Fossil casts and molds
When a creature dies and becomes buried, sometimes its remains can completely rot and dissolve away, leaving a hollow space within the rock called a mold. If that mold becomes filled with sediment and water, it can harden into rock, becoming a cast. Every so often, the same sort of process can happen within the body of an animal, providing a stone cast of some of its internal arrangements. If that happens within the brain case of a skull, it’s called a brain cast. In 1924, a brain cast found in South Africa caught the attention of British anatomist Raymond Dart. After removing the fossil from the rock with his wife’s knitting needles, he was left with the first few skull fragments of Australopithecus africanus, or “southern ape of Africa,” to be discovered.
Here’s a 25-minute video on The Taung Child, the fossil Raymond Dart discovered.
And here are a couple of articles on The Taung Child, and how it may have died.