Thomas Weidner

Upper Cambrian (Furongian) biostratigraphy in Scandinavia revisited: definition of superzones

The traditional Furongian trilobite biozones of Scandinavia, recently proposed abandoned due to inconsistent boundary definitions [Terfelt, F., Eriksson, M.E., Ahlberg, P. & Babcock, L.E., 2008: Furongian Series (Cambrian) biostratigraphy of Scandinavia – a revision. Norwegian Journal of Geology88, 73–87], are resurrected and elevated to superzonal rank. These superzones are usually readily recognized in the field, even by non-specialists, and for general correlation and mapping the more wide-ranging biozones are considerably more practicable than the very detailed zonation (formerly subzonation) introduced by Terfelt et al. (2008). Formal definition of the superzones is outlined including designation of stratotype sections. The superzones are each defined by the FAD of a characterizing species group and upwards delimited by the base of the succeeding superzone. The long used Olenus, Parabolina and Leptoplastus (super)zones as well as the recently introduced Acerocarina Superzone ( = Acerocare Zone of older literature) are maintained and formalized. The Protopeltura praecursor, Peltura minor and Peltura scarabaeoides zones are abandoned and replaced by two new units, named the Protopeltura and Peltura superzones, respectively. Accordingly, all Furongian superzones have a uniform naming style referring to a characteristic genus. The six Furongian superzones currently comprise 27 trilobite zones. The Paradoxides forchhammeri Superzone (Cambrian Series 3) is extended upwards to the base of the Olenus Superzone, thereby formally including the Agnostus pisiformis Zone.

A highly diverse trilobite fauna with Avalonian affinities from the Middle Cambrian Acidusus atavus Zone (Drumian Stage) of Bornholm, Denmark

A newly collected trilobite fauna from the lowermost part of the Alum Shale Formation at Øleå, Bornholm, Denmark, demonstrates the presence of a thin but richly fossiliferous Middle Cambrian Acidusus atavus Zone (Paradoxides paradoxissimus Superzone). The lower part of the zone (the Tomagnostus fissus–Ptychagnostus atavus Zone of older Scandinavian literature) is represented by a discontinuous limestone bed, up to 20 cm thick, whereas the upper part (Hypagnostus parvifrons Zone of older literature) is represented by shale, less than 80 cm thick, containing lenses of bituminous limestone. A total of 39 agnostid and 21 polymerid species are recorded from the A. atavus Zone (including 12 taxa treated under open nomenclature). One additional polymerid species collected from an ice-rafted boulder derived from the Bornholm area is also treated. The fauna is described and illustrated, with designation and reillustration of relevant lectotypes. The abundance of polymerid trilobites in comparison with nearby Scania, southern Sweden, where 25 agnostid and nine polymerid species have been reported from equivalent strata, is indicative of a less dysoxic environment in the Bornholm area, which was probably uplifted. The trilobite fauna resembles the coeval assemblages described from England, Wales, eastern Newfoundland and eastern Siberia, sharing several species not reported previously from Baltica, including Anopolenus sp., Agraulos longicephalus, Bailiella ornata, Clarella impar, Solenopleura? applanata, Acadagnostus aff. bulkurensis and Phalagnostus ovalis. A few faunal elements originally described from Bohemia and Australia are also present, viz. Hydrocephalus aff. carens, Phalagnostus nudus, Skryjagnostus pompeckji, Hypagnostus aff. clipeus, Euagnostus aff. interstrictus and Euagnostus? aff. glandifer.

The late Cambrian (Furongian) Acerocarina Superzone (new name) on Kinnekulle, Västergötland, Sweden

The traditional Furongian biozones of Scandinavia, based on olenid trilobites, are proposed elevated to superzonal rank. For the time being they are treated informally, except that the designation Acerocarina Superzone is proposed as replacement name for the uppermost superzone, formerly referred to as the Acerocare Zone. It comprises four zones. The lowest of these zones, hitherto referred to as the Peltura transiens Zone, is renamed as the Acerocarina granulata Zone. The distribution of the Acerocarina Superzone in Baltoscandia is mapped. A newly discovered 1.4 m thick succession on Kinnekulle, Västergötland, yielded Parabolina heres heres, P. transiens, Peltura costata, Peltura scarabaeoides westergaardi and Sphaerophthalmus alatus. In addition, A. granulata, Pelturina punctifera and Leptoplastides? sp. were collected from loose boulders deriving from the same interval. This is the first unambiguous record of P. costata and P. punctifera in Sweden. The fossil assemblage, assigned to the P. costata Zone, contains species hitherto considered characteristic of five different biozones, viz. the Ctenopyge tumida Zone, the Parabolina lobata Zone, the P. transiens Zone, the P. costata Zone and the Westergaardia scanica Zone. P. transiens, P. scarabaeoides westergaardi and S. alatus are assumed to have been reworked, whereas P. punctifera appears one zone earlier than in Norway. Parabolina (Parabolina) heres lata, not found during this study, may turn up in the P. costata Zone or even earlier. The Acerocarina Superzone is resting on the P. lobata Zone, and the intervening zone, characterized by Peltura paradoxa (uppermost “Peltura scarabaeoides” superzone), is missing in the succession on Kinnekulle.

