The Paleozoic assembly of the holocephalian body plan far preceded post-Cretaceous radiations into the ocean depths

Among cartilaginous fishes, Holocephali represents the species-depauperate, morphologically conservative sister to sharks, rays, and skates and the last survivor of a once far greater Paleozoic and Mesozoic diversity. Currently, holocephalian diversity is concentrated in deep-sea species, suggesting this lineage might contain relictual diversity that persisted in the ocean depths. Yet, the relationships of living holocephalians to their extinct relatives and the timescale of diversification of living species remains unclear. Here, we reconstruct the evolutionary history of holocephalians using comprehensive morphological and DNA sequence datasets. Our results suggest that living holocephalians entered and diversified in deep (>1000 m) ocean waters after the Cretaceous-Paleogene mass extinction, contrasting with the hypothesis that this ecosystem has acted as a refugium of ancient cartilaginous fishes. These invasions were decoupled from the evolution of key features of the holocephalian body plan, including crushing dentition, a single frontal clasper, and holostylic jaw suspension, in the Paleozoic Era, and considerably postdated the appearance of the living familes by 150 million years ago during a major period of biotic turnover in oceans termed the Mesozoic Marine Revolution. These results clarify the origins of living holocephalians as the recent diversification of a single surviving clade among numerous Paleozoic lineages.