An ammonite in Jurassic limestone (IMAGE) Proceedings of the National Academy of Sciences Caption An ammonite in Jurassic limestone which was searched for extraterrestrial spinels. Credit Image credit: Birger Schmitz. Usage Restrictions None License Licensed content Disclaimer: AAAS and EurekAlert! are not responsible for the accuracy of news releases posted to EurekAlert! by contributing institutions or for the use of any information through the EurekAlert system.