There is a series of open access papers about what
caused the Late Ordovician mass extinction. They are:
Bond, D.P. and Grasby, S.E., 2020. Late Ordovician
mass extinction caused by volcanism, warming, and
anoxia, not cooling and glaciation. Geology.
https://pubs.geoscienceworld.org/gsa/geology/article/doi/10.1130/G47377.1/586486/Late-Ordovician-mass-extinction-caused-by
Mitchell, C. E., and Melchin, M.J., 2020. COMMENT:
Late Ordovician mass extinction caused by volcanism,
warming, and anoxia, not cooling and glaciation. Geology