Warning: merfolk anatomy essay, skeleton and cartilage discussion
While drawing my merfolk OC's I discovered an interesting detail about how artists choose to depict merfolk. An ordinary fish-based mermaid has a horisontal fin, the movement of which corresponds to how human hip and knee joints move, and also to spine stability if the whole swim pattern is considered.
Shark-based merfolk on the other hand... Most if not all shark merfolk art I was able to find had a vertical tail fin, and were depicted swimming by swinging their hips sideways. (not that ideal for the spine of the human upper body, is it?)
And the thing is, both sharks AND most fish to my knowledge have a vertical tail fin. Only the non-shark merfolk have this detail changed to fit the anatomy of the upper body.
Now my goal isn't to criticize anyone or their art, all of this is fiction so do what you want! Both versions can definitely look awesome in drawing and movement. It's just so intriguing to see how our minds apparently have engrained the VERTICAL shark tail as a key characteristic of sharks, but not of other fish.
Then again, the issues with skeletal consistency all boil down to which bone structure do we imagine merfolk to have.
Are fish merfolk marine mammals? -That would be similar to whales, which have HORIZONTAL tail fins.
Do shark merfolk have a different bone structure compared to other fish merfolk? What if they have cartilage as at least a partial replacement for the bones of the human part?
Which half is the anatomically dominant part of the body for internal structures in merfolk, fish or human? Or do merfolk have chronic back and hip problems because of the inconsistensies? Or do they just have bodies with a long spine and no notable leg structures? That would make sense but wouldn't necessarily exclude back problems from sideways tail movement.
Personal headcanon from that: the human upper body was originally a decoy. It lures humans in like in mythology, or simply camouflages the intelligent fish so that people wouldn't want to fish for merfolk meat.(What benefit do clumsy human body parts add to an otherwise hydrodynamic fish? The only other plus seems to be hands with thumbs.)
Speaking of hydrodynamics, merfolk probably use their mouths more as a tool than humans on land. While swimming the head is often the first thing in front, unless the swimmer is reaching forward with their hands. Using the mouth to hold things would leave the hands free for swimstrokes.
Lastly, I present to you shark eyes + human eyes = a horrifying face or extremely cool looking eyes with active nictating membranes (third eyelids).
