The benthic graptolite Sphenoecium mesocambricus (Öpik, 1933) from the Middle Cambrian of Krekling, Oslo Region, Norway

Specimens of the Middle Cambrian benthic graptolite Sphenoecium have been discovered at two levels in the Alum Shale Formation at Krekling, Oslo Region, Norway. The material originated from an interval in the Goniagnostus nathorsti Biozone (upper Drumian stage of the Cambrian System) and represents only the second record of this genus from Norway. The specimens show the tubarium development more clearly than the type material of Sphenoecium mesocambricus, which was found in the same general region. Sphenoecium mesocambricus has a tubarium with slender creeping tubes, branching at close distances, and considerably widening erect thecal tubes with straight apertures. A much more slender specimen belonging to the same genus was also found, but cannot be referred to a described species.


Introduction
The Middle Cambrian graptolite Sphenoecium mesocambricus (Öpik, 1933) is one of the oldest benthic graptolites ever documented (see Maletz, 2014;Maletz & Steiner, 2015).Öpik (1933) originally described the species from the Paradoxides Shale of Krekling, based on a single complete specimen and a number of additional small fragments.He clearly identified it as a graptolite and named it Dendrograptus mesocambricus, even though little detail was recognisable in the available material.Maletz & Steiner (2015) redescribed the specimen and recognised a phosphatic brachiopod in the centre as the point of attachment of the specimen.Öpik (1933) originally reported the specimen to be associated with Ptychagnostus punctuosus (Angelin, 1851) and referred the material to the Paradoxides davidis Zone.A reidentification of the trilobites demonstrated the presence of Peronopsis fallax (Linnarsson, 1869), Goniagnostus nathorsti (Brøgger, 1878) and Lejopyge sp. on the slab (Maletz & Steiner, 2015, p. 1088), indicating the Goniagnostus nathorsti Zone.
Additional material had never been collected from the region and the holotype remained the only available specimen of this species.Bengtson & Urbanek (1986) described some much better preserved material of a Middle Cambrian benthic graptolite from the Eccaparadoxides pinus Zone of Närke, Sweden as Rhabdotubus johanssoni.The authors demonstrated the presence of irregular fuselli on the erect and creeping tubes and of branching in the creeping tubes, thus verifying the graptolitic origin of the material.Maletz & Steiner (2015) synonymised Rhabdotubus johanssoni with Dendrograptus mesocambricus and referred the material to the genus Sphenoecium Chapman & Thomas, 1936.The authors were able to show a vague indication of fusellar development also in type material of Sphenoecium mesocambricus from Krekling.

Locality
Krekling in the Eiker-Sandsvaer area of the Oslo Region in southern Norway is well known through the investigation of the Cambrian succession and the description of biostratigraphically important agnostid trilobites from the Cambrian Alum Shale Formation by Brøgger (1878Brøgger ( , 1882a, b), b).Brøgger (1878, p. 32) indicated the presence of c. 28 m of black shales of the Middle Cambrian Paradoxides paradoxissimus and Paradoxides forchhammeri zones at Krekling and described a rich trilobite fauna.Some of the trilobite species were revised by Bruton (1999) and Høyberget & Bruton (2008), who reillustrated some of the original specimens of Brøgger (1878).Brøgger also described a small number of hyolithids, revised by Berg-Madsen & Malinky (1999) and Malinky & Berg-Madsen (1999).Outcrops are poor in overgrown ravines, and commonly consist of temporary exposures of few metres of section.Öpik (1933) collected a single specimen of a benthic graptolite, identified as Sphenoecium mesocambricus, from the Krekling area and illustrated the exact locality in a photograph.He stated that the locality is opposite to and a few tens of metres from the train station (Fig. 1).The locality is now a protected nature reserve.The additional material described herein was collected from two localities along the slope not far from Öpik's locality (Fig. 1).Brøgger (1882a) described additional outcrops in the same unit farther away from the train station, but it has proven difficult to retrace his steps and to identify the smaller outcrops.Moreover, postglacial alluvium and vegetation generally makes an interpretation of the relationship between these outcrops impossible.

