Ammonite fossils. Ammonites, now extinct, were marine invertebrate molluscs with a spiral shell, most less than 20 centimetres across. These three ammonites are (left to right): Oistoceras wrighti; Asteroceras obtusum (this is the zonal index species); Tragophylloceras loscombi. All three are from Jurassic rocks near Lyme Regis, UK. They are part of the collections held at the Natural History Museum, London, UK.