An Orthothecid Hyolith with a Digestive Tract from the Early Cambrian of Bornholm, Denmark

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An orthothecid hyolith with a digestive tract from the early Cambrian of Bornholm, Denmark

Vivianne Berg-Madsen, Martin Valent & Jan Ove R. Ebbestad

To cite this article: Vivianne Berg-Madsen, Martin Valent & Jan Ove R. Ebbestad (2018) An orthothecid hyolith with a digestive tract from the early Cambrian of Bornholm, Denmark, GFF, 140:1, 25-37, DOI: 10.1080/11035897.2018.1432680 To link to this article: https://doi.org/10.1080/11035897.2018.1432680

Published online: 06 Apr 2018.

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Full Terms & Conditions of access and use can be found at http://www.tandfonline.com/action/journalInformation?journalCode=sgff20 GFF, 2018 VOL. 140, NO. 1, 25–37 https://doi.org/10.1080/11035897.2018.1432680

OPEN ACCESS An orthothecid hyolith with a digestive tract from the early Cambrian of Bornholm, Denmark

Vivianne Berg-Madsena, Martin Valentb and Jan Ove R. Ebbestada

aMuseum of Evolution, Palaeontology, Uppsala, Sweden; bDepartment of Palaeontology, National Museum, Cirkusová, Horní Počernice, Czech Republic

ABSTRACT ARTICLE HISTORY The hyolith assemblage from the early Cambrian of Bornholm, Denmark, shows a higher diversity than Received 7 September 2017 contemporary assemblages in Baltoscandia. The most common species in the Green Shales (Læså Accepted 22 January 2018 Formation, Norretorp Member, Cambrian Stage 3), is Hyolithes [=Hyolithus] (Orthotheca) johnstrupi Holm, KEYWORDS 1893. A specimen of this species shows a well-preserved and almost complete digestive tract, folded into Orthotheca johnstrupi an approximately 22 mm long chevron-like structure comprised of at least 20 arcuate loops on the ventral revised; digestive tract; side and a flattened, gently sinuous to straight anal tube on the dorsal side. The thin, phosphatic outer early Cambrian; Bornholm; shell layer of the conch is crushed under the digestive tract due to compaction while the digestive tract Denmark is preserved in three dimensions and appears undisturbed. The shape of the digestive tract is similar to that of the middle Cambrian Guduguwan hardmani (Etheridge) from Australia and the lower Cambrian specimens from Russia described by Meškova & Sysoev. The Danish specimen is probably an adult, lending support to the idea that the orthothecid digestive tract becomes more complex during ontogeny. Hyolithus (Orthotheca) johnstrupi is revised and here referred to Circotheca Sysoev, 1958.

Introduction Poulsen (1967) described further hyolith species from the Green Shales and confirmed that Hyolithus (Orthotheca) john- The hyolith assemblage from the early Cambrian of Bornholm, strupi was one of the dominant macro-fossils in the siltstone. Denmark, shows a higher diversity and contains better pre- Although he no doubt examined the specimen with the digestive served specimens compared to those of equivalent levels in tract preserved, it escaped his notice and he stated that “none Sweden. Three hyolith species from the lower Cambrian Green were better preserved than those described by Holm, for which Shales (Læså Formation, Norretorp Member, Cambrian Stage reason nothing new could be added” (Poulsen 1967, pp. 44–46). 3) of Bornholm, Denmark (Figs. 1 and 2) were included in The collections at the Geological Museum, Copenhagen, were the taxonomic study of Swedish Cambrian hyoliths by Holm examined in connection with the 1999 revision. A specimen, col- (1893): Hyolithus lenticularis, H. (Orthotheca) johnstrupi and H. lected at Læså near Vejrmøllegård, which revealed part of a diges- nathorsti. Re-examination of hyolith material from the Green tive tract was discovered by J.M. Malinky, who also confirmed Shales by V. Berg-Madsen confirms the presence of the hyolith- the identification as H. (Orthotheca) johnstrupi. The specimen ids Hyolithus nathorsti Johnstrup in Holm, 1893, Lovenedolithes was collected by K.A. Grönwall in 1892 who noted on the label groenwalli (Poulsen, 1967) including fragments of helens that it was a ‘specimen with an unusual structure’. Although this (see also Martí Mus & Bergström 2007) and an operculum, and was probably the earliest find of an orthothecid digestive tract a single specimen of Hexitheca teretiuscula (Linnarsson, 1877). its significance escaped notice. Thus, the first description of a Holm (1893) stated that H. (Orthotheca) johnstrupi was by far fossilized hyolith digestive tract became that of Thoral (1935) the most common species at the locality near Vejrmøllegård from the Lower Ordovician of France. (Fig. 1), and a great number of specimens were found in phos- Preserved soft parts in hyoliths are rare and only about a phoritic nodules, including 11 opercula. Through the courtesy dozen reports of are known with material from the Cambrian, of Professor J.F. Johnstrup in Copenhagen Holm had had access Ordovician and Devonian (see Devaere et al. 2014 and Moysiuk to all this material and also collected in situ himself. et al. 2017 for references). The soft part morphology is impor- Although Holm (1893, p. 56) stated most specimens were tant to distinguish between the order Hyolithida and the order found in phosphoritic nodules, only one of the here described Orthothecida. The orthothecid digestive tracts are the best docu- specimens figured by Holm comes from a nodule, all others mented and is complex and generally sinuously folded, while it is from the siltstone proper. Specimens supposedly deposited at the simple and U-shaped in the hyolithids, which may reflect differ- Geological Survey of Sweden (SGU) according to Holm (1893, ent feeding habits. However, Devaere et al. (2014) showed in an p. 4), where he was employed at that time, have not been located ontogenetic series of the orthothecid Conotheca subcurvata from (Malinky & Berg-Madsen 1999).

