End Ordovician Extinction

[TriloGuyKy]

While it could never be proven, what does everyone think about the proposed theory that a gamma ray burst around 6,000 light years from Earth caused the End-Ordovician extinction and the downfall of such creatures as the large Straight Nautiloid? I know there was a large ice-age at the end of the period, and invasive species. Perhaps a gammay ray burst precipitated some of these events? They say one hitting the earth is so rare it only happens once every billion years. The Ordovician extinction occured about 450 million years ago? Your thoughts?

http://www.aps.org/publications/apsnews/200407/extinction.cfm

Edited September 26, 2009 by TriloGuyKy

[Auspex]

I know only that I don't know nearly enough to comment on this, other than that I am predisposed to suspicion of exotic explanations where mundane processes will do.

Hopefully, others with some knowledge will weigh-in; I'd like to (maybe) learn something about this.

[siteseer]

I know only that I don't know nearly enough to comment on this, other than that I am predisposed to suspicion of exotic explanations where mundane processes will do.

Hopefully, others with some knowledge will weigh-in; I'd like to (maybe) learn something about this.

Yes, from what I've read about this, a fall in sea level and expansion of glaciers from the poles triggered by continental movements have been cited as the causes for the mass extinction since at least the 80's. Every once in a while someone proposes that a gamma ray burst from a "nearby" supernova could have caused one or more of the mass extinction events but clear evidence of it is never available, and as said, earthly explanations already suffice.

In "Out of Thin Air" by Peter Ward a time chart shows that oxygen levels were lower than they are today across the Ordovician, dipping to perhaps one-third of present levels around the middle of the period. Ward argued that the mass extinction (considered two rather closely-timed events by many) came as a result of the oxygen nadir but the mass extinction takes place at least 15 million years after that during a time when his chart shows oxygen levels were clearly on the increase. This would seem to eliminate "choking" as a cause.

A fall in sea level would be devastating. Continental shelves, the most popular real estate in marine environments across time, could be either fully exposed or greatly decreased in area, forcing increased competition for whatever's leftover. Taxa adaptable to deeper, cooler water or with close relatives that already lived in deeper water had an advantage. Creatures anchored to the bottom and adapted to life at a certain depth on the shelf would be particularly hard-hit in that case. What group was especially decimated? Brachiopods (shallow water animals anchored to the bottom). Bryozoans, corals, and echinoderms also suffered great losses.

[Guest solius symbiosus]

I read something about this several years ago. At that time, someone(???) had hypothesized that there should be isotopic anomalies in phyllosilicates(???) that formed after the event; they were searching for the anomalies. Unfortunately, I can't recall who was doing the work, or if anything was ever published.