Evolutionary dynamics of Cenozoic planktonic foraminifera: insights from biogeography, geochemistry, and morphology

The Earth is currently experiencing rates of environmental change unprecedented in the last 66 million years. As climate change accelerates, the need to quantify biotic responses associated with heightened extinction risk is becoming more urgent. The fossil record can provide a rich source of information about biotic responses to past environmental perturbations that can help ground truth predictions about future biodiversity responses. The marine microfossil record represents the most-complete biological archive available for this kind of study, with the macroperforate planktonic foraminifera having the most complete species-level fossil record of the last 66 million years. These organisms have a globally distributed fossil record and their readily fossilized calcium carbonate skeletons preserve a biogeochemical fingerprint of the environments in which they lived, as well as their ecological habits.
This thesis builds on this exceptional fossil record, first and foremost by assembling a new Cenozoic fossil occurrence database, Triton, the largest group specific fossil occurrence dataset ever created with 512,922 individual planktonic foraminiferal records.
Using Triton, the pre-extinction geographic range trajectories of Cenozoic planktonic foraminifera were largely demonstrated to show a reduction in geographic range prior to extinction. However, multiple taxa which speciate in the upper water column, and host photosynthetic algal symbionts exhibit pre-extinction range expansion, potentially indicating ecological resilience to selection pressures. Amongst significant climatic events through the Cenozoic, the Paleocene-Eocene Thermal Maximum (56 Ma) impacted pre-extinction geographic ranges most significantly, despite the muted effect of this event on planktonic foraminiferal species richness.
The investigation of the palaeolatitudinal dynamics of speciation and extinction shows that Cenozoic global temperatures are the primary control on the palaeolatitude of speciation, where a warmer world is typified by a greater proportion of speciation taking place at higher palaeolatitudes. Furthermore, speciation locations tend to be geographically isolated, despite
the interconnectivity of the pelagic ecosystem, and the majority of species exhibit an extinction palaeolatitude removed from their palaeolatitude of speciation.
Finally, high-resolution morphological, and geochemical examination of the planktonic foraminiferal record revealed a suite of pre-extinction responses during the extinction of two members of the genus Dentoglobigerina. Despite phylogenetic, morphological, and ecological affinity, these taxa exhibit species-specific phenotypic modifications which include permanent depth migration, “pre-extinction gigantism”, and photosymbiont bleaching.
The results generated through the construction of Triton, and high-resolution examination of the extinction of species of Dentoglobigerina, reveal a variety of spatiotemporal evolutionary dynamics with implications for improving our understanding of the nature of evolution within the largest ecosystem on Earth.