The early Rhuddanian survival interval in the Lower Silurian of the Oslo Region: A third pulse of the end-Ordovician extinction

• A complete record of end-Ordovician extinction events with thick ensuing Rhuddanian strata.
• A Lilliputian shelly fauna present in boundary layers and lower Rhuddanian strata.
• The dwarfing shows a two-step reduction in size.
• The first step includes a relict Ordovician fauna and lack some indicators of a survival interval.
• The second step in uppermost lower Rhuddanian shows all indications of a survival interval.
• This step coincide with a global negative ∂13C excursion and is possibly a third extinction pulse.
• The Benthic Assemblage in Rhuddanian is directly related to an earlier Ordovician Assemblage.

Ordovician/Silurian boundary layers with Rhuddanian strata are exposed as a long, continuously fossiliferous sequence in the Solvik Formation in the Asker area, central Oslo Region, Norway. Brachiopods belonging to Benthic Assemblage 5 are preserved in the lower parts of the formation. This level is investigated for the presence of a survival interval after the last end-Ordovician extinction event. The criteria for a survival interval include taxa that are dwarfed, long ranging, eurytopic, often opportunistic, as well as assemblages that show low density and diversity.Four species, Isorthis prima, Leangella scissa, Dicoelosia osloensis and Eoplectodonta duplicata, were collected and measured. The three former were dwarfed compared to a younger Aeronian fauna belonging to the same Benthic Assemblage. Detailed investigations showed a statistically significant two-step reduction in size for all but D. osloensis. The first interval, immediately above the last end-Ordovician extinction event, displays dwarfed brachiopods, but diversity is high and the number of long-ranging and eurytopic species is low due to the presence of globally and locally “relict” Ordovician species. The second interval, the upper parts of lower Rhuddanian, shows all the characteristics of a survival interval in which the sizes are statistically significantly smaller than both those in the interval below and those in the Aeronian interval above. No lithological change within lower Rhuddanian strata could be linked to the pronounced reduction in size. The second step in size reduction may be related to global occurrence of anoxia in the deep oceans with a pulse of anoxic water pushed onto the shelf at that time. It is coeval with a short negative ∂13C excursion found in some locales, signifying a brief period of global warming. The results suggest a protracted extinction event through parts of the early Rhuddanian and a third and final extinction event followed by a clear survival interval.