The ecological upheavals produced by the Cretaceous-Paleogene mass extinction event (K-Pg, -66 Ma) have been mostly studied at large scale with emphasis on clades' diversity dynamics. How this event affected the structure of paleocommunities is comparatively less investigated, especially within large vertebrate clades like fish. Here, we quantified changes in the contribution of elasmobranchs (sharks, skates, rays) and actinopterygians (ray-finned fishes) to the fish community across the K-Pg extinction by analyzing ichthyolith (fossil teeth and denticles) abundance through time. Based on extensive sampling of 20 horizons from two outcrops spanning the K-Pg event in Austria (>
4 tons of rock, >
9,000 ichthyoliths), we show that the K-Pg event fostered elasmobranch abundance while reducing actinopterygian density in the Tethys Ocean. Elasmobranch ichthyolith dominance in postextinction communities is not driven by estimated local environmental change (paleobathymetry, bottom-water oxygenation) and may relate to the greater independence of this clade from lower trophic levels in their ecology and early life stages than actinopterygians. We further measured the size structure of ichthyolith assemblages and found that the K-Pg event initiated an increase in the range of ecological niche space occupied by elasmobranchs simultaneously to the demise of actinopterygians in postextinction communities. Finally, using the fine taxonomic resolution of the elasmobranch fossil record, we demonstrate that local environmental fluctuations controlled elasmobranch community structure and richness, which are decoupled from global-scale upheavals. Our results challenge previous hypotheses and provide insights into global and regional environmental forcing over the structure of fish communities across a mass extinction event.