Trilobite biostratigraphy
As there are very few records of dendroid graptolites from Cambrian rocks, and most of these are poorly dated (see Maletz, 2014), a precise investigation of the faunal record associated with early dendroid graptolites from the Cambrian System is essential to understand the origin and early evolution of this group.The trilobite Høyberget & Bruton (2008) described the agnostid succession from a number of sections in Norway and commented upon the section at Krekling described by Brøgger (1878).The authors illustrated the fossil succession from a drillcore at Stavlum, c. 2.5 km northeast of Brøgger's (1878) section, and redescribed many of the taxa of Brøgger (1878).They did not, however, revise the agnostid biostratigraphy of the Krekling locality.
The earliest agnostids in the Krekling succession belong to Ptychagnostus gibbus according to Brøgger (1878), indicating the Ptychagnostus gibbus zone, but the base is not seen in this succession.Agnostids from this zone and the Ptychagnostus atavus Zone were not found at Stavlum, but Høyberget & Bruton (2008, p. 8, fig. 4) reported Hypagnostus parvifrons and Ptychagnostus affinis, which they referred to the top of the Ptychagnostus atavus Zone.Hypagnostus parvifrons appears to be found in the upper Ptychagnostus atavus to Ptychagnostus punctuosus zone of Brøgger's succession (see Fig. 3).Høyberget & Bruton (2008) recognised the base of the Goniagnostus nathorsti Biozone at 84.93 m, but did not separate the Goniagnostus nathorsti and Lejopyge laevigata biozones at Stavlum.The top of the Lejopyge laevigata Biozone was indicated by the FAD of Agnostus pisiformis at 68.30 m The thickness fits nicely with the succession shown by Brøgger (1878, table 1), in which the FAD of Goniagnostus nathorsti is about 10 m above the base of the Alum Shale Formation.The appearance and range of Lejopyge laevigata may be problematic, as Brøgger (1878) lumped a number of taxa under this name (see Høyberget & Bruton, 2008).Thus, the base biostratigraphy of the Middle Cambrian Series 3 (Fig. 2) is most precisely defined by the FADs of a number of agnostids; these have commonly been used to define chronostratigraphic units in the Cambrian (e.g., Peng & Robison, 2000;Peng et al., 2004Peng et al., , 2009aPeng et al., , b, 2012Peng et al., , 2014;;Babcock et al., 2005Babcock et al., , 2007Babcock et al., , 2015)).Fortunately, Cambrian agnostids are often abundant in shaly environments and can be associated with fossil graptolites.Thus, they can be used for precise dating of early graptolitiferous successions.Due to their wide distribution, these trilobites can also generally be used for intercontinental correlations (see Ahlberg, 2003;Babcock et al., 2005;Peng & Babcock, 2011;Peng et al., 2012).
of the Lejopyge laevigata zone is uncertain.According to Høyberget & Bruton (2008, p. 8), a more precise biozonation is not possible, as the flattened trilobites in the Alum Shale Formation commonly do not show any diagnostic features.
The type specimen of Sphenoecium mesocambricus is associated with a number of trilobites, based on which Maletz & Steiner (2015, fig. 11) referred the specimen to the Goniagnostus nathorsti Zone.The specimen is here referred to the interval of the upper Drumian, or the Goniagnostus nathorsti Biozone.Öpik (1933) originally identified the most common trilobites as Agnostus punctuosus, thus referring the material to an older biozone.A reidentification of the specimens refers the material to Goniagnostus nathorsti (Brögger, 1878), Peronopsis ferox (Tullberg, 1880) and Lejopyge calva Robison, 1964.According to the ranges provided by Høyberget & Bruton (2008), this association clearly identifies the Goniagnostus nathorsti Biozone.
The type material of Rhabdotubus johanssoni Bengtson & Urbanek (1986) originated from the Ptychagnostus praecurrens Biozone of Närke, Sweden (Fig. 2).This biozone is not recorded from the Krekling locality.Bengtson & Urbanek (1986, p. 294) did not discuss in detail the associated trilobite fauna, but mentioned Acado paradoxides (Baltoparadoxides) ex.gr.oelandicus (Sjögren, 1872) and Ellipsocephalus polytomus Linnarsson, 1877 as the most common species.Laibl et al. (2015) recently described Ellipsocephalus polytomus and indicated its occurrence in the provisional Cambrian Stage 5, the Acadoparadoxides pinus Zone in the polymerid zonation and the Ptychagnostus praecurrens Zone in the agnostid zonation of the Acadoparadoxides (Baltoparadoxides) oelandicus Superzone.Thus, the material of Rhabdotubus johanssoni is clearly older than the Krekling specimens.from which the growth of the colony started is visible.The type specimen of Sphenoecium mesocambricus was found to be attached to a phosphatic brachiopod (Fig. 3C), as are many specimens described by Bengtson & Urbanek (1986) as Rhabdotubus johanssoni.The holotype consists of numerous radiating thecal tubes, but interconnections between the tubes are not recognisable in the specimen, even though indications of branching of the tubes are visible.The specimen bears at least 30-35 thecal tubes that are apparently connected at the brachiopod shell in the centre (Fig. 3C).The new specimens clearly show the branching of the creeping, repent tubes which confirm the assignment of the taxon to the colonial Pterobranchia.
The new specimens are similar in their development and size to Sphenoecium fillicoides (Chapman, 1917) from the Cambrian of Victoria, Australia, but differences can be seen in the dimensions of the tubaria.The age of the Australian material was only indirectly established when Thomas & Singleton (1956) correlated the Heathcote fauna with the Ptychagnostus gibbus zone.As only a few specimens of Sphenoecium species have been collected, it is impossible at the moment to estimate the intraspecific variation of these taxa.Therefore, all the previously described taxa (see Maletz & Steiner, 2015, for an overview) are herein regarded as separate and valid taxa.

