Lateral cross sections of ammonite fossils.  A: The white arrows indicate the presence of pits.  B, C, and D show close up images of these pits.  Photo credit: Baets, K. D., Klug, C., & Korn, D. (2011). Devonian pearls and ammonoid-endoparasite co-evolution.  Acta Palaeontologica Polonica ,  56 (1), 159-180.