The Late Rodentocene: 20 Million Years Post-establishment

The Late Rodentocene: 20 million years post-establishment

Riffing the Reefs: Marine Life of the Late Rodentocene

HP-02017 is easily thought of as a planet of hamsters, but other life also thrives. The ecosystem's accessory organisms released onto the planet have since formed ecosystems of their own, equally players in the game of life as the hamsters are, and nowhere is this more evident than the shallow seas of the Late Rodentocene.

At first glance, the reefs that grow in the sunlit shallows of the planet's seas look incredibly like those of our own. Forests of algae and kelp grow in the rocks close to shore, as well as corals of all shapes and sizes that sprout in great masses, forming reefs that serve as a shelter for small, colorful sea creatures that thrive in abundance. Yet despite its initial familiarity, the marine biomes of HP-02017 are anything but: its similarities are superficial, and its creatures are something else entirely.

Only a choice few organisms were seeded into the seas: small mollusks such as sea snails and bivalves, as well as sponges and corals, which at first were from but a small collection that have since diversified into a dazzling array. But with so few creatures, and so many empty niches, it didn't take long for the choice few colonists to explode into a diversity rivalling that of Earth's oceans.

Corals, which reproduced via free-swimming planktonic larvae, have filled the empty spaces of their relatives the cnidarians, with some forms becoming tentacled stinging sessile hunters akin to sea anemones, while other drifting larvae become neotenic, remaining in their mobile forms into adulthood and become the transparent, drifting mock jellies.

The humble sea snail has also seen an extreme explosion of diversity in the past 20 million years, spawning thousands of species that came to fill nearly every marine invertebrate niche imaginable. Some lost their shells, coming to resemble sea slugs, and many of which would develop bright body colors for display or as warning coloration, converging heavily on nudibranches present in Earth's oceans. Other snails, developing flattened bodies and a unique vascular system in their belly-foot, become heavily convergent on echinoderms, with some being long-bodied bottom feeders like sea cucumbers, others developing venomous spines akin to urchins, and one strange lineage, developing vaguely-arm-like protrusions on their foot and a radula adapted for feeding on bivalves, becoming a bizarre analogue of a starfish. Others become tentacled swimmers resembling shelled cephalopods: the notiluses.

But by far the most diverse and successful invertebrate clade in the planet's oceans are descendants of planktonic krill, which, in the absence of fish, exploded in diversity to fill as many aquatic niches as they can. Known as shrish, these peculiar crustaceans first emerge as shrimp-like swimmers that propelled themselves through the water with a paddling array of feathery swimming legs.

As they evolved even further, however, they began taking on peculiar niches as time went on. Bottom feeders such as the trilobug became broad and flat, filling roles akin to flatfish or crabs, and some of these bottom-dwellers secondarily re-evolved to become active swimmers, such as the filter-feeding shringray that defends itself with venomous barbs on its tail. Others became elongated, flexible centipede-like predators that hunted other shrish, lurking in caverns in coral much like moray eels in wait to ambush their prey, known as the shreels.

Some shreels would eventually develop a shorter and more streamlined body, and give rise to active swimmers that propelled themselves with undulating waves of their abdomen and tail. Becoming a more efficient means of propulsion with larger or faster species, these paddletailed shrish would eventually modify their rearmost swimming legs along with their tail fan into a caudal fluke of sorts, while their thoracic limbs became used for catching food or filtering particles from water. This lineage of paddletailed shrish would eventually bring about the biggest top predators of the Late Rodentocene seas, the shrarks. Using their barbed rostrums and spiked forelimbs as three grabbing "jaws", the shrarks reach lengths of almost a meter: rivalling the largest marine arthropods of Earth's history, the eurypterids of the Devonian era.

The reefs of the Late Rodentocene are a vast and diverse ecosystem that flourishes in strange new ways independently of the world of rodents above. Life in the ocean takes on unusual new forms in a biome not yet invaded by the hamsters -- at least for the time being.

▪▪▪▪▪▪▪▪▪