Yūkō Era (幽光代)

The Yūkō Era — named Yūkōdai（幽光代）, the "Era of Twilight Light" — spans from 30 million years ago until the earliest emergence of tool-using hominins. The continents of Erthas are already in their modern positions, but the biosphere remains ancient and wild. This is an era of ecological refinement, lineage culmination, and evolutionary experimentation. It is a world where the stage is set, but the players of history have yet to arrive.  

Features of the Yūkō Era

• Modern faunal families solidify across every continent, with endemic megafauna dominating regional ecologies
• Many ancient lineages endure, having survived the Kasei extinction and its aftershocks
• Fluctuating climates shape biomes — warm periods interspersed with dry spells and glacial approaches near the end
• Microcontinents and collision zones lead to novel hybrid ecosystems where Laurasiatheres, Afrotheres, and Archontaglires coexist
• Birds and marine mammals flourish, filling a vast range of roles in both temperate and tropical regions
• Hominids evolve in Teiboshin during the final phase of the era

 

Continental Biogeography

  Daitō (Thādh, Shinyū, Konchū, Bōkō, Gōhyō, Chūin):

• Mammoths and ancestral rhinoceroses roam the boreal regions (Thādh, Chūin)
• Gliding triconodonts and tritylodontid relics persist in Konchū highlands
• Bōkō and Gōhyō teem with hoofed Laurasiatheres: early tapirs, deer, and perissodactyl browsers
• Predatory creodonts and saber-toothed felids fill the top predator niche
• Giant raptors and flightless land birds dominate open plains

  Daisai (Kiin, Tousai, Shinbō, Jinchū, Shinchū, Shinshi, Kyōshi, Jinshi):

• No placental mammals — marsupials, archontaglires, and xenarthran-like creatures dominate
• Kiin hosts cold-adapted mammals: woolly sloths and bear-like omnivores of marsupial descent
• Jinchiū and Jinshi remain forested refuges for glyptodontid analogues and large gliding rodents
• Shinshi and Kyōshi contain amphibian-rich wetlands with descendant temnospondyls and aquatic synapsids
• Shinchū and Tousai see marsupial herd grazers and strange armored browsing herbivores

  Dainan (Lomba, Ndali, Zhamo, Teigō):

• Zhamo's rainforests house the last large terrestrial crocodilians and therapsid carnivores
• Lomba supports woolly marsupials and hyena-like scavengers adapted to dry inland basins
• Teigō and Ndali harbor convergent afrotherian megafauna: small proboscideans, entelodonts, and armored sloth-kin
• Early elephants make forays into river valleys, while tapir-morphs dwell along springs

  Teiboshin (Boshi, Teishin):

• Cradle of hominid evolution late in the era
• No afrotheres — endemic monotremes, marsupials, and early hominiforms dominate
• Toothed whales exploit deep-sea currents offshore
• Boshi’s river systems teem with amphibious desmostylians and piscivorous synapsids
• Teishin hosts tree-dwelling insectivores and early omnivorous bipeds

  Teishugai (Chishu, Heiyū, Teigai):

• Highland Teigai supports venomous marsupials and carnivorous monotremes in dense rainforest biomes
• Chishu’s lakes host tritylodont-like herbivores and strange toothed amphibians
• Heiyū is home to lemur-analogues, burrowing xenarthrans, and long-beaked synapsids with moss-based diets

  Shinshin:

• No placental mammals — dominated by marsupial omnivores
• Upland regions harbor feathered gliders and nocturnal insect-hunters with long fingers
• Cold scrubland supports burrowers and arboreal foragers

  Kyōgai:

• Swamps teem with crocodyliforms and predatory salamanders
• Forest plateaus support marsupial grazers, arboreal gliders, and monotreme relicts
• Mild winters allow for year-round nesting of waders and fish-eating birds

  Kyōshu:

• Formed from Kyougai and Boshu — ecosystems reflect a tropical, marsupial-heavy fauna with early convergent predators
• Includes gliding monotremes, insectivores, and burrowing arboreal marsupials
• Wading birds and riverine crocodilians common in lowland basins

  Kōbiyū & Kōbō:

• Still cold and isolated — glacial species abound
• Furred multituberculates persist alongside boreal primate analogues
• Lakes contain ancient ray-finned fish and early freshwater cetaceans

  Chichū:

• No placental mammals — endemic pseudo-archontaglires dominate: squirrel-like seed dispersers, tree shrews, and gliders
• Temperate forests with brushland margins rich in grazing pseudo-deer and compact browsers
• Carnivorous mammals include nimravids and long-bodied mustelid analogues

  Chishi:

• Montane and coastal regions offer contrasting habitats: mossy pine glades and humid marshes
• Home to relic amphibians, burrowing synapsids, and four-winged climbing birds

  Boshu:

• Lush lowland jungles and volcanic slopes create an Eden of biodiversity
• Contains gliding monotremes, sap-drinking marsupials, and jungle crocodilians
• Floral diversity promotes coevolution — bird beaks and flowers evolve in tandem

 

Legacy

The Yūkō Era is remembered as the twilight before the rise of reason — an age where life surged, unfettered by civilization. While no true sapients yet walked the earth, many of the foundations of the living world were laid in these millions of years. From mountain to mire, forest to tundra, the world grew loud with breath and blood, waiting in silence for the stone to strike.