Yesterday I mentioned in passing some relatively small extinction events during the Silurian. There were at least three, which more or less mark the boundaries between the four epochs of the Silurian. The first one, at the boundary between the Llanderovy and Wenlock epochs, was pretty intense at some locations – more than half the trilobite species disappeared at a location in Sweden, and worldwide, 80% of conodont species died off. Conodonts are generally deeper-water animals, as are graptolites, which also suffered a lot in that 200,000-year event. Shallow water life, like corals, had little impact. Why is that? I don’t know.
The Mulde Event, about 427 million years ago, between the Wenlock and Ludlow Epochs, coincided with a global drop in sea level. What caused the sea-level drop is not known. The last extinction event also did a number on conodonts but had little effect on graptolites. Go figure. But the graptolites were decimated shortly after the sea-level change that marks this extinction, at a time when sea-water isotopes changed significantly. I hope it is evident that these things are complicated – most extinctions are way more complex, and less well understood, than the end of the dinosaurs.
One interesting aspect of these little extinctions is that we actually see an increase in stromatolites during the extinction periods. You remember stromatolites – buildups of calcareous material by algal mats. The theory seems to be that when animals that ate algae were decimated, it allowed for an expansion of the algal mats. But when life rebounded after the extinctions, the stromatolites declined.
—Richard I. Gibson