Origin and Development of Globigerina quinqueloba Natland in the North Pacific

Globigerina quinqueloba Natland is a characteristic planktonic foraminifer inhabiting temperate-subarctic water masses of the world oceans today. Morphotypes referred to Globigerina angustiumbilicata (Bolli) are common to abundant in Miocene deposits at the western and eastern margins of the North Pacific. Both these species are characterized by an average of five chambers in the final whorl and extremely simple morphology. Only subtle differences allow their separation when populations, rather than individuals, are examined. A direct lineage is proposed and illustrated from Globigerina angustiumbilicata to G. quinqueloba within the Middle Miocene to Recent interval. Populations of the Globigerina angustiumbilicata-G. quinqueloba lineage were studied from Middle Miocene ("Globorotalia fohsi barisanensis" Zone) through Pliocene deposits from Honshu Island, California, and the Mohole cores. Recent populations of Globigerina quinqueloba were studied from the eastern and western North Pacific as well as from the Sea of Japan. The following characteristics were measured on an average of 20 individuals from each fossil or Recent population; test diameter, diameter of the proloculus, total number of chambers, number of chambers in the final whorl, coiling direction, and character of the last formed chamber and aperture. Measurements and analysis demonstrate that only three morphologic characters within the lineage exhibit significant variation during the interval studied. These include test diameter, shape of the final chamber and aperture, and spinosity of the test surface. Average test diameter increases from less than 150 microns with in Middle Miocene populations to greater than 200 microns in Pliocene through Recent populations. Specimens with a simple aperture and final chamber are common in Middle Miocene horizons whereas individuals with an aberrant last chamber and lip comprise up to 50 percent of some Recent populations. A qualitative trend is apparent from smooth to hispid tests reflecting a possible change in wall microstructure with time. Oligocene populations have smooth test surfaces, Middle Miocene populations exhibit smooth to finely hispid surfaces, whereas Late Miocene through Recent populations display prominantly hispid surfaces. The study illustrates a long period gradational morphoseries from Globigerina angustiumbilicata to G. quinqueloba. Complications in interpretation of morphologic trends arise due to ecophenotypic variation within populations of a given horizon. The subtle nature of variation among simple morphologic characters during the Middle Miocene through Recent interval hinders a clearcut separation of the two species. Moreover, analysis of population characteristics, rather than analysis of individuals, demonstrates the difficulties inherent in the use of species limits defined on the basis of a single type specimen. Nevertheless, arbitrary statistical limits allow individuals assigned to Globigerina quinqueloba to be initially recognized within the Miocene "Globorotalia mayeri" Zone. Statistically defined populations of Globigerina quinqueloba appear initially within the Miocene "Globorotalia menardii" Zone