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supposed periodicity in mass extinctions

Not only was the periodicity not present in the geological record,
but the "companion star" (Nemesis) explanation doesn't stand up.
Dynamical analysis shows that an orbit so distant from the Sun isn't
stable enough to have a well-defined period. A 65-Myr period gives
an rbit with semimajor axis (more or less mean radius) greater
than 2 light-years, so that passing stars would strongly perturb
it. Even worse, the tidal field of the galaxy as a whole would
provide secular perturbations. Aside from this, deep infrared
searches have turned up no plausible companion. On the other 
hand, some folks have talked about triggering comet showers
as the sun bobs in and out of the galactic plane, which might
have a period comparable to that discussed for mass extinctions...

Bill Keel                       Astronomy, University of Alabama