Back home, the phylum Cnidaria (jellyfish, coral, anemone) and the class Amphibia are two of the most ancient groups of animals on the planet.
But let's say that, on an alternate Earth, many millions of years ago, the global climate became so hot and so dry (preferably from a volcanic eruption on the lines of the Siberian Traps) that all the amphibians and cnidarians went extinct with no modern legacy.
With that mindset, who would be the best candidates to fill the void?
First, let's analyse the roles these creatures play. Both are a lesser type of predator, and heavily connected to water. Interesting choice. I assume from "so hot and so dry" that the reduction in available water is key.
All adult amphibians are meat-eating predators. Their prey includes insects, slugs, worms, and even small mammals, such as mice. Aquatic amphibians eat water snails, insects, and small fish. Many amphibians hunt at night, using their sharp sight, smell, and hearing to track victims.
They are eaten by snakes and birds. Really, a change in the terrestrial ecosystem at this level would be drastic, and I would expect invertebrates to rise up and fill it. Look to the insects and creatures that live in arid places to fill this gap, like the wind scorpion / camel spider. That is a nasty-looking creature that would be up for the task. Though he might need some growth assistance by your narrative.
Jellyfish are opportunistic predators. This means that they are likely to eat whatever they come across, and though some jellyfish species are confined to eating only small fish eggs, others can kill and eat entire fish.
Other species of jellyfish are among the most common and important jellyfish predators, some of which specialize in jellies. Other predators include tuna, shark, swordfish, sea turtles, and at least one species of Pacific salmon.
With the recent jelly blooms, I would say that predators do not depend on eating jellies that much. So the jellies' disappearance would likely mean more for the small fish than the large ones. But since the available water is shrinking as well, your characters are not likely fishermen. Perhaps the removal of the jellies is just the right thing to keep the current balances in a smaller ocean.
Since your water has to go somewhere, is it going to be mostly overcast? A shimmery haze of a sky that rarely rains, locking in the heat like a dry sauna?
but it is believed there are organisms which have are still surviving in highly acidic environment check: https://en.wikipedia.org/wiki/Acidophile so it will cause evolution to change a little bit bu if this article: https://en.wikipedia.org/wiki/Thermoacidophile in wiki is correct then the were infact generators of life.
