Thread resurrect!
A recent paper in Science Advances,
UV-B radiation was the Devonian-Carboniferous boundary terrestrial extinction kill mechanism, supports the idea that severe ozone-layer depletion is sufficient to cause an extinction-level event. It mentions several additional effects, such as organic runoff from dead plant matter on land leading to algal blooms, oxygen depletion, and toxic/ozone depleting gas production in coastal ocean waters.