Asked ChatGPT what a capybara would look like 1 billion years in the future. Got this:
Hydrotherimys abyssoratus - “abyss-burrowing water rat”
Earth’s surface is mostly uninhabitable: a hyperarid supercontinent under a brighter sun, with low oxygen and high CO2. Mammals are nearly extinct, but this massive capybara descendant survives underground.
It lives in deep aquifer systems beneath the continent’s interior. About 2.4 meters long and 300 kg, it’s built like a cross between a capybara, a tank, and a mole rat. It has a flattened, armored skull for digging and keratinized scutes covering its back and tail. Eyes are vestigial - it relies on electroreception and subsonic vibrations to navigate.
Colonies are eusocial: one breeding queen, dozens of sterile workers. Communication happens through chemical trails and low-frequency pulses. They construct complex tunnel systems, complete with heat-exchange vents and mud siphons to manage water during dry centuries.
Their metabolism is adapted to low oxygen, using sulfur-based anaerobic respiration. A modified colon functions as a buoyancy organ in flooded tunnels. Diet consists of root starches, chemosynthetic algae, and decaying fungal mats.
The tunnels host a full biome: glowing fungi, parasitic arthropods, eel-like predators. They're hunted by blind, python-sized worm-lizards that track body heat.
Hydrotherimys isn’t intelligent, but its hive-level coordination and ecological influence make it one of the last successful mammalian lineages - a relic from Earth’s warm-blooded past, surviving in darkness beneath a dying sky.