255 Million Years PE
The Ultimocene epoch advances another five million years. Though on the surface the world is still hospitable, with thriving life from pole to pole, beneath the planet's crust the mantle is cooling down and the first hints of coming instability have come to light. The process of subduction, when two continental plates collide, is slowing a little every year, and with it slows the formation of mountain ranges. Far beneath the surface of Serina, things are beginning to change. Volcanism is decreasing overall, but not evenly. As the mantle cools, it is becoming unpredictable on very short time scales - lulls of a few thousand years result in global cooling and are followed by similarly brief periods of heavy volcanic emissions, resulting in a pulse of atmospheric carbon dioxide that warms the atmosphere several degrees and allows the spread of tropical rainforests north and south from the equator. In just a few thousand years more, however, the excess carbon is cycled back into the ground, and the climate returns to its default cooler state. Glaciers form around the south pole and then creep north across Serinaustra, pushing the forests back to a narrow equatorial band and restoring the extent of the Serinaustran steppe.
Ecosystems begin to change; not to be lost entirely, but to shift and move quickly in geologic time from one place to another, and back, in roughly predictable cycles. Grasslands and forests displace one another as the climate swings back and forth, followed by their inhabitant wildlife, yet some adapt better to the swing of the climate pendulum than others. Nomadic grazers, among them the circuagodonts and the giant archangels, follow the changing plains over tens of generations with little difficulty and continue to thrive. During warm periods forest-dwelling wildlife follows the expanding jungles north and south and experience population booms, but may find themselves isolated when climates cool and become drier, as the forests often recede unevenly. During each cycle some patches of this once contiguous forest, protected in a favorable microclimate, are left a little longer than the rest whilst the climate shifts around them. When the climate changes so that even these last holdouts can no longer support this now out of place forest, the animals find themselves trapped on what amount to islands on the plains and are unable to move back toward the equator. When the last trees die, these trapped organisms also perish. These micro-extinctions, most often occurring only upon a local, not a global scale, with organisms that still survive elsewhere, nonetheless provide a glimpse into a future where such changes have much wider, drastic repercussions.
But even with the first hints of an uncertain future ahead now on the horizon, Serina as a whole is still thriving in the early Ultimocene, and will continue for a while longer before the next great upheaval. Truly equatorial habitats, where the vast majority of biodiversity is found, are as yet little affected by the climate fluctuations that affect the polar regions. Here life continues as it has for tens of millions of years - and that life is thriving; yet this time, it is perhaps all the more important to cherish, because the clock is now definitively ticking, and these good times too will come to an end, as all things do.
Life on Serina has experienced many ups and downs in its history, but the current period is different. Every good time comes to its eventual end, but this time, the end will be final. When the curtains of past epochs of a similar grandeur closed - the Hypostecene as well as the Thermocene - times were trying, even devastating - but only for a time. There was loss, but life recovered afterwards to and even beyond its previous levels of biodiversity as conditions improved. But Serina was younger then, more stable. Now, as happens to everything in the cosmos from flowers to stars, Serina itself is dying.
It is not dying a sudden, explosive, or particularly violent death like the Pangeacene-Thermocene extinction event. There will be no sudden moment of clarity before all cuts to black. There shall be no definitive time where good times become bad times, as after an asteroid strike. But change is coming, bit by bit and year by year. The climate is going to get colder, first in the short term, but ultimately overtaking the world one day. As the early Ultimocene advances, the result will be a steady but inevitable decline of life from this point forward for the next few million years. There will be reprieves, where the world seems to recover, but all good times are temporary in the long term. Global cooling will eventually result in the end of all life on Serina some time after 300 million years of adaptation, innovation, and ultimately, extinction. 255 million years have already amassed; more than 50 million more remain. This is a very long time, almost as long as has amassed on Earth from the end of the dinosaurs until the evolution of modern man. The difference, however, is that throughout the remaining period, environmental conditions will become less predictable, and over the long scale less hospitable - not more so, as following most extinction events, because the Ultimocene is not merely an isolated extinction event, but simply the beginning of an end.
But the road ahead, to that final and inevitable end, is still a long one, with many stories left to be told.