Joseph John Sepkoski Jr. (July 26, 1948 – May 1, 1999) was a scientist who studied fossils at the University of Chicago. He researched how life on Earth has changed over time. Sepkoski worked with David Raup to create a new way of studying how species die out in large numbers, using numbers and patterns to understand how different types of life develop and change. They proposed that the extinction of dinosaurs 66 million years ago might have been part of a cycle of major extinctions that could have happened every 26 million years. Their most important work was identifying the "Big 5" mass extinctions, which are major events that have influenced the history of life on Earth.

Life and work

Sepkoski was born in Presque Isle, Maine. In 1970, he earned a B.S. degree with high honors from the University of Notre Dame. He studied under Stephen Jay Gould and received a Ph.D. in earth sciences from Harvard University in 1977. His doctoral research focused on the geology and fossils found in the Black Hills of South Dakota. From 1974 to 1978, he taught at the University of Rochester. In 1978, he began working at the University of Chicago and became a professor in 1986. He also worked as a researcher at the Field Museum of Natural History in Chicago. He died at the age of 50 due to heart failure caused by high blood pressure.

Sepkoski is most famous for creating detailed lists of marine animal families and genera. These lists are still used today in paleobiological research. He studied his own lists carefully and, in 1981, identified three major groups of marine animals in the fossil record. These groups, called the Cambrian, Paleozoic, and Modern Faunas, each include types of animals that share similar patterns of diversity, rates of change, and ecological roles. These groups replaced each other over time during the Phanerozoic era. Sepkoski used three mathematical models to describe these groups, but the reasons behind the major changes in species types remain unknown.

Sepkoski was married to Christine Janis, a paleontologist who studied fossil mammals. He had a son, David Sepkoski, from a previous marriage. David is a historian who studies the history of science.