CHITINOZOAN BIODIVERSITY IN THE ORDOVICIAN OF GONDWANA: AN INTERVAL-FREE APPROACH USING THE QUANTITATIVE STRATIGRAPHIC CORRELATION PROGRAM CONOP9

Chitinozoans are organic-walled microfossils that first appear in the Early Ordovician and diversify rapidly through the Paleozoic. They occur in a variety of marine paleoenvironments ranging from carbonate platforms to slope and basinal settings, but have an unknown taxonomic affinity. Along with graptolites and conodonts, chitinozoans are extremely important lower Paleozoic index fossils. The regional and global biodiversity patterns of Ordovician chitinozoans have been examined in detail, but typically within the traditional biostratigraphic temporal frameworks of unequal duration biozones or time slices (e.g., Paris et al., 2004). Recently, Goldman et al. (2008, 2013) and Hints et al. (2011) used the quantitative stratigraphic correlation program CONOP9 (Sadler et al., 2003) to examine Ordovician Baltoscandian chitinozoan biodiversity with both equal duration bins and interval-free approaches to temporal standardization.

In this study we used CONOP9 to construct an Ordovician composite range chart from the stratigraphic range data of 167 chitinozoan species from 65 boreholes and outcrops across Gondwana. These data were gathered from the published ranges of species in sections from Argentina, North Africa, the Middle East, Australia, and southern Europe. The CONOP9-derived results differ from previously published results in small but significant ways. Our diversity curve is generally similar to the global curve of Paris et al. (2004), which exhibits a broad diversity plateau that ranges from the upper Darriwilian to lower Katian, with a Sandbian dip between two peaks. Unlike the global pattern, however, the youngest peak is mid to upper, not lower, Katian and has greater diversity than the upper Darriwilian. The CONOP9-derived pattern lacks the very pronounced upper Darriwilian peak in the Paris et al. (2004) N. Gondwana curve. Chitinozoan diversity curves from Baltoscandia tend to have a broad Darriwilian to mid Katian diversity plateau followed by a long upper Katian - Hirnantian decline, a decline which is delayed and steeper in the CONOP Gondwana curve. These differences, particularly with the previous N. Gondwana curve could be attributable to CONOP-produced range extensions or perhaps from the substantial amount of post- 2004 data that tended to focus on the late Katian - Hirnantian interval.