WHEN IN ONTOGENY DO SPECIES DIFFERENCES EMERGE? A CASE STUDY OF A FOSSIL PLANKTONIC FORAMINIFERA LINEAGE

Variation among individuals forms the raw material for evolution. During their lifetime, individuals can often adjust their state in response to environmental conditions. This plasticity in developmental history is hypothesized to stimulate the expression of novel trait combinations, increasing mean population fitness and accelerating local adaptation. However, very few empirical data exist to test this hypothesis on macroevolutionary time scales, as developmental history is often impossible to reconstruct from fossilized remains.

Here, we use the exceptionally rich fossil record of planktonic foraminifera to study ontogenetic shifts in deep time. Foraminifera retain their entire life history inside their calcium carbonate shells, allowing for detailed morphometric analyses at different ontogenetic stages. We analyse morphological changes before, during and after speciation in the Menardella limbata – Menardella exilis – Menardella pertenuis lineage to study changes to the timing of developmental processes. Using micro-CT scans, we plot individual chamber coordinates in xyz-space and reconstruct 3-dimensional growth trajectories with a new custom-made R package, Foram3D. The package functions calculate distances and angles between subsequent chambers, quantify trochospirality and reconstruct “Raupian” coiling parameters at every growth stage, and determine the number of chambers per whorl at the time each chamber was built.

The resulting developmental trajectories show that three-dimensional growth varies among species from the neanic and adult stage onwards. The angles between subsequent chambers increase after a species-specific size threshold is crossed, resulting in an increase in chambers in the final whorl and reduced chamber growth rates. Similarly, trochospirality (‘steepness’ of the shell spire) decreases at different chamber numbers among species, resulting in species-specific adult shell shapes. These results suggest that ontogenetic constraints can be overcome, resulting in novel morphologies and the emergence of new species.