Plasticity in developmental trajectories has been proposed to contribute to species divergence but finding evidence for plasticity-led evolution in the fossil record remains challenging. Here we use high-resolution imaging techniques to map developmental change in Globorotalia plesiotumida–tumida lineage of planktic foraminifera from late Miocene until Recent. The unique mode of foraminiferal growth by the addition of chambers onto a calcite shell means that adult fossils retain information about their developmental history. All study specimens were obtained from the ODP in Western Caribbean, Leg 165, Sites 1000 and 999. We first assessed change in cumulative chamber volume and surface area at each chamber addition during the transition between the ancestral G. plesiotumida and its descendant G. tumida from 6.3 Ma to 5.3 Ma using five specimens reconstructed from Synchrotron X-Ray microtomography scans. This was complemented with measurements from 63 specimens reconstructed using X-Ray microtomography from five populations at 7.3 Ma, 6.3 Ma, 5.6 Ma, 5.3 Ma and 0.25 Ma. In addition, we characterised the external morphology of all study specimens by measuring their total length and coiling direction (n = 78). Our dataset shows that the transition interval in this lineage is characterised by an increase in variability in cumulative chamber volume compared to samples outside of this range. We also find that the transition is marked by a distinct shift in developmental trajectory and coiling direction in support of a rapid lineage division rather than gradual change. The large variation in developmental trajectories that we uncover emphasises the need for high-throughput approaches in studies of developmental change in the fossil record.