CONTACT Vivianne Berg-Madsen [email protected] © 2018 The Author(s). Published by Informa UK Limited, trading as Taylor & Francis Group. This is an Open Access article distributed under the terms of the Creative Commons Attribution-NonCommercial-NoDerivatives License (http://creativecommons.org/licenses/by-nc-nd/4.0/), which permits non-commercial re-use, distribution, and reproduction in any medium, provided the original work is properly cited, and is not altered, transformed, or built upon in any way.

Published online 06 Apr 2018 26 V. BERG-MADSEN ET AL.

(A) (B) (C)

(D)

Figure 1. A. Maps showing the position of Bornholm relative to the Danish mainland. B. The island with major roads and communities. C. Area with the locality Vejrmøllegård at Læså rivulet near Aakirkeby and Grødby rivulet, exact collecting locality unknown. D. Geological map of southern Bornholm showing sedimentary units and the position of Vejrmøllegård relative to the Læså rivulet and Læså Formation. Geological map based on Graversen (2009). the Cambrian of France that the digestive tract is U-shaped in Originally, the lower Cambrian strata were regarded as one uni- the earliest growth stages and becomes sinuous only in the adult form unfossiliferous quartzite and very little attention was paid to stage. They suggested that this could imply a distinct phyloge- the later called Green Shales (Pingel 1828; Forchhammer 1835). netic relationship between the two hyolith orders, and further Eventually, J.F. Johnstrup and his students collected fossils from speculated that the change in the morphology in the orthothe- the strata from 1869 onwards, and Johnstrup was the first to sub- cid digestive tract reflected a change in feeding habit during the divide the lower Cambrian on Bornholm (Johnstrup 1874, 1891). growth of the animal. The lower Cambrian of Scandinavia was revised by Nielsen & A description of the digestive tract from the lower Cambrian Schovsbo (2007), which is now divided in ascending order the of Bornholm is presented here, but our study also incited a rede- Hardeberg Formation overlain by the Læså Formation, which scription of Hyolithus (Orthotheca) johnstrupi Holm, 1893 and includes the Norretorp Member overlain by the Rispebjerg a reassignment of the species to Circotheca Sysoev, 1958 emend. Member (Fig. 2). The Norretorp Member, previously known as Berg-Madsen & Malinky 1999 which will be used throughout the Green Shales, comprises about 100 m of glauconitic siltstone the text. and storm generated sand beds (Nielsen & Schovsbo 2011). The siltstone is thinly bedded, bioturbated and the surface normally Geological setting of rusty brown colour due to the weathering of the glauconite and pyrite; the fresh colour is greenish-grey. Levels bearing phos- The geology of southern Bornholm is characterized by a number phoritic nodules are found throughout the stratum and hyolith- of fault blocks (Fig. 1) and exposures are primarily found only ids and orthothecids have been reported from both the nodules along rivulets or along the coast (Hansen 1936; Poulsen 1967). and surrounding siltstone. Poulsen (1967), and before him both GFF 27

Bornholm specimen is younger than hyoliths with preserved digestive tracts from the Terranuevian of Montagne Noire (Devaere et al. 2014).

