The bryozoan dispersion into the Mediterranean margin of Gondwana during the pre-glacial Late Ordovician

A quantitative palaeobiogeographical study of the late Katian bryozoans provides new insights into the debate on climatic changes immediately prior to the Hirnantian glaciation. It focuses primarily on the Mediterranean bryozoans, for which new data are now available, and compares their distribution with those from other Gondwanan terranes and the rest of the Late Ordovician palaeocontinents. The analysis shows that the bryozoans that invaded the Mediterranean margin of Gondwana, during the late Katian global transgression, were not a simple expansion of the range of those inhabiting the palaeocontinent of Baltica. The immigrant associations, although preserving Baltic affinities, developed an independent faunal signal, denoting clear environmental differences from those in the tropical palaeocontinents from which they came. The largest bryozoan biogeographical units underwent reorganization during the late Katian. They intermixed at low latitudes, making it impossible to distinguish between the former Siberian and North-American provinces, and a new independent Mediterranean Province emerged at mid latitudes. The late Katian loss of endemism amongst the bryozoans at tropical latitudes is counter to the increasing endemism reported for other tropical faunas in support of the hypothesis of a global warming event. However, the extinction of several bryozoan genera in tropical palaeocontinents, simultaneously with their immigration into the Mediterranean cool waters, is consistent with the hypothesis of a late Katian global warming: the Boda event. The Mediterranean Province bordered the southern half of Gondwana and could have extended from what is now Himalayan India to the Precordilleran terrane of Argentina. A palaeolatitude between 40 and 55° south is suggested for the province.