Late Ordovician graptolite mass mortality and subsequent early silurian re-radiation

Abstract

Late Ordovician graptolite mass mortality has been examined closely in only a few continuous stratal succession. These sections include those at Dob's Linn, Scotland; Anhui, China; and Mirny Creek, USSR. Correlations among these areas are not precise; however, many characteristic Late Ordovician graptolites appear to persist longer in the Anhui area than at either Dob's Linn or Mirny Creek. Graptolites disappear from the stratigraphic record more sharply at Dob's Linn than at the other localities. Lithologic and chemical aspects of the strata at each locality suggest that a significant environmental change occurred at the stratigraphic position at which the graptolites disappeared. That change appears to reflect both a diminution and a deterioration in environmental conditions under which the graptolites previously had flourished. The optimal conditions for graptolites appear to have been in low oxygen but bacteria-rich waters similar to those found in the modern Eastern Tropical Pacific. Areal reduction in those environments as well as reduction in food resources preferred by graptolites resulted in diminished graptolite populations. These reductions took place gradually in many parts of the world, generally commencing in high latitudes. The final Late Ordovician mortality, in the Tropics, may have been the result of introduction of toxins to graptolites or their food supply by local overturn and increased vertical advection from moderate depths. The timing of such events in the Tropics is non-synchronous, suggesting local environmental influences. Geochemical and lithologic evidence links the Late Ordovician graptolite mass extinction to progressive latitudinal habitat destruction commensurate with the final pulse of the Late Ordovician glaciation. Re-radiation of the surviving taxa in the Early Silurian followed deglaciation and redevelopment of marine environments preferred by graptolites.