A fish is living in a cool, safe environment. He is living his best life in this river.
Nemo & his Unique Discovery
One day, very nasty run-off consumes the water and the fish dies.
As time passes, the fish gets buried from deposition which covers the fish in little pieces of sediment. Sediment also pushes itself in between the bones and hardens which is an example of petrification.
As thousands of years pass, the water that was on top of the bones recedes. What is left, is the bones with hard compacted stone which is because of petrification. The bones soon await to be uncovered from weathering.
As water and wind erodes the soil that is burying the fossils, the bones/fossils/stone slowly peak through the ground .
Soon thereafter, a couple of paleontologists drive through the field where the bones are buried. Accidentally, they discover a dark thing sticking out of the ground and they go to investigate it. As they unbury the fossils, they come to the conclusion that it was a fish that used to live in the river that was there.