The digestive tract Hyolithids with soft parts preserved are generally rare but Moysiuk et al. (2017) reported more than 200 hyolithid specimens of Haplophrentis Babcock & Robison, 1988 from the Burgess Shale and Spence Shale Lagerstätten in western North America with preserved soft tissues. However, the digestive tract from Circotheca johnstrupi is only the third occurrence from the early Cambrian to be reported. Meškova & Sysoev 1981 isolated two fragments of digestive tracts from the early Cambrian of Siberia. These were extracted from the matrix by acetic acid and although associated with several hyolithids and orthothecids, were not referable to any genera or species. Recently, Devaere et al. (2014) recorded a substantial number of specimens of the species Conotheca subcurvata Yu, 1974 from the early Cambrian (Terraneuvian) of Montagne Noire, France with digestive tracts preserved. Several more orthothecids species, of middle Figure 2. Correlation between Siberian and Baltoscandian stages, acritarch zones Cambrian to Lower Devonian age, with preserved soft parts have and lithostratigraphic units on Bornholm. Modified from Moczydłowska & Vidal been recorded (see Devaere et al. 2014 for references). (1992) and Nielsen & Schovsbo (2011). The most complete specimen of Circotheca johnstrupi is MGUH 31848 (Fig. 3), but only about half the length of the specimen was visible when discovered. Careful preparation by V. Johnstrup (1891) and Grönwall (1899), noted the presence of Berg-Madsen subsequently revealed both the entire apical part specimens in a “grey limestone” within the Green Shales. This and the operculum. It was not attempted to free more of the ada- unit was later correctly identified as a thin bed of fine grained, pertural part of the conch as enough of the aperture was visible. very dense sandstone, at places with calcitic matrix, in the lower The conch measures 42 mm in length from the tip of the apex to part of the Green Shales at Læså and Grødbyå (Hansen 1936). the rim of the aperture and has an apertural diameter of about Hansen (1936) recognized levels especially rich in hyo- 7 mm (Fig. 3A). At least eight septa are present within the first liths and phosphoritic nodules in the Læså stream south-west adapical 10 mm, followed by 12 mm of ‘empty’ space now partly of Vejrmøllegård, belonging to the upper middle part of the filled with fragments of crushed shell and sediment grains (Fig. member. For details of localities and maps see Hansen (1936, 3A, B and H). The convoluted digestive tract is preserved in partly pp. 50–52, Fig. 8); subsequently used by Poulsen (1967) and phosphatized sediment, and folded into a chevron-like structure. Moczydłowska & Vidal (1992). It is about 22 mm long with a posterior width of 1.5 mm, the The Norretorp Member represents an offshore low-energy anterior width is 6 mm and the cross section is elliptical. Both the shelf environment and fairly deep water (Moczydłowska & Vidal anterior and posterior parts are slightly damaged and obscured 1992, Fig. 11). The siltstone sequence represents a shallow marine by microbial growth and consequently, the exact length cannot shelf and progressing coastline following transgressive and be measured (Fig. 3B, F and G). The position of the digestive regressive episodes. Occasional fluxes of more coarse-grained tract is slightly skewed from that of the operculum as judged storm deposits occur throughout the sequence. Judged from the from the axis of the conch, and with the ventral side facing the weathered surface of the samples the hyoliths most often lie par- flattened venter of the conch. On the ventral side, a series of at allel to the bedding plane but not in living position. least 20 arcuate loops can be counted, while on the dorsal side The lower Cambrian strata on Bornholm are equivalents of the central part is a 0.9 mm wide, flattened (empty?) and gen- the Siberian Nemakit-Daldynian to Atdabanian Stages (Mens tly sinuous (distortion?) straight anal tube with the posterior et al. 1987, 1990). The Læså Formation was deposited during u-shaped bend missing (Fig. 3B, G). A gap of 1.5 mm separates Cambrian Series 2 (Stage 3) (Fig. 2), and correlates with the the digestive tract from the in situ operculum preserved as a low Atdabanian Stage of Siberia. The lower part of the formation eccentric cone. Only the exterior of the operculum is preserved belongs to the Skiagia ornata-Fimbriaglomerella membranacea and sediment fills the anterior-most part of the conch. Acritarch Zone and the upper part the Heliosphaeridium dis- The thin, phosphatic outer shell layer is crushed under the similar-Skiagia ciliosa Acritarch Zone (Moczydłowska & Vidal digestive tract due to compaction but the digestive tract itself 1992). Specimens of Circotheca johnstrupi are found at several shows no sign of compaction, suggesting that phosphatization localities representing the lower and middle part of the Norretorp was finished long before the shell was compacted. Member (Poulsen 1967; Berg-Madsen pers. observation). Thus, The digestive tract, preserved in three dimensions, was the Bornholm specimen with the preserved digestive tract is of removed from the conch almost completely (broke in two pieces) the same age as the specimens with digestive tracts described and only the posterior 5 mm were impossible to extract without from the Atdabanian of Siberia (Meškova & Sysoev 1981). The damage. Once the digestive tract was removed its ventral side 28 V. BERG-MADSEN ET AL.

