We can understand that Shiva linga/lingum depicts entire universe or to say existence. But, Saligram is a fossile of a creature of a type Ammonite. What could be the possible reason behind considering ammonite fossil as symbol of lord Vishnu?
Also, it is said that Saligram found at Gandaki River are the only real Saligram used for worship of lord Vishnu? So, all other Ammonite fossils found across the world are not considered Saligram?