Test flattening in the larger foraminifer Heterostegina depressa: predicting bathymetry from axial sections

Previous attempts to quantify the test flattening trend in Heterostegina depressa with water depth have been rather unsuccessful. Due to its broad depth distribution, H. depressa is a perfect model species to calibrate test flattening as a bathymetric signal for fossil assemblages. This might enable us to better reconstruct palaeoenvironments of fossil larger foraminiferal communities or even provide clues on the degree of transport in allochthonous deposits. In this study we used growth-independent functions to describe the change of test thickness throughout ontogeny. Four growth-invariant characters, deriving from these functions, clearly quantify a transition of individuals with thicker to thinner central parts along the water depth gradient. This is probably controlled by light intensity because the photosymbionts of H. depressa (diatoms) are most effective at low irradiation levels. Thus, shallower specimens grow thicker to reduce light penetration, whereas specimens living deepe...