Figure 3. Circotheca johnstrupi (Holm, 1893). Specimen MGUH 31848 with preserved digestive tract. A. External mould with digestive tract removed. B. Specimen with digestive tract and anal tube in place. C. Latex cast of external mould. D. External mould showing external surface of operculum. E. Latex cast of external mould of operculum showing external surface of operculum. F. Ventral side of extracted digestive tract. G. Dorsal side of extracted digestive tract and anal tube. H. Posterior end of conch with septa. I. Detail of digestive tract in left ventral oblique view. J. Detail of digestive tract in right lateral view. Læså Formation at Vejrmøllegård, Læså rivulet. Collected by K.A. Grönwall. Scale bars = 0.1 cm. GFF 29 could be examined, and it also revealed the previously partly obscured operculum and ventral side of the conch, also allowing a latex cast to be made (Fig. 3C, E). The shape of the digestive tract of Circotheca johnstrupi is closer to that of Guduguwan hardmani (Etheridge in Foord, 1890) from the middle Cambrian of the Northern Territory in Australia (Kruse 1997) than those known from the early Cambrian of Montagne Noire, France (Devaere et al. 2014, Fig. 10). However, its morphology is similar to that described by Meškova & Sysoev (1981) from the early Cambrian specimens of Siberia. Devaere et al. (2014) suggested that a simple U-shaped digestive tract represents an early stage of development in orthothecid hyoliths, the folding advancing at later growth stages of the animal. All specimens of Circotheca johnstrupi from the lower Cambrian of Bornholm are remarkably uniform in size, and very few are so small that they can be regarded as juvenile. The specimen with the digestive tract is most likely an adult individual, which lend support to the suggestion by Devaere et al. (2014). Although without early stages with a preserved digestive tract, we can only see the final development. The discussion of dorsal/ventral position in hyoliths is closely related to the mode of life and in several hypotheses, among others discussed by Yochelson (1961), Fisher (1962), Marek (1963), Runnegar et al. (1975), Marek & Yochelson (1976), Sysoev (1973, 1976, 1984), Dzik (1980), Kruse (1997), and Marek et al. (1997). The latter authors regarded the inferred dorsal/ventral position and orientation of the gut in the single Australian specimen described by Kruse (1997), where the flat side was interpreted as dorsal, as a ‘preservational oddity’. The view of Kruse (1997) has not since been endorsed and the inflated part of the conch in both hyolithids and orthothecids has been considered as dorsal. In a conch with a nearly circular cross section, the maximum width of the digestive tract must be less than the diameter of the conch. Once free of assumed organic filaments keeping it in place, the intestine has a much greater chance to move in a nearly circular conch if this is being turned upside down or rolled. This means it can end up in any position relative to the operculum. The conch coquina on the bedding plane shown by Kruse (1997, Fig. 2A) indicates a high energy environment. Thus, it is likely that tumbling of the intestine took place, which explains its upside down orientation.

Systematic palaeontology Remarks. – In a footnote Holm (1893, p. 7) explains the reason for spelling the genus Hyolithus rather than the nowadays still widely used Hyolithes introduced by Eichwald (1840), pointing out that it was entymologically incorrect, and he would use ‘the only correct form of the name Hyolithus’ (translated here). That the latter term is still used, as exemplified for instance by the Figure 4. Circotheca johnstrupi (Holm, 1893). Lectotype MGUH 1506. A. Partial synonymy list presented below, could be because the footnote steinkern and partial external mould in right oblique view. B. Reproduction of original figure in Holm (1893, pl. 1, Fig. 28). C. Right lateral view of latex cast may have escaped notice. Holm’s work was as well written in of external mould. D. Detail of ornamentation. Line across the conch may mark Swedish which could also be a hinder for the understanding by possible shell breakage. Læså Formation at Vejrmøllegård, Læså rivulet. Collected non-Scandinavian readers. by J.F. Johnstrup. Scale bars in A–C = 0.5 cm. Scale bar in D = 0.1 cm. For the transliteration of Russian names (for example Syssoiev; Sysoev; Sysoyev) and references, we use the ISO 9:1995 translit- Family Circothecidae Missarževskij 1969, emended Berg- eration of the Cyrillic characters (i.e. Sysoev). Madsen & Malinky 1999 Class Hyolitha Marek 1963 Genus Circotheca Sysoev, 1958, emended Berg-Madsen & Order Orthothecida Marek 1966 Malinky, 1999 30 V. BERG-MADSEN ET AL.

Shale Formation (Paradoxides paradoxissimus Super Zone, Lejopyge laevigata Zone) in Västergötland, Sweden.

Circotheca johnstrupi (Holm, 1893) emended Figs. 3–11

1893 Hyolithus (Orthotheca) johnstrupi n. sp. – Holm, p. 56, pl. 1, Figs. 28–32, 71. Non 1893 Hyolithus (Orthotheca) johnstrupi n. sp. – Holm, p. 56, pl. 1, Fig. 33. 1902 Hyolithus johnstrupi Holm – Grönwall, p. 18, Fig. 4. 1930 Hyolithus johnstrupi Holm – Milthers p. 33, Fig. 9. Reproduced from Holm. 1938 Hyolithes johnstrupi Holm – Rosenkrantz, p. 5, pl. 2, Fig. 1. 1946 Hyolithes (Orthotheca) johnstrupi Holm – Sinclair, p. 77. 1962 Orthotheca johnstrupi Holm – Sysoev, p. 10. 1966 Hyolithes johnstrupi Holm – Rasmussen, p. 27, text-Fig. 2 (pars)., left image only. 1967 Hyolithes johnstrupi Holm – Schiønning & Wagner, p. 66, Fig. 58. Redrawn from Grönwall. 1967 Orthotheca johnstrupi Holm – Poulsen, p. 21. Non 1988 Hyolithes (Orthotheca) johnstrupi Holm – Landing, p. 675.

