One good way to preserve a fossil is to make a wash with wood glue and water. This works for porous, fragile materials.
Get any wood glue that is not waterproof. The waterproof glues are different and will not mix or penetrate well.
In a disposable cup mix a little water to some of the glue to make it into a thin paint consistency.
Brush it over the entire surface and allow to dry for 12 ...