes the former.
A unique feature of the tuatara is a "third eye" on the top of the head. The "eye" has a retina, lens, and nerve endings, but is not used for seeing. It is visible under juvenile tuatara skin but after a few months becomes covered with scales and pigment. The unique eye is sensitive to light and is believed to help the tuatara judge the time of day or season. Additionally, it has two parallel rows of teeth in its upper jaw, and the gap between these rows are where the teeth from the lower jaw fit to perform special grinding/sawing motion to crush prey. Furthermore, the tuatara has a diapsid skull, but lacks a complete lower temporal bar, which separates it from other species as well as its acrodont dentition and overhanging pair of incisor-like teeth. Tooth shape was originally designed for a strictly insectivore diet with piercing teeth. Later on, the teeth became more diversified for various ancestors of the tuatara, which included herbivores, carnivores, and omnivores. The teeth were complex enough to crush crab shells, while others maintained continuous growth in the lower jaw for the breakdown of plant material. Their current dental structure is specialized for grinding up prey after capture. The fossil record shows the tuatara lineage separating from squamates approximately 240 million years ago.
Tuatara were originally classified as lizards in 1831, when the species was discovered by John Edward Gray and the British Museum received a skull. The fossil records show that they have been around since the Middle Triassic, approximately 240 million years ago. The tuatara is traditionally called a "living fossil". This statement has been challenged, as some studies have considered it a model of evolutionary adaptation well adapted to its current conditions, rather than an unchanging remnant of the rest of Sphenodontia. A further major difference from lizards is the row of spine-like structures running part way down the back at least in the New Zealand species of Tuatara.
The genus remained misclassified until 1867, when Albert Günther of the British Museum noted features similar to birds, turtles, and crocodiles. He proposed the order Rhynchocephalia (meaning "beak head") for the tuatara and its fossil relatives.  Many disparately related species were subsequently added to the Rhynchocephalia, resulting in what taxonomists call a "wastebasket taxon". These include the superficially similar (both in shape and name) but unrelated rhynchosaurs, which lived in the Triassic. Williston proposed the Sphenodontia to include only tuatara and their closest fossil relatives in 1925. Sphenodon is derived from the Greek for "wedge" (σφήν/sphen) and "tooth" (ὀδούς/odous). However, today Rhynchocephalia is used to include Gephyrosaurus and Sphenodontia, while Sphenodontia excludes the former.
Rhynchocephalia