Ammonites with the centre whorls gone crop up occasionally on the beach, and I'd wondered what had caused this phenomenon. The curator of Whitby Museum (who has a fine selection of fossils from this coast) has supplied the answer: it's jurassic marine reptile vomit.
He has an icthyosaur skeleton on his wall with several fossilized ammonites where you'd expect its digestive tract to be. We know that herbivorous land dinosaurs used 'gizzard stones' to help grind up and digest their food and would regurgitate these once they had ben worn smooth.
It seems that icthyosaurs would eat ammonites and after a while would need a clear out, honking up the partly digested ammonite shells.