Anyone know the details to the third eye in most early tetrapods and
even in todays reptiles.

It seems like the strangest turn in evolution.

On Saturday, January 7, 2023 at 2:13:56 PM UTC-8, Popping Mad wrote:
> Anyone know the details to the third eye in most early tetrapods and
> even in todays reptiles.
>
> It seems like the strangest turn in evolution.

Wikipedia has an interesting entry for this; lots of details of which I was unaware. It seems
to a very primitive feature, having develope long before vertebrates, possibly primitive to Bilateria(?).

https://en.wikipedia.org/wiki/Parietal_eye

erik simpson <eastside.erik@gmail.com> wrote:
> On Saturday, January 7, 2023 at 2:13:56 PM UTC-8, Popping Mad wrote:
>> Anyone know the details to the third eye in most early tetrapods and
>> even in todays reptiles.
>>
>> It seems like the strangest turn in evolution.
>
> Wikipedia has an interesting entry for this; lots of details of which I
> was unaware. It seems
> to a very primitive feature, having develope long before vertebrates,
> possibly primitive to Bilateria(?).
>
> https://en.wikipedia.org/wiki/Parietal_eye
>
Recently PZ Myers got into the molecular evolution of photoreception and
how it differs across bilaterian phyla. He touched on circadian rhythms
which though neontological in coverage might be relevant here:

https://youtu.be/jhexF9rEo-M

On Sunday, January 8, 2023 at 9:11:39 AM UTC-8, *Hemidactylus* wrote:
> erik simpson <eastsi...@gmail.com> wrote:
> > On Saturday, January 7, 2023 at 2:13:56 PM UTC-8, Popping Mad wrote:
> >> Anyone know the details to the third eye in most early tetrapods and
> >> even in todays reptiles.
> >>
> >> It seems like the strangest turn in evolution.
> >
> > Wikipedia has an interesting entry for this; lots of details of which I
> > was unaware. It seems
> > to a very primitive feature, having develope long before vertebrates,
> > possibly primitive to Bilateria(?).
> >
> > https://en.wikipedia.org/wiki/Parietal_eye
> >
> Recently PZ Myers got into the molecular evolution of photoreception and
> how it differs across bilaterian phyla. He touched on circadian rhythms
> which though neontological in coverage might be relevant here:
>
> https://youtu.be/jhexF9rEo-M

Outstanding. Informative not only in the chemical details diverging from Ur-bilaterian to
Protostomes and Deuterostomes, but in illustrating how evolutionary changes can be
inferred where there are no fossil eyes.

On 1/7/23 19:10, erik simpson wrote:
> On Saturday, January 7, 2023 at 2:13:56 PM UTC-8, Popping Mad wrote:
>> Anyone know the details to the third eye in most early tetrapods and
>> even in todays reptiles.
>>
>> It seems like the strangest turn in evolution.
>
> Wikipedia has an interesting entry for this; lots of details of which I was unaware. It seems
> to a very primitive feature, having develope long before vertebrates, possibly primitive to Bilateria(?).
>
> https://en.wikipedia.org/wiki/Parietal_eye

https://academic.oup.com/biolinnean/article/101/4/870/2450636

The parietal, or ‘third’, eye is a photosensory organ that occurs as a
part of the pineal complex (i.e. the pineal gland and associated
structures) in some vertebrates, most notably in many lizards and in the
tuatara (Eakin, 1973). Almost all vertebrates, except crocodilians and a
few mammals, have a pineal complex that is generally involved in the
endocrinal regulation of circadian and seasonal cycles, reproduction,
and body temperature (Quay, 1979; Ralph et al., 1979); its main
secretory product being the hormone melatonin (Lutterschmidt,
Lutterschmidt & Hutchison, 2003). The pineal complex is considered to be
most developed in ‘lower’ vertebrates and to show a tendency for
evolutionary reduction in size and functionality (Edinger, 1955; Eakin,
1973; but see also Quay, 1979). The ancestral state is presumed to have
been a possibly paired photosensory organ, as seen in some extant
cyclostomes. Possibly, the parietal eye and the pineal gland of
tetrapods are the descendants of the left and right parts of this organ
(Eakin, 1973). In the pineal gland, the endosecretory pinealocytes
appear to be derived from photoreceptor cells (Collin, 1971; Kappers,
1971; Ralph et al., 1979) and, in many taxa, including lizards, the
pineal gland retains photosensory capability (Edinger, 1955; Gundy &
Wurst, 1976b; Quay, 1979).

The parietal eye shows a phylogenetic distribution that reflects
frequent reduction, loss or rudimentation. Indeed, several Permian
reptiles, including some therapsids, had parietal foramina that are
relatively much larger than those of any extant taxa (Edinger, 1955;
Quay, 1979). The parietal eye is lost in birds, turtles, crocodilians,
snakes, and mammals, as well as in many individual species, genera, and
families of lizards. Gundy & Wurst (1976a, b) report that about 60% of
all lizard genera include species with an externally visible parietal
eye. Among amphibians, a frontal eye occurs in ranid frogs, although
parietal eyes appear absent or vestigial in other taxa (Ralph, 1975).
The loss of parietal eyes is also supported by reports of possible
developmental vestiges in some snakes, birds, and mammals (Stebbins &
Eakin, 1958; Quay, 1979).

