The final 10 Myr of the Paleozoic saw two of the biggest biologic crises in Earth history: the Middle Permian extinction (often termed the Guadalupian-Lopingian extinction, GLE) that was followed 7–8 Myr later by Earth’s most catastrophic loss of diversity, the Permian-Triassic mass extinction (PTME). These crises are not only manifest as sharp decreases in biodiversity and - particularly for the PTME - total ecosystem collapse, but they also drove major changes in biological morphological characteristics such as the Lilliput effect. The evolution of test size among different clades of foraminifera during these two extinction events has been less studied. We analyzed a global database of foraminiferal test size (volume) including 20226 specimens in 464 genera, 98 families, and 9 suborders from 632 publications. Our analyses reveal significant reductions in foraminiferal mean test size across the Guadalupian-Lopingian boundary (GLB) and the Permian-Triassic boundary (PTB), from 8.89 to 7.60 log10 μm3 (lg μm3), and from 7.25 to 5.82 lg μm3, respectively. The decline in test size across the GLB is a function of preferential extinction of genera exhibiting gigantism such as fusulinoidean fusulinids. Other clades show little change in size across the GLB. In contrast, all Lopingian suborders in our analysis (Fusulinina, Lagenina, Miliolina, and Textulariina) experienced a significant decrease in test size across the PTB mainly due to size-biased extinction and within-lineage change. The PTME was clearly a major catastrophe that affected many groups simultaneously, and the GLE was more selective, perhaps hinting at a subtler, less extreme driver than the later PTME.

We constructed a global database of foraminifer test sizes from the Early Permian to the Late Triassic illustrated in 632 published papers. Our database includes 20,226 individual specimens representing 464 genera, 98 families, and 9 suborders. We applied the timescale and ages provided in the IUGS Geological Time Scale (2018) (http://www.stratigraphy.org/ICSchart/ChronostratChart2018-08). The systematic classification of foraminifers has been controversial, and here we follow Tappan and Loeblich (1988), Mikhalevich (2000), Armstrong and Brasier (2004), and Groves et al. (2005). We have performed additional standardization for some controversial and uncertain species (including synonyms) (by Haijun Song).

Foraminiferal test morphologies are widely variable and include cones, columns, disks, spheres, hemispheres, and ellipsoidal shapes. We use test volume as a proxy for body size to mitigate against biases caused by shape changes over time. Test symmetries were used to calculate test volume from the two measurable axes in thin-sectioned specimens. The logarithmic scale with base 10 of test volume has been chosen because the rate of biological evolution is an inverse power function of time, it is often used for statistical comparison of right-skewed distributions (Gingerich 1983). In addition, the value is reduced on a linear scale after logarithmization, making it easier to compare test sizes across a broad range of sizes (Payne 2005).

We chose the t-test method to compare the differences in the mean test values of foraminifera between adjacent stages, and this analysis is mainly performed using the Statistical Package for the Social Sciences (SPSS).