comets and extinctions

On a similar theme, at http://phys.org/print364584736.html … according to a study in the Monthly Notices of the Royal Astronomical Society, Michael Rampino (a geologist) and Ken Caldeira (ecologist) claim mass extinctions can be linked to known craters over the last 260 million years (including the Chicxulub crater in Yucatan). Specifically, they set out to show a cyclical pattern over the studied period with both impact craters and extinction events juxtaposed close to each other every 26 million years of time. This cycle is perhaps related to our solar system passing through dense mid-plane regions of our galaxy. The pertubations in the Oort Cloud led to periodic comet showers, some of which threatened the Earth.