Sepkoski and David Raup produced a new understanding of extinction events, by developing a statistical approach to the study of taxonomic diversification.
But his most important contribution was the identification of the "Big 5" mass extinctions, events that have shaped the evolution of life on earth.
Under Stephen Jay Gould he earned a Ph.D. in geological sciences from Harvard University in 1977.
Sepkoski was also a research associate at the Field Museum of Natural History in Chicago.
Sepkoski is perhaps best known for his global compendia of marine animal families and genera, data sets that continue to motivate a tremendous amount of paleobiological research.