Recognition of a probable secundo–primo event in the Early Silurian

The early Telychian graptolite extinctions discussed by Loydell in the Stimulograptus utilis Sub-zone agree completely with the predicted consequences of a secundo–primo event, the first to be identified. The event would have included brief inhibitions in vertical circulation, causing a drop in primary planktic productivity from an already low level and causing extinctions; less well-oxygenated deep waters would enhance preservation of organic-walled fossils. The succeeding primo episode would instigate more vigorous vertical circulation with increased primary planktic productivity and higher downward transport of oxygen.