Material. – Lectotype, here designated, MGUH 1506 (Holm 1893, pl. 1, Figs. 28–29; here Fig. 4). Paralectotypes MGUH 1507 (Holm 1893, pl. 1, Fig. 30; here Fig. 5A), MGUH 1509 (Holm 1893, pl. 1, Fig. 32; here Fig. 6E, F) from Læså, Bornholm, collected by Johnstrup. MGUH 1514 (Holm 1893, pl. 1, Fig. 31; here Figs. 5C and 11D). Paralectotype? MGUH 1508 (Fig. 6A, B). Additionally, figured MGUH specimens are MGUH 31848– MGUH 31857. All specimens are housed in the Natural History Museum of Denmark, University of Copenhagen (MGUH).

Stratigraphy. – Læså Formation, Norretorp Member, upper Skiagia ornata–Fimbriaglomerella membranacea to lower Heliosphaeridium dissimilare–Skiagia ciliosa acritarch zones. Cambrian Series 2 (Stage 3).

Diagnosis. – Circotheca having ventral apertural edge with short, ligula-like projection; shell with transverse rugae only and with subtriangular cross section with highly rounded edges; exterior of operculum with prominent concentric rugae and interior with finer rugae.

Description. – Orthoconic conch with small apical angle and slender appearance; cross section round at apex growing increas- ingly subtriangular with highly rounded edges; venter flat and grading through rounded lateral edges into inflated dorsum that Figure 5. Circotheca johnstrupi (Holm, 1893). A. Paralectotype MGUH 1507, lacks a clearly defined median ridge. Aperture almost planar with partial steinkern, left lateral view. White infilling on either side is old ‘latex’. B. a faint ligula-like projection along ventral rim. Surface of shell, Reproduction of original figure in Holm (1893, pl. 1, Fig. 30). C. Paralectotype MGUH 1514, partial steinkern in dorsal view with phosphatized bacterial matter at least on part of dorsal and part of lateral margins, covered along the centre and septa at the apex. D. Reproduction of original figure in Holm with low transverse rugae but with regions of irregular width (1893, pl. 1, Fig. 71). Læså Formation at Vejrmøllegård, Læså rivulet. Collected by J.F. and of irregular spacing containing growth lines; on venter lines Johnstrup. Scale bars = 0.5 cm. and rugae are convex towards aperture to follow edge of ventral ligula-like projection. At least eight septa present in apical region; Type species. – By original designation of Sysoev (1958), Hyolithus internal mould smooth. (Orthotheca) stylus Holm, 1893, p. 52, pl. 1, Figs. 16–20; pl. 6, Operculum heavily rounded subtriangular in outline. Conical Figs. 6–9, from the mid-Cambrian Series 3 part of the Alum shield broad and flat with very gentle fold in central portion; GFF 31

Figure 6. Circotheca johnstrupi (Holm, 1893). A, B. Steinkern of MGUH 1508 purportedly the original of Holm (1893, pl. 1, Fig. 31). C, D. Steinkern of MGUH 31849 proposed here as true original of Holm (1893, pl. 1, 31). E, F. Steinkern of MGUH 1509, original of Holm (1893, pl. 1, Fig. 32). Læså Formation at Vejrmøllegård, Læså rivulet. Collected by J.F. Johnstrup. Scale bars in A, B, D = 0.5 cm. Scale bars in C, E = 0.1 cm.

