ONTOGENETIC MORPHOMETRIC COMPARISON OF MIDDLE-LATE EOCENE PSEUDOHASTIGERINID PLANKTONIC FORAMINIFERA USING X-RADIOGRAPH IMAGE ANALYSIS

Middle to late Eocene species of the planispiral planktonic foraminiferal genus Pseudohastigerina include P. micra and P. naguewichiensis, which are widely recognized and easily distinguished species. Less readily distinguished are Pseudohastigerina acutimarginata, which resembles P. micra but has a more pinched peripheral margin, and a previously unrecognized morphotype that resembles P. micra in its early ontogeny but becomes more evolute later in its ontogeny. To test whether or not these latter forms should be synonymized under P. micra, we obtained high-resolution X-radiograph images of specimens mounted in axial and edge views to characterize ontogenetic changes in shell and chamber size and shell coiling. Populations of Pseudohastigerina were selected from a geographic array of sites and from several stratigraphic levels within the middle and upper Eocene. The biometric analyses demonstrate significant differences in chamber growth trajectories, chamber coiling, maximum test size, and roundness of the ultimate chamber face between populations of P. naguewichiensis and the other three pseudohastigerinid morphotypes. Populations assigned to P. micra and P. acutimarginata are statistically indistinguishable in all coiling and size measurements, and comparison of ultimate chamber roundness shows a continuous range of circular to peripherally elongate shapes. The strongly evolute pseudohastigerinids are similar to P. micra in all measured features except final chamber number and maximum test size. We conclude that P. micra and P. acutimarginata are synonymous and probably represent ecophenotypes, whereas the evolute pseudohastigerinid forms are closely related to P. micra but may represent a new species. Stable isotope data will also be presented in order to infer the depth ecologies of the various morphotypes.