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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
© 2018 The Author(s). Published by Informa UK Limited, trading as Taylor & Francis Group
Published online: 06 Apr 2018.
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https://doi.org/10.1080/11035897.2018.1432680
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
The hyolith assemblage from the early Cambrian of Bornholm, Denmark, shows a higher diversity than contemporary assemblages in Baltoscandia. The most common species in the Green Shales (Læså Formation, Norretorp Member, Cambrian Stage 3), is Hyolithes [=Hyolithus] (Orthotheca) johnstrupi Holm, 1893. A specimen of this species shows a well-preserved and almost complete digestive tract, folded into an approximately 22 mm long chevron-like structure comprised of at least 20 arcuate loops on the ventral side and a flattened, gently sinuous to straight anal tube on the dorsal side. The thin, phosphatic outer shell layer of the conch is crushed under the digestive tract due to compaction while the digestive tract 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
The hyolith assemblage from the early Cambrian of Bornholm, Denmark, shows a higher diversity and contains better pre-served specimens compared to those of equivalent levels in Sweden. Three hyolith species from the lower Cambrian Green Shales (Læså Formation, Norretorp Member, Cambrian Stage 3) of Bornholm, Denmark (Figs. 1 and 2) were included in the taxonomic study of Swedish Cambrian hyoliths by Holm (1893): Hyolithus lenticularis, H. (Orthotheca) johnstrupi and H.
nathorsti. Re-examination of hyolith material from the Green
Shales by V. Berg-Madsen confirms the presence of the hyolith-ids Hyolithus nathorsti Johnstrup in Holm, 1893, Lovenedolithes
groenwalli (Poulsen, 1967) including fragments of helens (see also Martí Mus & Bergström 2007) and an operculum, and a single specimen of Hexitheca teretiuscula (Linnarsson, 1877). Holm (1893) stated that H. (Orthotheca) johnstrupi was by far the most common species at the locality near Vejrmøllegård (Fig. 1), and a great number of specimens were found in phos-phoritic nodules, including 11 opercula. Through the courtesy of Professor J.F. Johnstrup in Copenhagen Holm had had access to all this material and also collected in situ himself.
Although Holm (1893, p. 56) stated most specimens were found in phosphoritic nodules, only one of the here described specimens figured by Holm comes from a nodule, all others from the siltstone proper. Specimens supposedly deposited at the Geological Survey of Sweden (SGU) according to Holm (1893, p. 4), where he was employed at that time, have not been located (Malinky & Berg-Madsen 1999).
Poulsen (1967) described further hyolith species from the Green Shales and confirmed that Hyolithus (Orthotheca)
john-strupi was one of the dominant macro-fossils in the siltstone.
Although he no doubt examined the specimen with the digestive tract preserved, it escaped his notice and he stated that “none were better preserved than those described by Holm, for which reason nothing new could be added” (Poulsen 1967, pp. 44–46).
The collections at the Geological Museum, Copenhagen, were examined in connection with the 1999 revision. A specimen, col-lected at Læså near Vejrmøllegård, which revealed part of a diges-tive tract was discovered by J.M. Malinky, who also confirmed the identification as H. (Orthotheca) johnstrupi. The specimen was collected by K.A. Grönwall in 1892 who noted on the label that it was a ‘specimen with an unusual structure’. Although this was probably the earliest find of an orthothecid digestive tract its significance escaped notice. Thus, the first description of a fossilized hyolith digestive tract became that of Thoral (1935) from the Lower Ordovician of France.
Preserved soft parts in hyoliths are rare and only about a dozen reports of are known with material from the Cambrian, Ordovician and Devonian (see Devaere et al. 2014 and Moysiuk et al. 2017 for references). The soft part morphology is impor-tant to distinguish between the order Hyolithida and the order Orthothecida. The orthothecid digestive tracts are the best docu-mented and is complex and generally sinuously folded, while it is simple and U-shaped in the hyolithids, which may reflect differ-ent feeding habits. However, Devaere et al. (2014) showed in an ontogenetic series of the orthothecid Conotheca subcurvata from
© 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.
