PALEOBIOGEOGRAPHY OF ORDOVICIAN CALCAREOUS ALGAE

From the study of Ordovician algal microfloras the following can be concluded: during the Early Ordovician, the algal microflora is scanty, but cosmopolitan, and different from that of the Upper Cambrian. Its cosmopolitan character seems to denote the absence of a climatic gradient during that time. By Middle Ordovician times, new algal taxa appear. The algal palaeogeographic distribution suggests a possible climatic zonation. No algae are known from Gondwana, which was located at high latitudes. For the same reason, calcareous algae are absent (or not recorded) from South Europe. In contrast, the occurrence of calcareous algae and stromatoporoids of Middle Ordovician age provides support for inferring that the North Europe plate has reached low latitudes by Middle Ordovician times. During the Upper Ordovician, some groups of calcareous algae crossed the Proto-Atlantic or Iapetus ocean, and migrated towards North European and North American regions, according to the general pattern of oceanic surface currents. This crossing and this migration are the result of the narrowing of the Proto-Atlantic or Iapetus ocean. At that time, like other pelagic and benthic organisms, many groups of calcareous algae are declining. During the Middle and Late Ordovician, the migration and the distribution of some groups of calcareous algae resulted from asynchronous oceanic surface currents close to the palaeoequator, probably mainly in an eastward direction in the Late Middle Ordovician (from North America to Euroasia), and mainly in a westward direction in the Late Ordovician-Lower Silurian (from Euroasia to North America).