J. John Sepkoski Jr.; Devised New Theory on Evolution

J. John Sepkoski Jr., University of Chicago geophysicist and paleobiologist whose research on marine fossils demonstrated catastrophic and cyclical mass extinctions of life on Earth, including the dinosaurs, has died at 50.

Sepkoski died Saturday in Chicago of heart failure.

Along with colleague David M. Raup, Sepkoski provided evidence supporting the theory of catastrophism over the long-held concept of gradualism in evolution. Their work indicated that periodic catastrophes made evolution more episodic and subject to chance than the earlier theory of gradual change.

“There wasn’t an evolutionary march toward greater and greater complexity, toward modernity,” Sepkoski told The Times in 1989, “but a record of jumps and starts.”

Their conclusions, first announced in 1983, were based on Sepkoski’s six-year analysis of global extinctions of marine organisms for the last 250 million years. Life forms vanished in staggering numbers, they found, about every 26 million years.

Best known is the still-mysterious destruction of the dinosaurs during the Cretaceous geologic period 65 million years ago. Other major extinctions included vast numbers of plankton and shellfish 240 million to 250 million years ago, and half of all animal families in the Triassic period 220 million to 225 million years ago.

“With that long a cycle,” Sepkoski said in 1983, “we suspect that the forcing agent will not be terrestrial but rather solar or galactic.” His conclusion fueled scientific views that dinosaurs became extinct because a huge asteroid struck Earth, raising clouds of dust that blocked out the sun and destroyed their food sources.

Sepkoski, who worked with data reaching back 600 million years, in 1993 completed a long study showing that insect species are the greatest survivors on Earth.

Born in Presque Isle, Maine, Sepkoski earned his degrees at the University of Notre Dame and Harvard University. He taught four years at the University of Rochester but spent most of his career at the University of Chicago.

He was a research associate in Chicago’s prestigious Field Museum of Natural History and had served as president of the Paleontology Society in 1995-96. That organization gave him its Charles Schuchert Award in 1983 for his catastrophic evolution theory.

Survivors include his wife, Christine, and son, David.