KEYWORDS Orthotheca johnstrupi revised; digestive tract; early Cambrian; Bornholm; Denmark
ARTICLE HISTORY Received 7 September 2017 Accepted 22 January 2018
CONTACT Vivianne Berg-Madsen Vivianne.Berg-Madsen@em.uu.se
OPEN ACCESS
Originally, the lower Cambrian strata were regarded as one uni-form unfossiliferous quartzite and very little attention was paid to the later called Green Shales (Pingel 1828; Forchhammer 1835). Eventually, J.F. Johnstrup and his students collected fossils from the strata from 1869 onwards, and Johnstrup was the first to sub-divide the lower Cambrian on Bornholm (Johnstrup 1874, 1891). The lower Cambrian of Scandinavia was revised by Nielsen & Schovsbo (2007), which is now divided in ascending order the Hardeberg Formation overlain by the Læså Formation, which includes the Norretorp Member overlain by the Rispebjerg Member (Fig. 2). The Norretorp Member, previously known as the Green Shales, comprises about 100 m of glauconitic siltstone and storm generated sand beds (Nielsen & Schovsbo 2011). The siltstone is thinly bedded, bioturbated and the surface normally of rusty brown colour due to the weathering of the glauconite and pyrite; the fresh colour is greenish-grey. Levels bearing phos-phoritic nodules are found throughout the stratum and hyolith-ids and orthothechyolith-ids have been reported from both the nodules and surrounding siltstone. Poulsen (1967), and before him both the Cambrian of France that the digestive tract is U-shaped in
the earliest growth stages and becomes sinuous only in the adult stage. They suggested that this could imply a distinct phyloge-netic relationship between the two hyolith orders, and further speculated that the change in the morphology in the orthothe-cid digestive tract reflected a change in feeding habit during the growth of the animal.
A description of the digestive tract from the lower Cambrian of Bornholm is presented here, but our study also incited a rede-scription of Hyolithus (Orthotheca) johnstrupi Holm, 1893 and a reassignment of the species to Circotheca Sysoev, 1958 emend. Berg-Madsen & Malinky 1999 which will be used throughout the text.
Geological setting
The geology of southern Bornholm is characterized by a number of fault blocks (Fig. 1) and exposures are primarily found only along rivulets or along the coast (Hansen 1936; Poulsen 1967).
(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).
Johnstrup (1891) and Grönwall (1899), noted the presence of specimens in a “grey limestone” within the Green Shales. This unit was later correctly identified as a thin bed of fine grained, very dense sandstone, at places with calcitic matrix, in the lower part of the Green Shales at Læså and Grødbyå (Hansen 1936).
Hansen (1936) recognized levels especially rich in hyo-liths and phosphoritic nodules in the Læså stream south-west of Vejrmøllegård, belonging to the upper middle part of the member. For details of localities and maps see Hansen (1936, pp. 50–52, Fig. 8); subsequently used by Poulsen (1967) and Moczydłowska & Vidal (1992).
The Norretorp Member represents an offshore low-energy shelf environment and fairly deep water (Moczydłowska & Vidal
1992, Fig. 11). The siltstone sequence represents a shallow marine shelf and progressing coastline following transgressive and regressive episodes. Occasional fluxes of more coarse-grained storm deposits occur throughout the sequence. Judged from the weathered surface of the samples the hyoliths most often lie par-allel to the bedding plane but not in living position.
The lower Cambrian strata on Bornholm are equivalents of the Siberian Nemakit-Daldynian to Atdabanian Stages (Mens et al. 1987, 1990). The Læså Formation was deposited during Cambrian Series 2 (Stage 3) (Fig. 2), and correlates with the Atdabanian Stage of Siberia. The lower part of the formation belongs to the Skiagia ornata-Fimbriaglomerella membranacea Acritarch Zone and the upper part the Heliosphaeridium
dis-similar-Skiagia ciliosa Acritarch Zone (Moczydłowska & Vidal
1992). Specimens of Circotheca johnstrupi are found at several localities representing the lower and middle part of the Norretorp Member (Poulsen 1967; Berg-Madsen pers. observation). Thus, the Bornholm specimen with the preserved digestive tract is of the same age as the specimens with digestive tracts described from the Atdabanian of Siberia (Meškova & Sysoev 1981). The
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 spec-imens 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 Cambrian to Lower Devonian age, with preserved soft parts have been recorded (see Devaere et al. 2014 for references).
