The lineage that led to the modern Cave Rat (Troglorattus ultimus) is one of the oldest on Apterra, having diverged from other rodents only about 70,000 PA. Throughout the Early and Middle Muricene, they lived in relative peace, safe in their isolated home within a dangerous and ever-changing world. They lost their sight entirely about 1,000,000 years ago, for the surface world was becoming increasingly deadly and choked with ratweasels and other threats that forced them to remain in the dark permanently. Retreating into their oasis of unending night, they continued to feed on the fish, isopods, and mosquitoes that shared this habitat.
For many years, this new way of life suited them well, but the changes facing their planet have finally caught up to them. As alpine glaciers expanded a millimeter at a time down the slopes of the mid-Abelian ridge, walls of ice gradually came to encompass even the low-lying mountain pass the cave was situated in. As the years wore on, fewer and fewer plants grew atop the permafrost, reducing the amount of energy that trickled down into the caverns, while more and more openings to the outside world became sealed shut. Cold drafts blew into the vents that remained, and rats who'd become used to the cave's stable temperature were driven deeper and deeper. These factors increased in severity over many years; populations entered a slow but steady decline as the quantity of available food decreased, while a drop in oxygen levels caused the death of many pups, whose respiratory systems weren't yet developed enough to avoid suffocation.
Now, there is only one left. She wasn't the last to be born; in fact, dozens of young were born and died during her three-year life, none of which could handle the increasingly stale underground air. She'd even given birth to a litter of her own just before the her mate - the last male - succumbed to the cold. That was over a month ago now. Since then, she has been truly on her own, calling out and feeling around in the dark for a family she'll never find.
It has now been two months since this final individual was left alone. The stream that brought life to the cave for millions of years now runs dry, its source having frozen over. The last cavern-minnows wriggle around in tiny remnant pools before these too solidify, preserving their frozen corpses. The mosquitoes last a bit longer, clinging to the rocky walls and pursuing their sole remaining target. If they'd lasted a bit longer, they might've killed the rat from exsanguination, but in the end they die off before this can happen, finding nowhere to reproduce and reaching the end of their natural lives.
It's three months after the second-to-last rat died, and the detritivorous woodlice are entering a state of population collapse as well. Having eaten through the last of the organic material that blew in on the freezing wind, they slowly starve to death. Only the archaea-eaters, supported by chemosynthetic microbes with no need for outside nutrients, are still going strong. With their extremely slow metabolism, they don't mind the mild anoxia, and they alone stand a chance of surviving long enough to see the ice sheets melt away some day. It is these crustaceans that support the rat in her final days. She makes regular trips into the deepest parts of the cave, eating her fill but quickly retreating to the few remaining entryways due to the anoxic conditions further down. This journey occurs several times each day, but for now it sustains her.
The final member of this species has now been the sole survivor for three months and two weeks. Feeling a pang of hunger, she descends once more to find a few woodlice. Even the trip down has left her out of breath, and the ascent is even more grueling. She scampers up a rock face towards the sole remaining exit to the surface, a vital source of breathable air that she's visited more than a hundred times over the past few weeks. As she closes in, she hears - and feels - a deep rumbling. She'll never know it, but a herd of hardy migratory mountain kiwis has just started an avalanche high on the slope a few kilometers uphill. She retreats a few steps as the sound grows louder, hiding in a small crevice while fluffy snow begins to fill in the opening.
It is now exactly one hundred and sixteen days since the endling became the last of her kind. She can't go down to eat pill bugs anymore; the last time she tried, she almost couldn't make it back. Even huddled around the wall of frozen water that was once her lifeline, she begins to feel weary. Perhaps if she'd started digging as soon as the avalanche occurred, she might've made her way out and survived for a few more days above ground. Now, though, the snow has compacted into ice, entombing her beneath the mountain. She lies down, too weak to stand anymore, drifts off to sleep, and within a few minutes breathes her last.