CONSEQUENCES OF PLATE MOTIONS FOR MID AND LATE ORDOVICIAN ENVIRONMENTAL AND BIODIVERSITY CHANGES: A LAURENTIAN PERSPECTIVE

Ordovician research has drawn attention to two intervals of marine organismal biodiversity change: an early mid-Ordovician radiation, perhaps the most significant in the Phanerozoic, and the Late Ordovician extinction, one of the five most severe in the Phanerozoic. Sepkoski analyzed Ordovician generic diversity for a 1995 Ordovician symposium and pointed out that "more than three times more biodiversity was added" to marine ecosystems in the early mid-Ordovician radiation than in the better-known Early Cambrian metazoan radiation. The Late Ordovician extinction reduced marine organismal biodiversity to the level that it had been prior to the early mid-Ordovician radiation. Major plate motions, notably those involving Laurentia, may have been primary forces leading to these biodiversity changes. During Early and early mid-Ordovician, small plates drifted northward from Gondwana, the Baltic Plate moved into the tropics, and Laurentian Plate motion resulted in relative sea level fall that led to exposure of most former shallow marine environments. Karsts formed widely across the Laurentian Plate. Shallow marine environments were diminished to small sites along plate margins. Mid-Ordovician sea level rise expanded these environments, providing suitable habitats for organismal radiations seen in Laurentian stratigraphic sections. Potentially, Late Ordovician Gondwanan glaciation was initiated after a highlands-bearing plate drifted into one margin of Laurentia. Weathering of that highlands generated the Queenston deltaic sedimentary complex. Glaciation-linked ocean circulation changes diminished platform margin upwelling and its related oxygen minimum zones, leading to graptolite extinctions. Fragmented and reduced benthic organismal habitats resulting from glacio-eustatic sea level fall appear to have resulted in the Late Ordovician benthic marine organismal biodiversity changes.