Spine-like structures in Paleogene muricate planktonic foraminifera

Abstract. Muricate planktonic foraminifera comprise an extinct clade that was diverse and abundant in the Paleogene oceans are widely used palaeoclimate research as geochemical proxy carriers for upper oceans. Their characteristic wall texture has surface projections called “muricae” formed by upward deflection mounding of successive layers test wall. The group is generally considered to have lacked “true spines”: is, acicular calcite crystals embedded projecting from such occur many modern some groups. Here we present evidence polished sections, scanning electron microscope images dissections, showing radially orientated crystalline spine-like structures centre muricae various species Acarinina Morozovella projected life. morphology placement suggest they evolved independently true spines. Nevertheless, may served a similar range functions spines species, including aiding buoyancy predation especially harbouring algal photosymbionts, function which probably first evolved. Our observations strengthen analogy between mixed-layer-dwelling their spinose counterparts.