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. Berg-Madsen subsequently revealed both the entire apical part and the operculum. It was not attempted to free more of the ada-pertural part of the conch as enough of the aperture was visible. The conch measures 42 mm in length from the tip of the apex to the rim of the aperture and has an apertural diameter of about 7 mm (Fig. 3A). At least eight septa are present within the first adapical 10 mm, followed by 12 mm of ‘empty’ space now partly filled with fragments of crushed shell and sediment grains (Fig.
3A, B and H). The convoluted digestive tract is preserved in partly phosphatized sediment, and folded into a chevron-like structure. It is about 22 mm long with a posterior width of 1.5 mm, the anterior width is 6 mm and the cross section is elliptical. Both the anterior and posterior parts are slightly damaged and obscured by microbial growth and consequently, the exact length cannot be measured (Fig. 3B, F and G). The position of the digestive tract is slightly skewed from that of the operculum as judged from the axis of the conch, and with the ventral side facing the flattened venter of the conch. On the ventral side, a series of at least 20 arcuate loops can be counted, while on the dorsal side the central part is a 0.9 mm wide, flattened (empty?) and gen-tly sinuous (distortion?) straight anal tube with the posterior u-shaped bend missing (Fig. 3B, G). A gap of 1.5 mm separates the digestive tract from the in situ operculum preserved as a low eccentric cone. Only the exterior of the operculum is preserved and sediment fills the anterior-most part of the conch.
The thin, phosphatic outer shell layer is crushed under the digestive tract due to compaction but the digestive tract itself shows no sign of compaction, suggesting that phosphatization was finished long before the shell was compacted.
The digestive tract, preserved in three dimensions, was removed from the conch almost completely (broke in two pieces) and only the posterior 5 mm were impossible to extract without damage. Once the digestive tract was removed its ventral side
Figure 2. Correlation between Siberian and Baltoscandian stages, acritarch zones and lithostratigraphic units on Bornholm. Modified from Moczydłowska & Vidal (1992) and Nielsen & Schovsbo (2011).
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.
Family Circothecidae Missarževskij 1969, emended Berg-Madsen & Malinky 1999
Genus Circotheca Sysoev, 1958, emended Berg-Madsen & Malinky, 1999
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 diges-tive tract represents an early stage of development in orthothe-cid 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 spec-imen 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 oth-ers 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 synonymy list presented below, could be because the footnote may have escaped notice. Holm’s work was as well written in Swedish which could also be a hinder for the understanding by non-Scandinavian readers.
For the transliteration of Russian names (for example Syssoiev; Sysoev; Sysoyev) and references, we use the ISO 9:1995 translit-eration of the Cyrillic characters (i.e. Sysoev).
Class Hyolitha Marek 1963
Order Orthothecida Marek 1966
Figure 4. Circotheca johnstrupi (Holm, 1893). Lectotype MGUH 1506. A. Partial 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 of external mould. D. Detail of ornamentation. Line across the conch may mark possible shell breakage. Læså Formation at Vejrmøllegård, Læså rivulet. Collected by J.F. Johnstrup. Scale bars in A–C = 0.5 cm. Scale bar in D = 0.1 cm.
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, col-lected 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 lacks a clearly defined median ridge. Aperture almost planar with a faint ligula-like projection along ventral rim. Surface of shell, at least on part of dorsal and part of lateral margins, covered with low transverse rugae but with regions of irregular width and of irregular spacing containing growth lines; on venter lines and rugae are convex towards aperture to follow edge of ventral ligula-like projection. At least eight septa present in apical region; internal mould smooth.
