Abstract Eunicidan polychaetes formed a significant part of Early Palaeozoic marine invertebrate communities, as shown by the abundance and diversity of scolecodonts (polychaete jaws) in the fossil record. In this study we summarize the early radiation and biodiversity trends and discuss the palaeobiogeography of these fossils. The oldest (latest Cambrian–Early Ordovician) representatives had primitive, usually symmetrical, placognath/ctenognath type jaw apparatuses. The first more advanced taxa, possessing labidognath-type jaw apparatuses or placognath apparatuses with compound maxillae, are first recorded in the Middle Ordovician. The most significant increase in generic diversity occurred in the Darriwilian, when many common taxa appeared and diversified. The Ordovician and Silurian scolecodont occurrences allow some palaeobiogeographical units and distribution patterns to be explored and outlined. The most robust data presently at hand derive from successions in Baltica and Laurentia. That information, together with new records from other palaeocontinents, reveals a wide distribution for the most frequent and species-rich genera and families, similar to the biogeographical patterns of extant polychaetes. Like many other benthic and pelagic fossil groups, scolecodont-bearing polychaetes show an increased cosmopolitan character in the Silurian as compared with the Ordovician. Species-level endemism appears to be relatively common, inferring a potential for scolecodonts as biogeographical tools in the future.

Abstract Microbially induced sedimentary structures (MISS) in siliciclastic shallow-marine strata occur in the lower and middle Cambrian and the Silurian of southern Sweden. These are typically transparent wrinkle structures with a wide range of morphologies. They are exclusívely associated with shoreface to lower shoreface environments, characterized by fine-grained sandstone interbedded with mudstone and a Cruziana ichnofacies. Thicker, non-transparent forms with high-relief crinkled surfaces occur in the same paleoenvironments. The landward sand-dominated facies belt with Skolithos ichnofacies (upper shoreface–foreshore) lack preserved wrinkle structures. Evidently, wrinkle structures are more common than previously thought in the Swedish Paleozoic and provide an important tool for understanding paleoenvironments and benthic paleoecology in strata largely devoid of body and trace fossils.