HIGH PALEOLATITUDE (HODH, MAURITANIA) RECOVERY OF GRAPTOLITE FAUNAS AFTER THE HIRNANTIAN (END ORDOVICIAN) EXTINCTION EVENT

Marine shales directly overlying lower Hirnantian (uppernost Ordovicia n) glacially related sediments in Mauritania (northwest Africa) have p roduced a rich graptolite fauna spanning the Ordovician-Silurian bound ary in an area of high palaeolatitude. The lowermost transgressive san dy shales are barren of graptolites, but overlying shales show a sudde n appearance of a diverse fauna indicative of the terminal Ordovician persculptus Zone, suggesting that with improving conditions, colonisat ion by a relatively cold-tolerant fauna was possible. This. fauna is r eplaced by a low-diversity assemblage dominated by long-ranging taxa, probably representing the basal Silurian acuminatus and atavus Zones. With the extinction of the persculptus Zone fauna, conditions were sti ll hostile to warm water Silurian graptolites, and a Normalograptus fa una was again established. A sudden influx of fairly diverse taxa mark s the base of the acinaces Zone and the establishment of a typical Low er Silurian fauna with the establishment of warmer water conditions. ( C) 1998 Elsevier Science B.V. All rights reserved.