Olle Hints

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.

Acritarchs from the Ordovician–Silurian boundary beds of the Valga-10 drill core, southern Estonia (Baltica) and their stratigraphical and palaeobiogeographical implications

Fourteen samples of the Valga-10 drill core, south Estonia, from the lower Jelgava Formation (middle Pirgu Regional Stage, Upper Katian) to the lowermost Õhne Formation (lowermost Juuru Regional Stage, Lower Rhuddanian) were investigated for acritarchs. The section is biostratigraphically and chemostratigraphically well constrained; it comprises the rugata, taugourdeaui and scabra chitinozoan zones, the ordovicicus and giradeauensis conodont zones and the Hirnantian Isotopic Carbon Excursion (HICE). The good preservation allowed the identification of three prasinophyte phycomata and 52 acritarch species including the four new species Evittia porkuniensis, Helosphaeridium tongiorgii, Nexosarium leherissei and ?Veryhachium bulliferum. One new combination is proposed: Poikilofusa obliquipunctata (Uutela & Tynni 1991) comb. nov. Comparison with contemporaneaous palynofloras shows that eastern Laurentia and Baltica share a high number of species during the latest Katian—Hirnantian. Some of these species show a potential for future improvement of biostratigraphical correlation between the uppermost Katian— Hirnantian strata of low to mid-latitude carbonate platforms in eastern Laurentia and Baltica. Conversely, significant taxonomic differences exist between the assemblage studied and typical Gondwanan palynofloras. These results suggest that the Laurentian/Baltic and Gonwanan phytoplanktonic palaeoprovinces existed during latest Ordovician times. Published data reveal similar distribution pattern for chitinozoans and graptolites during the Hirnantian. A bathymetric ridge rise associated with the opening of the Rheic Ocean, coupled with the Hirnantian glacially-driven sea-level fall might have prevented water mass exchange and mixing/migration of phytoplankton between Gondwana and Laurentia/Baltica, thus being at the origin of the observed acritarch bioprovincialism. Additionally, distribution and diversity patterns of acritarchs are compared to those of other microfossils in the Valga-10 section. Near the base of the Hirnantian (Porkuni Regional Stage), benthic organisms (ostracods and scolecodonts) and phytoplankton (acritarchs) show increasing diversity with appearances of new taxa and new morphologies. Planktonic (chitinozoans) and nektonic (conodonts) organisms show a different pattern, with a decline in diversity during the earliest Hirnantian and a marked increase in the later part of the stage. Two alternative hyptotheses are proposed to explain these differences in diversification patterns: (1) the development of a shallower, proximal environment in the locality studied during the Hirnantian glaciation may have been more favourable to the diversification of benthonic organisms; (2) the planktonic and nektonic organisms suffered the effects of glaciation more than benthonic ones.

Two new polychaete families from the upper Ordovician of Estonia

Study of three upper Ordovician borehole sections from Estonia has revealed abundant and well-preserved scolecodonts representing more than 50 species of jaw-bearing eunicid polychaetes. In this paper, two monotypic families, Conjungaspidae and Tretoprionidae, are introduced, based on two new species (Conjungaspis minutus gen. et sp. nov. and Tretoprion astae gen. et sp. nov.). The most distinctive features of conjungaspids are the small, distally rounded and long-horned carriers, merged with large basal and laeobasal plates, and symmetrical jaw apparatus. Conjungaspids are supposedly a primitive group displaying common features with some placognaths and labidognaths. Tretoprionids are characterized by sub-transversely prolonged and strongly elevated scraper-like denticles in the anterior part of the posterior maxillae, unusual anterior maxillae composed of several weakly fused teeth, and occurrence of a hole or large incision in the outer face of the left posterior maxilla.

Ordovician polychaeturid polychaetes: Taxonomy, distribution and palaeoecology

The fossil polychaete family Polychaeturidae is considered as monogeneric and comprises four species of the genus Pteropelta. Pteropelta, originally established on isolated scolecodonts (the carriers), is revised and the apparatus-based Polychaetura is shown to be a junior synonym of Pteropelta. In addition to Pteropelta gladiata and Pteropelta kielanae, Pteropelta huberti sp. nov., and Pteropelta sp. A are herein described from the Upper Ordovician of Estonia and Sweden. Polychaeturids include some of the most common and characteristic scolecodont-bearing polychaetes in the Ordovician of Baltoscandia. They first appeared in the early Darriwilian (Mid Ordovician), flourished in the Late Ordovician and disappeared in the early Silurian. The distribution patterns of individual polychaeturid species infer regional biostratigraphical potential. Polychaeturids were geographically widespread during the Ordovician and have been recorded from at least three palaeocontinents.

