Joseph John Sepkoski Jr. (July 26, 1948 – May 1, 1999) was a University of Chicago paleontologist. Sepkoski studied the fossil record and the diversity of life on Earth. Sepkoski and David Raup produced a new understanding of extinction events, by developing a statistical approach to the study of taxonomic diversification. He suggested that the extinction of dinosaurs 66 mya was part of a cycle of mass extinctions that may have occurred every 26 million years. But his most important contribution was the identification of the "Big 5" mass extinctions, events that have shaped the evolution of life on earth.

Sepkoski was born in Presque Isle, Maine. In 1970, Sepkoski received a B.S. degree, magna cum laude, from the University of Notre Dame. Under Stephen Jay Gould he earned a Ph.D. in geological sciences from Harvard University in 1977. His Ph.D. was on the field geology and paleontology of the Black Hills of South Dakota. From 1974 to 1978, Sepkoski taught at the University of Rochester. In 1978, he joined the University of Chicago and became a professor in 1986. Sepkoski was also a research associate at the Field Museum of Natural History in Chicago. He died of heart failure related to high blood pressure at the age of 50.

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. Sepkoski himself explored his compendium very thoroughly. In 1981, he identified three great Evolutionary Faunas in the marine animal fossil record. Each of his Evolutionary Faunas, the Cambrian, Paleozoic, and Modern Faunas, is composed of Linnean classes of animals that have covarying diversity patterns, characteristic rates of turnover, and broadly similar ecologies. Most importantly, they sequentially replaced one another as dominant groups during the Phanerozoic. Sepkoski modeled the Evolutionary Faunas using three coupled logistic functions, but the underlying drivers of the prominent shift in taxonomic composition represented by the three faunas remains unknown.

Sepkoski was married to paleontologist Christine Janis, a specialist in fossil mammals. His son (from a previous marriage) is the historian of science David Sepkoski.

• (1983). Charles Schuchert Award, Paleontological Society
• Elected foreign member of the Polish Academy of Sciences

• Sepkoski, J. John Jr. (1978). "A kinetic model of Phanerozoic taxonomic diversity. I. Analysis of marine orders". Paleobiology. 4 (3): 223–251. doi: 10.1017/s0094837300005972. JSTOR  2400203. S2CID  87102033.
• Sepkoski, J. John Jr. (1979). "A kinetic model of Phanerozoic taxonomic diversity. II. Early Phanerozoic families and multiple equilibria". Paleobiology. 5 (3): 222–251. doi: 10.1017/s0094837300006539. JSTOR  2400257. S2CID  89475468.
• Sepkoski, J. John Jr. (1981). "A factor analytic description of the Phanerozoic marine fossil record". Paleobiology. 7 (1): 36–53. doi: 10.1017/s0094837300003778. JSTOR  2400639. S2CID  133114885.
• Raup, David M.; Sepkoski, J. John Jr. (19 March 1982). "Mass extinctions in the marine fossil record". Science. 215 (4539): 1501–3. Bibcode: 1982Sci...215.1501R. doi: 10.1126/science.215.4539.1501. PMID  17788674. S2CID  43002817.
• Sepkoski, J. John Jr. (1984). "A kinetic model of Phanerozoic taxonomic diversity. III Post-Paleozoic families and mass extinctions". Paleobiology. 10 (2): 246–267. doi: 10.1017/s0094837300008186. JSTOR  2400399. S2CID  85595559.
• Raup, David M.; Sepkoski, J. John Jr. (Feb 1984). "Periodicity of extinctions in the geologic past". Proceedings of the National Academy of Sciences of the United States of America. 81 (3): 801–805. Bibcode: 1984PNAS...81..801R. doi: 10.1073/pnas.81.3.801. PMC  344925. PMID  6583680.
• Sepkoski, J. John Jr. (1988). "Alpha, beta, or gamma: where does all the diversity go?". Paleobiology. 14 (3): 221–234. doi: 10.1017/S0094837300011969. JSTOR  2400884. PMID  11542147. S2CID  2826581.
• Sepkoski, J. John Jr. (1996). "Patterns of Phanerozoic extinction: a perspective from global data bases". In Otto H. Walliser (ed.). Global Events and Event Stratigraphy. Berlin: Springer. pp. 35–51. doi: 10.1007/978-3-642-79634-0_4. ISBN  978-3-642-79636-4.
• Sepkoski, J. John Jr. (2002). "A compendium of fossil marine animal genera". Bulletins of American Paleontology. 363. Ithaca, NY: Paleontological Research Institution: 1–560. ISBN  978-0-87710-450-6.

• "Sepkoski, J. John, Jr.". American National Biography. Oxford University Press. Subscription needed.

• Miller, Arnold. (Sep. 1999). Memorial: J. John Sepkoski, Jr.: A personal reflection Archived 2007-02-07 at the Wayback Machine. Journal of Paleontology.
• NASA, Near Earth Object Program
• Sepkoski's Online Genus Database