ORDOVICIAN CHITINOZOAN BIOGEOGRAPHY AND PALEOECOLOGY: EXAMINING THE EFFECT OF HABITAT ON SPECIES LONGEVITY

The relationships between geographic range and evolutionary dynamics, including taxonomic duration, have been analyzed for a number of fossil groups, particularly benthic macrofossils. Ordovician boreholes from the East Baltic region and Scandanavia are extremely fossiliferous and well sampled, providing an excellent microfossil data set for evolutionary studies. In this study we examined the relationship between habitat preference and species longevity in chitinozoans. Baltoscandian boreholes span three confacies belts, the Scanian (slope, black shale), Central Baltoscandian (outer shelf, argillaceous limestones), and North Estonian (carbonate platform) belts. We first used chitinozoan distribution patterns across the 3 confacies belts to develop a model of chitinozoan paleoecology. We found that chitinozoan biotopes tended to reflect onshore/offshore distribution as opposed to depth stratification. Species were grouped into three biotopes, generalist (taxa occurring in all three confacies belts), platform to outer shelf (taxa occurring only in the North Estonian and Central Baltoscandian confacies), and platform restricted (taxa occurring only in boreholes from the North Estonian Platform). We then used the quantitative stratigraphic correlation program CONOP9 to construct a Middle and Upper Ordovician composite range chart from the stratigraphic range data of 127 chitinozoan species from 13 boreholes in Estonia, Poland, Latvia, and Sweden. We converted the CONOP composite into a timescale using the geochronologic ages for chitinozoan biozone boundaries from Webby (2004). We used this timescale to calculate individual taxon durations in millions of years in order to test the hypothesis that members of different biotopes would have different average species longevities. Generalist species that occurred in all confacies belts had an average longevity of 8.6 million years, species restricted to platform and outer shelf environments had an average longevity of 2.5 million years, and species restricted to the North Estonian platform had durations of 0.8 million years. The mean longevities of members of these biotopes were all significantly different from each other (p < .05).