The Ordovician chitinozoan biodiversification and its leading factors
Résumé
Regional and global dataset now available for almost all of the Ordovician fossil groups have led to a search for the causes controlling the major biodiversification events that occurred during the Ordovician period. A review of the physico-chemical state of the Ordovician world is presented, with an emphasis on the regional or global changes contemporaneous with the origination and extinction events that could have influenced the chitinozoan diversity. This study focuses on the chitinozoans because these enigmatic organic-walled microfossils are an important component of the Ordovician palaeoplankton and provide one of the best documented dataset. The intrinsic factors that initiated the Ordovician biodiversification of this group are not discussed because the chitinozoans are regarded as reproduction stages of cryptic "chitinozoan animals", whose biological characteristics are speculative, at best. This study has two main goals: i) the evaluation of the impact of regional and global physico-chemical events on chitinozoan diversity, ii) the comparison of the biodiversification patterns of the chitinozoans with other selected benthic and pelagic Ordovician fossil groups. The chitinozoan diversification was progressive and showed similar patterns in Laurentia, Baltica and northern Gondwana from the Tremadocian to the late Darriwilian, when the group reached its acme in Baltica and northern Gondwana. From the MiddleUpper Ordovician boundary onward, the biodiversification pattern documented in Laurentia diverged drastically from the two other regions. In the Late Ordovician, the contribution of the Laurentian chitinozoans to the global curve was high, suggesting major faunal inputs from the two other regions, which significantly lowered the endemic character prevalent in the EarlyMiddle Ordovician. In the Late Ordovician, large anti-clockwise oceanic currents developed as the result of a thermohaline circulation. This marine circulation was likely to have been driven by a global cooling concomitant with a major palaeogeographic reorganisation of the southern hemisphere. These oceanic/climatic changes intervened in the breakdown of the existing chitinozoan endemism. Globally, the chitinozoan biodiversity was not much higher in the mid Late Ordovician than in the late Darriwilian diversity peak. The most obvious feature is the progressive decrease in diversity during the Late Ordovician, long before the Hirnantian glaciation. The influences of cosmic and volcanic parameters are excluded as they appear to have had too low impact on the Late Ordovician decrease of chitinozoan diversity. Correlation is noticed between some diversification events and sea-level changes, at least on a regional scale. More globally, however, a climatic control is favoured. A durable greenhouse environment gave an efficient support to the diversification. Conversely, the onset of an icehouse environment in the early Late Ordovician onward, culminating with the Hirnantian glaciation, is interpreted as a limiting factor for the chitinozoan diversification. The reasons behind the onset of this icehouse episode are not fully understood, but they appear to be linked to changes in the palaeogeography (e.g., progressive closure of the Iapetus, northward drift of Avalonia and its docking with Baltica) and in the carbon cycle (e.g., high organic carbon burial with a lowering of the pCO2).