Operculum heavily rounded subtriangular in outline. Conical shield broad and flat with very gentle fold in central portion;
Type species. – By original designation of Sysoev (1958), Hyolithus (Orthotheca) stylus Holm, 1893, p. 52, pl. 1, Figs. 16–20; pl. 6, Figs. 6–9, from the mid-Cambrian Series 3 part of the Alum
Figure 5. Circotheca johnstrupi (Holm, 1893). A. Paralectotype MGUH 1507, partial steinkern, left lateral view. White infilling on either side is old ‘latex’. B. Reproduction of original figure in Holm (1893, pl. 1, Fig. 30). C. Paralectotype MGUH 1514, partial steinkern in dorsal view with phosphatized bacterial matter along the centre and septa at the apex. D. Reproduction of original figure in Holm (1893, pl. 1, Fig. 71). Læså Formation at Vejrmøllegård, Læså rivulet. Collected by J.F. Johnstrup. Scale bars = 0.5 cm.
the fold in the growth lines and rugae on the venter (Fig. 9), and from the fact that the operculum is not planar, with the cardi-nal and main shields intersecting at an oblique angle. Circotheca
johnstupi differs from the mid-Cambrian C. smetanai Valent et
al., 2012 (Barrandian Area, Czech Republic) in the conch sculp-ture where C. smetanai has very narrow longitudinal ribs and in the cross section where C. smetanai shows a circular cross section. It should be noted that the cross section of C. johnstrupi (Holm, 1893, pl. 1, Fig. 29) represents the posterior subcircular part of the internal mould of the conch, which otherwise gradu-ally becomes more subtriangular anteriorly as seen by the shape of the opercula (Fig. 6).
Holm (1893) noted that this species was similar to Hyolithus (Orthotheca) degeeri Holm, 1893, but types of the latter are incompletely preserved, and indeed, Holm himself even sug-gested that better preserved specimens of that species may show it to be the same as Circotheca johnstrupi. Ironically, it is the name of the former that has come to be widely used for conical fossils in the Cambrian (see Malinky & Berg-Madsen 1999, pp. 54, 55 for details of ‘O.’ degeeri), whereas it is the later species that is far better preserved. Cobbold (1919) noted that the cross section of surface of cardinal shield rounded and slightly convex with
con-stant slope throughout; transition between conical and cardinal shields forms an angle of about 8 degrees. Exterior with con-centric rugae but fainter and fewer in number than on interior.
Remarks. – This species was originally based on three internal
moulds of conchs (MGUH 1506, MGUH 1507, MGUH 1514) and three opercula (MGUH 1508–MGUH 1510) (Holm 1893, pl. 1, Figs. 28–33, 71). MGUH 1506 is selected as the lectotype (Fig.
4). It is preserved as an internal and external mould which meas-ure 40 mm long, and has an apertural width of 5 mm. Additional specimens furnish details of the shell, the digestive tract and in particular of the operculum (Fig. 8).
The species is assigned to Circotheca Sysoev, 1958 under the revised concept of this genus given by Berg-Madsen & Malinky (1999). This species differs from the type species C. styla, in hav-ing transverse ornament only, and from both the Ordovician
C. caperai Marek, 1983 from Montagne Noire, France, and C. neptis Marek, 1989 from the Barrandian Area, Czech Republic,
by having a ventral ligula-like apertural projection. Although the aperture on all specimens of C. johnstrupi is incomplete, the presence of the projection is certain, having been inferred from
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.
& 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 locali-ties 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 of Circotheca
john-strupi 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-rior of the shell, and the convex side pointing towards infill-ing of the conch (Figs. 6 and 8B–D, F–H). The operculum in Fig. 10B is preserved with the apex pointing towards the interior of the shell. Holm (1893, p. 57) mentioned the simi-larity of these opercula with the “brachiopods Acrothele and
Discinella” (=Mobergella but referred to Discinella by Moberg
(1892)). From the figures (Holm 1893, pl. 1, Figs. 31, 32) the shape seems close to that of the lingulate brachiopod Acrothele whereas neither shape, ornament nor interior are close to that of the enigmatic fossil Mobergella.