A paradoxidid moult ensemble from the Cambrian of Sweden

A mass aggregation of 148 paradoxidid trilobites and associated specimens of the agnostoid Pentagnostus praecurrens is preserved on a surface of a split orsten lens from the Middle Cambrian Series 3 Acadoparadoxides pinus–P. praecurrens Zone in Jämtland, Sweden. Most specimens are complete or nearly complete, lying parallel to the sediment surface and seem unaffected by currents or sorting. The association is interpreted as a moult ensemble. Paradoxidid specimens are represented by two taxa, identified as paradoxidid sp. 1 (n = 28 specimens) and Eccaparadoxides sp. 2 (n = 45 specimens). The species are preserved both dorsum up and dorsum down, in about equal number, which may reflect a natural tendency to moult in either posture. They probably moulted by opening the cephalic sutures along the wide rostrum. The dorsal sutures are invariably open and the librigenae are very often displaced, commonly backwards in relation to the cranidium, but not symmetrically in relation to the axial shield. The glabella of the paradoxidids is often crushed, so that sometimes the underlying hypostome is outlined. Very few examples show the joint between the cranidium and the trunk being broken. Facies interpretation suggests deposition below storm wave base. Rapid burial, possibly by blanketing from hypopycnal flows followed by an extended period of slow sediment input (Type 1 facies of Brett et al. 2012) may explain the unusual preservation. The animals may have lived in an ex-aerobic environment, but evidence to support this is at the moment insubstantial.

The Furongian trilobite Irvingella from Jämtland and Ångermanland, Sweden

New material of Irvingella from central Sweden includes a cranidium of Irvingella major which is associated with fragmentary olenid trilobites that represent the Parabolina brevispina Subzone of the P. spinulosa Zone. This affords a direct correlation between the olenid trilobite zonation of Baltica and the Irvingella major Zone at the base of the Sunwaptan Stage in Laurentia. Several topotypic cranidia of Irvingella suecica Westergård confirm the features of that species but do not provide a direct indication of its position within the olenid zonal succession.

A paradoxidid–agnostoid fauna from the mid-Cambrian (Stage 5) of the Caledonian Lower Allochthon on Tåsjöberget, Ångermanland, Sweden

We describe faunas, formerly very little known, from allochthonous mid-Cambrian strata in the Fjällbränna Formation (Tåsjön Group) in Ångermanland that are equivalent to the “Paradoxides oelandicus Beds” of the autochthon. Paradoxidid trilobites are by far the commonest forms but are represented only by dissociated sclerites; 7–10 taxa appear to be present, but most of them are so incomplete that they cannot be identified with certainty. They are accompanied by rare agnostoids of two taxa, Pentagnostus praecurrens and Acadagnostus acadicus. The assemblages lack eodiscoid or other polymerid trilobites, molluscs and brachiopods, and in this respect contrast with shallower-water shelf faunas in the “Oelandicus Beds” of the Swedish autochthon, but bear comparison with shelf-edge or slope faunas from Novaya Zemlya (Arctic Russia), the Moesian Platform (Romania) and South Carolina (USA).

Tomagnostus sibiricus Pokrovskaya & Egorova, 1972 (Trilobita) from the Middle Cambrian Exsulans Limestone of Scania, Sweden

Species assigned to the Middle Cambrian agnostid genus Tomagnostus are briefly reviewed and a large number of Tomagnostus sibiricus found in Scania, Sweden, are described. The material derives from the Triplagnostus gibbus Zone in the Exsulans Limestone. Several morphological features support an assignment of the species to Tomagnostus: occasional scrobiculation of the cephalon, half of the cephala have an incipient frontal sulcus, pygidia have well-developed F2 furrows, well-defined transverse depression on posteroaxis and occasionally minute marginal spines or swellings. One pygidium of the biostratigraphically important Pentagnostus praecurrens and one pygidium of the rare Onymagnostus seminula are also described from the Exsulans Limestone of Brantevik, Scania. The new findings expand the stratigraphic ranges of these species in Scandinavia.