Stoneworm

When a mutation caused a crusher worm to be born with an extra pair of legs, nature took advatage of this unique opportunity. The stoneworm evolved from this, replacing it's ancestor in the high desert. The most obvious change is that their hollow shells, which have grown to resemble a rock. The stoneworm's have evolved this to hide from predetor. They are able to carry these shells because of their third pair of legs. They have also evolved shrunken forearms because it no longer latching onto desert shellwalkers.

They are only capable of reproducing from the small organ hidden in their protective rock-shaped shell. The shell isn't filled by the stoneworm, but by its slowly growing young. When the young are still in their protective shell they eat the iron rich waste of their parent, but when the parent dies the young will have to escape by eating their parent, which also gives them the vital nutrients to grow larger.