BIOGEOGRAPHY OF ORDOVICIAN OSTRACODS: PALAEO-CONTINENTAL OR CLIMATIC CONTROLS?

MEIDLA, TÕNU ; TINN, OIVE; SALAS, MARIA JOSE; WILLIAMS, MARK; SIVETER, DAVID; VANDENBROUCKE, T.; SABBE, KOEN

Londres

Congreso; IPC 3 International Palaeontological Congress; 2010

International Paleontological Association, Natural History Museum

We present a pilot study of the global biogeography of shelf marine ostracod genera for two Ordovician time-slabs, thought to reflect greenhouse and glacial climate states respectively. These correspond to the earliest Late Ordovician (Nemagraptus gracilis interval) and to the latest Ordovician (Hirnantian Stage). At present, the palaeolatitudinal range of the material includes equatorial to high latitude (> 60°S) for the gracilis-interval assemblages, but is mainly limited to low latitudes for the Hirnantian. Cluster and correspondence analyses of ostracod datasets from the gracilis-interval reveal five distinct biogeographical provinces: Baltican, Laurentian, Gondwanan, Siberian and Kazakhstanian. The biogeographical affinities of the Argentine Precordillera ostracod assemblage are poorly determined. Ostracod biogeography appears 275 dominantly controlled by palaeocontinental affinities and less influenced by the latitudinal surface temperature gradient. Thus, Laurentia and Siberia may have been positioned in the tropics during the Ordovician, but have taxonomically distinct ostracod faunas. The situation with the benthic ostracod fauna contrasts with that determined for planktonic ‘mixed-layer’ organisms (chitinozoans, graptolites), where palaeolatitude has been a more significant control. The Hirnantian interval contains ostracod data from a limited number of localities, mainly in Baltica. Two groups revealed by cluster analysis could be tentatively interpreted as tropical faunas (upper-middle ramp localities of Baltica, together with Yukon) and calm water faunas (lower ramp localities of Baltica, and Carnic Alps from Peri-Gondwana). An overall decrease of ostracod provincialism is suggested for the Hirnantian but a response to global cooling is not clear from the distribution pattern, mainly because of the scarcity of high-latitude data.