A geopetal is in fact a fossil spirit level and is extremely useful for mapping purposes
On death the crinoid sinks to the sea bed. The skeleton is broken down sometimes allowing mud held in suspension to fill in available cavities. Sometimes the stem and / or the crinoid calyx collects mud as it settles out of suspension. Ideally any remaining space would need to be filled with sparry calcite. If subsequently the rocks were tilted, the geopetal will preserve the horizontal of when it was formed.
The crinoid calyx in the photograph is a good example of a fossil spirit-level. The top half of the cavity has been completely recrystallised with sparry calcite, whilst the bottom half contains sediment.