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.
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