The lizard parietal eye (Fig. 1) clearly has photosensory capability as
indicated by its structure with a somewhat cup-shaped photosensory
retina usually below a translucent ‘lens’ and a ‘cornea’, and by direct
evidence for electrophysiological response to light (Hamasaki, 1969;
Solessio & Engbretson, 1999). Nevertheless, its specific ecological
functions are enigmatic (Eakin, 1973). The frequent evolutionary losses
could mean that its functionality is easily dispensable, and one
hypothesis is that the parietal eye is a functionless vestige that is
maintained with some degree of complexity in some taxa as a result of
unknown constraints. As a result of its complex cup-eye design, however,
the parietal eye must have had an adaptive ancestral photosensory
function, and alternative adaptive explanations of its maintenance can
be found either in continuation of ancestral function(s), or in the
exaptation of new functions. Evidence for current adaptation in the
broad sense can either be found in direct demonstration of ecologically
relevant functionality, or indirectly through meaningful covariation of
parietal-eye traits with ecological variables on low phylogenetic levels.

On 1/8/23 09:11, *Hemidactylus* wrote:
> erik simpson <eastside.erik@gmail.com> wrote:
>> On Saturday, January 7, 2023 at 2:13:56 PM UTC-8, Popping Mad wrote:
>>> Anyone know the details to the third eye in most early tetrapods and
>>> even in todays reptiles.
>>>
>>> It seems like the strangest turn in evolution.
>>
>> Wikipedia has an interesting entry for this; lots of details of which I
>> was unaware. It seems
>> to a very primitive feature, having develope long before vertebrates,
>> possibly primitive to Bilateria(?).
>>
>> https://en.wikipedia.org/wiki/Parietal_eye
>>
> Recently PZ Myers got into the molecular evolution of photoreception and
> how it differs across bilaterian phyla. He touched on circadian rhythms
> which though neontological in coverage might be relevant here:
>
> https://youtu.be/jhexF9rEo-M

Very interesting.

The things I get out of it after some reflection are:

1. The patterning of activities differently for day
versus night or circadian rhythms, like mentioned earlier,
may go back all the way to the divergence of bilaterans
from jellyfisn, if not earlier. Some animals might be
nocturnal, some diurnal, and some crepuscular, but modifying
activities based upon day and night in multicellular animals
with nervous systems is very ancient.

2. Since this divergence is so ancient, it probably occurred
in the oceans, where the sunlight penetrates to the photic
layers but is less significant in the deeper layers.

On 1/8/23 12:11, *Hemidactylus* wrote:
> Recently PZ Myers got into the molecular evolution of photoreception

which has nothing to do with this, fwiw.

https://www.youtube.com/watch?v=_JxI1XxHXrc
https://www.youtube.com/watch?v=ZEL68kt1Q-8

https://www.science.org/doi/abs/10.1126/science.1123802
https://link.springer.com/chapter/10.1007/978-3-642-65495-4_4

Visual Centers in the Brain pp 113–140Cite as

The Parietal Eye (Pineal and Parietal Organs) of Lower Vertebrates
Eberhard Dodt
Chapter
315 Accesses

23 Citations

3 Altmetric

Part of the Handbook of Sensory Physiology book series (1536,volume 7 /
3 / 3 B)

Abstract
In the course of evolution the median eyes, in addition to paired
lateral eyes, have independently developed in several classes of animals
including crustaceans, insects and vertebrates. Within the order of
vertebrates some lower classes including fishes, amphibians and reptiles
possess photosensitive structures within the epiphyseal complex deriving
ontogenetically from the diencephalon. While the gross anatomy of the
median eyes has been well examined for over 100 years, the fine
structure and knowledge of the physiological properties of median
photoreceptors has remained remarkably incomplete. Until recently their
functional role was based mainly on indirect evidence and speculation,
except for the classes of crustaceans and insects in which both the
functional operation and the sensory significance of the median eyes
(ocelli) are well known.

On 1/8/23 09:11, *Hemidactylus* wrote:
> erik simpson <eastside.erik@gmail.com> wrote:
>> On Saturday, January 7, 2023 at 2:13:56 PM UTC-8, Popping Mad wrote:
>>> Anyone know the details to the third eye in most early tetrapods and
>>> even in todays reptiles.
>>>
>>> It seems like the strangest turn in evolution.
>>
>> Wikipedia has an interesting entry for this; lots of details of which I
>> was unaware. It seems
>> to a very primitive feature, having develope long before vertebrates,
>> possibly primitive to Bilateria(?).
>>
>> https://en.wikipedia.org/wiki/Parietal_eye
>>
> Recently PZ Myers got into the molecular evolution of photoreception and
> how it differs across bilaterian phyla. He touched on circadian rhythms
> which though neontological in coverage might be relevant here:
>
> https://youtu.be/jhexF9rEo-M

I wonder if there could be something like some sort of
'living fossils' among some of the advanced jellyfish
or worms.

One set of sensors for night, another set of sensors for
day. The relative output of the sensor sets changes the
activity patterns.

Simple inhibitory rather than stimulatory receptors at
the interface between the two nerve nets or day/night
nerve fibers determine whether the organism has day or
night patterned activity.

On Saturday, January 7, 2023 at 5:10:01 PM UTC-7, erik simpson wrote:
> On Saturday, January 7, 2023 at 2:13:56 PM UTC-8, Popping Mad wrote:
> > Anyone know the details to the third eye in most early tetrapods and
> > even in todays reptiles.
> >
> > It seems like the strangest turn in evolution.
> Wikipedia has an interesting entry for this; lots of details of which I was unaware. It seems
> to a very primitive feature, having develope long before vertebrates, possibly primitive to Bilateria(?).
>
> https://en.wikipedia.org/wiki/Parietal_eye

I suppose the answer to this is obvious but I’d rather not speculate, so here goes: is the 3rd eye more useful for ”cold-blooded” animals than endothermic ones? In other words, might it do more than just regulate sleep and have some sort of thermoregulatory function?