BIOGEOGRAPHY OF ORDOVICIAN OSTRACODS: PROGRESS AND PROBLEMS

MEIDLA, TÕNU ; TINN, OIVE ; SALAS, MARIA JOSE; SIVETER, DAVID ; WILLIAMS, MARK

Graz

Congreso; 7º European Ostracodologist Meeting; 2011

Our current knowledge on global biogeography of Ordovician ostracod faunas is based on data that are unequally distributed over different palaeocontinents. Most Ordovician ostracod genera have been described from Baltica and Laurentia, and some other areas have revealed a rather continuous record of lower diversity faunas (e.g. Bohemia, Kazakhstan, Siberian Platform and adjacent areas). Many potentially important regions for ostracod studies are characterized by sparse and discontinuous records (e.g. China, several terranes from Peri-Gondwana). Our current reconstruction (Meidla et al, 2011, in press) is based on the record of genera from different areas within two narrow intervals (time slabs) which are thought to reflect different climate states, comprising the earliest Late Ordovician and the latest Ordovician. Cluster and correspondence analyses of the data sets from the N. gracilis interval reveal a subdivision into five distinct biogeographical provinces, namely Baltican, Laurentian, Gondwanan, Siberian and Kazakhstanian provinces. The results show that the distribution of genera is mainly controlled by palaeocontinental affinities and less affected by a latitudinal gradient.             This pilot study was based on a limited number of most important monographic papers published within the last sixty years. Such data have several limitations: (1) Use of old published data means that some ostracod-bearing strata from immediately below and immediately above this interval may have been incorporated into the dataset. In some cases, the correlation problems may be even more difficult to solve. In the pilot study, some records of possibly Hirnantian age (Himalayas, Anticosti) were disregarded because of uncertainty of age of the records. (2) Several taxonomic problems were encountered, but they are difficult to solve without thorough revision of original collections. Because of taxonomic uncertainty, about ten percent of the records of genera in the particular areas remained tentative only in our pilot study.             The pilot study was originally expected to reveal changing biogeographical patterns of ostracods in the Late Ordovician, but the results are difficult to interpret, mainly because of a restricted geographic range of the Hirnantian data. Better resolution could become available if the number of analyzed time slices with good geographic coverage could be increased. More promising intervals could be the Middle Darriwilian (equivalents of the D. artus graptolite zone in Britain) and early Katian (equivalents of the D. clingani graptolite zone).