Three dimensional analysis of ontogenetic variation in fossil globorotaliiform planktic foraminiferal tests and its implications for ecology, life processes and functional morphology

Abstract

Ontogenic trajectories recorded within the fossilized calcite tests of planktic foraminifera are significant for understanding their paleoecology and evolution. High-resolution 3-D imaging techniques enable quantitative measurement of test calcite and enclosed cavity volumes allowing a detailed characterization of the ontogenetic variations in the biomass to calcite ratio. Here we used micro-CT scanning to inspect ontogenetic variations among four globorotaliform planktic foraminiferal morphospecies from the late Miocene of International Ocean Discovery Program Site U1490 in the tropical Western Pacific. A conservative log-linear growth pattern is observed for all specimens. The proloculus size has an important control on the growth trajectory of the inspected specimens, and specimens with smaller proloculus volume generally have more chambers at the end of their life. The overall pattern of deviations from the log-linear growth trajectory among inspected specimens is well constrained, which varies between different groups in magnitude. The ontogenetic mean density, which is proposed here as an indicator of the degree of difficulty in producing positive buoyancy, shows a generally decreasing trend before the terminal stage in all specimens. We propose that the decreasing mean density (MD) as the test enlarges is an important adaption for planktic foraminifera to maintain an optimal habitat for nutrition and reproduction. The value of the mean density at the two-chambered stage exhibits a linear relationship with chamber volumes among inspected specimens with different habitats and different coiling forms, for which the ecological implications are explored.