No abstract

Global integrated biochemostratigraphic studies of Silurian conodont biostratigraphy and carbon isotope chemostratigraphy typically focus on the biogeochemical events of the Wenlock through Pridoli epochs (Ireviken, Mulde, Lau, and Klonk events). The global change events that took place during the Llandovery Epoch (early Aeronian, late Aeronian, and Valgu events) are poorly understood in comparison. One major contributing factor is the fact that Llandovery strata are often shale-dominated or replete with unconformities in carbonate-dominated successions. As a result, only a handful of stratigraphically complete carbon isotopic and conodont biostratigraphic data sets are presently available from this interval worldwide. The Knapp Creek Core, Johnson County, east-central Iowa, preserves 228 feet (69.5 m) of dolomitic Silurian strata from the East-Central Iowa Basin and contains the Blanding, Hopkinton, Scotch Grove, and Gower formations. Integrated high-resolution carbon isotope chemostratigraphy and conodont biostratigraphy produced here demonstrate the presence of the late Aeronian, early Telychian ʻValguʼ, and early Sheinwoodian ʻIre-vikenʼ positive carbon isotope excursions in the core. Biostratigraphically significant conodonts recovered from the core include Pseudolonchodina (formerly Aspelundia) fluegeli, Distomodus staurognathoides, Aulacognathus kuehni, Aulacognathus bullatus, Wurmiella? polinclinata polinclinata, Pterospathodus eopennatus, Pterospathodus amorphognathoides angulatus, Pterospathodus amorphognathoides amorphognathoides, and Kockelella ranuliformis. Biochemostratigraphic data from the core indicate that the late Aeronian to early Sheinwoodian interval is well represented in the Iowa stratigraphic succession, help to validate recent and ongoing revisions to the Welsh Basin and base Telychian GSSP, and demonstrate that the East-Central Iowa Basin is an important region that provides the opportunity to study Llandovery biogeochemical events in an expanded carbonate setting.

The numerical calibration of the base of the Devonian is poorly constrained due to several factors. Few precise radioisotopic age determinations are available from the late Silurian and Early Devonian, and the limited published data carry large error bars from older analytical methodologies. Volcanic ashfalls suitable for dating occur in the Lower Devonian of the Appalachian Basin, but have not been precisely correlated into the global chronostratigraphical scheme because of limited bio‐ and lithostratigraphical information. Here, we report a new U‐Pb zircon radioisotopic age determination of 417.61 ± 0.12(0.23)[0.50] Ma and improved chronostratigraphical context, including revised biostratigraphy, for an ash bed in the New Scotland Formation, Helderberg Group, from the Lochkovian Stage that was previously identified as the Kalkberg K‐bentonite. This new information helps to integrate the classic New York Appalachian Basin succession into global Siluro‐Devonian stratigraphy, refine the calibration of the Silurian–Devonian boundary and more precisely estimate the duration of both time periods.

X-ray diffraction data from Silurian conodonts belonging to various developmental stages of the species Dapsilodus obliquicostatus demonstrate changes in crystallography and degree of nanocrystallite ordering (mosaicity) in both lamellar crown tissue and white matter. The exclusive use of a single species in this study, combined with systematic testing of each element type at multiple locations, provided insight into microstructural and crystallographic differentiation between element type ( S a , S b - c , M ) as well as between juveniles and adults. A relative increase in the unit cell dimensions a / c ratio of nanocrystallites during growth was apparent in areas demonstrating single-crystal behaviour, but no such relationship was seen in dominantly polycrystalline areas. Systematic variations in mosaicity were identified, with mosaicity (as a proxy for disorder) increasing during growth, as well as along elements from tip to base. These results provide potential insight into the integrity of conodont apatite as a recorder of palaeoseawater chemistry, as well as demonstrate the need to consider the influence of ontogeny and element type on the use of conodonts in palaeothermometry and geochemical investigations.

The previously monotypic bathyurid trilobite Aponileus Hu, 1963, was known only from poorly preserved material of itstype species, A. latus, and was interpreted as a junior subjective synonym of Psephosthenaspis Whittington. New speciesfrom the upper Floian (Blackhillsian) Fillmore Formation of western Utah belong to Aponileus and help demonstrate thatthe genus is an entirely Lower Ordovician (upper Floian) clade phylogenetically separate from the Middle Ordovician(Dapingian) Psephosthenaspis. Species of either group have broadly similar morphology, but the species of Psephosthe-naspis which most closely resembles those of Aponileus is the youngest and most derived member, and the similarities areconvergent. Psephosthenaspis roots among a group of heavily tuberculate, mostly undescribed, upper Floian species, oneof which is briefly illustrated for comparison. The species most resembling some species of Aponileus, P. glabrior, is il-lustrated on the basis of new silicified material which extends its range from the upper Juab Formation into the base of theKanosh Formation. New Blackhillsian species are A. laikaae (Fillmore Formation; Presbynileus ibexensis Zone), A. aasei(Fillmore Formation; probably Pseudocybele paranasuta Zone), A. belkaae, A. strelkaae, and A. ugolekae (all Wah WahFormation; "Pseudocybele nasuta Zone"). Aponileus? veterokae n. sp., from high in the "Pseudocybele nasuta Zone" ofthe Wah Wah Formation, is tentatively assigned. Bolbocephalus glaber Poulsen, 1927, from the Nunatami Formation ofnorthwest Greenland, is poorly known but is also a member of Aponileus, and is similar in morphology to A. latus and A.aasei; it is revised on the basis of reillustrated type material. Phylogenetic analysis indicates that A. laikaae is the basalspecies, followed by a sister pair of A. belkaae and A. strelkaae. These are sister to a pair of subclades, the sister pair of A. ugolekae and A.? veterokae, and an effaced group lacking genal spines including A. aasei, A. latus, and A. glaber.