  The Late Devonian was a protracted period of low speciation resulting in
biodiversity decline, culminating in extinction events near the
Devonian-Carboniferous boundary. Recent evidence indicates that the final
extinction event may have coincided with a dramatic drop in stratospheric
ozone, possibly due to a global temperature rise. Here we study an alternative
possible cause for the postulated ozone drop: a nearby supernova explosion that
could inflict damage by accelerating cosmic rays that can deliver ionizing
radiation for up to $\sim 100$ kyr. We therefore propose that the end-Devonian
extinctions were triggered by supernova explosions at $\sim 20$ pc, somewhat
beyond the "kill distance" that would have precipitated a full mass extinction.
Such nearby supernovae are likely due to core-collapses of massive stars; these
are concentrated in the thin Galactic disk where the Sun resides. Detecting
either of the long-lived radioisotopes Sm-146 or Pu-244 in one or more
end-Devonian extinction strata would confirm a supernova origin, point to the
core-collapse explosion of a massive star, and probe supernova nucleosythesis.
Other possible tests of the supernova hypothesis are discussed.
