DIVERSITY CHANGES IN UPPER ORDOVICIAN GRAPTOLITES: MORE EVIDENCE FOR A MID-CARADOC BIOTIC EVENT IN NORTH AMERICA

The early Late Ordovician (Middle Caradocian) has recently been recognized as an interval that contains important regional and perhaps global biotic events. Patzkowsky and Holland (1996) and Emerson et al. (2000) documented regional brachiopod extinctions in the Appalachian Basin and Upper Mississippi Valley, respectively. Kaljo et al. (1996) and Ainsaar et al. (1999) reported ostracode, achritarch, and chitinozoan extinctions from the Middle Caradocian strata of Baltoscandia. Biostratigraphic and chemostratigraphic evidence indicates that the North American and Baltoscandian extinctions are approximately synchronous, prompting suggestions that these regional extinctions are part of a global mid-Caradoc bioevent.

A preliminary examination of the pattern of graptolite faunal changes across this interval provides comparative data from another group of fossils, and from a different lithofacies - basinal black shales. In eastern North America, both the brachiopod extirpations and the Climacograptus bicornis - Corynoides americanus graptolite zonal boundary occur close to Holland and Patzkowsky's (1996) M4-M5 sequence boundary, just above the Millbrig K-bentonite. Graptolite species diversity drops dramatically across this zonal boundary. The C. bicornis Zone contains a relatively high diversity fauna of 18 genera and 38 species. In the overlying C. americanus Zone, diversity drops to 10 genera and 13 species. Only 11% of the species present in the C. bicornis Zone carry over into the C. americanus Zone. Unfortunately, no section in eastern North America spans the C. bicornis - C. americanus zonal boundary and some of the faunal disparity may be related to the boundary hiatus. In the Australasian and British successions, diversity remains fairly constant across this interval, although the percentage of carryover taxa remains low. One the most interesting aspects of the graptolite turnover in North America is the loss of nearly all the dicellograptids, dicranograptids, and members of the Orthograptus calcaratus species group - taxa that are present in the other successions. The graptolite data provide additional evidence that a significant bioevent occurred in North America during the Mid-Caradoc.