I don't think there is any symbol, message or underlying meaning behind how they are arranged. I haven't read the book so I don't know if the book sheds more light or how differing the film is from the book. And I haven't seen the question posed to the director in any interviews.
At the very least in the script it appears that there isn't much describing how they are laid out. Script link (page 105 of the pdf)
EXT. BEACH/LIGHTHOUSE - DAY
LENA stands on the beach.
The LIGHTHOUSE is still a few hundred metres away.
The area here is littered with bones and scraps of clothing.
The scene of a massacre, many years ago.
Blown and blasted by the wind, sand, sun, and salt.
The bones look like ivory.
A bright red backpack has been
bleached pink. The body beneath it has almost been
completely submerged into the dune.
LENA reaches down and picks up the top half of a skull.
The back of the skull morphs into the star-shape of a
vertebrae, as it had been fused with its own backbone.
LENA drops it back on the sand.
Then continues towards the tall white lighthouse building.
Just based on that there doesn't appear to be anything specific about how its laid out in the script, "scene of a massacre", "area here is littered with bones".
I think a possible reason why they are laid out like that is perhaps Kane, having lost his mind, laid out his dead crew members bones out like that.
As for lack of mutations in these human remains I think we can put it down to that whosever these human remains are weren't alive long enough for the mutations to affect them.
I think I'd need to rewatch the movie again to give a better answer on that but hopefully the script link helps somewhat.