Here in NE North Carolina for years farmers clearing and tilling land 40 and more miles from the ocean, have uncovered shells, including conchs. This area was once the ocean floor.
I have ‘clam shells’ I personally picked up in 2001 just south of Fallon ,Nevada. Altitude about 4000’. I picked them off the ground near the site of an old Pony Express station, just east of US 95. There were quite a few of them.