The mysterious pineal/parietal eye/opening.
Some tetrapods have one between the parietals (Fig. 1). Others seal off that opening. Some either greatly expand the pineal or reduce the length of the parietal creating a ‘large’ opening relative to the length of the parietal. The pineal body varies greatly in size. In humans it is tucked in below the cerebrum and cerebellum. By contrast, in lampreys it extends high above the brain and is light sensitive (Fig. 2). According to Wikipedia, “The [third] eye is photoreceptive and is associated with the pineal gland, regulating circadian rhythmicity and hormone production for thermoregulation.
“The tuatara has a third eye on the top of its head called the parietal eye. It has its own lens, a parietal plug which resembles a cornea, retina with rod-like structures, and degenerated nerve connection to the brain. The parietal eye is only visible in hatchlings, which have a translucent patch at the top centre of the skull. After four to six months, it becomes covered with opaque scales and pigment. Its purpose is unknown, but it may be useful in absorbing ultraviolet rays to produce vitamin D, as well as to determine light/dark cycles, and help with thermoregulation. Of all extant tetrapods, the parietal eye is most pronounced in the tuatara. It is part of the pineal complex, another part of which is the pineal gland, which in tuatara secretes melatonin at night. Some salamanders have been shown to use their pineal bodies to perceive polarised light, and thus determine the position of the sun, even under cloud cover, aiding navigation.“
The distribution pattern in extinct tetrapods
is readily apparent in a broad sense ( Fig. 1). Even so, exceptions appear often. Reversals appear rarely. It is interesting to note the last time a third eye opening appeared in the skulls of various lineages. The large reptile tree (LRT, 1413 taxa) scores for 231 traits, one of which is #39, the pineal foramen. Scoring choices include:
- Present and tiny <.20 parietal length
- Present and large ≥ .20 parietal length
- Between the frontals (= anterior to the parietals)
The primitive state is: ‘between the frontals’.
This occurs in basal fish, prior to sarcopterygians. This state also occurs by reversal in several iguanid squamates related to Chlamydosaurus.
The derived state is ‘absent’.
This occurs in a wide variety of taxa from derived eryopids to mammals, several squamates, macrocnemids (including fenestrasaurs) and euarchosauriformes among others.
Most basal tetrapods
have a ‘tiny’ pineal opening.
Most basal reptiles
have a large pineal opening.
According to Wikipedia
“The pineal gland is a small endocrine gland in the brain of animals with backbones. The pineal gland produces melatonin, a serotonin-derived hormone which modulates sleep patterns in both circadian and seasonal cycles.”
“The results of various scientific research in evolutionary biology, comparative neuroanatomy and neurophysiology, have explained the phylogeny of the pineal gland in different vertebrate species. From the point of view of biological evolution, the pineal gland represents a kind of atrophied photoreceptor. In the epithalamus of some species of amphibians and reptiles, it is linked to a light-sensing organ, known as the parietal eye, which is also called the pineal eye or third eye.”
Zhu M, Yu X-B, Ahlberg PE, Choo B and 8 others 2013. A Silurian placoderm with osteichthyan-like marginal jaw bones. Nature. 502:188–193.