How fossils form: fish die their soft parts decompose and the hard part get covered by small particles of rock (sediments) as more layers of sediment build up on top, the sediment around the skeleton begins to compact and turn to rock.
How it got in Kansas: one way is that animals took it there, another one is there was sea there before and because of tectonic plates moving ( moving on top of each other) land is created and the fossils where on that land