Claimed Periodicity of Mass Extinctions
In 1984, paleontologists David Raup and Jack Sepkoski published a paper claiming that they had identified a statistical periodicity in extinction rates over the last 250 million years using various forms of time series analysis. They focused on the extinction intensity of fossil families of marine vertebrates, invertebrates, and protozoans, identifying 12 extinction events over the time period in question. The average time interval between extinction events was determined as 26 million years. At the time, two of the identified extinction events (Cretaceous-Paleogene and Late Eocene) could be shown to coincide with large impact events. Although Raup and Sepkoski could not identify the cause of their supposed periodicity, they suggested a possible non-terrestrial connection. The challenge to propose a mechanism was quickly addressed by several teams of astronomers.
In 2010, Melott & Bambach re-examined the fossil data, including the now-improved dating, and using a second independent database in addition to that Raup & Sepkoski had used. They found evidence for a signal showing an excess extinction rate with a 27-million-year periodicity, now going back 500 million years, and at a much higher statistical significance than in the older work. The change from 26 to 27 million years is expected based on a 3% "stretch" in the geological timescale since the 1980s.
Read more about this topic: Nemesis (hypothetical Star)
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