Do coelacanths have knees?
Veeerrrry primative ones.
Well, from the art gallery referenced above, it seems the artist is working under the “tails begin beneath the knee” theory. (I’d link to the specific examples, but they involve full frontal nudity, I suppose that’d be frowned on by the “aussie mermaid” PTB.)
However, I can’t tell from the paintings if the mermaids’ thighs are actually two separate limbs that re-join into a single tail, or if it just looks like the beginnings two limbs, but it’s really just the upper part of a solid tail.
In any case, my tireless and painstaking research on mermaid anatomy* has yielded at least a slight tidbit: the blog of some fellow who visited Copenhagen, including a museum** that featured a mermaid skeleton. Which, as you can see, features a pelvis, but no human-like leg bones. However, it may be possible that mermaids normally possess some free-floating vestigial femurs in the tail that could, under the right position, give the mermaid the appearance of having a humanoid leg structure under the flesh of the tail. Such bones might be left out of mermaid skeletal displays by mistake, misunderstanding, or some misguided sense of aesthetics on the part of anatomists or anthropologists.
And yes, for those who were wondering, I am indeed a weird, pathetic excuse for a human being.
*Meaning, I searched for “Mermaid skeleton” on Google images. And what of it?
**A modern art museum, anyway.
They lay egg sacs in the ocean, and then the mermen come and spray semen to fertilize 'em. How mermaid bukkake works is left to your imagination…