Introduction

This project is part of a larger endeavor aimed at restructuring the classification of Cretaceous age planktonic foraminifera. The restructuring is based on observed morphological changes through time.

In this investigation, two species of Late Cretaceous, planktonic foraminifera, Heterohelix rajagopalani and Gublerina cuvillieri, were studied from Ocean Drilling Program Hole 761B on Exmouth Plateau in order to determine if Heterohelix rajagopalani is the ancestor of Gublerina cuvillieri.

Gublerina cuvillieri was first described in 1948 by Kikoine. Subsequently, Govindan described G. rajagopalani in 1972. Nederbragt (1991) re-assigned the latter species to the genus Heterohelix. According to Nederbragt, the two species share no phylogenetic relationship.

Methods

• H. rajagopalani and G. cuvillieri were picked from Hole 761B (Exmouth Plateau) and their stratigraphic distribution was recorded

• Specimens were x-rayed using a Faxitron X-ray machine

• Shell growth measurements of the two species were obtained using ImagePro Plus biometric software

• Measurements obtained include the following (see image below):

• Scanning Electron Microscope images were taken to observe differences in test ornamentation and external morphology

• Samples were picked for isotope analysis from Hole 761B and sent to Dr. Ken MacLeod at the University of Missouri for analysis

• The external morphology was observed from the SEM images. The observations are presented below.

Heterohelix rajagopalani (Govindan 1972)

• test periphery is strongly costate
• test walls are microperforate
• test ornamentation is usually concentrated on top of the earlier formed chambers, resulting in a raised, fused mass.
• biserial test
• chambers reniform in shape
  
• test angle from proloculus to widest margins average 54°
• zig-zag suture pattern is distinctly depressed through ontogeny
• periphery is truncate
• aperture is interiomarginal and is situated at the base of the last formed chamber
• average number of chambers is 16
  
  

Gublerina cuvillieri (Kikoine 1948)

• chambers finely costate
• ornamentation elements are concentrated on chamber surface and wrap around to the periphery of the test
• inner flaring area of test is free of ornamentation
• test walls are microperforate
  
• biserial test with a mutiserial stage later in ontogeny
• chambers flare and diverge rapidly in later stages of ontogeny
• chambers are sub-rectangular and wider than they are tall, they have more arch than those of H. rajagopalani
• aperture is interiomarginal and is situated at the base of the last formed chamber
• average number of chambers is 19
• test angle from proloculus to widest margins average 55°

Results

Quantitative data from x-ray image measurements provide evidence that H. rajagopalani is the ancestor of G. cuvillieri.

The measurement data for the two species are virtually identical for the biserial portion of the tests, indicating a close phylogenetic relationship.

Transitional specimens with characteristics of both species were also found, giving further evidence for a common ancestral relationship.

Heterohelix rajagopalani and G. cuvillieri diverge later in their ontogenetic history as expressed in G. cuvillieri by 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 will reveal the two species’ depth ecologies and may provide further insight into the evolution of H. rajagopalani and G. cuvillieri.

Acknowledgments

I would like to thank Dr. Kenneth MacLeod for his help with isotopic analysis and Dr. Dan Georgescu for all the assistance he provided during the research process.

I would also like to thank Mr. Max Berry for his generous donation that made this research possible. Special thanks to the NMNH RTP program.

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