PATTERN AND PROCESS OF DIVERSITY CHANGE IN UPPER ORDOVICIAN (MID-CARADOC) GRAPTOLITES

The early Late Ordovician (Middle Caradoc) has been recognized as an interval that exhibits a significant drop in marine biodiversity. Sepkoski (1995) noted that further research was needed to discover if this decline was a real phenomenon or an artifact of relatively few studies devoted to this interval. We examined of the pattern of graptolite faunal changes across the Climacograptus bicornis - Corynoides americanus graptolite zonal boundary in North America and compared the diversity changes to published data from sections that span this interval on other continents. Biostratigraphy and chemostratigraphy were used to correlate the various sections. A sharp decline in graptolite species diversity occurs in North America but not on other continents. In eastern North America, diversity drops from 18 genera and 38 species in the C. bicornis Zone to 10 genera and 13 species in the overlying C. americanus Zone. Only 11% of the species present in the C. bicornis Zone carry over into the C. americanus Zone. A similar pattern occurs in the graptolite-bearing rocks of Oklahoma. In the Australasian and British successions, however, diversity remains fairly constant across this interval, although the percentage of carryover taxa remains low. The North American decline in graptolite diversity is mirrored by local extinctions in benthic invertebrates (Emerson et al., 2001; Patzkowsky and Holland, 1996; Sloan, 1987). Several geologic and environmental events have been suggested as possible causes for these biotic changes. These include changes in basin circulation associated with the Taconic Orogeny, extensive explosive volcanism and ash deposition, and a drop in seawater temperature (Patzkowsky and Holland, 1996; Sloan, 1987). There is some additional evidence to suggest that graptolite species diversity may be sensitive to seawater temperature changes or other physical and chemical changes that might accompany temperature fluctuation. Testing of this hypothesis may help elucidate the mechanisms of graptolite extinction during the Hirnantian glaciation and the subsequent Early Silurian recovery.