Morphometric Analysis of Planktonic Foraminifera

Raw data were obtained from Hole 865B of the Ocean Drilling Program (equatorial mid-Pacific Ocean). The entire data set consists of 51 time slices through a stratigraphic interval spanning around 13 million years. We focus on two samples here: one middle Eocene and one upper Eocene; a future contribution will analyze many more time slices and discuss the pattern of evolution in detail. Samples of 10 cm³ were taken from the sediment and washed over a 63-micron sieve to remove fine particles (mainly coccoliths). The sieved residue is >99% planktonic foraminifer shells. All specimens of the Turborotalia cerroazulensis group of morphospecies were identified by eye using the taxonomic criteria of Pearson et al. [40 ] from multispecies assemblages and picked without further reference to species designation. Most other groups of foraminifera are easily distinguished, although rejection of specimens belonging to T. altispiroides and T. ampliapertura required a greater degree of expert discrimination [40 ]. The first 200 specimens encountered were manually separated and mounted on cardboard slides in a standard orientation (edge on, aperture facing). For each specimen, fine adjustments were made using a universal stage to achieve as consistent a standard orientation as possible. The choice of orientation in side view and measurements were carefully designed to capture the greatest range of morphological variability in the group, including the characters that are used in qualitative discrimination of the morphospecies by working taxonomists [40 ].
Measurements were made from photographs of each individual using Image Pro+ (Image Software, UK). The following morphological traits were incorporated in analysis: area, 'filled' (the proportion of a circle of an individual's radius filled by that individual), final chamber inflation (chamber width scaled by length), final chamber and aperture aspect ratio (the height: width ratio of the final chamber and aperture, respectively), test height (axis/radius), test expansion (diameter/radius), umbilical angle, chamber number and chirality. See fig. 2 for more information.