surface of cardinal shield rounded and slightly convex with con- the fold in the growth lines and rugae on the venter (Fig. 9), and stant slope throughout; transition between conical and cardinal from the fact that the operculum is not planar, with the cardi- shields forms an angle of about 8 degrees. Exterior with con- nal and main shields intersecting at an oblique angle. Circotheca centric rugae but fainter and fewer in number than on interior. johnstupi differs from the mid-Cambrian C. smetanai Valent et al., 2012 (Barrandian Area, Czech Republic) in the conch sculp- Remarks. – This species was originally based on three internal ture where C. smetanai has very narrow longitudinal ribs and moulds of conchs (MGUH 1506, MGUH 1507, MGUH 1514) in the cross section where C. smetanai shows a circular cross and three opercula (MGUH 1508–MGUH 1510) (Holm 1893, pl. section. It should be noted that the cross section of C. johnstrupi 1, Figs. 28–33, 71). MGUH 1506 is selected as the lectotype (Fig. (Holm, 1893, pl. 1, Fig. 29) represents the posterior subcircular 4). It is preserved as an internal and external mould which meas- part of the internal mould of the conch, which otherwise gradu- ure 40 mm long, and has an apertural width of 5 mm. Additional ally becomes more subtriangular anteriorly as seen by the shape specimens furnish details of the shell, the digestive tract and in of the opercula (Fig. 6). particular of the operculum (Fig. 8). Holm (1893) noted that this species was similar to Hyolithus The species is assigned to Circotheca Sysoev, 1958 under the (Orthotheca) degeeri Holm, 1893, but types of the latter are revised concept of this genus given by Berg-Madsen & Malinky incompletely preserved, and indeed, Holm himself even sug- (1999). This species differs from the type species C. styla, in hav- gested that better preserved specimens of that species may show ing transverse ornament only, and from both the Ordovician it to be the same as Circotheca johnstrupi. Ironically, it is the name C. caperai Marek, 1983 from Montagne Noire, France, and C. of the former that has come to be widely used for conical fossils neptis Marek, 1989 from the Barrandian Area, Czech Republic, in the Cambrian (see Malinky & Berg-Madsen 1999, pp. 54, 55 by having a ventral ligula-like apertural projection. Although for details of ‘O.’ degeeri), whereas it is the later species that is far the aperture on all specimens of C. johnstrupi is incomplete, the better preserved. Cobbold (1919) noted that the cross section of presence of the projection is certain, having been inferred from 32 V. BERG-MADSEN ET AL.

& Bengtson 1989; Malinky & Berg-Madsen 1999 and references therein). Many of these fossils are poorly preserved moulds and casts of uncertain affinity. Use of the name Circotheca for any of them distorts the geographical and stratigraphical distribution of that genus. However, C. johnstrupi (Holm, 1893) demonstrates that authentic Circotheca has a range of at least lower to middle Cambrian. Circotheca johnstrupi (Holm, 1893) is by far the most common hyolith in the Norretorp Member but has so far not been found elsewhere in Scandinavia. The species is found at several localities representing the lower and middle part of the member. The specimens are preserved either as internal moulds, partly empty conchs or as external mould of the conchs. The conchs often con- tain single strands or sheets of microbial growth which eventually develop to solid phosphate filling the conch completely, as also demonstrated by Devaere et al. (2014). Reconstructions after Holm’s illustrations ofCircotheca johnstrupi have frequently been published, but most authors did not distinguish between hyolithids and orthothecids (see synonymy list). A reconstruction showing Hyolithes johnstrupi with helens (exclusively found in hyolithids) was published by Rasmussen (1966), which since then has been reproduced in textbooks, pop- ular literature and recently on internet at least 10 times in most cases as Hyolithes or a hyolithid (see for instance Rasmussen 1969, p. 290; Poulsen & Poulsen 1975, p. 57, text-Fig. 43). We refrain from listing all these publications, except for those works where the reconstruction is illustrated next to a photograph of the lectotype MGUH 1507.

Opercula Most opercula have a height between 4 and 6 mm and a width between 5 and 7 mm. Almost all are found well inside the conch and usually with the concave side lying against the inte- Figure 7. Orthothecid? MGUH 1510. A. Internal surface of supposed operculum. B. Enlarged reproduction of original figure in Holm (1893, pl. 1, Fig. 33). Læså rior of the shell, and the convex side pointing towards infill- Formation at Vejrmøllegård, Læså rivulet. Collected by J.F. Johnstrup. Scale ing of the conch (Figs. 6 and 8B–D, F–H). The operculum bar = 0.1 cm. in Fig. 10B is preserved with the apex pointing towards the interior of the shell. Holm (1893, p. 57) mentioned the simi- H. (O.) degeeri is similar to that of H. (O.) johnstrupi, and he later larity of these opercula with the “brachiopods Acrothele and figured (Cobbold 1931, pl. 41, Fig. 5) an operculum Hyolithes Discinella” (=Mobergella but referred to Discinella by Moberg (Orthotheca) sp. indet. (MGUH 31857), noting similarities with (1892)). From the figures (Holm1893 , pl. 1, Figs. 31, 32) the those of C. johnstrupi figured by Holm. Whereas C. johnstrupi shape seems close to that of the lingulate brachiopod Acrothele is founded on a sound morphological basis with features sup- whereas neither shape, ornament nor interior are close to that porting placement under Circotheca, Malinky & Berg-Madsen of the enigmatic fossil Mobergella. (1999) judged ‘O.’ degeeri to be unrecognizable. Holm (1893, pl. 1, Figs. 31, 32) figured two opercula taken Holm (1893) included both Circotheca. johnstrupi and H. (O.) out of their context as the position inside the conch is omitted degeeri under the division Complanati, in which the ventral side is (Fig. 6). We have some doubt that the operculum MGUH 1508 slightly flattened. The incomplete preservation of the latter species labelled as part of the type series is the one figured in Holm precludes further comparison (Malinky & Berg-Madsen 1999). (1893, pl. 1, Fig. 31). It is slightly chipped, positioned so that Holm (1893, pl. 1, Fig. 71) also noted that the conch (i.e. it is difficult to angle for a drawing and it is difficult to see any MGUH 1514, Fig. 5C, D) may have as many as seven septa, but details without colouring it dark and whitening with ammonium because of preservation, the exact number may be difficult to chloride sublimate (Fig. 6A, B). Another specimen available to determine. Several other specimens of Circotheca johnstrupi also Holm, but not labelled as part of the type series, MGUH 31852 reveal two or more septa of which the best preserved are those (Fig. 6C, D), seems to be a better match as it is perfectly preserved in MGUH 31848 (Fig. 3A, H). and details would be easy to delineate in a drawing. The size of Circotheca johnstrupi is a definitive representative of the two afore-mentioned specimens is about equal and matches Circotheca from the lower Cambrian. The nameCircotheca has that shown by Holm (1893). been widely used for tubular fossils from this level, particularly The only operculum showing the interior (MGUH 31850, in the Tommotian of Siberia and Meischucunian of China (Qian Figs. 8A, E and 11A) is free of overlying sediment, the exterior GFF 33

