Chronologic Relationships of Orbulines to the Globorotalia Fohsi Lineage

In tropical areas four important planktonic foraminiferal datum planes serve to delimit and subdivide the Miocene. The Globigerinoides trilobus datum defines the base of the Lower Miocene, the Orbulina datum defines the base of the Middle Miocene, the Globorotalia menardii datum defines the base of the Upper Miocene, and the Sphaeroidinella dehiscens datum defines the top of the Miocene (Bandy, 1963, 1964). In tropical areas, Praeorbulina and orbulines appear initially at the base of tlie four Globorotalia fohsi zones whereas the Globorotalia menardii datum is at or near the upper limit of the G. fohsi zones. By contrast in temperate areas of the eastern Pacific, off northern Mexico and in California, Praeorbulina and orbulimes appear initially in the upper p a ri of the G. fohsi zones (G. praefohsi zone); Orbulina universa usually appears at the base of the Upper Miocene or with the Globorotalia menardii group. In the Aquitaine Basin of France, orbulines first appear at a midpoint in the Globorotalia fohsi lineage. In the Western Garpathians, orbulines and Praeorbulina appear first just below the Upper Miocene Globigerina nepenthes zone of the Tortonian. It is suggested th a t the Karpatian and Sallomacian Stages of the Garpathians are approximately equivalent to the Middle Miocene zones of the G. fohsi lineage. The later appearance of orbulines in higher latitudes of some areas than in the tropics is attributed to a polyphyletic origin. Differences in chronology of oppearance is supported by comparing appearances in different areas with other planktonic foraminiferal lineages. A polyphyletic origin of orbulines is supported by the development of different species of Globigerina and Globigerinoides inside of orbuline chambers, by the differences in wall structure of orbulines, and by laboratory cultures and the biology of the living forms of orbulines. Most temperate orbulines probably arose from Globigerina bulloides, Globigerina praebulloides, or Globigerina quinqueloba in the later Middle Miocene. Most tropical orbulines probably arose from Globigerinoides sicanus originally at the base of the Middle Miocene and from still ottier species of Globigerinoides subsequently