There have been five mass extinctions since the divergent evolution of early animals 600 -450 million years ago. The cause of the third and fourth was volcanic activity, while an asteroid impact led to the fifth. But triggers of the first and second mass extinctions had, until now, been unknown.
From here.The first mass extinction occurred at the end of the Ordovician. This age is between the divergence of the Ordovician and land invasion of vascular land plant and animals. Animals in the Ordovician-Silurian comprised marine animals like corals, trilobites, sea scorpion, orthoceras, brachiopods, graptolite, crinoid and jawless fish. Approximately 80% of species disappeared at the end of the Ordovician.
The Ordovician spans 41.2 million years from the end of the Cambrian Period 485.4 million years ago (Mya) to the start of the Silurian Period 443.8 Mya (per Wikipedia). The cause of the Devonian extinction event ca. 365 million years ago, which killed many tropical marine species is still not known.
Elevated mercury levels at the transition point measured in two locations suggest a volcanic trigger for the extinction, although it isn't clear why these volcanic eruptions took place at this time. The paper is:
David S. Jones, Anna M. Martini, David A. Fike, Kunio Kaiho. "A volcanic trigger for the Late Ordovician mass extinction? Mercury data from south China and Laurentia." Geology (May 2017) G38940.1