Meltdown

The year is now 3,000,000 Post-Abandonment. The vast continental ice sheets are, after hundreds of thousands of years of continuous growth, finally beginning to retreat. Hybrid taigas are reclaiming land from the tundra, and temperate forests and pseudoforests are in turn taking over the mid-latitudes. New and more diverse grasslands are growing, and former deserts bloom with lush foliage as rain returns after millennia locked away in ice. The equatorial Medithalassic is warm once again, its waves finally free of snow and slush. The seabed has a strange, pockmarked appearance, with divots, scrapes, and tunnels dug into the muck. The culprit is a newcomer to the seabed, but its actions have at last reversed Apterra's cooling trend, returning carbon to the atmosphere for the first time since the planet was seeded. It's far from the first detritivore to take up residence in the depths of the sea. To understand what sets it apart from the other bottom-dwellers, we must trace its history, from its origin to its current form.

Scuttleworms are a group of neotenic mosquitoes that evolved from an ancestor that regained adult-like legs after adapting to a permanently aquatic life cycle. Such evolutionary throwbacks are common across many lineages of Apterran animals, in this case being caused by a mutation that caused the legs to mature while the rest of the body remained larval in appearance. The first scuttleworms were generalists, as their bodies allowed them to take advantage of many freshwater food sources. As time went on, levels of specialization increased, with some taking to active predatory lifestyles, using hooked claws on their long forelegs to catch slow-moving prey whilst hidden under lakebed sediments. Others became more herbivorous, feeding on aquatic grasses. Sometime around a million years ago, Canistroculex established itself as the most common plant-eating scuttleworm genus, feeding primarily on water-lily-like basket-grasses that often choked the surface of ponds and slow-moving streams.

This food source afforded the mosquitoes an opportunity to expand their niche. The first salt-loving basket-grasses began to colonize shallow, brackish estuaries around 2.3 million PA. With them came the hardy scuttleworms; within another 50,000 years, the species C. terminofluvius diverged from its relatives. The basket-grasses, of course, soon evolved into the first true seabaskets, creating a new biome that, while it was not destined to last, was incredibly biodiverse in its time. The Estuarine Scuttleworms were not so quick to evolve a tolerance to seawater, remaining restricted to areas of moderate salt concentrations until the end of the Late Muricene. Finally, just before the time of the Great Decarbonisation, they arrived in the seabasket forest, finding great success in a region of abundant food and hiding places. The golden age was not to last; within sixty millennia after the end of the Muricene, the seabaskets would be extinct. Many of the species that relied on these forests died as well, such as the grazing grumbletoads, which were unable to survive in any other habitat. The adaptable scuttleworms, though, were not going anywhere.

With the shallows decimated, the survivors soon made their way into the deep ocean, picking away at the rotting algae alongside filthfish and mudeater crabs. Unlike the others, though, scuttleworms had a unique trait - they could dig deep into the carbon-rich sediment, eating debris buried deep underground. The ability to burrow was one they inherited from the very earliest scuttleworms, which buried themselves to hide from predators. In carnivorous genera, this was further adapted into a form of camouflage that allowed them to more easily catch their prey. For herbivores, tunnels could serve as a safe place to lay eggs, a strategy employed to this day by freshwater, brackish, and marine species. When they arrived on the seabed, burrows also served as a means of food acquisition, enabling them to feed while remaining far from the prying jaws of shellcracks and other benthic hunters. 

By 2.8 million PA, the Sludgescuttler (C. putrivora) was a common sight across all of Apterra's oceans. Generations of tunnels wove throughout the mud, with huge populations living underground. Mudeater crabs began to occupy the widest passageways, eating previously inaccessible food exposed by the scuttleworms. The overall detritus consumption rate increased drastically. For the first time, bottom-feeders were eating enough to match the algae that rained down each year, returning carbon to the water column and, in time, to the air above. Starting about 100,000 years ago, more carbon was being released than sequestered, and atmospheric carbon concentrations began to gradually rise. Ice melted, snow became rain, and plants could grow freely again. Global temperatures rose quickly as the greenhouse effect restarted. Sea levels rose, though some of the newly-exposed land remained above the water due to tectonic uplift. After losing more than half of all its species, Apterra's biosphere began to increase in diversity once again. This was not a simple return to Muricene-esque ecosystems. The loss of major keystone species from that epoch, and the evolution of new ones over the last half-million years, have changed the world permanently. Though life will soon flourish in a post-glacial Apterra, the Muricene is dead. The Arthrocene has arrived.