Planet of Coasts

It's ten million years Post-Abandonment, and Apterra has only become richer and more diverse than ever. The complexity of the planet's biomes has never been greater, and conditions are hospitable across nearly its entire surface. Changes in all the major landmasses have reshaped the globe, while changes in living organisms have allowed untold numbers of species to flourish. The land, seas, and sky are coming into their own as stable and productive ecosystems. Looking at the life that exists in this age, one might hardly believe how few creatures it all originated from. The Ice Age is a distant memory, and the niches of all its victims have long since been filled. Apterra in the Middle Arthrocene shows few outward signs that its life didn't evolve naturally; the lack of pre-Muricene fossils is the only thing that gives it away. Yet the ongoing adaptive radiation that has characterized the planet's history isn't done yet, as speciation still outstrips extinction by a wide margin. That same diversification is responsible for the creation of new habitats and new ways of living completely alien to the world from which Apterra's inhabitants all descend.

Climate and Atmosphere

Apterra has continued on an overall warming trend over the last four million years. This increase in temperature has resulted in more frequent and powerful cyclones and tropical storms, especially in the balmy Medithalassic. While these are destructive forces in the short term, over time they help the planet as a whole. As they travel poleward, they transport heat energy from the equator to higher latitudes, causing arctic and antarctic temperatures to rise. This means that, while the mean temperature of the equator has dropped slightly since the Early Arthrocene, the poles have gained about five degrees, extending temperate climes as far as the tip of the Northern Isle. 

Meanwhile, the same storms bring precipitation farther inland than was common in previous ages, turning deserts into savannas and savannas into rainforests. Loxodia's once-great Ten-Year Desert, for instance, now receives regular monsoons for about two months every year, promoting tree growth, which in turn helps it retain water in between rains. The middle of Abeli, once a "Patchland" of scattered grasslands, scrubby trees, and deserts, is now the Seamless Savanna, a huge and continuous stretch of sparsely-wooded terrain crisscrossed by bridge-like vines and carpeted with basket- and barrel-grasses. It and analogous habitats in Loxodia and Ailuropia support megafauna on a scale that would've been impossible under the fragmented conditions of the past. 

Oxygen levels are still on the rise as plants work their way through the still-rich supply of atmospheric carbon produced at the end of the Ice Age. Oceanic creatures work hard to return this essential molecule to the air just as quickly, preventing a repeat of the cataclysm that kicked off this epoch. With the seabed long since cleaned of the muck that once choked it, Apterra's main reservoirs of solid carbon are now found on land, buried in meters-deep soil that promotes lush vegetation worldwide. Arthropods benefit the most from the oxygen-rich air, growing as large as the biggest Carboniferous bugs from Earth's history. Even the small ones are adapting novel behaviors and niches never before seen on either planet. The plants and small invertebrates that thrive in the Middle Arthrocene environment, in turn, support avian, reptilian, and mammalian life on a tremendous scale.

Panapterra

The supercontinent has seen its borders shift in a few ways over the past 4,000 millennia. The rising sea level has subsumed many coastlines, from the lowland river valleys of Abelox to the far-northern reaches of the Ailuropian Streamlands. The Founding Plains, once a land of swaying grasses dissected by a single, massive river and its tributaries, has lost much of its area to the sea, including the site where vertebrate life was originally seeded. A huge inlet reaches hundreds of kilometers inland, and the raised sandbars that used to force all of the region's water to flow through a single outlet have now collapsed, allowing dozens of small and mid-sized rivers to pour into the Medithalassic at numerous points. Western Ailuropia has similarly washed away, with soft sediment eroding into the ocean until the waves began to brush against the harder bedorck of the Quartzkarst, halting its progression.

The decreased prevalence of glaciers is not only due to Apterra's global warming trend, but is also a result of rapidly shrinking peaks. This began when the continental glaciers wore thousands of cubic kilometers off every mountain range, and it has continued for the last seven million years because of the Arthrocene's heavy rainfall, which causes them to weather away much quicker than before. There are only two active mountain-building regions at the moment (though this is set to change in future millennia): one massive volcanic peak between Sub-Abeli and Abelox, as well as the South Abelian Ridge. The latter is bolstered by the collision between Abeli and Sub-Abeli, the smaller of which is still moving to the north and west. 

