The basic idea is that a massive organism with a bone structure similar to a tortoise's dies in an open area. Then, once only bones are left, they become an open cavern with fertile soil layered on the bottom. How long might these bones last in the environment described below?
On a world similar to Earth minus size, gravity, and a mildly different atmosphere. The weather is much more violent, causing frequent dust storms. The planet is larger than Earth and is mostly scrublands. However, there are certain geological formations, bowl mountains, that provide adequate shelter for forests. There are two large oceans on either side of the planet as well. They have much lower salt content than Earth's oceans do. The planet is made of lighter elements than Earth is, resulting in slightly stronger gravity.
The planet was once lush with many thick forests that could support the massive organisms. The organisms grew too big for the planet to support them and it became how it is described in the quote above.
This image is poorly edited and does not represent proper anatomical proportions. Namely, the legs, which should be much thicker and stockier. As well as the head and neck vertebrae. It should also be noted that the shell is made of bone rather than keratin like Earthean tortoises.