Here's an excerpt from The Story of Time, by Jose Arguelles, on Turtle:

"... Why Turtle Has A Shell

Once time had woven the Biosphere into a living web of ocean and land life, Great Grandmother Galaxy recalls, Earth was happy. Since Turtle already contained the lore of time, Earth chose Turtle to be the guardian of the animal kingdom. Since Tree was already the keeper of time, Earth chose Tree to be the guardian of the plant kingdom.

Now Earth spoke to Turtle...“Turtle, on your back are the thirteen scales which hold the knowledge of time called the thirteen moons. This is a cosmic power that you carry on your back. Your shell holds the knowledge of the whole Earth. Your shell is a demonstration of the cosmic power of time which the whole Earth weaves into the living mantle of the Biosphere. The top of your shell with its thirteen scales shows how the Biosphere is ruled by the Law of Time.”

Thirteen scales to reflect The path of thirteen moons Thirteen moons to guide Earth’s spiral journey But once around the Sun Thirteen moons, one Earth, one Sun, one Turtle... Constant to the Law of Time Turtle, you are wonderful In your shape and form in Time!"