The Time Traveler stops his machine, now at roughly 850 million years in the future. He steps outside and is hit with a massive blast of heat. He slowly stepped out on an extremely different world than before. The once warm and pleasant world had now become incredibly unbearable and desert-like. The temperature was very high, at least 60°C (140°F). The time traveler tries to take a breath and quickly begins to gag. There's almost no oxygen in the air. He quickly rushes back into his machine to grab his oxygen and heat suit. Once re-pressurized, he set back out onto this world. The sky was no longer it's recognizable blue, but an opaque grayish brown. The ground is covered in dust and sandblasted rock. The land is nothing more than giant dune fields of bare rock and dirt, with a relentless Sun in the sky above.
The traveler walks through the desert wasteland and spots his first sign of life when he reaches the shore. The ocean is surrounded by crusty salt, clearing showing the water is evaporating. The oceans are hot and salty. Only animals with efficient kidney systems like mollusks and crustations are left. The corals are gone. The fish are gone. Whole groups of phyla are disappearing. Looking to the beach, he spots small armored animals knee-high that resembles armadillo. Near the water, there are small amounts of lichens and moss adapted to the high heat. There are some lizards, insects, and arthropods. All living under rocks or in burrows underground, temporarily safe from the murderous heat and radiation.
Returning to his machine and continuing forward, the time traveler wondered if the animals of this dying Earth knew that soon, their evolutionary adaptations would soon be to no avail…

Over the past 4.1 billion years of life, animals have only been there for the last 700 - 600 million years of Earth's history. Before around 550 million years ago, animals were mostly invisible in the fossil record. Based on the tiny amount of animal fossils, the animals before 550 million years ago were very soft and transparent. However, around 550 million years ago there was a massive rise in animal biodiversity that lead to nearly all the animal phyla we have today. This event lasted only 10 million years and is a major part of geologic history, known as the “Cambrian Explosion”. The exact cause is unknown, but one possible factor was a rise in oxygen in the water, which allowed animals to become more complex. Mabey in the future, a similar crash in oxygen from the death of complex plant life will cause animals to die in the same way.
Now, in this future world (roughly 800 - 900 million years from now) the loss of complex plants on land and in the sea will rapidly speed up the slow rise in global temperatures, increasing 10 - 20°C over 100 million years. Therefore, the time with no plants but animals may be as long as 200 million years, but it may be as short as 10 or 20 million years. Now, some form of photosynthesis will still be happening after the loss of complex plants, but how much will determine the future of animal life.
If simple plant life is unable to produce enough oxygen for animals to survive, then the age of animals will end in less than 20 million years. However, if simple plants are able to keep oxygen around, then rising global temperatures will have a profound effect on future biota.
Once the global average temperature reaches 38°C (100°F) on average, animal life will begin to die at the equator and begin migrating to the poles. Flying creatures will fare better than walking animals, as they are able to travel large distances looking for cooler regions. The equator will become devoid of most animals and be home to most bacteria and fungi. The subtropics will become the new tropics and the polar regions and high latitudes will become the new subtropics. There will be no more naturally occurring snow or ice caps. The oceans will also change. Most species not tolerant of warmer waters will die out. The coral reefs will disappear or many adapt to grow in deeper, cooler water. Nearly all marine communities will be disrupted. Only the creatures in deep ocean trenches around hydrothermal vents will be immune. At least for now.
When global temperatures reach 40°C (104°F) on average, dramatic changes should occur in animal life. The change in temperature rise will be relatively slow, about 1°C rise over 100,000 to 1 million years to allow evolutionary adaptations. There will be new body plans, new ways of protecting from UV damage and high heat. During this time, bacteria that are adapted to high heat will flourish. Most of the remaining ponds of water will have stunning hues of blues, yellows, reds, and greens as bacteria, once restricted to hot springs are able to fill the niche that plants left behind. For remaining animals, it will be a time where life scrambles to evolve life jackets on a sinking ship. Even burrowing underground or swimming deeper underwater will eventually be to no eval, as the humid heat cooks the Earth from the outside in.
Most studies conclude that animal life has an upper-temperature limit of around 45°C (113°F). At that point, the mitochondria in cells stop functioning. Once global temperatures are this high, the equator will be far hotter still. Even at the poles, animals will be nocturnal, hiding from the deadly breath of our dying star. These animals will likely hibernate during the polar day and be active during the polar night when it's cooler. It’s likely very few animals will stay on the surface, with most hiding underground. Termite and Mole-like creatures may dominate the Earth, feeding off bacteria and fungi. Ultimately, the land will progress back to its pre-animal state.
When global temperatures reach 50°C (122°F), whole-scale extinction will occur on the land. Animal life in the seas will be different, however. While ocean temperatures will rise to levels not seen since the Hadean Eon, the insulating properties of water will allow some kind of animal life to continue deep underwater. However, like on land, most animal species will be extinct. Only communities deep underwater will be largely unaffected, as most surface water communities will likely perish or migrate underwater.
At around 60°C (140°F), animal life on land will be essentially extinguished. Only fungi, algae, lichens and bacteria will be on the land and possibly some lower invertebrates like flatworms and nematodes. At this point, the land will have largely completed its transformation back to its pre-animal state. The toadstools and mushrooms may be the new redwoods of this world. The two-inch tall masters of once forested land.
Eventually, the global temperature will reach 70°C (158°F), all animal life on land will go extinct. Only the bottom, hydrothermal vent communities will still harbor animal life, the tiny oasis in a barren ocean, that is until the ocean itself evaporates...

This is my final part of this small series (for now). I will continue it eventually. If you have any more questions about the far future, check out the links below or comment...
https://www.researchgate.net/public.....able_lifetimes
https://www.researchgate.net/public.....able_lifetimes
https://www.amazon.com/Life-Death-P...../dp/0805075127
https://en.wikipedia.org/wiki/Future_of_Earth
https://en.wikipedia.org/wiki/Timel.....the_far_future