Vagrant benthos (Annelida; Polychaeta) associated with Upper Ordovician carbonate mud-mounds of subsurface Gotland, Sweden

Micropalaeontological investigations of Upper Ordovician carbonate mud-mounds and enclosing strata of subsurface Gotland, Sweden, demonstrate that jaw-bearing polychaetes formed the most diverse faunal element associated with these build-ups. Although not present within the mound cores (intra-mound facies), scolecodonts, or polychaete jaws, occur abundantly immediately below and particularly above the mounds; the supra-mound facies also has the most diverse fossil assemblages. By contrast to the scolecodont distribution, the most diverse conodont faunas were recorded in the intra-mound facies. This reinforces the fact that scolecodont and conodont abundance and diversity numbers are commonly inverse to one another, suggesting that these metazoans occupied different niches and responded differently to taphonomical processes. The polychaete assemblage has no less than 27 species belonging to 12 genera, of which Oenonites, Mochtyella and Pistoprion are the most abundant. The assemblage has a characteristic Baltic signature and is similar in taxonomic composition to coeval ones from other areas of the Baltoscandian palaeobasin, such as that of present-day Estonia. A principal component analysis clusters the Gotland assemblage most closely to those recorded from shallow to transitional shelf environments of Estonia, indicatino that the mud-mounds were formed in such environments. (Less)

New data on Ordovician stable isotope record and conodont biostratigraphy from the Viki reference drill core, Saaremaa Island, western Estonia

The Viki drill core is one of the reference sections for the Estonian bedrock succession, characterizing strata of Mid Ordovician through early Silurian age in the eastern part of the Baltoscandian basin. In this article, we present the hitherto missing Ordovician stable carbon isotope curve and Middle Ordovician conodont biostratigraphy from the Viki core to complement the previous lithological, geochemical, geophysical and biostratigraphic studies. For the first time, the Hirnantian sulfur isotope excursion is reported from the eastern Baltic region, showing high amplitude and tight coupling with carbon cycling.

Reconstructing the environmental conditions around the Silurian Ireviken Event using the carbon isotope composition of bulk and palynomorph organic matter

The carbon isotope composition (δ13C) of bulk organic matter and two palynomorph groups (scolecodonts and chitinozoans) from the Llandovery-Wenlock strata of Gotland (E Sweden) are compared to gain knowledge about carbon cycling in the Silurian (sub)tropical shelf environment. The δ13C values of the palynomorphs are mostly lower than the δ13C values of the bulk organic matter, and the δ13C values of the benthic scolecodonts are lower than those of the planktonic chitinozoans. While the difference between bulk and palynomorph δ13C may be in part a function of trophic state, the lower values of the scolecodonts relative to those of chitinozoans, which are assumed to live in the well-mixed water column, might imply an infaunal mode of life for the polychaetes that carried the scolecodonts. Lower δ13C for the scolecodonts in the middle of the section may represent variations in primary marine productivity (supported by acritarch abundance data), oxidation of organic matter in the bottom waters, or genera effects. In general, however, trends between the three data sets are parallel, indicating similarities in the low frequency, environmentally forced controls. The δ13C data show a decreasing trend from the base of the section, up to a horizon well below the base of the Upper Visby Formation. At this level, and therefore probably several 10 kyr before the δ13C increase in the carbonates, the δ13C organic values increase by ~1‰. This perhaps is an expression of a changed composition of the bulk organic matter associated with the extinction events prior to the Llandovery-Wenlock boundary.

Paranomenclature and the rules of zoological nomenclature; with examples from fossil polychaete jaws (scolecodonts)

Names of zoological taxa are governed by a set of laws, the International Code of Zoological Nomenclature or ICZN. In contrast to taxonomy, where decisions are based on interpretations, the laws of nomenclature are explicit and must be obeyed. One of the consequences, as discussed herein, is that paranomenclature, that reflects parataxonomic classifications based on parts of organisms independently of other parts, is not recognized under the ICZN and thus cannot be applied to formally diagnosed taxa. In the case of jawed polychaete annelids, parataxonomic treatments of the jaw elements as well as ICZN-governed naming of the reconstructed apparatus has divided their fossil record into separate but biologically overlapping groups of element-based and apparatus-based taxa. The independent treatment of groups created in this way has the potential for much confusion, if their names are used in formal systematics. Other recommendations of ICZN for effective diagnosis and illustration should also be considered in proposing names within groups such as this, where paranomenclature has been customary in the past.

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.