Katian prelude to the Hirnantian (Late Ordovician) mass extinction: a Baltic perspective

Abstract

The diversity of several groups of Late Ordovician fossils in the Baltic is discussed and corresponding curves are presented. The curves are correlated with the latest d 13 C trend and probable sea surface temperatures of the equatorial ocean in order to link biodiversity and environmental changes. Most groups of macrofossils and acritarchs showed a high-level biodiversity in the Sandbian and early Katian during a sea level high stand and optimal temperatures. Conodont diversity peaked in the mid-Katian, scolecodonts in the late Katian and rugose corals even in the Hirnantian, indicating their different ecological demands and the evolutionary status of each group. The diversity peak close to the Guttenberg d 13 C excursion marks a turning point in environmental evolution when a new global cooling began but tropical conditions persisted in Baltica due to its northward drift. Biodiversity fluctuated during the Katian. Two diversity peaks noted in the Katian are followed by low-diversity episodes showing a general step by step decrease in diversity, and a correlation with variable sea level and episodes of climatic cooling. It suggests that the Hirnantian mass extinction was preceded by a rather prolonged period of biodiversity change here called the Katian prelude. Katian biodiversity curves and their links with different facies parameters show that environmental conditions within the ecosystem were the main drivers of change, not solely temperature or more generally climate, but also sedimentary environments including water dynamics together with depth and substrate conditions. The biotic component is less well known, but also important.

Ordovician conodont diversity in the northern Baltic

“…Higher, in the lowermost Porkuni Stage, conodont diversity drops rapidly and reaches its minimum (in both the number of species and genera) in the uppermost Ordovician ( Fig. 3; Kaljo et al 2011). The early Silurian (Rhuddanian and early Aeronian; Juuru and lower Raikküla stages in Estonia, respectively) was a time of rapid radiation and almost complete turnover of conodont faunas.…”

“…Roman numbers to the right of the diversity curve of genera: characteristic intervals of diversity changes discussed in the text. Basal Tremadoc, Basal Arenig, Basal Llanvirn and Basal Caradoc events after Barnes et al (1996); Mid-Caradoc Event after Männik (2004); End-Ordovician Event after Kaljo et al (2011) (the Upper Asgill event sensu Barnes et al (1996) corresponds to its upper part); Sandvika Event after Aldridge et al (1993). Black arrows to the right of the supersequences indicate the sequence boundaries (vertical ruling) corresponding to regional gaps and characterized, as a rule, by increased input of terrigenous material into the basin.…”

CHITDB: a database for documenting and analysing diversification of Ordovician–Silurian chitinozoans in the Baltic region

“…The start of the Late Homerian excursion (LHCIE) closely matches the beginning of a crisis in modelled chitinozoan diversity (Mulde Event), whereas in the Ireviken Event interval the onset of the chitinozoan extinction seems to precede the early Sheinwoodian carbon isotope excursion (ESCIE; note that a similar pattern was reported also in case of conodonts, see Loydell 2007 for details). The correspondence is less straightforward in the Ordovician, but overall the Katian series of isotope excursions starting with the Guttenberg excursion (GICE) is characterized by declining diversity of chitinozoans as shown earlier (Kaljo et al 2011). It follows that the Baltic chitinozoan fauna and its diversity were intimately linked to global environmental changes and only the smaller-scale fluctuations could to be attributed to regional variables.…”

“…The correspondence is less straightforward in the Ordovician, but overall the Katian series of isotope excursions starting with the Guttenberg excursion (GICE) is characterized by declining diversity of chitinozoans as shown earlier (Kaljo et al . ). It follows that the Baltic chitinozoan fauna and its diversity were intimately linked to global environmental changes and only the smaller‐scale fluctuations could to be attributed to regional variables.…”

New Hirnantian orthide brachiopods from the type section of the Porkuni Stage (Porkuni quarry, northeastern Estonia)

“…The immigration of the new Hirnantia fauna into the Baltic Basin increased brachiopod diversity. At the same time the habitable areas and faunal diversity decreased in the Estonian facies belt due sea level fall caused by the development of the Gondwana ice cap (Hints & Harper 2003;Kaljo et al 2011).…”