99% of all marine fossil generating species on Earth that ever existed have gone extinct. There are periods in the fossil record that show massive extinction rates. For example, the K-T (now the K-Pg) boundary marks the extinction of dinosaurs in the fossil record along with marine animals, at the Cretaceous- Paleogene boundary. The cause of the major mass extinctions varies. And there has always been debate because geologic evidence can be subducted by plate tectonics, for example. Things change over 250 million years. So naming a culprit is difficult even when there is geologic evidence. In the case of K-Pg, current thinking has the smoking gun as a large meteor impact, preceeded by a long period of a major volcanic outflow at the Deccan Traps. A double whammy. My question is about the Ordovician-Silurian extinction event ~425mya, two major events very close together, so it's another possible double whammy: http://en.wikipedia.org/wiki/Ordovician–Silurian_extinction_event (Note the graphic at the top of the article. Off topic: hidden in there is a ~62my cycle of recurring ME's.) The primary cause was an ice age, but there is a proposed extra-terrestrial culprit: a gamma ray burst. http://en.wikipedia.org/wiki/Gamma-ray_burst http://arxiv.org/abs/astro-ph/0309415 This is posited because the distribution of extinctions was very largely in genera that lived near the surface of the ocean. And it happened quickly geologically. IMO, it is also interesting to consider. I believe the Mellott paper started consideration of GRB's for the culprit. GRB's apparently occur only for a short period after some types of massive stars implode. Let's assume the culprit object was within 6000ly of Earth 425mya. The search radius will have increased over time. What kind of object would a viable candidate now be? Is it currently possible to analyze existing Chandra survey results for possible candidate objects? http://www.nasa.gov/mission_pages/chandra/main/index.html#.Uim9sTaUSwA Is this even a remotely feasible path to take to provide some sort of evidence for the GRB hypothesis?