While their cousins take to the "trees", another branch of the massively-successful Armadillidium genus has expanded in the opposite direction. Though terrestrial isopods are commonly thought of as insects (one of their common names even has the word "bug" in it), they are, in fact, crustaceans. If one takes a peek beneath the well-armored exoskeleton of any woodlouse on Earth or Apterra, it is possible to see the animal's gills, an evolutionary leftover from the days when they were aquatic animals. Indeed, while they get little fanfare, the isopods' move to land from the sea is an evolutionary innovation on par with the emergence of tetrapods in the Carboniferous. On this world, however, there are no oceanic isopods; A. vulgare is a dedicated terrestrial species. But now it's been 100,000 years since Apterra was abandoned, and these ancient arthropods are once again claiming the waterways as their home.
There is a large and highly productive temperate peninsula jutting out from the northeast corner of Loxodia. It is here where we'll find our pioneering pill bugs. A quiet stream runs through a valley between two rolling hills. Basket-grasses grow along its banks like reeds, while a more standard turfgrass covers the slopes. Scattered across the tops of the hills, there are a few towering palm-grasses, their spindly stalks carrying their blades far above their surroundings. Inspecting the creek closer, there are some large, smooth stones half-submerged in the slow-moving current. A kiwi trots by, looking over its shoulder. While this Apteryx descendant is generally small and graceful, this one has just heard the growl of a predatory rat. It can be forgiven, then, for not noticing how slippery the rocks are. It trips, falling headfirst into the shallows, tipping the rock on its side in the process.
Over a hundred one-centimeter-long isopods scurry out. They'd been clinging to the underside of the stone, feeding on whatever debris and detritus floated their way. Now, though, drop into the water, each landing with a tiny, nearly-inaudible splash. Their four rearmost pairs of legs paddle frantically, flinging themselves forwards. They dive down, using their front limbs to dig into the soft, silty riverbed. Here they can stay for up to an hour without issue; like many aquatic creatures, they still require occasional supplements of oxygen from the air. They quickly realize there is no danger, cautiously make their way back to their rock, flip upside-down, and return to their feeding.
The kiwi, a young individual of a species called A. xanthopteryx, drags itself onto the riverbank. Its foe snarls at it but decides to turn back, noticing the scent of a rival male in the area. The kiwi walks along the sandy shore of the creek. It soon joins with a larger river, home to millions more A. subfluvius. woodlice. They dare not venture far into the water, for the rapids are much stronger here and their swimming abilities are still rudimentary. In a waterway of this size, the isopods also have to fear predation from the local Gambusia populations. There is such a wealth of untapped resources in their riverbed environment, though, that none of this matters. This species increases its range with every passing year. As their numbers grow larger and more widely distributed, each local population adapts to the particular environment it finds itself in. The individuals that dwell in the river, for instance, are on average 10% bigger and faster than their counterparts in the backwater streams that feed into it. With time, this one species will become many, spreading far and wide across all of Panapterra and beyond.