The birthplace of life on Apterra continues to be a hub of vertebrate diversity. Over the past few dozen millennia, its climate has shifted slightly, with coastal regions around the Medithalassic now receiving increased rainfall. This has resulted in the takeover of wetland grasses, and the mammalian inhabitants of the region have adapted in turn. Semi-webbed toes are nearly universal, as no rat can make a living in this area without regular trips through its waterways. In terms of plant life, the estuary vaguely resembles the Ailuropian swamps of the previous age. However, basket-grasses have yet to colonize this region, meaning their niches are instead occupied by the now cosmopolitan rat-grasses.
At the waterline of every stream, pond, and inlet, these rat-dispersed plants dominate the waterlogged soil, forcing out all competition until they form a monoculture spanning thousands of kilometers of coast. Their mutualists find this habitat quite friendly, as it provides for them a steady supply of food year-round. The seeds of these grasses are quite large, up to half a centimeter in diameter. This excess of nutrients is not just for the seedling's benefit, for it can successfully sprout using just a fraction of the energy the seed provides. It also serves as an incentive for the rats; the size and convenience of this food source keeps them coming back day after day. With the sheer quantity of seeds available on a regular basis, some are certain to sprout and carry on this cycle of mutualism.
Not all rats are so kind to these plants, though. One local species by the name of Rattus gramenicisor has no need for high-calorie rat-grass seeds. Instead, it simply grazes on the abundant grasses themselves. Moving in vast herds, this two-kilo creature travels about a kilometer a day, mowing the shore grasses down to their roots as they go. Like other nomadic grazers, this species can't visit the same place twice in a short period, nor can it remain anywhere long. Like its close cousin R. migratorius, it stays on the move, raising its well-developed young as it migrates. This is not to say parental care is absent; on the contrary, mothers and fathers both pitch in, as competition is fierce and their pups need all the help they can get if they're to survive to adulthood.
Predatory rats live here too, but unlike the den-building mesocarnivores of the jungle, these hunters follow the herds. Chief among this group is Rattus unguivenator, a species that is, in fact, a close relative of the various nomadic grazers, sharing their stamina and ability to cross long distances quickly. It does not, however, have precocial young, and therefore cannot pursue its main prey while its pups are small. It gets by during this time by scavenging and rooting around for isopods and nests of small rat species. Once the young are weaned, no further care is given. Since its specialized predatory behavior evolved only in the last few millennia, R. unguivenator's prey have not yet evolved defensive strategies. R. gramenicisor (and especially their own offspring) are such easy, naïve targets that even a one-month-old carnivore has little trouble taking one down.
While this predator-prey relationship plays out along the shore, the hills are a safe haven for herbivores that otherwise couldn't survive the onslaught of R. unguivenator hunters. The seed-eaters retreat here to nest once they've eaten their fill, while others spend nearly their whole lives above the waterline. Rattus sordosiccus is one such form, dwelling in burrows dug into small islands. Family colonies numbering up to 20 or so establish a single patch of land as their territory, barring conspecifics from setting foot there except during the breeding season. On these islets, which are just a few meters in height and width at high tide, a less aquatic species of rat-grass grows. This one does not produce seeds continuously; rather, every individual plant on an island will set seed at the same time, generally during the late summer. Over the next week or so, the local colony must collect as many of these as possible, storing them away in the dryest chambers of their burrows. Whatever seeds they fail to find will germinate within a few months. The rats spend the rest of the winter and spring in an inactive state of torpor, only occasionally wakening to feed on their collected seeds. In the productive months, fish, woodlice, and various plant matter are also eaten, and the colony spends much of its time weeding out all other grasses from its island to ensure the next crop is large enough to sustain it for another year.
Under the surface of the slow-moving waterway below, a small greyish-brown rat has a lifestyle completely opposite to that of its burrowing cousins. R. platycauda is the most aquatic species in this area, feeding primarily on the Gambusia that populate these waters in massive numbers. They are adept swimmers, using their vertically-flattened tails to propel themselves in quick bursts. Their jaws are long and narrow; along with their flexible necks, this allows them to snap at prey before it has a chance to react. They spend most of their time under the water, breathing every minute or two. They still sleep, mate, and raise their pups on land, remaining relatively fast runners.
In this world of plentiful food and ecological niches, each of these species has great odds of evolving into ever-more derived forms as the Muricene continues. Some of them will even go on to establish long legacies in the megafaunal communities of Apterra's future.