Because I’m a person who can’t stand not having some kind of explanation in-universe for stories involving the mythological / cryptozoological / etc., I’ve made a pair of evolutionary trees for sirens and merfolk, and sphinx and gryphons, respectively. I’ll talk about my rationalizations and all below the read more breaker. I might update this post later with example drawings for the gryphons and sphinxes, as they’re rather complicated.
Sirens & Merfolk
Evolving from a shared ancestor with the parvorder Odontoceti (toothed whales), sirens are carnivorous, and took on human-like appearances to lure sailors and beachgoers close enough to pull them into the water. Sirens diversified into near-shore, ocean, and deep-dwelling groups. Merfolk evolved from a shared ancestor with the ocean and deep-dwelling sirens, taking on a more herbivorous or scavenging feeding strategy. The merfolk diversified into deep-dwelling, shallow-dwelling scavengers, and shallow-dwelling herbivores. All groups have extant species, with ocean merfolk and shore sirens being the most common, and deep-dwelling sirens and merfolk being rarest.
Gryphons & Sphinxes
Evolving from a shared ancestor with the family Felidae, gryphons are carnivorous, evolving flight and shark beaks (along with front talons, rather than paws) to occupy an otherwise empty niche (that is, very large flying predator. As human development continued, it became advantageous for gryphons to be faster/more mobile walking, as well, so their front limbs changed back to paws, while keeping the wings and beak. Eventually, airborne predation became more dangerous than it was worth (as gryphons are massive targets for archers), and returning to the land became ideal - from here sprouted the winged lion (wings mostly used for cooling down though use of their high surface area), hieracosphinx (beak still valuable for tearing flesh), and Greek sphinx (wings mostly ornamental, though allowing short gliding flight). Wings, though useful in their own way, take a lot of energy to maintain (particularly with molting), so some parts of the groups lost their wings entirely, as well as beaks (favoring teeth for grinding and chewing food), giving rise to the European, Egyptian, and Indian sphinxes. All groups have extant members, with non-winged and non-beaked being the most prevalent forms. This explanation’s (more than) a bit of a stretch, but I’m at a loss for any other way(s) I could possibly work out this tree.
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