Estimation of ancestry in planktonic foraminifera:Globoquadrina Dehiscens

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Abstract
The utility of comparative morphology in estimating ancestry in a group that evolved iteratively is examined in a study of Globoquadrina dehiscens (upper Oligocene Miocene) and Truncorotaloides primitiva (Paleocene-middle Eocene). Gross shape of shells in axial and spiral orientations is closely comparable as is the globorotalid-like surface topography of chambers. Similar ontogenetic changes in aperture shape occur although the flap-like extension characteristic of apertures formed late in the ontogeny of G. dehiscens is absent in T. primitiva. Globoquadrina dehiscens might be interpreted as the more advanced shell because of somewhat greater compression of lateformed chambers (possibly signifying improved form resistance late in ontogeny), but to infer that it descended from T. primitiva is at variance with the stratigraphic data. These show that the taxa are in separate lineages that may have differentiated in the Paleocene. Although some systematists urge the use of comparative morphology in estimating ancestry, stratigraphic data are especially valuable when character suites are limited and homoplastic.