Lizards possess two distinct types of openings on their heads, each serving a specialized anatomical function. The first is a small, scale-covered spot on the top of the head, and the second involves larger gaps located on the sides of the skull behind the eye. These openings are not accidental; they represent distinct evolutionary adaptations that serve either a sensory purpose or a structural function.
The Sensory Spot: The Parietal Eye
The tiny, almost translucent spot visible on the forehead of many lizard species is a specialized organ known as the parietal eye, or sometimes the pineal eye. This structure is housed within a small opening in the skull bones called the parietal foramen. It is a photoreceptive organ that is sensitive to light, but unlike the main eyes, it does not form images.
The primary function of this “third eye” is to detect changes in light intensity and duration, particularly overhead sunlight. This capability is crucial for thermoregulation. Since lizards are ectothermic, the parietal eye helps them identify appropriate basking locations and maintain an optimal internal temperature.
Studies show that covering the parietal eye affects thermoregulatory behavior, often leading lizards to select higher body temperatures. Signals from this organ travel to the pineal gland, which synchronizes the animal’s internal biological clock. This connection helps regulate circadian rhythms, aligning daily cycles of activity and rest with the external day-night cycle.
The parietal eye also measures the overall duration of daylight, contributing to seasonal awareness, or photoperiod sensing. This awareness influences hormonal activity, which governs important life cycle events. These events include the timing of reproduction and preparation for hibernation.
Structural Gaps: Temporal Fenestrae
The other set of openings are the temporal fenestrae, which are large gaps in the bone structure located on the sides of the skull behind the eye socket. These openings do not house a sense organ but serve fundamental mechanical purposes related to feeding and skull design. Their presence is a defining trait for a large group of reptiles.
A primary function of the fenestrae is to accommodate the powerful jaw muscles used for biting. As these muscles contract, they expand in volume, and the gaps provide necessary space for the muscle bulk to operate efficiently. The openings also offer a greater surface area for the attachment of these jaw adductor muscles.
These expanded attachment points allow for stronger, more efficient jaw closure and increased bite force. Furthermore, replacing solid bone with open space reduces the overall weight of the skull. This weight reduction is achieved without compromising structural integrity, as the remaining bone forms struts that reinforce the skull against biting stresses.
Why Lizards Are Diapsids
The temporal fenestrae are a defining characteristic that places lizards within the evolutionary group known as Diapsida. The name Diapsida, meaning “two arches,” refers to the presence of two distinct openings, one upper and one lower, on each side of the skull behind the eye. This skull design is a shared ancestral trait among many reptiles.
This diapsid condition was inherited from early amniotes, the ancestors of all modern reptiles and birds. Other amniotes are categorized by different skull patterns, such as Synapsids (mammals), which have a single lower opening, and Anapsids (earliest reptiles), which originally lacked temporal openings.
The diapsid skull structure represents a successful evolutionary blueprint for specialized feeding mechanisms. This anatomical feature groups lizards, snakes, crocodiles, and birds together in this major classification of vertebrates.