THE TERMINAL ORDOVICIAN EVENT AND PALEOZOOGEOGRAPHY OF BENTHIC AND PLANKTONIC FAUNAS (BRACHIOPODS AND GRAPTOLITES)

Climatic changes that resulted from the Late Ordovician glaciation coi ncided with the mass extinction and drastic alterations of the paleozo ogeography of marine biota. The extinction of warm-water faunas in epi continental seas, as well as some brachiopod assemblages on the open s helf at the beginning of the Hirnantian, apparently resulted from the glacio-eustatic regression and decrease of water temperature in low la titudes favored by the concentration of major continents in the southe rn hemisphere. The Hirnantian was a time when new paleozoogeographic r elations were formed, which extended the distribution areas of short-l ived cold-water faunas of the temperate belt, namely, of the Hirnantia n brachiopod assemblage. After their extinction at the beginning of th e Rhuddanian, the vacant ecologic space was occupied by impoverished a ssemblages of warm-water brachiopods, namely, virgianids and atrypids, which survived the biocoenotic crisis. With the advance of Llandoveri an transgression, they occupied epicontinental seas and assumed a cosm opolitan character. The provincial differences of Graptoloidea were su bstantially obliterated before the initial Ashgillian. After the extin ction event in the Hirnantian and reduction of black-shale sedimentati on areas, the impoverished postcrisis diplograptid assemblages develop ed. The beginning of the adaptive radiation of graptolites corresponde d to the end of the Hirnantian; however, its global effect was manifes ted later in the Rhuddanian, when Graptoloidea once again assumed plan etary distribution areas.