The larger continent, on the other hand, is moving south on its eastern edge and north at its westernmost extent, creating a tension that will soon lead to all of Abeli gradually rotating in a clockwise direction as the Arthrocene progresses. Loxodia has changed little, as its plate isn't moving much and its coasts have been largely protected from the storms that batter its neighbors. Ailuropia, conversely, has started on an eastward trajectory, causing Ailox to compress into a kinky zigzag that, at several points, threatens to sink away and cut the two continents apart from one another entirely. Luckily, the land bridge is still intact, so Panapterra has retained its integrity for the time being, even if a few of its constituent parts seem a bit worse for wear.

Islands and Island Continents

The smaller landmasses have not been spared the wrath of rising tides, but most have weathered the changes without major losses. Aglirium, for example, has lost a lot of its northern lowlands, but its southern half has grown towards Abeli after the hotspot that created the Gecko Isles reawakened. It has even connected with the smaller island to its south that served as a stepping-stone for the lizards that first colonized the landmass, bringing it still closer to the shore. Its highest mountains are now as tall as those of the mainland ranges, which, in combination with Apterra's cooling equator, means that there now exists a small but productive temperate zone high on its slopes.

The already temperate Unciolis has warmed along with the rest of the planet, and there are now just a few remaining patches of taiga forest deep in its interior. The tectonic plate it rests upon has taken it to the north and east, further increasing its temperature as it decreases in latitude. To its northwest, Choeropica has not seen the same good fortune, losing nearly a third of its area and seeing its tenuous connection to Panapterra become impassible for terrestrial organisms. Smaller islets like those that bridged this gap have fared poorly, with thousands having been reclaimed by the waves. Perinesia has been lucky in that, though more than half its Early Arthrocene land is now gone, continued volcanic activity has replaced what was lost with newly-emerged islands, and the fast-moving Sub-Abeli has also left behind a handful of broken pieces in its wake.

Post-Abeli, with its protective tideslopes that accrete mass around its edges, has expanded in area despite no geologic uplift having occurred. Because the biome now extends more than 200 kilometers inland, water can no longer simply wash in and out freely. Instead, channels have formed, funneling the tides in and out at speeds exceeding 30 km/h at their centers. This means that, while the margins of Post-Abeli are largely safe, the dry land within is abnormally exposed to erosion and has consequently begun losing elevation quickly, with areas nearest the channels seeing a meter of soil washed away every few millennia. This will continue until the channels grow deep and wide enough that the fastest tidal flow can restrict itself to the center, insulating the banks with buffers of slower backwaters.

The Oceans

The land's loss is the sea's gain. Hundreds of thousands of square kilometers have been added to Apterra's oceans, mostly in the form of shallow near-shore meadows of plumekelp that support life in abundance. The remnants of sunken islands, coastlines with too shallow a grade, and valleys that plunge far into every continent have all been converted into lush underwater fields and forests that thousands of species of fish, insects, crustaceans, hydrozoans, algae, and vascular plants all call home. Larger vertebrates can then feed on the massive quantities of forage fish (or equivalents), growing to sizes that rival the fearsome predators of Earth's seas. But the oceans haven't only taken area from the land; their own internal structure has also changed radically.

Four million years ago, the barriers between the warm, shallow Medithalassic and the cool, deep Perithalassic first began to break down, with a conveyor-belt-like interchange transporting nutrients and living things from one to the other. This is no longer the case. Starting at the mouth of the Medithalassic and bordered by Choeropica, Unciolis, Perinesia, and an assortment of smaller islands and submerged seamounts, a large area of water has become a distinct and somewhat isolated environment of its own. It thus deserves its own name as a distinct ocean - the Interthalassic.

The Interthalassic might appear to have many connections to the larger ocean that surrounds it, but in fact most of these are shallow and block the movement of water masses, preventing most organisms from passing through. Practically speaking, it only mixes freely with the Perithalassic through the passage south of Abeli, with the rest of its boundary largely repelling any currents, plants, and animals that approach. Despite this, it still serves as something of a melting pot with an ecosystem intermediate between those of its neighbors. Though its waters are deep, they maintain a comfortable tropical temperature year-round, fed by Medithalassic outflow and warm winds from Abeli. This makes the Interthalassic ocean a new hub of speciation, outpacing both others in terms of sheer species-richness. Through its porous undersea borders, its endemic inhabitants are already beginning to spread to the rest of the world.