Late Neogene planktonic foraminiferal assemblages of the California margin are consistently of low diversity. Diversity remained relatively low even during warmer intervals. Assemblages are dominated throughout the interval by Globigerina bulloides and a number of species of Neogloboquadrina. Other persistent forms occurring throughout in varying frequencies are Globigerina quinqueloba, Globigerinita glutinata, Orbulina universa, and Globorotalia scitula. Globorotalia (Globoconella) spp. also occurs throughout, although in relatively low frequencies. Globigerinoides ruber occurs fairly persistently, although in low frequencies in the southern sites of the transect. Reasons for this persistently low diversity are unclear, but it probably resulted from the relatively cool sea-surface temperatures that marked the California Current system during the late Neogene in combination with strong seasonal upwelling. High variability in the modern surface waters results in part from the strong coastal upwelling. Planktonic foraminiferal assemblages of the California margin mostly include generalist forms that must be adapted to high variability of near sea-surface conditions.
The Neogloboquadrina plexus exhibits a dynamically changing succession of forms, with one or two species dominating at any particular interval. This group includes relatively large forms such as Neogloboquadrina kagaensis and Neogloboquadrina asanoi, forms of intermediate size that include Neogloboquadrina humerosa praehumerosa and Neogloboquadrina dutertrei, and smaller forms that include a number of taxa that were previously considered to represent variants of N. pachyderma (Kennett, 1976; Keller, 1978c). These morphotypes include both sinistral- and dextral-coiled forms as well as those exhibiting differences in general shape, numbers of chambers in the final whorl, apertural characteristics, and other features. A long-standing question has been whether the different forms of N. pachyderma represent distinct species or ecophenotypic varieties of the same species (Bandy, 1959; Kennett, 1976; Keller, 1978c). However, recent molecular biological studies (DNA) of modern forms (Darling et al., 1998; Stewart et al., 1998) have demonstrated that at least some of these morphotypes, including sinistral- and dextral-coiled forms, are distinct species.
We have discovered that several morphotypes of N. pachyderma have distinct biostratigraphic ranges in the late Neogene sequences of the California margin. Furthermore, their appearance and disappearance in the sequence appear to represent unique events. This suggests that several of the morphotypes are distinct and separate forms within the phylogeny of the neogloboquadrinid plexus. From an evolutionary perspective, these forms act as distinct species, an interpretation that supports the general results of the molecular biological investigations (Darling et al. 1998; Stewart et al., 1998). Nevertheless, in this investigation we continue to classify them as forms of N. pachyderma—but in doing so, we recognize that their future classification will need to refer to them as separate species. The taxonomy of each of the stratigraphically important forms is discussed below.