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A cool thing to imagine would be a world where the land was dominated by megafaunal frog-descendants, as it is/was by mammals today, dinosaurs before the mammals, and non-mammalian synapsids and crocodylomorphs before the dinosaurs.

However, frogs aren't exactly the kind of creature that would be in the position to seize the megafauna niches, compared to other clades like mammals, birds, turtles or squamates. They're not particularly generalistic, poorly suited to terrestrial locomotion and could probably be seriously diminished in the future (due to chytridiomycosis), all that being relative to the aforementioned clades.

So, the playing field would need be leveled to allow them to dominate. However, my question comes here; what natural extinction event could seriously damage the like of mammals, squamates and turtles, but spare frogs? Come to think of it, even within Amphibia salamanders would probably be better equipped to rise.

I'm asking this for a friend, who was wondering. Well, technically, he said he'd be my friend if I figured this out for him, so no pressure...

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    $\begingroup$ You know, since reptiles evolved from amphibians, and mammals from reptiles, we could argue we are all frogs #JeSuisLaGrenouille $\endgroup$ – Renan Apr 18 at 17:13
  • $\begingroup$ How similar to modern frogs do this hypothetical clade's members need to be? I ask because the wide range of mammals that we see today share very little visually and behaviorally with the mammals that survived the extinction of the dinosaurs. $\endgroup$ – David Coffron Apr 18 at 17:52
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    $\begingroup$ @Renan Well, not really, I'm afraid. First of all, frogs didn't appear until way after the appearance of amniotes. Secondly, reptiles didn't evolve from amphibians (amphibians referring to Lepospondyls, Temnospondyls and Lissamphibians), they evolved from stem-tetrapods, and mammals didn't evolve from reptiles (otherwise Sauropsida would not be considered monophyletic), they evolved from stem-amniotes. $\endgroup$ – SealBoi Apr 18 at 17:55
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    $\begingroup$ I think that this is particularly demeaning to all french people. $\endgroup$ – Mathaddict Apr 18 at 18:02
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    $\begingroup$ Sorry, I learned biology by playing videogames :D $\endgroup$ – Renan Apr 18 at 18:21
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Well, amphibians in general already had a run as the dominant species of land vertebrates, although they were also the first so I guess there wasn't much competition for that spot.

So as most other tetrapods are going to be too much competition, we first need to wipe almost all of them out, down to the smallest. This means something like the Great Dying, where most biodiversity is wiped out completely. In considering what kind of event would work best I think it's useful to make a list of what amphibians in general have going on for them:

  • They have a lower metabolic requirements than most other tetrapods, probably tied with reptiles, so they can handle a sudden food scarcity a little better. They also feed mostly on arthropods, many species of which are likely to survive even a massive extinction.

  • They can handle the cold better than reptiles while remaining active, but not to the extent of some birds and mammals. However, birds and mammals both rely on their metabolism to keep this advantage, as without it the insulation provided by hair, feathers and fat wouldn't really work. Amphibians have a more passive resistance, some of them even being able to survive freezing, returning active when they thaw.

  • They are amphibious. Duh. This however isn't such a big advantage in surviving an extinction event, as most living amphibians tend to live only in freshwater, and ponds, rivers and lakes are going to fare much worse in the face of an extinction compared to seas and oceans.

Considering the previous points, I would suggest the following options. None are flawless and most require very contrived coincidences in order to work, but I do believe that frogs and amphibians in general might survive them and diversity in the following eons.

  • Viral epidemic: reptiles, birds and mammals are all fairly close in terms of evolutionary relationships, amphibians are the most removed tetrapod, cladistically speaking. Therefore, if some kind of horrible virus started spreading among say birds, it would have an easier time jumping to reptiles and mammals than to amphibians. This hypothetical virus could cause catastrophic symptoms for long term survival of the aforementioned clades, maybe imparting a mutation that heavily favors the carrier in the short term but dooms it in the long run, allowing it to spread and not be selected against quickly. Meanwhile frogs and other amphibians are just removed enough that the virus doesn't manage to make the jump before most of its viable hosts have gone extinct.

  • Terrible global cooling: some kind of super ice age with a fast onset could wipe out most animals from the face of the earth, but a lucky amphibian with a genetic knack for hibernation or an innate resistance to cold might manage to endure the worse winters. Toads and newts are the most likely winners among amphibians. This also has the benefit of removing the amphibians most fierce competition for inheriting the earth, rodents, as small animals actually do much worse at lower temperatures and if this hypothetical ice age happens quickly enough they wouldn't have a chance to adapt. Meanwhile larger animals die simply because they require more food, which will quickly become scarce. I will point out that such an ice age would need to happen like with timing like clockwork in order to work. Too slow and other tetrapods adapt just like they already did in the past. It also needs to last just long enough to avoid making it a TPK for the entire ecosystem. Something like repeated meteor impacts might do the trick.

This is all I can think of, but I will say this: in speculative zoology the "how" is quite often the least interesting part, it rarely holds up to scrutiny and it gets in the way of the premise, so if you want to do Frogworld, do it and drop just a few hints in regards to what made it possible, without going in too much detail.

