There is an article on BBC discussing a paper in “Geology” claiming that the extinction of the Trilobites (and much else in the oceans) during the Permian-extinction (-250 million yrs) was due to massive soil erosion following huge volcanic episode (Siberian Traps). Nothing to do with asteroids, gamma ray bursts, comets, interstellar dust clouds or the Wrath of Khan.

http://news.bbc.co.uk/1/hi/sci/tech/4499722.stm

To quote:-
“The authors of the Geology paper believe volcanic gases from the eruption would have depleted Earth's protective ozone layer and acidified the land and sea, killed rooted vegetation.
This would have hindered the retention of soil and allowed it to be washed into the surrounding oceans.
Soil materials in the oceans would have blocked out light and soaked up oxygen. Analysis of rock chemistry suggests that after the so-called "soil crisis" on land, the marine ecosystem succumbed to the stresses of environmental change.
Oceanic life faltered, completing a global catastrophe.”