Cardioceras praecordatum Jurassic ammonite fossil collected from the Oxfordonian Oxford Clays of Huntingdonshire, UK. Cardioceras are an genus of ammonite which existed during the Oxfordinian stage of the Jurassic approximately 154 to 146 Mya. They were nektonic carnivores which fell to the sea floor upon death. These examples were preserved within an oxygen starved deep marine shelf which gave rise to the Oxford Clay Formation. These specimens exhibit prominent, slightly sinuous ribs with a relatively pronounced ventral keel. Their evolute shell measures approximately 3 - 4 cm and it is supplied in a small plastic bag with an information label.