Figure 8. Circotheca johnstrupi (Holm, 1893). A, E. MGUH 31850. External mould with bacterial mat near operculum and internal surface of operculum. B, F. MGUH 31851. External mould with impressions of ornamentation and external surface of operculum. C, D, G. MGUH 31852. External mould, latex cast of external mould and external surface of operculum. H. MGUH 31853. Operculum showing external surface. Læså Formation at Vejrmøllegård, Læså rivulet. Collected by J.F. Johnstrup. Scale bars in = 0.5 cm.

side obscured by microbial growth. The operculum is almost the operculum of representatives of the genus Circotheca shows circular in outline with a broad and flat main shield with a distinct paired bilobate cardinal processes (see type species very shallow fold in the central portion, eccentric apex and a Circotheca styla (Holm, 1893) in Berg-Madsen & Malinky 1999 rounded cardinal shield, the shields intersecting at an oblique (text-Fig. 9E, F and R) or Circotheca smetanai Valent et al. 2012 angle. It has a maximum width and height of 4.9 and 4.7 mm, (Fig. 2A, B)) which are not present in any form at Holm’s speci- respectively. It is notable that the operculum lacks internal men. Although conchs are present in the same sample, the oper- characters like clavicles and cardinal processes. The conch is culum is not connected with any of these. If indeed an operculum incomplete and it is difficult to make out whether or not the and not a piece of isolated shell, the possibility exists that it is an operculum is withdrawn, but a slightly skewed position may operculum belonging to one of the hyolithid species (for instance be possible. Hyolithus nathorsti Johnstrup in Holm) present in the Norretorp The record of this species allows for additional documentation Member, the interior of which are equally poorly known (Holm of the fact that the orthothecid operculum was narrower than 1893; Poulsen 1967). the aperture, and therefore could be withdrawn into the conch The apparent lack of cardinal processes on the internal sur- as suggested by Marek (1966, Fig. p. 91; 1967) and illustrated by face of the operculum of Circotheca johnstrupi may suggest that Devaere et al. (2014, Fig. 4I, K, p. 5). We exclude the specimen it was fastened in the epidermis much like in gastropods. This MGUH 1510 described by Holm (1893, pl. 1, Fig. 33, here Fig. may also explain why almost all opercula referable to Circotheca 7) from the genus Circotheca. Holm illustrated what he believed johnstrupi are found inside the conch, suggesting that they were to be the interior of the operculum possessing a deep, oval pulled in by the visceral mass. Sediment infilling the apertural depression surrounded by cardinal processes. It cannot be com- part of the conch would prevent the operculum from disap- pletely ruled out that the structures are malformations caused pearing. Sediment infill could also explain why some opercula by secondary growth. The usual morphology of the interior of were kept in place allowing development of microbial growth 34 V. BERG-MADSEN ET AL.

