Fossil occurrence data Fossil data used to calculate diversity variation were obtained from a previously published database of Permian‒Triassic marine fossils (Song et al., 2018; Song et al., 2020). The database contains 52,322 occurrences at the generic level from 1,768 published papers and the Paleobiology Database, spanning the Late Permian Changhsingian to the Late Triassic Rhaetian (Data 1). Our analysis is based on the occurrences of genera, as taphonomy prevents species-level identifications. Within the considered interval, a total of 1,097 genera belong to 13 major clades, including two clades of protozoa (foraminifera and radiolarians), nine clades of invertebrates (corals, sponges, brachiopods, bryozoans, ostracods, cephalopods, gastropods, bivalves, and echinoderms), and two clades of vertebrates (conodonts and fishes). For marine arthropods, we used only ostracod data because ostracods are abundant in the fossil record during the late Permian. Other marine arthropods are very rare in this time interval. For example, only two genera of trilobite, one genus of chelicera, and one genus of decapod are recorded in the Changhsingian bin compared to > 100 genera of ostracods in the Paleobiology Database. We did not consider background extinction in the Late Permian because many studies have shown that the background extinction rate of marine taxa in the Changhsingian is negligible compared to the mass extinction interval around the Permian‒Triassic boundary (Yin et al., 2007; Shen et al., 2011; Song et al., 2013; Fan et al., 2020). Therefore, the results using the Changhsingian and Induan fossil data reflect a selectivity pattern of the Permian‒Triassic mass extinction rather than background extinction. Body size data We used the comprehensive database of Schaal et al. (2016) to assign body size expressed as the maximum length for each species. Using the maximum size for each taxon is a common approach for body size studies, as the effects of juvenile specimens in the database can be avoided (Stanley, 1973; Jablonski, 1997; Lockwood, 2005; Heim et al., 2015; Payne et al., 2016; Schaal et al., 2016). We followed the same methods to compile additional data for taxa not included in this database. A number of recently published databases were used to compile the size data, including references (Romano et al., 2016; Shi et al., 2016; Foster et al., 2018; Chen et al., 2019; Feng et al., 2020; Foster et al., 2020). Other size data were mainly obtained from the published taxonomic literature (see Data 2). Only common taxa from both Changhsingian and Induan are included because these taxa have abundant fossil data to study their size change during the Permian-Triassic interval, i.e., foraminifera, brachiopods, ostracods, gastropods, cephalopods, bivalves, conodonts, and fishes. Other taxa including corals, sponges, radiolarians, bryozoans, and echinoderms are absent/very rare in the Induan bin (see Data 1), and accordingly are not included in this study. The Changhsingian and Induan body size dataset is composed of 1495 species in 635 genera belonging to eight common clades. Other data were obtained from the above fossil occurrence and body size datasets. References Chen, J., Song, H., He, W., Tong, J., Wang, F., Wu, S., 2019. Size variation of brachiopods from the Late Permian through the Middle Triassic in South China: Evidence for the Lilliput Effect following the Permian-Triassic extinction. Palaeogeography, Palaeoclimatology, Palaeoecology, 519: 248–257. Fan, J.-x., Shen, S.-z., Erwin, D.H., Sadler, P.M., MacLeod, N., Cheng, Q.-m., Hou, X.-d., Yang, J., Wang, X.-d., Wang, Y., 2020. A high-resolution summary of Cambrian to Early Triassic marine invertebrate biodiversity. Science, 367(6475): 272–277. Feng, Y., Song, H., Bond, D.P.G., 2020. 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