Comment on The world’s oldest known vertebrates had two pairs of eyes
lvxferre@mander.xyz 5 days ago
The association with the pineal gland bugged me (what does a pair of eyes have to do with a gland responsible for sleep cycles), so I went for a wiki walk and found this page, on the “parietal eye”. It’s present in quite a few cold-blooded vertebrates, and responsible for both thermoregulation and sleep cycles.
I’d going to take a guess here and say it’s an intermediate stage between the second pair of eyes and pineal glands, something like
- secondary pair of eyes with a similar function to the primary one →
- secondary pair gets specialised into detecting near infra-red →
- secondary pair merges and gets protected by a membrane, forming the parietal eye →
- parietal eye specialises further, producing hormones for thermoregulation and sleep cycles →
- IR detection gets fucked up in hot-blooded animals (NB: parallel development for archosaurs and mammals) →
- the thing “hides” itself inside the cranium (less likely to be damaged), becoming the pineal gland
…or something like this. Herpetologists can probably come up with a better hypothesis than I do.
megopie@beehaw.org 5 days ago
It seems likely that its external sensing function faded before the development of “hot blood” (endothermy) as it’s vestigial even in very basal reptiles like the tuatara, so likely it was already disappearing as a sensory organ fairly early in quadruped evolution. Snakes, crocodiles and turtles (all exothermic) all lost it completely as an external feature, snakes are particularly notable as they’re in the same branch as tuataras and lizards, many of whole still have it as a vestigial external structure. It also appeared in some extinct branches of therapsids(many appear to have been endothermic) in some form, but is completely absent in mammals, the only surviving branch of therapsids.
It does function as a sensing organ in many amphibians, suggesting that it became vestigial for sensing some where in the early evolution of amniotes, but stuck around as an external structure across multiple branches but many have since convergently evolved to loose it as an external structure.
lvxferre@mander.xyz 5 days ago
Got it - thanks for the info!