Abstract
Understanding heterogeneity in species richness between closelyrelated clades is a key research question in ecology and evolutionarybiology. Multiple hypotheses have been proposed tointerpret such diversity contrasts across the tree of life, with moststudies focusing on speciation rates to explain clades’ evolutionaryradiations, while often neglecting extinction rates. Here we studya notorious biological model as exemplified by the sister relationshipsbetween mackerel sharks (Lamniformes, 15 extant species)and ground sharks (Carcharhiniformes, ∼290 extant species). Usinga comprehensive fossil dataset, we found that the diversity dynamicsof lamniforms waxed and waned following repeated cycles ofradiation phases and declining phases. Radiation phases peaked upto 3 times the current diversity in the early Late Cretaceous. In thelast 20 million years, the group declined to its present-day diversity.Along with a higher extinction risk for young species, we furthershow that this declining pattern is likely attributed to a combinationof abiotic and biotic factors, with a cooling-driven extinction (negativecorrelation between temperature and extinction) and cladecompetition with some ground sharks. Competition from multipleclades successively drove the demise and replacement of mackerelsharks due to a failure to originate facing the rise of ground sharks,particularly since the Eocene. These effects came from ecologicallysimilar carcharhiniform species inhibiting diversification of mediumandlarge-sized lamniforms. These results imply that the interplaybetween abiotic and biotic drivers had a substantial role in extinctionand speciation, respectively, which determines the sequentialrise and decline of marine apex predators.