Middle and Late Ordovician Biodiversity Dynamics in Marine Microfossils from Baltoscandia

The early Late Ordovician was an interval of significant decline in marine biodiversity that has been attributed to sea level, facies, and climatic changes. In the East Baltic area several workers have described a significant diversity decline and faunal turnover in marine microfossils at the Keila-Oandu Stage boundary, an event called the Oandu Crisis. To get a more complete understanding of microfossil diversity dynamics in the Middle and Upper Ordovician rocks of Baltoscandia we used constrained optimization (CONOP9) to construct a composite range chart from the stratigraphic data of 455 chitinozoan, conodont, ostracod, and graptolite species from 14 boreholes and five outcrops in Baltoscandia. We used the CONOP composite as a timescale in which to calculate biodiversity, extinction, and origination rates through the Middle and Late Ordovician. Traditional biodiversity metrics and more recent probabilistic methods based on capture-mark-recapture analysis were used to estimate biodiversity and fossil recovery patterns. We were particularly interested in faunal dynamics across the late Keila and early Oandu stages, an interval associated with a prominent positive carbonate δ¹³C isotope excursion known as the GICE. We divided the CONOP composite into 500,000 year intervals spanning the Lasnamagi through Porkuni stages. Our data show that overall biodiversity increases steadily from the base of the Keila to the middle Rakvere, mainly due to an increase in ostracod diversity. Chitinozoan diversity reaches a zenith in the Late Keila, drops through the Oandu Stage coincident with the GICE, and then gradually declines across the rest of the Ordovician. Conodonts have diversity peaks in the lower Uhaku and lower Kukruse Stages, and then decline gradually through the Late Ordovician. Microfossil diversity changes across the Late Sandbian and Early Katian are driven by low origination rates and not higher extinction rates, as extinction rates tend to drop in this interval as well.