Logarithmic expression of Globigerina bulloides shell evolution through the biometric analysis: Paleoceanographic implications for the late Quaternary

Fossil foraminifera are a treasure trove of information in applications ranging from microevolution to paleoclimatology. The architecture of their tests is of the key importance in systematic and phylogenetic studies and can reveal micro-evolutionary traits through the biometric analyses. In the present paper, we analyze the laws of growth that control planktonic foraminifera shell morphology. We report the results of a biometric study of the temporal variation in the shell shape and chamber size of the cosmopolitan, subpolar to temperate species Globigerina bulloides d'Orbigny from core top sediments in the eastern tropical Atlantic Ocean. Morphological variation in terms of test shape and adult chamber size in G. bulloides, has been measured in 116 down core sediment samples from the tropical waters (19°N) of the northern Antarctic Ocean and has resulted in a model that simulates the basic morphology (chamber size and spatial arrangement) of planktonic foraminiferal shells of that species. The investigated samples comprise a continuous record that spans the last 200 kyrs. The specimens for this morphometric study were picked from a restricted sieve fraction and were mounted for Scanning Electron Microscopy (SEM) analysis. The restricted size of the specimens constrained the analysis to adult specimens and minimized ontogenic effects while allowing the documentation of very small overall changes in the parameters under investigation in time. The dimensions that were measured for each test were its height, width and the diameters of the last seven chambers. This allowed the determination of chamber centers and their analogies resulting in a mathematical model based on a logarithmic spiral equation that describes the evolution of the test with the growth during their adult phases. The model presented herein belongs to a family of so-called "fixed-axis" coiling models.