Conodont biostratigraphy, taxonomy and palaeoecology in the Darriwilian (Middle Ordovician) of Baltoscandia - with focus on meteorite and extraterrestrial chromite-rich strata

Abstract: This thesis deals with conodont biostratigraphy and palaeoecology in the Darriwilian (Middle Ordovician) of Baltoscandia, with focus on limestone strata showing traces of the ~ 470 Ma L-chondrite parent body break-up. The evidence for this cosmic event includes abundant centimeter to decimeter-sized fossil meteorites and elevated levels of sediment-dispersed extraterrestrial L-type chromite found in a stratigraphical interval corresponding to the uppermost Lenodus antivariabilis, the L. variabilis, and the Yangtzeplacognathus crassus conodont biozones, and the lower Microzarkodina hagetiana Subzone of the Lenodus (Eoplacognathus?) pseudoplanus Zone. Whereas these fossil meteorites are found only on freshly-sawed limestone-slabs at the active Thorsberg quarry on Mount Kinnekulle, southern Sweden, the occurrence of chromite has been recorded from various localities in Sweden, as well as in Russia and south-central China. A high-resolution conodont biostratigraphy over the c. 2.5 m thick, chromite-rich interval at the abandoned Hällekis quarry, Kinnekulle, was established in order to improve correlation between chromite-yielding sequences. Since the meteorite/chromite-rich interval here directly associates with a conspicuous rock sequence known as the ‘Täljsten’ interval, these strata were also investigated for palaeoecological and environmental changes. This anomalous interval has previously been suggested to represent a relative sea-level fall, but it has also been speculated that its formation could have been linked to the L-chondrite parent body break-up. Hence, the nature of the ‘Täljsten’ needed to be further explored in terms of faunal dynamics and depositional setting. The conodont fauna demonstrated continuous faunal reorganizations throughout the sampled sequence, as did additional microfaunal components. Along with observed changes in microfacies, it could be verified that the chromite-rich interval coincides with a gradually progressing regression-transgression cycle. An interesting find of tiny Panderodus conodont elements, including a serrate arcuatiform element, was made in a bed deposited just prior to the inferred maximum lowstand of the ‘Täljsten’ interval. Such serrate elements have previously been referred to as ecophenotypes of non-serrate forms, but sometimes also as distinctive species or subspecies. The Hällekis specimens, described as Panderodus serratus hallekisensis, represent one of the earliest records of the genus and hence provided clues to the primitive morphology and suprageneric relationship of Panderodus. Due to an observed tendency for overlapping morphologies and inconsistencies in identifications, the shapes of dextral Pa-elements from the zonal indices L. variabilis, Y. crassus and L. (E?) pseudoplanus were quantitatively compared using landmark-based geometric morphometrics. Linear shape of the posterior and posterio-lateral denticle rows combined was considered to best reflect the overall shape of the element-type in question, and 23 landmarks were used for the analysis; 4 fixed and 19 sliders (semi-landmarks). The analyses highlighted large-scale shape and symmetry related differences between the three species, but also revealed some overlaps. It also became clear that the three taxa display notable shape-variation of the dextral Pa element, a matter only looked into closer for L. variabilis. The Y. crassus elements included in the analysis suggest that the Baltoscandian and Chinese forms cannot be regarded as equivalents. The contingencies regarding the zonal indices naturally influence, e.g., the potential for a high-resolution correlation of chromite distribution between Kinnekulle and coeval sedimentary sequences at other localities. It was recently shown that the >1000km2 Osmussaar Breccia in north-western Estonia contains non-reworked L-type chromite, suggesting a connection to an L-chondritic impactor. An assessment of the conodont fauna of the limestones above the brecciate layers indicates that the formation of the breccia concurred with the deposition of the meteorite/chromite-yielding strata in Sweden. The limestone bed immediately superposing the breccia could be brought to the Microzakodina ozarkodella Subzone of the L. (E?) pseudoplanus Zone, i.e. only slightly higher stratigraphically than the recognized upper limit for high abundances of extraterrestrial material. This stratigraphical divergence as compared to the Swedish interval can be explained by a period of non-deposition and a resulting hiatus, a conceivable scenario considering the numerous sedimentation-gaps characterizing the early Darriwilian of north-western Estonia.