The largest radiations of freshwater fishes initiated at the Cretaceous-Paleogene boundary
Freshwater biodiversity is disproportionally high relative to the limited availability of freshwater habitats. This pattern is exemplified by freshwater fishes. Over 50% of freshwater fish species are concentrated in a single clade, Ostariophysi, including the 5000 species of minnows, carps, and loaches, the 4500 species of catfishes, and the over 2000 species of tetras, pirahnas, and characins. However, the relationships and ages of ostariophysans remain uncertain. We show that the initial diversification of ostariophysans involved only two freshwater invasions and was driven by the strikingly rapid origination of major crown clades, including Neotropical electric fishes, lutefishes, and multiple major living clades of catfishes, carps and minnows, and tetras and characins, within five million years of the Cretaceous-Paleogene mass extinction. This result is congruent with the record of well-preserved body fossils of ostariophysans, but contrasts with the controversial assignment of isolated teeth and bones from the Cretaceous to nested lineages of this set of freshwater fish radiations. Although we confirm that Alepocephaliformes, an obscure marine lineage, is the living sister to Ostariophysi, our results demonstrate that the former clade only recently invaded the deep ocean, a transition that involved the loss of structures essential for enhanced auditory capabilities in ostariophysans and the related herrings and anchovies. These results establish a surprisingly young age for the major lineages of living freshwater fishes.