The morphological evolution was investigated in the tropical Neogene planktonic foraminiferal lineage Globorotalia menardii, G. limbata and G. multicamerata during the past 8 million years at ODP Hole 806C (Ontong-Java Plateau). This research is an extension of previous studies from the Caribbean Sea, the tropical Atlantic and the Eastern Equatorial Pacific.
The peripheral influence of Agulhas Current faunal leakage of Indian Ocean or even Pacific menardiforms into the South Atlantic is suspected to be responsible for a transgressive, transatlantic expansion of large menardiforms from 2.3-2.06 Ma to 2.58-1.7 Ma, which installed after a Northern Hemisphere Glaciation (NHG) size incursion of menardiforms around 2.6 Ma (Knappertsbusch, 2007 and 2016; Knappertsbusch & Friesenhagen 2018). The investigation from Western Pacific Warm Pool (WPWP) ODP Hole 806C, i.e. from an area outside reach of Agulhas Current, serves as a blind test for this szenario. Here, stable warm environments prevailed back to Pliocene times, and influences of NHG are expected to bear less severely on shell size evolution than in the Atlantic Ocean.
For this study >5250 specimens comprising G. menardii, G. limbata and G. multicamerata from 33 stratigraphic levels were morphometrically investigated using imaging- and microfossil orientation robot AMOR. Attention was given to trends of spiral height (δX) versus axial length (δY) in keel view, for which bivariate contour- and volume density diagrams were constructed for visualization of evolutionary patterns.
In WPWP Hole 806C G. menardii evolved in a more gradual manner than in the Atlantic. Plots of δX versus δY reveal bimodality between 3.18 Ma – 2.55 Ma with a dominant mode of smaller G. menardii (δX<~300 μm) persisting until the Late Quaternary, and a weak mode of larger G. menardii (δX>~300 μm) until 2.63 Ma. Up-section, bimodality vanished but G. menardii populations shifted towards extra large shells between 2.19-1.95 Ma supporting the possibility of long-distance diversal in this group. Morphological evolution of G. limbata and its evolutionary successor G. multicamerata in the WPWP are also different from those in the tropical Atlantic, but analyses need still further investigation.
In summary, Pacific menardiform globorotalid patterns contrast those in the Atlantic realm. There is inter-oceanic morphological asymmetry with considerable regional environmental control over shell evolution and indication of long-distance dispersal of G. menardii, both with implications for biostratigraphic applications.
Knappertsbusch, M. and Friesenhagen, T. (2018). Prospecting patterns of morphological evolution in menardiform globorotalids along Agulhas‘ trackway: Review and research in progress. Abstract. FORAMS 2018 Symposium, 17-22 June 2018, Edinburgh, UK., Session IX, temporary abstracts, 331.
Knappertsbusch, M. (2016). Evolutionary prospection in the Neogene planktic foraminifer Globorotalia menardii and related forms from ODP Hole 925B (Ceara Rise, western tropical Atlantic): evidence for gradual evolution superimposed by long distance dispersal ? Swiss Journal of Palaeontology, 135, 205-248.
Knappertsbusch, M. (2007). Morphological variability of Globorotalia menardii (planktonic foraminifera) in two DSDP cores from the Caribbean Sea and the Eastern Equatorial Pacific. Carnets de Géologie / Notebooks on Geology, Brest, Article 2007/04. http://paleopolis.rediris.es/cg/CG2007_A04/index.html.
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