No consistent size responses in radiolarians to the climatic changes and mass extinctions during the Paleozoic-Mesozoic transition

Plankton are important indicators of ocean health and are crucial for the support of ecosystems. Few analyses have been carried out on the morphological responses of planktonic organisms to the greatest mass extinction in the Permian-Triassic transition. Here, we explore a large database on the middle Permian to Middle Triassic Radiolaria, which is one of the most important planktonic bio-indicators of primary productivity from those times. Our study encompasses the Guadalupian-Lopingian extinction (GLE, 260 Ma) and the Permian-Triassic mass extinction (PTME, 252 Ma), as well as the substantial ocean warming after the late Permian and perturbed ocean conditions from the Early to early Middle Triassic. Through these mass extinctions, we would predict extensive species loss for radiolarians coupled with substantial reductions in the size of survivors. However, we find significant size reductions of all radiolarians only through the GLE, whereas the four radiolarian orders show different trends through the PTME: the deep-water Albaillellaria and Latentifistularia survivors show size reduction, but the shallow-water Entactinaria and Spumellaria survivors increased in size. In addition, radiolarian size changes varied geographically: low-latitude forms decreased in size, whereas mid- to high-latitude forms increased in size in both the GLE and the PTME, reflecting differences in primary productivity. Size change through time shows a significant correlation with Sr isotopes, which is an indicator of continental weathering, and a correlation with seawater temperature. These findings provide novel evidence for predicting ecological responses of modern plankton to future climatic and environmental changes.