Associated fauna.
The specimens are preserved in a fossil-rich, bituminous, black shale in which other fossils are not uncommon, but generally are fragmentary where they occur.Remains of trilobite exuviae are the most common and a small number of agnostid trilobites have been identified in association with the pterobranch remains.They belong to Diplagnostus planicauda (Angelin, 1851) and Valenagnostus marginatus (Brøgger, 1878).Diplagnostus planicauda was not mentioned or illustrated by Brøgger (1878) and was first described and illustrated from Norway by Høyberget & Bruton (2008).A single, poorly preserved complete specimen (Fig. 4A) and a second pygidium (Fig. 4B-C: counterparts) was found.Valenagnostus marginatus is present as a single pygidium (Fig. 4E).

Description.
A single laterally preserved specimen without counterpart (Fig. 3D) is available from Krekling.Branching in this specimen appears to be irregularly developed, but this may be due to the unfortunate preservation in which the thecae overlap considerably and many parts are covered.The colony is about 7 mm

The new material
Three new specimens of the pterobranch genus Sphenoecium Chapman & Thomas, 1936, were collected from the Middle Cambrian Goniagnostus nathorsti Biozone of the Alum Shale Formation of Krekling.The material can be referred to two different taxa on the basis of the dimensions of their thecal tubes.
Description.Both colonies are small and completely flattened.Part of the silvery-shining, somewhat thermally altered fusellum is preserved (see Maletz & Steiner, 2015), but fusellar construction is not visible.The colonies are up to 13 mm in diameter and show a circular form, which may be based on the small size and the original attachment point.Three or four tubes appear to originate from a common point and branch further at close distances of less than 1 mm.The tubes are initially about 0.3-0.4mm wide and an increase in tube width is not seen in the encrusting, creeping parts.Individual erect tubes slowly widen to about 1.2-1.3mm at the aperture.The apertures are straight and simple.None of the observed tubes is more than 5 mm long, but it cannot be ruled out that the tubes are incomplete or immature.The largest specimen bears nine tubes of various lengths.
Remarks.The original attachment of the colonies cannot be estimated, as neither of the two new specimens show any indication of attachment, but still a common point wide and 5-6 mm high, but measurements are difficult due to the poor preservation.The individual thecal tubes are about 0.4-0.5 mm wide and more than 3 mm long.There are c.16-20 thecal tubes visible, but most of them are partly covered by others and the exact number is impossible to tell.
Acknowledgements.Agnes Lyche Melvaer, Kirsten Lyche Baastad and Maximo Alfonso Rojo are thanked for their hospitality in providing accommodation to HMW during the geological field trips to Norway in 2013 and 2014.HMW acknowledges the assistance and company of Erik Brett Jacobsen, Jens Koppka, Maximo Alfonso Rojo and Roger Smeets in the field.Roger Smeets and Maximo Alfonso Rojo collected and donated the specimens to the authors.D. L. Bruton (PMO Oslo, Norway) is acknowledged for his encouragement to HMW to continue the exploration of the Cambrian succession in the Krekling area.JM acknowledges the support of F.-J. Lindemann (PMO, Oslo, Norway) in obtaining type and other material for comparison from the Natural History Museum, Oslo, Norway.Per Ahlberg (Lund, Sweden) and Magne Høyberget (Mandal, Norway) provided additional information on agnostid biostratigraphy and species identifications.The manuscript was improved considerably through the reviews of Magne Høyberget and Lucy Muir.

Figure 1 .
Figure 1.Map of the Krekling area and the locations of the investigated outcrops.Map based on http://geo.ngu.no/kart/berggrunn/.