CHITINOZOAN BIODIVERSITY DURING THE ORDOVICIAN AND ITS RELATION TO THE EVOLUTION OF LAURENTIAN MARGIN

The rapid evolution of chitinozoan microfaunas from Laurentia has led to the establishment of well-defined regional biozonations. To document the changes of these microfaunas during the Ordovician, information related to 142 chitinozoan species reported from 109 localities and 1391 samples has been stored in a database. The stratigraphic occurrence of each species is expressed in terms of the 19 Ordovician time-slices used for the IGCP no. 410 project. The rates of species origination/extinction and the total species diversity for each time slice can be expressed in curves which illustrate the evolution of the microfauna and highlight the major turnover events. The Ordovician chitinozoan radiation process is well represented by these curves with fluctuating origination and extinction ratios and with total diversity values which increase continuously form late Early to late Middle Ordovician (Darriwilian). The diversity is even higher in the Late Ordovician with a maximum peak attained in the Mohawkian. The Cincinnatian is marked by an important decrease in diversity, mirroring an increase in the extinction rate. These variations can be related to the geological evolution of the Laurentian margin and to concurrent global events. Lower Ordovician passive margin sediments contain a chitinozoan fauna characterized by moderate species diversity, and the fluctuation of the origination and extinction rates may reflect the prevalent eustatic sea level variations. The change in diversity at the end of the Middle Ordovician is related to the change from a passive to an active margin setting and corresponds to the beginning of the Taconic Orogeny and the formation of an associated foreland basin. The deepening of this basin from the late Whiterockian to the early Cincinnatian is marked by an increase in chitinozoan species diversity and by more or less constant extinction and origination rates. From the early Cincinnatian onwards, the drastic decrease in the biodiversity and the corresponding increase in the extinction rate may reflect the Late Ordovician ice-cap development in Gondwana and the associated sea-level fall.