| Summary: | Restructuring the classification of Cretaceous age planktonic foraminifer requires studying morphological changes through time and phylogenetic relationships between species. In the following study, Gublerina cuvillieri and its proposed ancestorHeterohelix rajagopalaniwere picked from Ocean Drilling Program Hole 761B (Exmouth Plateau), and their relationship was evaluated using biometric analyses and morphological observations from SEM images. The relationship and taxonomy betweenG. cuvillieri and H. rajagopalani has been the subject of some debate. Gublerina cuvillieriwas first described by Kikoine (1948) from southern France, and Govindan (1972) described Gublerina rajagopalanifrom southern India. Nederbragt (1991) has since suggested that G. rajagopalani actually belongs in another genus, Heterohelix. In this study, quantitative data from x-ray image measurements suggests H. rajagopalani resembles G. cuvillieri. The measurement data for the two species are virtually identical for the biserial portion of the tests, indicating a close phylogenetic relationship. Heterohelix rajagopalani and G. cuvillieridiverge later in their ontogenetic history as expressed in G. cuvillieriby separation of the serial chambers, development of a multiserial stage, greater chamber number, and concentration of ornamentation elements in bands extending around the chamber edge. Isotope data indicates the two species lived at relatively the same water depth.
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