Spine-like structures in Paleogene muricate planktonic foraminifera

Abstract

Abstract. 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.

Revealing Their True Stripes: Mg/Ca Banding in the Paleogene Planktonic Foraminifera GenusMorozovellaand Implications for Paleothermometry

“…The exact Mg distribution we see in our maps depends on where the polished surface dissects the muricae. Our maps appear to mostly reveal off‐center dissections of the muricae where bands and laminae bend upward from secondary layers with newer layers onlapping against them (Figure S9 in Supporting Information ; Pearson et al., 2022). This is relevant for paleothermometry as it sheds light on how morozovellids might have harbored symbionts within an external spine array very much like modern spinose species, strengthening the comparison of Paleogene mixed layer species to modern spinose species occupying the same realm (Pearson et al., 2022).…”

“…aragonensis and M . crater support a modification of the classic growth model whereby multiple thinner “adult” layers, each layer consisting of a high/low Mg/Ca band pair, are deposited over the test after the organism reaches maturity (Pearson et al., 2022). This additional layering is also seen in N .…”

“…They suggested that these rods projected through the test surface in life, forming spines, which were analogous but not homologous with the so-called "true spines" of various modern genera such as Globigerina, Globigerinoides, Trilobatus, and Orbulina (see Figure S9 in Supporting Information S1 for schematic). We cannot expect the spine structures themselves to be continuously resolved as they are typically only 1-2 μm in diameter (Pearson et al, 2022), but linear high Mg/Ca anomalies orthogonal to the test surface in some of our images (e.g., Figures 3f and 4a) may occasionally trace their path. The exact Mg distribution we see in our maps depends on where the polished surface dissects the muricae.…”

“…Pearson et al. (2022) showed evidence that muricae typically contain a central rod made of a single calcite crystal that projects through the test layers, which onlap onto them. They suggested that these rods projected through the test surface in life, forming spines, which were analogous but not homologous with the so‐called “true spines” of various modern genera such as Globigerina , Globigerinoides , Trilobatus, and Orbulina (see Figure S9 in Supporting Information for schematic).…”

“…Multi-species stable isotope and trace element analysis show that morozovellids lived in the upper ocean (Anagnostou et al, 2016;John et al, 2013;Pearson et al, 2001Pearson et al, , 2007 and carbon isotope analysis shows they hosted algal photosymbionts like many modern species (D'Hondt et al, 1994;Luciani et al, 2017;Norris, 1996). However, morozovellids show visible differences to extant forms: for example, their "muricate" test morphology ("muricae" = distinct calcite protrusions containing spine-like structures; Figure 1; Figure S1 in Supporting Information S1) (Blow, 1979;Pearson et al, 2022) is not found today, thus challenging analogies with modern species upon which Mg/Ca-temperature calibrations are based. We focused on M. aragonensis and M. crater, two common morozovellids from the early-middle Eocene.…”