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  • $\begingroup$ Note those early amphibians were not frogs, frog are latecomers. $\endgroup$ – John Apr 20 at 13:22
  • $\begingroup$ Absolutely, though I figured that once you get amphibians in general to become dominant again, selecting frogs as the centerpiece is much easier and can be pretty much handwaved. They are after all the amphibian group that colonised the largest variety of environments, from ponds to treetops. $\endgroup$ – Inquisitive Geek Apr 20 at 14:25
  • $\begingroup$ Salamanders have done better, All that plus invading caves and glacial streams. $\endgroup$ – John Apr 20 at 15:44
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Honestly, another meteor, and a bit of luck

I know this is the boring answer, but it seems the most plausible. The reasons mammals were able to thrive after the K-T extinction event is that they could survive off of insects and aquatic plants in the absence of the larger flora the dinosaurs relied on. Amphibians fill a similar role in ecosystems: eating lots of insects. The only edge frogs have over these small mammals is that they serve the first part of their life underwater.

So, if another meteor hit the earth (perhaps one made of a lot of ice to increase the amount of water on the planet), most of the larger animals die off, leaving behind small mammals and reptiles, small amphibians (like frogs), and aquatic or semi-aquatic creatures (just like before). Perhaps one key aspect of this changing world is the abundance of small waters where tadpoles can thrive. Then, because the aquatic creatures didn't suffer as much, it would be better for adult individuals to leave the water (to avoid getting eaten by things like crocodiles). In this environment, amphibians begin to fill the various niches that had previously been owned by mammals.

While this is a possible case, it is equally if not more likely that mammals just take over again. There is really nothing that frogs do better than small mammals after an extinction event other than lay their eggs in water. So you need a world with enough water that having young on land is detrimental to growing populations (as it causes too much competition for land-based food among the young).

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    $\begingroup$ A meteor with enough ice to turn the Earth back into a water world would be enormous. The volume of the earth's oceans is larger than an asteroid the size of Texas or France. Whereas the K-T extinction meteor was only about the size of the U.S. city of San Francisco. $\endgroup$ – Jasper Apr 18 at 18:57
  • $\begingroup$ @Jasper I did not suggest one of that size. Just one that would increase the amount of water somewhat. $\endgroup$ – David Coffron Apr 18 at 20:14
  • $\begingroup$ Amphibians do even poorer after a bolide impact, they are too vulnerable to PH change which impacts create. $\endgroup$ – John Apr 20 at 13:24
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A common approach is to just show the creatures that dominate the modern world, highlight their features that are relevant to the plot, and discuss whether those features are well- or ill-suited to the environment. The reader will know from their real-world knowledge that other kinds of creatures might have been possible, but the author has no need point out whether those kinds of creatures never evolved, or whether they died out, or whether they were killed off. If the creatures have extra limbs, it is common for the author to state that multi-limbed sea creatures beat tetrapods to colonizing the land.

John Ringo's series of novels set on Marduk uses this approach. Marduk's native sentients are 6-limbed descendants of amphibians; many of the large creatures are 6-limbed analogs of dinosaurs. The Empire of Man volume's free sample includes half of the first book, March Upcountry.

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Chytridiomycosis saves them.

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

The disease in its epizootic form was first discovered in 1993 in dead and dying frogs in Queensland, Australia. It had been present in the country since at least 1978 and is widespread across Australia. It is also found in Africa, the Americas, Europe, New Zealand, and Oceania. In Australia, Panama, and New Zealand, the fungus seemed to have suddenly 'appeared' and expanded its range at the same time frog numbers declined...] However, it may simply be that the fungus occurs naturally and was only identified recently because it has become more virulent or more prevalent in the environment, or because host populations have become less resistant to the disease. The oldest documented occurrence of Batrachochytrium is from a Japanese giant salamander collected in 1902, although this strain of the fungus belongs to an endemic lineage that has not been implicated in any mass-mortality events.

The frogs wrestle with this new virulent chytrid, as their ancestors have for millennia. Eventually they come to peaceful coexistence, and the current chytrid adds itself to other "endemic lineages" among amphibians.

Other vertebrates have historically not had such trouble with fungi. Then they do. A pneumocystis-like fungal lung pathogen spreads like wildfire through the vertebrates, infecting and then rapidly killing anything that breathes air with lungs. As with pneumocystis, the natural habitat and mode of transmission of this fungus is not clear but (also as with pneumocystis) it is apparently everywhere. The amphibians survive because many can respire to some extent through their skins, and because their coexistence with chythrids has granted them a measure of immunity.

Besides amphibians, fish do fine. Also some sea snakes survive by virtue of their skin-breathing ability.

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  • $\begingroup$ Chytridiomycosis attacks wet keratinized skin, and spreads via water, that is why it only bothers amphibians, lungs contain no keratin and more importantly can release some nasty digestive enzymes to protect themselves. fungal lung infections already exist, they just don't do well. $\endgroup$ – John Apr 20 at 13:31
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Perhaps consider the dual of your solution. Instead of killing off the mammals, give the frogs an extra bonus.

In Australia, Cane Toads are a major pest. They're an invasive species we brought along, virtually invulnerable, and poisonous to boot. They damage the ecosystem by consuming resources and getting eaten (poisoning the mammals). When we were there, some of our drivers would literally swerve out of their way to hit these guys. One of them admitted it was really more for their own satisfaction. The Cane Toads apparently can regurgitate their own guts to survive getting crushed, so they don't always die.

Why did they become such a pest? Because we brought them to Australia suddenly. They have no natural predators. No natural predators is an I-WIN button for most species.

Evolve a frog like the Cane Toad in an isolated region. Then have a major event (earthquake, volcano, etc.) break the isolation. Enter the toads, and they start to take over!

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