Probably an unprepared nothosaurus vertebra. The vertebra is broken in half and needs to be glued.  A small piece is missing on the back side.

 Location: Heilbronn
Epoch/Age: Triassic, "Oberer Muschelkalk, "Grenzbonebed",~ 235 Million years
Size of the stone: 21 × 13 cm
Size of the fossil (estimated) : 3,2 × 2,5 cm