Why has no tetrapod re-evolved gills?
(writing in progress)
Various lineages of tetrapods, after extensive modification for terrestrial life, have evolved to re-enter the water (foraging entirely underwater).
These include
- Serpentes (https://en.wikipedia.org/wiki/Sea_snake)
- Sauropterygia
- Ichthyopterygia
- mosasaurs
- Hupehsuchia
- Pleurosauridae
- Thalattosauria
- Thalattosuchia
- Chelonia
- mustelid Carnivora https://en.wikipedia.org/wiki/Enhydra
- Sirenia https://en.wikipedia.org/wiki/Sirenia
- Cetacea
- Sphenisciformes
All of these forms have evolved streamlining and fin-like flippers. However, all retain lungs, and none has re-evolved gills.
Therefore, all aquatic tetrapods are limited by the need to return to the surface each time they inhale.
Why have gills not re-evolved?
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https://richarddawkins.net/2013/08/why-didnt-cetaceans-didnt-evovle-into-having-gills/
https://en.wikipedia.org/wiki/Sea_snake
https://evolution-outreach.biomedcentral.com/articles/10.1007/s12052-009-0139-y
Why gills? Why not any other form of water gas exchange - e.g. via the anus or tail, or elsewhere on the body?
Because gills are too leaky. Too high osmotic costs. Air is far more efficient. And oxygen can readily be stored in blood and muscles ...
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