Abstract
Combined graptolite range data through the late Llandovery and early Wenlock of the Canadian Arctic Islands, the British Isles, and Lithuania reveal that survivorship rates from one zone to the next are between 56 and 85%, except at the Llandovery—Wenlock boundary, where only 19% (8 of 42) of the taxa of the upper sakmaricus—crenulata Zone survive into the centrifugus Zone. The high rate of graptolite extinction appears to coincide with glacioeustatic sea-level fall and may be attributed to changing oceanic temperatures, salinity, circulation patterns, oxygenation, and/or productivity associated with the onset of a glacial maximum event. □Graptolite, extinction, Silurian, Llandovery, Wenlock.
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