THE GREAT ORDOVICIAN BIODIVERSIFICATION EPISODE: ORGANIC MICROFOSSILS FROM PLATTEVILLE FORMATION CARBONATES PROVIDE NEW INSIGHT INTO TERRESTRIAL ECOSYSTEMS

Modern peatlands dominated by peatmosses store a very large proportion of global soil carbon as peat, such long-term sequestration fostering global climate homeostasis. Peatmosses harbor methanotrophs and nitrogen fixers that play globally important roles in Earth’s biogeochemical cycles. Although fossils attributed to peatmosses have been reported from the Permian and Carboniferous, recent molecular clock analyses suggest that the peatmoss lineage is considerably older. Because certain Caradocian microfossils had been interpreted as the resistant remains of sporophytic capsule epidermis of peatmosses and certain late Ordovician fossil spores resemble those of peatmosses, we examined mid-Ordovician dolomites (Sinnipee Group), located in Dane Co., WI, dated at 455-460 Mya, for evidence of peatmosses. Acid macerations performed by different investigators on collections made 12 years apart yielded organic microfossil remains closely resembling portions of peatmoss leaves and stems. Morphometric analyses indicated that the two sets of samples likely represent the same plant. Co-occurring foraminifera resembled early-diverging types previously described from this formation; no indicators of modern contamination (e.g. pollen) were observed. Both plant and foraminifera remains exhibited low thermal maturity. Electron-dispersive X-ray spectroscopy of plant remains revealed mineral coatings associated with reduced degradation that are not typical for modern materials. The sample location was near a site from which the oldest known fossil glomalean fungi were reported, their presence suggesting a vegetated terrestrial source (Redeker et al. 2000, Science 289:1920), possibly the erosion-resistant quartzite Baraboo Hills, which may have been emergent through much of the Ordovician. The new remains, consistent with time-scale phylogenetic evidence for earliest appearance of mosses, add to accumulating evidence that bryophytes were important components of Ordovician landscapes and that peatlands similar in biogeochemical function to those of modern times may have existed since the mid-Ordovician, a much longer time period than previously thought.