other isotope geochemistry samples are measured, for both carbon isotopes and oxygen isotopes. Some belemnoids (such as Belemnites of Belemnitida) serve as index fossils, particularly in the Cretaceous Chalk Formation of Europe, enabling geologists to date the age the rocks in which they are found. Classification Note: all families extinct Clade Belemnoidea Basal and unresolved Genus Jeletzkya Genus Belemnotheutis Order Aulacocerida Family Aulacoceratidae Family Dictyoconitidae Family Hematitidae Family Palaeobelemnopseidae Family Xiphoteuthidae Order Belemnitida Suborder Belemnitina Family Cylindroteuthidae Family Hastitidae Family Oxyteuthidae Family Passaloteuthidae Family Salpingoteuthidae Suborder Belemnopseina Family Belemnitellidae Family Belemnopseidae Family Dicoelitidae Family Dimitobelidae Family Duvaliidae Suborder Belemnotheutina Family Belemnotheutidae Family Chitinobelidae Family Sueviteuthidae Order Diplobelida Family Chondroteuthidae Family Diplobelidae Order Phragmoteuthida Family Phragmoteuthidae Family Rhiphaeoteuthidae See also Nautiloidea Ammonoidea List of belemnites References External links TONMO.com Cephalopod Fossils articles and discussion forums Mesozoic cephalopods Paleozoic cephalopods Cephalopods by classification Cretaceous cephalopods Carboniferous cephalopods Devonian cephalopods Permian cephalopods Jurassic cephalopods Triassic cephalopods Devonian first appearances Maastrichtian extinctions