Ammonite fossils collected from the bed of the Gandaki River at a place close to Saligrama or Muktinath (literally place of salvation) in the Mustang district of Nepal are known as shaligrams or shilas and are considered aniconic representations of Vishnu. These are first mentioned in Devi Bhagavath completely. If we want to refer puranas, it is as so there was king name Dharmadwaja who was initially a devotee of goddess Lakshmi who later became rival of the same and completely ruined. His son Padmadwaja realized his father's mistake became a devotee of Lakshmi, pleased to his devotion Lakshmi herself incarnated as Tulasi.