EVOLUTIONARY DYNAMICS OF FISHES THROUGH THE LATE PERMIAN MASS EXTINCTION

Mass extinction events near the Palaeozoic/Mesozoic boundary are evidence of a major upheaval in the biosphere. While patterns and processes of extinction and recovery of Permian–Triassic marine invertebrates and terrestrial vertebrates are relatively well studied, little is known about fishes during this critical interval. Here we present an updated analysis of the diversity dynamics and size changes of Permian–Triassic cartilaginous fishes (Chondrichthyes) and bony fishes (Osteichthyes). We show that chondrichthyan genus diversity declined during the Middle–Late Permian, supporting a combined overall extinction as a result of the end-Guadalupian and Late Permian events. Many Palaeozoic groups (e.g., Cladodontomorphi, Petalodontiformes) were largely replaced by hybodonts and modern sharks (Neoselachii). Holocephalans and xenacanths survived into the Triassic but at low diversity levels. Eugenodontiformes went extinct in the Early Triassic. A significant overall decrease in tooth size and body length observed across the Permian/Triassic boundary suggests a selective loss of large-sized chondrichthyans. The largest extinction occurred amongst marine groups, with benthic and pelagic groups suffering most, but selectivity for these palaeoecological traits is not evident. In contrast to Chondrichthyes, Osteichthyes experienced diversification events during the Early Triassic and Middle Triassic, resulting in a general pattern of low diversity during the Permian to higher levels in the Triassic. This Triassic radiation of bony fishes led to increased diversity among the modern ray-finned fishes (Neopterygii), which comprise the bulk of today’s fish faunas. Furthermore, Palaeopterygii experienced a significant increase in body size across the Middle/Late Permian boundary and these fishes remain among the larger fishes during the Triassic. Neopterygians mostly remained smaller and, thus, mostly occupied lower positions within aquatic food webs. Our data supports previous findings of an important evolutionary turnover among fishes, changing from chondrichthyan-dominated communities of the Carboniferous–Permian to osteichthyan(actinopterygian)-dominated associations of the Meso- and Cenozoic.