http://science.slashdot.org/science/07/08/02/1923251.shtml this describes a theory that galactic cosmic rays cause problems on a 65 MY timescale, causing biodiversity drops. Interesting stuff... between impact events (and the "nemesis" theory explaining their periodicity), this new theory, orbital theories, and the changes in oxygen levels, there seem to be more mass extinction theories than there have been mass extinctions. What to we blame for the demise of the ammonites? I forget if I've mentioned this more than in passing, but I recently read Peter Ward's book Out of Thin Air which presents hypotheses of oxygen levels as major drivers for evolutionary events, including a theory that some early pre-ceph molluscs developed their gill and circulatory systems for much more efficient oxygen extraction first, but it led to powerful mantles to push water past those gills quickly, which led to the jet propulsion, which led to free swimming, which drove a need for neutral buoyancy, hence the chambered shells. Rather backwards from the traditional view that the chambered shell came first and then jet propulsion became a benefit. Ward admits that this needs to be investigated before being accepted, but it makes for a very interesting read, and is yet another explanation for extinctions, speciations, radiations, and whatnot. It also has some interesting stuff about dinosaurs, birds, air sacks, and such.