Today you can watch as cheetahs try to run down gazelle on the African plains. As one animal becomes better at what it does, then the other must improve or suffer hunger or death. The invertebrates in the seas have exactly the same struggles going on. Ammonites were preyed upon by the 'sea dragons' and thus animals thicker shells and improved their jet systems to allow them to escape the jaws of the predator were favoured. One ammonite shell that had been bitten sixteen times and survived is known.
Predators come up with new tricks too.
The gastropods started to bore into the shells of their bivalve prey during the Cretaceous. Bivalve shells are thinner and more vulnerable to boring than gastropods and a combination of a rasping tongue and the use of acid, which dissolved the shell of the victim allowed gastropods to become more efficient feeders. The final step the process was to suck out the flesh of the bivalve by inserting a mouth tube. These 'arms races', as they are sometimes called, have gone on since Cambrian times. The meat-eating dinosaurs and their prey would have had similar struggles, showing that very different types of animals are governed by similar rules.