A scynthetusk is predatory octodactylopod which lives 100 million years in the future, in rainforests of eastern Siberia. Big as leopard, strong and well armed, it hunts many smaller arboreal animals with ease. But there is a species of small hunter, which scent fills its hearts with terror. Cunning and agile marsupial descendant, known as vampire possum. Octodactylopod on the picture is now attacked by one. Vampire possum is not a predator but omnivore similar to large dormouse. It has however very good reason to hunt octodactylopods and to be master at it. This female is pregnant, but it has no pouch and will not feed her babies with milk like her ancestors did long ago. It needs a host to nourish her joeys. The only place where they can develop is inside octodactylopod respiratory system (known as labyrinth organs and located inside its skull), where she will try to inject them trough “nostrils” on creature back. If this unlucky scynthetusk would not manage to shake off or kill her quickly, vampire possum larvae would live inside its respiratory system during next three painful months which may end by death if host becomes to emaciated. Most of octodactylopod females have brooding pouch inside their labyrinth organs. They use it to gestate their own larvae which feed on milk-like secretion produced there. This scynthetusk don’t have anything like this because it is a male. For vampire possum it makes no difference. Her ancestor larvae few million years ago were feeding on octodactylopod milk, so they were limited to infest only brooding females. Vampire possum larvae feed on blood instead, so they can parasite on every octodactylopod if only it is big enough, no matter if it is a brooding female or not. Its descendants will later evolve very different, non-parasitic larvae forms.
Scynthetusk is descendant of cichlid (here you can find more about octodactylopods fav.me/d5kggry ), vampire possum is descendants of sugar glider (which was shipped to Eurasia and released to the wild long ago by humans).
Vampire possum biology is of course inspired by awesome changelings of Serina. If you know them, you can probably guess some of the ways which evolution of vampire possum descendants might go. I was wondering if changeling vertebrate-holometabola physiology can be applied to mammals too. Marsupials are group which could go this way, making use of octodactylopods to free larva from mothers care (it would enable evolution from parasite, to parasitoid, scavenger and thereafter free living insect-hunting larva which would probably be imposible in placentalia). Also marsupials are group which is quite outshined by placental mammals and birds today. Similar way like archosaurs in triassic and mammals in cretaceous were. If they will not go completely outcompeted, there still can be a quite interesting future waiting for them. If so, they can evolve into something very different before it happens.