When the BigL Coast closed off a large area and inadvertently created the Hydro-Nuke Salt Lake, a number of megaorthoceros segnoneusteses were trapped within. Luckily for them, segnoneustes fed off of chemicals and dead matter, which allowed them to continue to survive where other species died as the salt levels rose.
These stranded microorganisms quickly adapted to this new environment; they have gained a new pair of long swimming paddles for turning at quicker speeds, while the increased amount of salt has increased the buoyancy, and has kept the species from requiring an immediate advancement in this manner.
In order to aid in survival in this empty zone, the females have grown to be able to feed through commensal parasitism off of the local river swarmers, which are also known to feed off of the megaorthoceros salisvimmia.
Like their ancestors, salisvimmias swim using their long, thickened tail-flagellum, turn using their long paddle-fins, feed using a massive proboscis. They also retain the sexual dimorphism, mating habits, and reproduction style of their ancestors.
Because they were accidentally closed off from the rest of the megaorthoceros segnoneustes population, they did not have a chance to replace their ancestors, and have instead grown into an offshoot.