A COMPOSITE TAXON RANGE CHART AND CONODONT BIODIVERSITY DYNAMICS FROM THE ORDOVICIAN OF BALTOSCANDIA

Both CONOP 9 and a new method called Horizon Annealing were used to construct composite range charts from the stratigraphic range data of 159 conodont species in 24 boreholes and outcrops in Baltoscandia. We converted the composite sections to timescales in which to calculate biodiversity, extinction, and origination rates through the Early, Middle and Late Ordovician. The two methods produced broadly similar range charts and diversity curves that differed in small but interesting ways. We divided the composites into 1.15 my intervals (a temporal resolution twice that of the median zone duration) spanning the Paltodus deltifer through Amorphognathus ordovicicus conodont zones Our data show that overall biodiversity increases steadily from the base of the P. deltifer Zone to the base of the E. suecicus Zone, and then steeply declines throughout most of the remaining Ordovician. Interestingly, the start of this decline is coincident with the mid-Darriwilian (Kunda) regression and δ13C isotope excursion. Extinction rates climb steadily through the Early Ordovician, fluctuate around higher values during much of the Middle Ordovician, before reaching a peak low in the E. suecicus Zone. Extinction rates then drop again to pre-E. suecicus Zone values for the remainder of the Ordovician. Origination rates are very low across the Billingen-Volkhov boundary (base of the Dapingian) and climb to a peak in the late Lenodus variabilis Zone. Origination rates crash in the E. suecicus Zone and remain low until the late A. tvaerensis Zone when they begin to slowly rise again. Thus, the dramatic late Middle and Late Ordovician drop in conodont diversity in Baltoscandia appears to be attributable to depressed origination.