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Morphometric Analysis of Sphaeroidinellopsis: New Biostratigraphic Information from Gradually Evolving Lineages

A. C. Gary, N. Healy-Williams, D. F. Williams

Gradually evolving lineages have not been routinely used biostratigraphically because morphological changes are difficult to define through time. However, using sophisticated shape-analysis systems, we developed a zonation based on gradual evolutionary changes in morphology.

In this study, we analyzed morphologic changes in specimens of the Sphaeroidinellopsis lineage in middle Miocene to Pleistocene sediments from the DeSoto Canyon area of the Gulf of Mexico. We quantified morphology with an automated video digitizer and Fourier analysis, which breaks the total shape into specific shape components. Fourier analysis accurately and objectively quantifies the morphology of various microfossils. We used EXTENDED CABFAC/EXTENDED Q-MODEL to define the morphologic substructure (i.e., existence of distinct morphotypes) of the Sphaeroidinellopsis population within time intervals. Comparing morphotypes between samples, we found the following. (1) Morphotypes are time continuous. When a morphotype disappears it does not reappear up section. (2) The progressive chan e in morphotypes through time follows a shape continuum. Morphotypes become increasingly more spherical with decreasing age. (3) The number of morphotypes present in any given sample is generally greater than the number of species possibly present. A zonation scheme based on gradual evolutionary changes in the morphology of foraminiferal lineages could complement traditional biostratigraphic approaches by significantly increasing the resolution of correlations in late Tertiary sediments.

AAPG Search and Discovery Article #91043©1986 AAPG Annual Convention, Atlanta, Georgia, June 15-18, 1986.