Early land plant spores from the Paleozoic of Sweden – taxonomy, stratigraphy and paleoenvironments

Abstract: The Silurian through earliest Devonian, between ca 444 and 412 million years ago, was characterized by significant changes in global climate and environment. At that time, plants and animals had begun to expand the colonization of previous relatively desolate terrestrial landscape. Macrofossils of land plants are very rare from this period because these early land plants lacked large and robust tissues that could be easily fossilized. The spores produced by these early land plants are, therefore, an important tool for resolving the establishment of the early vegetation on Earth. Spore walls are composed of sporopollenin, a complex mix of biopolymers incorporating long chains of fatty acids, phenolics, phenylpropanoids and traces of carotenoids that are, collectively, very resistant to desiccation, pressure and high temperatures. These properties make spores easily preserved as fossils in the sedimentary deposits. Fossil spores can be used as a tool to: study diversity of the terrestrial vegetation, date and correlate sedimentary successions (biostratigraphy), make paleoenvironmental and paleoclimatic interpretations and, finally, assess the potential for hydrocarbon (gas, oil and coal) generation in the sediments. During the Silurian, the paleocontinent Baltica, of which Sweden was a part, was covered by shallow seas. However, the study area in central and southern Skåne was not too distant from the coast. The sediments here were deposited in foreshore to shoreface environments with water depths no more than a few tens of meters, and in some areas, represented by Lower Devonian successions, possibly even in non-marine environments such as rivers and lakes. Spores from the early land plants were transported by river water into the shallow sea and sank to the bottom as they lost buoyancy. Today these spores from early land plants are preserved in the mid-Paleozoic sedimentary rocks exposed in present day Skåne and on Gotland. In this project, sedimentary rock samples obtained from drill cores and outcrop at several localities in Skåne and Gotland, were processed by acid dissolution techniques to recover assemblages of organic-walled palynomorphs – mainly plant spores, detrital organic material, and marine palynomorphs. These palynological assemblages were studied by light microscopy to determine the diversity of spore taxa through several short stratigraphic intervals at various sites. Many of the bore cores were drilled by the Swedish Geological Survey during the 1960s and have never been investigated for their fossil spore content. Overall, there have been very few studies of spores from the Silurian of the Baltic region and the knowledge about the composition of the early land floras from this part of the world is sparse. In this thesis, rich and well-preserved assemblages of early land plant spores are described from both drill core and outcrop samples from Skåne. Both cryptospores (produced by the earliest land plants) and trilete spores (produced by vascular plants) have been recovered. In total, 66 species of both cryptospores and trilete spores were identified in the upper Silurian and Lower Devonian successions of Skåne. These results reveal a rather diverse early terrestrial flora previously unrecognized in the region. Further, a revised stratigraphy for the Öved Sandstone Formation is presented as the c. upper 100 metres of the succession is based on palynostratigraphy of this study shown to represent Devonian strata. From the Burgsvik Formation, on Gotland, sediments coeval to the studied sections in Skåne, also yielded spores from early land plants. In these beds, the earliest land plant fossil with in situ cryptospores from Baltica was discovered. These remains consist of spore masses with part of the sporangium still preserved, a possible land plant axis, and also more poorly preserved spore masses interpreted to be coprolites (fossil feaces) from arthropods. This last category represents one of the earliest examples of plant-animal interactions globally. Further, this study reports the oldest evidence of land plants in Sweden. This is in the form of spores identified in Upper Ordovician sedimentary successions in drill core material from Röstånga, Skåne.