A statement in the presentation said there was some indication the Ordovician-Silurian extintion was possibily caused by a Gamma-Ray Burst. What are those unique clues that Paleontologists have developed from that period that astronomers suggest best fits a GRB event? It is well-established that the extinction at the end of the Cretaceous was due in large part to a comet or asteroid. A similar event may be a factor in the Permian extinction. Too many SUVs and people are being held accountable for today's rapid die-off. What factors in each major extinction has Paleontology uncovered that Astronomy can provide a possible cause?