Biotic cycles in the early Silurian correlate broadly with postulated sea-level changes, but are better explained by a model that involves episodic changes in oceanic state. Primo episodes were characterized by cool high-latitude climates, cold oceanic bottom waters, and high nutrient supply which supported abundant and diverse planktonic communities. Secundo episodes were characterized by warmer high-latitude climates, salinity-dense oceanic bottom waters, low diversity planktonic communities, and carbonate formation in shallow waters. Extinction events occurred between primo and secundo episodes, with stepwise extinctions of taxa reflecting fluctuating conditions during the transition period. The pattern of turnover shown by conodont faunas, together with sedimentological information and data from other fossil groups, permit the identification of two cycles in the Llandovery to earliest Wenlock interval. The episodes and events within these cycles are named: the Spirodden Secundo episode, the Jong Primo episode, the Sandvika event, the Malmøykalven Secundo episode, the Snipklint Primo episode, and the Ireviken event.

Biotic and sedimentological cycles in the Wenlock are related to episodic changes in oceanic state. These may be explained by alternations between primo and secundo conditions, the former being characterized by cold oceanic bottom waters and high nutrient supply, and the latter by salinity-dense bottom waters and diminished nutrient supply. The lithological pattern, together with the turnover of conodont faunas and of other fossil groups, permits the identification of a sequence of episodes and events in the Wenlock, following the extinctions associated with the earliest Wenlock Ireviken Event: the Vattenfallet Secundo Episode, the Sanda Primo Episode, the Boge Event, the Lansa Secundo Episode, the Allekvia Primo Episode, the Valleviken Event, the Hellvi Secundo Episode, the Mulde Event and the Klinte Secundo Episode. The Mulde Event differs from others in separating two secundo episodes, and may represent an interval when deep waters remained saline and vertical circulation virtually ceased. The effects of the events and episodes were stronger in the early and late Wenlock than in the middle part of the epoch.

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An acritarch assemblage from beds beneath the Ogof Hen Formation near Carmarthen (the 'Login beds') resembles assemblages from the Watch Hill Grits and underlying sediments of latest Tremadoc or earliest Arenig age in the Skiddaw Group of the English Lake District. We conclude that the Login acritarch assemblage is younger than at least some part of the late Tremadoc Angelina sedgwickii Biozone, but older than the early Arenig Didymograptus deflexus Biozone. Its occurrence may therefore coincide with that of the graptolite Tetragraptus approximatus. This is of particular significance in any attempt to define an internationally correctable base for the Arenig Series in Britain.