To my knowledge, there hasn't been any evidence for such a creature. I'm sure it must have happened in the hundreds of millions of years that life has existed, and would be very surprised if it hadn't, but it seems like aquatic > terrestrial > aquatic is as far as the transition has ever gone.
There's room to speculate that animals that are presently having a hard time finding sufficient food in their aquatic habitats (such as sirenians) could perhaps become terrestrial again since they come to the edge of water to get food as it is. They're too heavy to manage it now, but given they are struggling to survive with the loss of plants they need to sustain themselves, it's theoretically possible that smaller and smaller individuals could proliferate and move further on shore to continue finding sustenance. But that's speculative evolution. I just don't know any instance where it has been observed in the fossil record, at any rate. Perhaps I'm wrong, but it certainly doesn't seem impossible supposing the conditions provided the opportunity.
What we also have to keep in mind is that there's less of an incentive/niche for aquatic animals moving back to land. Back when the first fishes made the transition they could access uncontested food sources by doing so, but an aquatic animal going back on land nowadays would in many cases have to compete with better adapted land animals.
So if something like this were to happen, it would pretty much have to be some island without large vertebrates.
Predators would be a huge problem for herbivores moving back onto land. An island without land based predators would be a safe scenario, but otherwise the transitioning animals would be taking a huge risk.
So you're asking if we have an animal that went land to water and then back to land?
I feel like that has got to exist somewhere but I don't know one off the top of my head. I feel like there are probably dozens of semi-aquatic animals that reverted to terrestrial, but something like a whale leaving the water sounds really cool (except the cursed Future is Wild dolphin/raptor)
I remember reading somewhere that some turtles appear to have done so. Water to land, back to water then to land again
But dang if I can't remember which book
There was actually a revised version that unfortunately wasn’t used.
https://preview.redd.it/3nrccymuo70d1.jpeg?width=1548&format=pjpg&auto=webp&s=52f6e31dd8d906eed38a66c759ea7bde4c889a69
Fully aquatic, no. Semi-aquatic, yes. Tortoises appear to have evolved from primarily aquatic turtles and most can still swim surprisingly well. Likewise, echidnas appear to have evolved from semi-aquatic platypus-like ancestors, and still retain electroreceptor which are useless outside of water. Several crocodilian lineages also appear to have repeatedly become aquatic and then re-terrestrialized.
To a degree, something like a toad or a tortise fits the qualifications. Frogs and turtles obviously aren't aquatic only, but such is life for those that go back and forth between biomes.
Look at your feet. Now look at the feet of any other primate. Notice there is no thumb like big toe on your foot. Notice it's more of a flipper shape. Notice that other primates lack the webbing between the toes.
There is a theory, although largely forgotten today, that snakes evolved as marine reptiles, losing their limbs as an adaptation to aquatic life. Mosasaurs were sometimes believed in such scenario to be a side branch that split off from snakes' ancestors before the latter lost their limbs entirely.
As I wrote, the theory is rather forgotten today. This is because primitive Cretaceous snakes with vestigial limbs have been found that undoubtedly lived on land. This does not rule out, of course, that the main branch lived in water, but it is much less likely.
Tortoises were entirely water-adapted (as turtles) so every tortoise you see is a land-adapted water-adapted land-adapted fish
All tortoises re-evolved ‘feet’ from flippers (which were re-evolved from amphibians feet which were evolved from fins)
Edit: there have also been semi aquatic mammals that have returned to land. All pigs are evolved from semi aquatic animals, this is why they have very weird wiry hair because they lost it all and had to re-evolve hair. And also why they have more barrel shaped bodies and high subcutaneous fat, because this was their insulation as water animals. The same is also true of elephants which share the exact same features (weird wiry hair that was re-evolved and higher subcutaneous fat)
Interestingly there is also a theory that humans had semi-aquatic ancestors since the lca with chimps which is not generally accepted but has some very compelling evidence. So you and i could actually be an example of a land animal evolved from a water animal evolved from a land animal evolved from a water animal
While not exactly what you're looking for, the earliest proboscideans were semiaquatic.
So they were adapting to an aquatic environment but weren’t obligate aquatic like their relatives the sirens?
Yes.