Holm (1893, pl. 1, Figs. 31, 32) figured two opercula taken out of their context as the position inside the conch is omitted (Fig. 6). We have some doubt that the operculum MGUH 1508 labelled as part of the type series is the one figured in Holm (1893, pl. 1, Fig. 31). It is slightly chipped, positioned so that it is difficult to angle for a drawing and it is difficult to see any details without colouring it dark and whitening with ammonium chloride sublimate (Fig. 6A, B). Another specimen available to Holm, but not labelled as part of the type series, MGUH 31852 (Fig. 6C, D), seems to be a better match as it is perfectly preserved and details would be easy to delineate in a drawing. The size of the two afore-mentioned specimens is about equal and matches that shown by Holm (1893).
The only operculum showing the interior (MGUH 31850, Figs. 8A, E and 11A) is free of overlying sediment, the exterior
H. (O.) degeeri is similar to that of H. (O.) johnstrupi, and he later
figured (Cobbold 1931, pl. 41, Fig. 5) an operculum Hyolithes (Orthotheca) sp. indet. (MGUH 31857), noting similarities with those of C. johnstrupi figured by Holm. Whereas C. johnstrupi is founded on a sound morphological basis with features sup-porting placement under Circotheca, Malinky & Berg-Madsen (1999) judged ‘O.’ degeeri to be unrecognizable.
Holm (1893) included both Circotheca. johnstrupi and H. (O.)
degeeri under the division Complanati, in which the ventral side is
slightly flattened. The incomplete preservation of the latter species precludes further comparison (Malinky & Berg-Madsen 1999).
Holm (1893, pl. 1, Fig. 71) also noted that the conch (i.e. MGUH 1514, Fig. 5C, D) may have as many as seven septa, but because of preservation, the exact number may be difficult to determine. Several other specimens of Circotheca johnstrupi also reveal two or more septa of which the best preserved are those in MGUH 31848 (Fig. 3A, H).
Circotheca johnstrupi is a definitive representative of Circotheca from the lower Cambrian. The name Circotheca has
been widely used for tubular fossils from this level, particularly in the Tommotian of Siberia and Meischucunian of China (Qian
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 Formation at Vejrmøllegård, Læså rivulet. Collected by J.F. Johnstrup. Scale bar = 0.1 cm.
the operculum of representatives of the genus Circotheca shows distinct paired bilobate cardinal processes (see type species
Circotheca styla (Holm, 1893) in Berg-Madsen & Malinky 1999
(text-Fig. 9E, F and R) or Circotheca smetanai Valent et al. 2012
(Fig. 2A, B)) which are not present in any form at Holm’s speci-men. Although conchs are present in the same sample, the oper-culum is not connected with any of these. If indeed an operoper-culum and not a piece of isolated shell, the possibility exists that it is an operculum belonging to one of the hyolithid species (for instance
Hyolithus nathorsti Johnstrup in Holm) present in the Norretorp
Member, the interior of which are equally poorly known (Holm
1893; Poulsen 1967).
The apparent lack of cardinal processes on the internal sur-face of the operculum of Circotheca johnstrupi may suggest that it was fastened in the epidermis much like in gastropods. This may also explain why almost all opercula referable to Circotheca
johnstrupi are found inside the conch, suggesting that they were
pulled in by the visceral mass. Sediment infilling the apertural part of the conch would prevent the operculum from disap-pearing. Sediment infill could also explain why some opercula were kept in place allowing development of microbial growth side obscured by microbial growth. The operculum is almost
circular in outline with a broad and flat main shield with a very shallow fold in the central portion, eccentric apex and a rounded cardinal shield, the shields intersecting at an oblique angle. It has a maximum width and height of 4.9 and 4.7 mm, respectively. It is notable that the operculum lacks internal characters like clavicles and cardinal processes. The conch is incomplete and it is difficult to make out whether or not the operculum is withdrawn, but a slightly skewed position may be possible.
