Explain how the turtle’s evolutionary process evolved the function of turtles shells?

The explanation for the turtle's evolutionary process was quite a challenging one for evolutionists.

Two categories of paradigms have come to offer their ideas about how they think the turtle came about its amour.

Palaeontologists (that is, those who examine and report on the history of life here on earth using fossil records) state that the turtle's shell developed into what it is now over time from bony scales same as those which can be found on armadilloes and crocodiles.

Development biologists have a  very different opinion. They theorize that the shell evolved out of ribs. The ribs, according to them due to the need for environmental adaptation, became wider, and flatter and eventually merged to form shells.

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the answer is:

carbon dioxide