- Puerocene Era
Entire animal kingdom possesses unusually and uniformly long lives that are spent primarily in a state of relative immaturity, followed by a final growth spurt into adulthood and sexual maturity, followed by death shortly after. Ancestral demi-humans first emerge. War sparked by actual humans in rebellion against their youthful ancestors. War ended when violent death and natural disasters forestalled the demi-human’s centennial planet wide mating period (seemed like a good idea at the time), leading to generational collapse and utter capitulation. Mass extinction follows as any animals with previously dominant reproduction cycle hunted to extinction by now dominant humans.
- Dustball Earth
A slight slowing in the planet’s rotation caused by an approaching planetoid increased average wind speed while debris from said approach burned away most vegetation, leading to runaway erosion that eventually grinds the tops of continents down to mostly featureless dune deserts. The seas rise but become shallower overall as sand from the continents covers the sea floors and the displaced water spreads out over former landmasses. Continental dust storms rack the dusty continents, while ocean currents kick up silt storms in the brown waters. All the producers are kelp or really shitty palm trees that grow in the occasional equally shitty oasis. Competition between remaining animals elevates to heights of ludicrous cruelty. Fucking miserable.
- Purging Sprite Bombardment
Passing through the polar jets of an active galactic nucleus causes a profusion of magical spirits to generate spontaneously in the surrounding space. The entire rock layer from this period is covered in the soot left by their corpses- once they pass through the atmosphere, they tend to decay quickly unless actively combatting magic. Their purging abilities cancel magic; their large numbers lead to the extinction of all overtly magical species, the general cancellation of many naturally occurring magical effects. Every once in a while, the fossilized ashes of one of their orbital webcities is unearthed.
- Tholocene Occlusion
Entire planetary system passes through cold nebula. Gas and dust eventually fills the stars system and effectively occludes all other stars and planets. Sun still visible, but so dim that daylight is more like twilight while night is dark as pitch. Nocturnal predators develop extremely fine tuned organs for hunting without sight and half the ecosystem is totally blind. Most producers don’t even use sunlight, and just break down rocks instead or use thermal vents. Reddish-brown sky feared and worshipped as nightmarish, hateful god by dominant civilization of proto-illithids, who survived the end of the Occlusion by turning to the consumption of other species brains.
- Astrogenocene Renaissance
Entire planetary system passes through an active star nursery. Ecosystem explodes in diversity and size, as the glowing gas, young hot stars, and polyplods all provide more energy than the producers know what to do with. Biosphere covers every corner of the earth and extends several kilometers up from the surface. Every plant layer of the planet-wide jungle absorbs a different type of light. Many creatures are totally unique, though some still belong to species. Some of those unique organisms are still around, and are without exception hungry, insane, and comparatively blind.
- Paramalekene Supremacy
A species of parasitic mollusc, resembling a cuttlefish, becomes intelligent and successful enough to start building societies and civilizations. Golden Age follows global unification of all city states, but is itself followed swiftly by sudden population reduction and extinction. The cultural norms of the unified society saw parasitism as a disgusting bodily function that was tragically necessary for reproduction. The vestigiality of the associated mechanisms becomes so desirable that cosmetic maiming becomes normal. Those with naturally almost useless parasitic organs become paradoxically the only ones capable of getting someone to breed with them. Eventually entire species ceases to function as parasites and therefore unable to reproduce. Final generation of scholars admits begrudgingly that overweening pride might not have been the best look.
- Postcambrian Implosion
Early in the history of complex life, an extremely successful species of protozoa runs amok and out-competes the rest of the entire ecosystem. Giant herbivorous (actually, they’ll try to eat anything) segmented sea cucumber things, up to miles long, with four chubby arms positioned radially on each segment. Actually a massive colony of smaller organisms. They ended up eating just about everything on the planet, then gallingly proceeded to form their equivalent of an endospore before starving to death. Entire rock layer is still filled with the damn things, technically alive and just waiting for a chance to ruin everything. Someone accidentally revived one of their heads by spilling a cup of water on it, and found out that they can talk.
- Equatorial Schism
Poorly understood tidal forces caused by even less well understood gravitational interactions possibly coupled with also unfathomable geological processes cause entire equatorial region to erupt in in gouts of flame. A massive impassable equatorial ridge that girdles the entire world spews constant flows of lava and deadly gases. Northern and Southern hemispheres totally isolated from each other, and develop ecologically/culturally totally independent of each other, until someone finds a way through and everything goes to hell. The continental Southern Ridge Worshippers see insular Northern sea worshippers as faithless usurpers occupying their gods’ lands. Northerners see the Southerners as demonic hordes emerging from their hell. Neither species/society is biologically capable of speaking or hearing the complete range of the other’s language, but both are terrible at fighting each other, so I guess it works out?
Alternatively, pick your two favorite time periods and throw them together.