A quantitative study of Late Eocene Nummulites (Foraminiferida), Jackson Stage, Southeastern United States



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Paleontological analysis of extensive new collections from surface exposures of the Jackson Stage in the eastern Gulf Coast Province has determined the presence of five nummulitid species: Nummulites willcoxi Heilprin, 1882; N. heilprini Hantken, 1886; Operculina floridenis (Heilprin), 1884; 0. mariannensis (Vaughan), 1928; and Heterostegina ocalana Cushman, 1921. N. willcoxi is redescrihed and N. heilprini reinstated. Three sample groups of N. willcoxi and one of N. heilprini were examined by factor and canonical variate analysis. The twenty nine quantitative test variables used in the analyses include external dimensions, embryonic dimensions, and dimensions and counts recorded in the first four volutions of each test. Principal component factor analysis yielded eight or nine common factors which account for over 80 per cent of observed test variability. Four well resolved factors explaining over 60 per cent of test variability are associated with the embryonic growth stage, mature growth stage, external size and form, and chamber volume in relation to size. The extra factors are not clearly identifiable, probably reflect observational error, and may be correlated with the first four. Canonical variate analysis very effectively distinguished ten essential test variables out of the original twenty nine, and gave three canonical variates which collectively account for all the observed test variability. Two canonical variates account for about 95 per cent of test variability. The first canonical variate diagnoses morphospecies. The second canonical variate distinguishes between populations of megalospheric N. willcoxi. Two form groups of N. willcoxi are recognized by their different patterns of increase in chamber volume. The thin lenticular form grew rapidly in chamber height in its outer two or three volutions. The more inflated, biconvex form accommodated relatively more of its increase in chamber volume in alar prolongations. The two form groups of N. willcoxi are from different stratigraphic levels, but from very similar carbonate shoal habitats, as indicated by the lithology of the enclosing limestones of the Ocala Group. Specimens from a transitional lithofacies differ in their smaller external dimension, and a lower, less variable growth rate following the thin lenticular pattern. Test variation in larger Foraminiferida is the product of interacting heriditary and environmental factors. This systematic quantitative examination of variation, correlation and interrelations in fossil Nummulites demonstrates that canonical variate analysis can show how populations of a single species differ from one another. When the effects of sedimentary variables on fossil test morphology can be quantified, it will be possible to objectively discriminate between adaptive and evolutionary trends in test variation.