Provincialism of bivalves across the K/Pg mass extinction boundary

Mass extinctions have played a crucial role in shaping the biogeographic structure of the marine biota throughout the Phanerozoic. Multiple clades exhibited pronounced cosmopolitanism following the Permian-Triassic (P/T) and Triassic-Jurassic (T/J) mass extinctions, characterized by sharp increases in biogeographic connectedness (BC). Here, we analyze global occurrence data of bivalves to examine their biogeographic dynamics across the Cretaceous-Paleogene (K/Pg) mass extinction boundary. In striking contrast to the patterns observed during the P/T and the T/J events, BC declines significantly from the Maastrichtian (0.049 ± 0.002) to the Danian (0.024 ± 0.002), signaling a shift toward heightened provincialism. Although geographically widespread genera exhibited selective survival through the K/Pg crisis, these survivors underwent substantial range contraction across the boundary, resulting in a fragmented post-extinction bivalve biogeography dominated by geographically narrow-ranging taxa. Quantitative comparisons across the P/T, T/J, and the K/Pg mass extinctions indicate that Maastrichtian genera possessed markedly lower within-genus species richness across the localities, compared to pre-P/T (Changhsingian) or pre-T/J (Rhaetian) levels, which might had limited species-level buffering of geographic ranges of genera, preventing the cosmopolitanism events seen after earlier events. Our findings highlight how taxonomic structure within clades influences biogeographic resilience, with implications for understanding macroevolutionary consequences of mass extinctions.