Figure 9. Circotheca johnstrupi (Holm, 1893). A, B. MGUH 31854, latex cast of ventral side of external mould showing well-preserved ornamentation. Læså Formation at Grødby rivulet. Collected by J.F. Johnstrup. C, D. MGUH 31855. Dorsal, left lateral and ventral views of latex cast of external mould showing well-preserved ornamentation. Scale bar in A = 0.5 cm. Scale bars in B–E = 0.1 cm.

in the reducing conditions inside the conch developed during tissue (see discussion in Zhao & Bengtson 1999; Devaere et al. decomposition. 2014). The posterior septated part of the conch is always devoid of microbial growth. Shell Summary Already Holm (1893, p. 1) noted three to four layers in the shell, assuming it consisted of calcium carbonate; the calcium The digestive tract documented in the Cambrian orthothecid carbonate has secondarily been replaced by calcium phosphate. herein is the first such from Denmark (and Scandinavia), and He also believed the conch and the operculum consisted of two only the third known with certainty from the lower Cambrian. different types of material and compared this with the situation It is preserved in three dimensions and can be mechanically in certain gastropods corneous opercula. In several conchs, two detached from the conch allowing inspection of all sides. The and perhaps three layers can be distinguished (MGUH 31854, importance of such a specimen in understanding hyolith pal- Fig. 10D; MGUH 1514, Fig. 11B). The variable expression and aeobiology and palaeoecology cannot be overstated, given that numbers of rugae on the exterior relative to the interior of the few other such specimens have been found. Its significance may operculum shows that there are distinct differences in the respec- be judged in part from the general reconstruction of hyolith soft tive shell layers. The present material is too scarce and badly part morphology given by Runnegar et al. (1975), who conferred preserved to provide further information. separate phylum status on the hyoliths because of presumed differences between them and the Mollusca, where hyoliths had Microbial growth been traditionally placed (Marek & Yochelson 1964, 1976). The systematic position of hyoliths was summarized and dis- Phosphatized microbial growth is often found around the oper- cussed by Malinky & Yochelson (2007) who regarded these as a cula, and in some cases the internal mould seems to consist of class within the molluscs. Sun et al. (2016) mentioned the phy- more or less dense filaments of phosphatized bacteria (Fig. 11). logenetic relationships of molluscs, sipunculans, hyolithids and Holm (1893) described this infill as being like cinder (Holm orthothecids based mainly on the first appearance of group and 1893, pl. 1, Fig. 71; here Figs. 5C, D and 11B). Microbial growth development of the gut and intestine and orthothecids are treated is represented by botryoidal, uniseriate and branching filaments here as a more primitive/ancestral group of hyoliths. Based on as well as diagenetic overgrowth, especially below the only oper- excellent material with preserved soft tissues of the hyolithid culum revealing the interior the filaments form film or sheets Haplophrentis Babcock & Robison 1988 from the Burgess Shale, MGUH 31850 (Fig. 11A). Some of these filamentous structures Canada and the Spence Shale, USA, Lagerstätten, Moysiuk et al. may reflect direct phosphatization of the partly decomposed soft (2017) published a new hypothesis that hyoliths can be placed GFF 35

Figure 11. Circotheca johnstrupi (Holm, 1893). A. MGUH 31850. Detail of phosphatized bacterial mesh below operculum. See Fig. 8A for entire specimen. B. MGUH 1514. Detail of posterior part of conch with phosphatized bacterial infilling. Numbers 1–3 indicate three possible shell layers. See Fig. 5C for entire specimen. Scale bars in = 0.1 cm.

tract to be preserved, it must filled with sediment; and the fact that intestines are in some cases preserved fully three-dimen- sional supports the notion that these animals were deposit Figure 10. Circotheca johnstrupi (Holm, 1893). A, B. MGUH 31856. Steinkern of conch in ventral and left lateral views with operculum resting on floor of conch feeders ingesting sediment. In contrast, experimental evidence with external side down. C. MGUH 31857. Ventral side of nearly complete conch. D. showed hyolithids were suspension feeders. Lastly, the remains of MGUH 31854. Detail of latex cast of conch showing two shell layers. See Fig. 9A for microbial growth beneath the operculum and within the conch entire specimen. Scale bars in A, C = 0.5 cm. Scale bars in B, D = 0.1 cm. demonstrate the early microbially mediated phosphatization of this material which was also shown in the early Cambrian material from France described by Devaere et al. (2014). among the Lophophorata. Based on their findings, this affinity seems to be clear for the hyolithids but such high taxonomic Acknowledgement placement for orthothecids is not at the moment possible as the organization of the internal organs remains unknown. J.M. Malinky is thanked for his initial enthusiastic involvement in this study about 20 years ago. Original specimens of Holm and subsequent Marek et al. (1997) used the rare preservation of the intes- material from the Norretorp Member, Bornholm, were kindly made avail- tines in orthothecids for a palaeoecological interpretation. They able from the Geological Museum in Copenhagen and museum numbers naturally reasoned that in order for the orthothecid digestive were provided by A.R. Bashforth. M. Valent was financially supported by 36 V. BERG-MADSEN ET AL. the Ministry of Culture of the Czech Republic (DKRVO 2018/06, National Kruse, P.D., 1997: Hyolith guts in the Cambrian of northern Australia – Museum, 00023272 – M. Valent). 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