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The origin of the theory that major geologic events may occur at regular intervals can be traced back not to a study of volcanism or plate tectonics but to an investigation of marine extinctions. In the early 1980s, scientists began to look closely at the question of how these extinctions occur. Two paleontologists, Raup and Sepkoski, compiled a master list of marine species that died out during the past 268 million years and noted that there were brief periods during which many species disappeared at once. These mass extinctions occurred at surprisingly regular intervals. Later studies revealed that extinctions of terrestrial reptiles and mammals also occurred periodically. These findings, combined with the research of Raup and Sepkoski, led scientists to hypothesize the existence of some kind of cyclically recurring force powerful enough to affect living things profoundly. Speculation that so powerful a force might affect geologic events as well led geologists to search for evidence of periodicity in episodes of volcanism, seafloor spreading, and plate movement. 17. According to the passage, Raup and Sepkoskis research was concerned with learning more about the habitats of marine species studying plate tectonics and the occurrence of volcanism over the past 268 million years examining extinctions of marine species over the past 268 million years | 
