The Cretaceous-Palaeogene (K-Pg) mass extinction profoundly altered vertebrate ecosystems and prompted the radiation of many extant clades [1, 2]. Sharks (Selachimorpha) were one of the few larger-bodied marine predators that survived the K-Pg event and are represented by an almost-continuous dental fossil record. However, the precise dynamics of their transition through this interval remain uncertain [3]. Here, we apply 2D geometric morphometrics to reconstruct global and regional dental morphospace variation among Lamniformes (Mackerel sharks) and Carcharhiniformes (Ground sharks). These clades are prevalent predators in today's oceans, and were geographically widespread during the late Cretaceous-early Palaeogene. Our results reveal a decoupling of morphological disparity and taxonomic richness. Indeed, shark disparity was nearly static across the K-Pg extinction, in contrast to abrupt declines among other higher-trophic-level marine predators [4, 5]. Nevertheless, specific patterns indicate that an asymmetric extinction occurred among lamniforms possessing low-crowned/triangular teeth and that a subsequent proliferation of carcharhiniforms with similar tooth morphologies took place during the early Paleocene. This compositional shift in post-Mesozoic shark lineages hints at a profound and persistent K-Pg signature evident in the heterogeneity of modern shark communities. Moreover, such wholesale lineage turnover coincided with the loss of many cephalopod [6] and pelagic amniote [5] groups, as well as the explosive radiation of middle trophic-level teleost fishes [1]. We hypothesize that a combination of prey availability and post-extinction trophic cascades favored extant shark antecedents and laid the foundation for their extensive diversification later in the Cenozoic [7-10].

No abstract

Highlights d Shark tooth morphologies track changing habitats and resource availability d Tooth shape correlates with diet in extant shark species d Declines in lamniform disparity can be linked with dietary ''specialization'' d Modern lamniforms are more disparate than coeval carcharhiniforms

Although a diverse range of aquatic vertebrates are documented from the Upper Cretaceous (mid-Campanian) marine strata of the Kristianstad Basin in southern Sweden, only chondrichthyans and marine amniotes have been described in detail to date. In contrast, coeval actinopterygians are virtually unreported, yet their remains are extremely abundant at most sampled localities. A comprehensive assessment of these fossils has identified the first Late Cretaceous actinopterygian fauna from the Fennoscandian Shield, incorporating indeterminate lepisosteids, the durophagous pycnodontid Anomoeodus subclavatus, the predatory pachycormid Protosphyraena sp., a large ichthyodectid, pachyrhizodontids resembling Pachyrhizodus, the enchodontid Enchodus cf. gladiolus and indeterminate small teleosts. These taxa are diagnosed mainly from isolated teeth and scales, implying substantial taphonomic loss prior to burial. Moreover, the prolific recovery of actinopterygian skeletal remnants in recent excavations suggests that historical collecting biases, rather than ecological paucity, have contributed to their under-representation in the Swedish Cretaceous record. Palaeobiogeographically, the Kristianstad Basin actinopterygians show compositional resemblance to assemblages from the Northern European Platform and the Western Interior Seaway of North America, advocating distributional communication across the Boreal proto-Atlantic Ocean.