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High palaeolatitude (Hodh, Mauritania) recovery of graptolite faunas after the Hirnantian (end Ordovician) extinction event

1998; Elsevier BV; Volume: 142; Issue: 3-4 Linguagem: Inglês

10.1016/s0031-0182(98)00070-4

1872-616X

Charlie J. Underwood, Max Deynoux, Jean‐François Ghienne,

Paleontology and Evolutionary Biology

Marine shales directly overlying lower Hirnantian (uppermost Ordovician) glacially related sediments in Mauritania (northwest Africa) have produced a rich graptolite fauna spanning the Ordovician–Silurian boundary in an area of high palaeolatitude. The lowermost transgressive sandy shales are barren of graptolites, but overlying shales show a sudden appearance of a diverse fauna indicative of the terminal Ordovician persculptus Zone, suggesting that with improving conditions, colonisation by a relatively cold-tolerant fauna was possible. This fauna is replaced 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 still hostile to warm water Silurian graptolites, and a Normalograptus fauna was again established. A sudden influx of fairly diverse taxa marks the base of the acinaces Zone and the establishment of a typical Lower Silurian fauna with the establishment of warmer water conditions.