The Ordovician chitinozoan biodiversification and its leading factors

International audienceRegional 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)

Palaeogeography and palaeoclimatology as main controlling factors for the Ordovician chitinozoan diversification

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A Cenozoic-Style Scenario for the End-Ordovician Glaciation

International audienceThe end-Ordovician (Hirnantian) was an enigmatic interval in the Phanerozoic, known for massive glaciationpotentially at elevated CO2 levels, biogeochemical cycle disruptions recorded as large isotope anomalies and adevastating extinction event. Such linkage of eustatic, biological and isotopic records to the climatically forceddevelopment of an ice sheet can only be contemplated within a framework of high-resolution sequence stratigraphythat integrates allo-, chemo- or biostratigraphic markers. We develop sequence stratigraphic correlations fortwo superbly exposed and exceptionally well-developed latest Ordovician successions, the Anti-Atlas of Moroccoand Anticosti Island in Canada. Both offer sections, on a 100-km scale, from the basin edge to the axis of activesedimentary depocentres. Relative to the end-Ordovician ice-sheet centre (present-day north-central Africa), theyprovide a near-field (Anti-Atlas, siliciclastic platform) and a far-field (Anticosti Island, mixed carbonate and siliciclastic)stratigraphic records. These two successions, up to 300 and 100m thick, respectively, were deposited inbasins with notable subsidence rates and significant (ca. 100 m) initial water depths, enabling the development ofcomprehensive archives of the latest Ordovician glaciation. This framework, driven by glacio-eustatic cycles tiedto the evolution of polar continental-scale ice sheets over west Gondwana, enables the correlation of eustatic cyclesat a level that is beyond the resolution capability of most absolute dating methods and of biozones, the latter typicallyof Myr duration. A proposed Cenozoic-style scenario including three main glacial cycles and higher-orderphenomena necessitates the revision of the end-Ordovician, glaciation-related sequence of events

Chitinozoan-based calibration of Early-Mid Ordovician transgressive events on northern Gondwana

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Chitinozoans

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