What were the characteristic of life forms that had been fossilised ?

Explanation

The organism with hard part likely to be fossilised, than those who do not have such parts. The harder the material, better it would be preserved soft parts fossils occur rarely, e.g., birds and pterosaurs have very light bones, hollowed out and specialised for flight.
So, they have sparser fossil records a compared to mammals, whose bones are partially mineralised during life.