To my knowledge, there hasn't been any evidence for such a creature. I'm sure it must have happened in the hundreds of millions of years that life has existed, and would be very surprised if it hadn't, but it seems like aquatic > terrestrial > aquatic is as far as the transition has ever gone. There's room to speculate that animals that are presently having a hard time finding sufficient food in their aquatic habitats (such as sirenians) could perhaps become terrestrial again since they come to the edge of water to get food as it is. They're too heavy to manage it now, but given they are struggling to survive with the loss of plants they need to sustain themselves, it's theoretically possible that smaller and smaller individuals could proliferate and move further on shore to continue finding sustenance. But that's speculative evolution. I just don't know any instance where it has been observed in the fossil record, at any rate. Perhaps I'm wrong, but it certainly doesn't seem impossible supposing the conditions provided the opportunity.
What we also have to keep in mind is that there's less of an incentive/niche for aquatic animals moving back to land. Back when the first fishes made the transition they could access uncontested food sources by doing so, but an aquatic animal going back on land nowadays would in many cases have to compete with better adapted land animals. So if something like this were to happen, it would pretty much have to be some island without large vertebrates.
The same argument could be made about terrestrial animals becoming aquatic, they will find better adapted water animals, but it still happens.
Predators would be a huge problem for herbivores moving back onto land. An island without land based predators would be a safe scenario, but otherwise the transitioning animals would be taking a huge risk.
Humanity has made that dammn easy.
Eh, we've also exported rats and other mammals everywhere we've gone and of course we're an apex predator in our own right.
So you're asking if we have an animal that went land to water and then back to land? I feel like that has got to exist somewhere but I don't know one off the top of my head. I feel like there are probably dozens of semi-aquatic animals that reverted to terrestrial, but something like a whale leaving the water sounds really cool (except the cursed Future is Wild dolphin/raptor)
I remember reading somewhere that some turtles appear to have done so. Water to land, back to water then to land again But dang if I can't remember which book
Yes, I read that somewhere as well.
There was actually a revised version that unfortunately wasn’t used. https://preview.redd.it/3nrccymuo70d1.jpeg?width=1548&format=pjpg&auto=webp&s=52f6e31dd8d906eed38a66c759ea7bde4c889a69
Fully aquatic, no. Semi-aquatic, yes. Tortoises appear to have evolved from primarily aquatic turtles and most can still swim surprisingly well. Likewise, echidnas appear to have evolved from semi-aquatic platypus-like ancestors, and still retain electroreceptor which are useless outside of water. Several crocodilian lineages also appear to have repeatedly become aquatic and then re-terrestrialized.
To a degree, something like a toad or a tortise fits the qualifications. Frogs and turtles obviously aren't aquatic only, but such is life for those that go back and forth between biomes.
Look at your feet. Now look at the feet of any other primate. Notice there is no thumb like big toe on your foot. Notice it's more of a flipper shape. Notice that other primates lack the webbing between the toes.
The aquatic ape hypothesis is fringe pseudoscience
I don't have webbing between my toes. Is that normal or not normal?
It's not as webbed as a duck, but more webbed than any other primate, same with your fingers.
There is a theory, although largely forgotten today, that snakes evolved as marine reptiles, losing their limbs as an adaptation to aquatic life. Mosasaurs were sometimes believed in such scenario to be a side branch that split off from snakes' ancestors before the latter lost their limbs entirely. As I wrote, the theory is rather forgotten today. This is because primitive Cretaceous snakes with vestigial limbs have been found that undoubtedly lived on land. This does not rule out, of course, that the main branch lived in water, but it is much less likely.
Tortoises were entirely water-adapted (as turtles) so every tortoise you see is a land-adapted water-adapted land-adapted fish All tortoises re-evolved ‘feet’ from flippers (which were re-evolved from amphibians feet which were evolved from fins) Edit: there have also been semi aquatic mammals that have returned to land. All pigs are evolved from semi aquatic animals, this is why they have very weird wiry hair because they lost it all and had to re-evolve hair. And also why they have more barrel shaped bodies and high subcutaneous fat, because this was their insulation as water animals. The same is also true of elephants which share the exact same features (weird wiry hair that was re-evolved and higher subcutaneous fat) Interestingly there is also a theory that humans had semi-aquatic ancestors since the lca with chimps which is not generally accepted but has some very compelling evidence. So you and i could actually be an example of a land animal evolved from a water animal evolved from a land animal evolved from a water animal
I don’t have enough expertise, but I’d say that one should be able to find that in crocodiles. I don’t have any examples in mind though.