Petra Tonarová

Ordovician and Silurian polychaete diversity and biogeography

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.

Environmental changes close to the Lower–Middle Devonian boundary; the Basal Choteč Event in the Prague Basin (Czech Republic)

The Basal Choteč or jugleri Event, close above the Lower–Middle Devonian boundary, has been regarded as a minor but important eustatic transgressive event, which is characterized by significant environmental changes, faunal extinction, appearance of new forms, and maximum radiation, particularly among the goniatites. This study contributes to a more precise stratigraphic allocation of the event, and provides a reconstruction of paleoenvironmental settings in the type area of the event, the Prague Basin (Czech Republic). The beginning of a transgression is recorded already in the Třebotov Limestone (partitus Zone, Eifelian, early Middle Devonian). The basin-wide change in the sedimentation pattern (onset of peloidal and crinoidal grainstones (calciturbidites) of the Choteč Formation) corresponding to the uppermost partitus and costatus conodont zones, base of Nowakia (Dmitriella) sulcatasulcata dacryoconarid Zone, and Pinacitesjugleri goniatite Zone is interpreted here to be linked to a maximum flooding of the basin. A hypothesis of enhanced nutrient load during sedimentation of the Choteč Formation is suggested here as a triggering mechanism for intense micritization and peloid formation and prasinophyte blooms, which could be, along with a greater depositional depth, responsible for oxygen deficiency and consequent reduction of diversity and habitat tracking among benthic invertebrates.

Palynology, microfacies and biostratigraphy across the Daleje Event (Lower Devonian, lower to upper Emsian): new insights from the offshore facies of the Prague Basin, Czech Republic

The Zlíchovian/Dalejan boundary interval (Emsian, Lower Devonian) of the Pekárek Mill section was studied employing biostratigraphy (dacryoconarid tentaculites, conodonts) and palynology (chitinozoans, prasinophytes, scolecodonts) and microfacies analysis in order to shed more light on the timing and characteristics of the Daleje Event. The results of our study stress the great importance of the base of the Nowakia elegans Zone for the substage level division of the Emsian. Onset of the Daleje transgression is linked with higher terrigenous input, and coinciding changes in the chitinozoan assemblages were recorded at this level. The transgression at the base of the N. elegans Zone preceded the main transgression taking place in the N. cancellata Zone; it can be correlated with the Upper Zlíchov Event. For the first time, Emsian chitinozoans and a jawed polychaete fauna are described in detail from the Prague Basin and can be correlated with other northern Gondwanan regions. The family-level composition of scolecodont assemblage confirms the dominance of paulinitids in the peri-Gondwanan realm.

Darriwilian (Middle Ordovician) worms of southern Sweden

Abstract The record of scolecodonts (polychaete jaws) from the Ordovician of Sweden is very poor. In this paper, we document a Darriwilian (Middle Ordovician) assemblage recovered from palynological samples from the “orthoceratite limestone” (Lanna and Holen limestones) of Mount Kinnekulle, Västergötland, southern Sweden. The collection of diminutive specimens forms an assemblage taxonomically composed mainly of simple placognath (mochtyellids, xanioprionids) and ctenognath (tetraprionids) taxa, whereas labidognaths (polychaetaspids) and taxa with other evolutionary grade-type apparatuses are very rare or absent. In addition, putative priapulid (penis worm) teeth were identified, possibly representing the first fossil representatives recorded in Sweden. The highest scolecodont abundance coincides with the lower to middle part of the “Täljsten” interval (lower Kunda Baltoscandian Stage). These strata are interpreted as having been formed during a marked regressional phase, suggesting that the palaeobathymetry and/or bottom substrate was optimal for polychaete colonization at that time. This new assemblage from Kinnekulle adds to the global scolecodont record in which data on Middle Ordovician and older specimens are still rudimentary but of importance for understanding early polychaete phylogeny.

Impact of the Silurian Ireviken Event on polychaete faunas: new insights from the Viki drill core, western Estonia

The impact of the Llandovery–Wenlock Ireviken Event (IE) on the jawed polychaete faunas is explored in this study, based on new data on scolecodonts from the Viki drill core of western Estonia. A distinct faunal reorganisation is observed, with the most abrupt changes recorded between datum points 4 and 6 of the IE, which coincides with the major turnover interval of other fossil groups. Based on what is now known, polychaetaspids and mochtyellids suffered most severely during the event, whereas the members of other families seem to have been less affected.