(This art is not owned by me. I do not take any credit for this artwork in any way and are simply using it as a way to show a possible future biosphere. Please support the original creator, Dragonthunders, by following his DeviantArt and or other services if available and supporting his series "The Future is Far".
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Regardless of what shape the supercontinent will be, there are many things about these supercontinents that are the same. For one, subduction zones will completely surround these landmasses, meaning mountain ranges will encircle these continents at the coast, walling off their interior. The vast middle parts of these continents will likely become arid grassland and dry desert. Most of the forests of today will be restricted to the mountainous coasts, which will lower biodiversity overall. The greater land to sea ratio will also increase rainfall and erosion, meaning more organic material will be squelched into the crust. Sea levels will likely drop due to less tectonic activity and the planet may enter into another glacial period. Oxygen levels may increase, lowering temperatures even further. This could allow animals themselves to become much larger than they are today.
Besides just land life, there will be another change that could bring upon disaster for life on Earth. During the formation of the supercontinent, rising temperatures could cause the ocean currents to slow down or collapse completely. This could cause ocean water to be depleted of oxygen and become anoxic. These periods of anoxic oceans have occurred in the past, during the Paleozoic and Mesozoic periods. During that time, sea life almost completely retreated to the poles due to the anoxic equatorial waters. Sealife on the ocean floor almost went extinct as oxygen levels became too low, forced to stay in the upper 200 meters of ocean. Not only did this lower biodiversity, but it may also have caused something even worse. Carbon Dioxide and Methane dissolve better in anoxic water, so millions of tons of greenhouse gases became dissolved in the anoxic deeper water, making the oceans extremely unstable. The longer the oceans remain anoxic, the more unstable they become. Eventually, all it will take is to trigger an event that could cause an extinction event of epic proportions.
In fact, this has already happened. 252 million years ago, the Earth experienced the worst mass extinction to date. Up to 96% of sea life and 70% of land life went extinct and took over 10 million years to recover. While it is still unknown what the exact cause was, it may have been the release of greenhouse gases from an anoxic ocean. Once the trigger event happens, the oceans erupt huge amounts of carbon dioxide, hydrogen sulfide, and methane. The methane would have triggered continent-wide firestorms that would generate massive soot that would have blocked the Sun and caused global cooling. The hydrogen sulfite would have poisoned most animal life. Once the soot cleared, the added Carbon Dioxide and Methane would have caused runaway global warming. Such a mass extinction is unlikely to cause all plants and animals to go extinct, but it would be enough to cause a significant amount of the life on Earth to die, changing the course of evolution forever.
Even without a mass extinction, models show that global productivity, a measure of how productive life is, will be much lower than it is today. It is estimated that biodiversity may be cut by 25% or more in this future. Yet even this future Earth bears some familiarity, but soon there will be an even stranger world ahead. A world without plants...

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