Evolutionary trends of the genus Bullatimorphites (Ammonitina) during the Bathonian-Callovian (Middle Jurassic) in Western Europe

The genus Bullatimorphites, (macroconchs Bullatimorphites, Kheraiceras; mic roconchs Bomburites, Schwandorfia, Sphaeroptychius, Treptoceras), is charac terized by an ontogenesis with three morphologic phases: serpenticone, spha erocone and ellipticone. Within the successive populations, the relative de velopment of these morphologies allows thr defined species to be placed in an evolutive and stratigraphic lineage. The history of the genus comprises five steps: 1) emergence (Early Middle Bathonian), 2) first gradual evoluti on (Middle-Upper Bathonian), 3) first morphological diversification (Upperm ost Bathonian), 4) second gradual evolution, 5) second morphological divers ification and extinction (Lower Callovian). Such events are biostratigraphi c datum lines which coincide with the palaeogeographic evolution of the Wes t Peri-Tethyan platforms during the Middle Jurassic. ((C) Academie des scie nces / Elsevier, Paris,).