Spine-like structures in Paleogene muricate planktonic foraminifera

• Published in: Journal of Micropalaeontology
• Main Authors: Pearson, Paul N., John, Eleanor, Wade, Bridget S., D'HAENENS, Simon, Lear, Caroline H.
• Format: Article in Journal/Newspaper
• Language: English
• Published: COPERNICUS GESELLSCHAFT MBH 2022
• Subjects: Planktonic foraminifera
• Online Access: http://hdl.handle.net/1942/38039; https://doi.org/10.5194/jm-41-107-2022

Muricate planktonic foraminifera comprise an extinct clade that was diverse and abundant in the Paleogene oceans and are widely used in palaeoclimate research as geochemical proxy carriers for the upper oceans. Their characteristic wall texture has surface projections called "muricae" formed by upward deflection and mounding of successive layers of the test wall. The group is generally considered to have lacked "true spines": that is, acicular calcite crystals embedded in and projecting from the test surface such as occur in many modern and some Paleogene groups. Here we present evidence from polished sections, surface wall scanning electron microscope images and test dissections, showing that radially orientated crystalline spine-like structures occur in the centre of muricae in various species of Acarinina and Morozovella and projected from the test wall in life. Their morphology and placement in the wall suggest that they evolved independently of true spines. Nevertheless, they may have served a similar range of functions as spines in modern species, including aiding buoyancy and predation and especially harbouring algal photosymbionts, the function for which we suggest they probably first evolved. Our observations strengthen the analogy between Paleogene mixed-layer-dwelling planktonic foraminifera and their modern spinose counterparts. This research has been supported by the Natural Environment Research Council (grant nos. NE/P019102/1, GR3/12618, NE/X509345/1, and NE/B503225/1) Funding was provided by NERC grant NE/P019102/1 to Caroline H. Lear and Paul N. Pearson as part of the SWEET (Super-Warm Early Eocene Temperatures and climate) consortium (https://www.deepmip.org/sweet/, last access: 22 July 2022). We are grateful to Chris Poole for Fig. 3b and c. We thank Dan Lunt (University of Bristol) for coordinating that project and funding the artwork in Fig. 12. The Belgian American Educational Foundation and the Fulbright Commission of Belgium and Luxemburg provided support to Simon D’haenens. Tanzanian ...