While Ant-eater [the echidna] goes out to look for food, Turtle looks after baby Ant-eater. When Ant-eater has not returned by the third day, Turtle eats baby Ant-eater. Ant-eater returns and, in anger, begins to throw stones at Turtle. Turtle throws spears at Ant-eater. This is why the echidna's back is covered with spines, and the turtle's back is covered with stones.
According to this Dreaming story, 'If you look carefully you can see the little baby ant-eater in the turtle's mouth'. Source: Aboriginal Children's Stories (1968)