The Wikipedia page on Mayan numerals mentions the symbol for zero is an upside down turtle shell. It is not clear from the symbol itself that it is a turtle shell, and I'm not able to find any source explaining why we believe it is a turtle shell. I've seen other references (1 and 2) saying it is an olive (or oliva) seashell.
What is the evidence that this post-classical Mayan symbol for zero is a shell and is it actually a turtle shell or some other kind of shell?
The reason why I'm interested is that the Mayans used dots and bars for the rest of this variant of their number system, which could have come from calculating using rocks and sticks on the ground (this is just a conjecture by me). I can imagine they might have also picked up olive shells along the beaches and used that to represent zero. That would seem to be a better size to manipulate for calculations than a big turtle shell (as an avid seashell collector, it's neat to think that the olive shell could have been associated with an early concept of zero).
From the paper Anna Blume: "Maya Concepts of Zero" (March 2011, Proceedings of the American Philosophical Society 155(1)) I referenced above as (1) it says:
In his 1886 commentary on the Dresden Codex, Förstemann originally thought these oval forms “looked like a symbol for the human eye”. Twenty-four years later, in their 1910 study “Animal Figures in the Maya Codices,” it was Tozzer and Allen who identified the standard oval form that the Maya used to represent zero in the codices as a stylized oliva shell.
So it seems like there is some guesswork here, and I'm not sure where the turtle shell theory originated.