The record of this species allows for additional documentation of the fact that the orthothecid operculum was narrower than the aperture, and therefore could be withdrawn into the conch as suggested by Marek (1966, Fig. p. 91; 1967) and illustrated by Devaere et al. (2014, Fig. 4I, K, p. 5). We exclude the specimen MGUH 1510 described by Holm (1893, pl. 1, Fig. 33, here Fig.
7) from the genus Circotheca. Holm illustrated what he believed to be the interior of the operculum possessing a deep, oval depression surrounded by cardinal processes. It cannot be com-pletely ruled out that the structures are malformations caused by secondary growth. The usual morphology of the interior of
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.
tissue (see discussion in Zhao & Bengtson 1999; Devaere et al.
2014). The posterior septated part of the conch is always devoid of microbial growth.
Summary
The digestive tract documented in the Cambrian orthothecid herein is the first such from Denmark (and Scandinavia), and only the third known with certainty from the lower Cambrian. It is preserved in three dimensions and can be mechanically detached from the conch allowing inspection of all sides. The importance of such a specimen in understanding hyolith pal-aeobiology and palaeoecology cannot be overstated, given that few other such specimens have been found. Its significance may be judged in part from the general reconstruction of hyolith soft part morphology given by Runnegar et al. (1975), who conferred separate phylum status on the hyoliths because of presumed dif-ferences between them and the Mollusca, where hyoliths had been traditionally placed (Marek & Yochelson 1964, 1976).
The systematic position of hyoliths was summarized and dis-cussed by Malinky & Yochelson (2007) who regarded these as a class within the molluscs. Sun et al. (2016) mentioned the phy-logenetic relationships of molluscs, sipunculans, hyolithids and orthothecids based mainly on the first appearance of group and development of the gut and intestine and orthothecids are treated here as a more primitive/ancestral group of hyoliths. Based on excellent material with preserved soft tissues of the hyolithid
Haplophrentis Babcock & Robison 1988 from the Burgess Shale, Canada and the Spence Shale, USA, Lagerstätten, Moysiuk et al. (2017) published a new hypothesis that hyoliths can be placed in the reducing conditions inside the conch developed during
decomposition. Shell
Already Holm (1893, p. 1) noted three to four layers in the shell, assuming it consisted of calcium carbonate; the calcium carbonate has secondarily been replaced by calcium phosphate. He also believed the conch and the operculum consisted of two different types of material and compared this with the situation in certain gastropods corneous opercula. In several conchs, two and perhaps three layers can be distinguished (MGUH 31854, Fig. 10D; MGUH 1514, Fig. 11B). The variable expression and numbers of rugae on the exterior relative to the interior of the operculum shows that there are distinct differences in the respec-tive shell layers. The present material is too scarce and badly preserved to provide further information.
Microbial growth
Phosphatized microbial growth is often found around the oper-cula, and in some cases the internal mould seems to consist of more or less dense filaments of phosphatized bacteria (Fig. 11). Holm (1893) described this infill as being like cinder (Holm
1893, pl. 1, Fig. 71; here Figs. 5C, D and 11B). Microbial growth is represented by botryoidal, uniseriate and branching filaments as well as diagenetic overgrowth, especially below the only oper-culum revealing the interior the filaments form film or sheets MGUH 31850 (Fig. 11A). Some of these filamentous structures may reflect direct phosphatization of the partly decomposed soft
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.
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 feeders ingesting sediment. In contrast, experimental evidence showed hyolithids were suspension feeders. Lastly, the remains of microbial growth beneath the operculum and within the conch demonstrate the early microbially mediated phosphatization of this material which was also shown in the early Cambrian mate-rial from France described by Devaere et al. (2014).
Acknowledgement
J.M. Malinky is thanked for his initial enthusiastic involvement in this study about 20 years ago. Original specimens of Holm and subsequent material from the Norretorp Member, Bornholm, were kindly made avail-able from the Geological Museum in Copenhagen and museum numbers were provided by A.R. Bashforth. M. Valent was financially supported by among the Lophophorata. Based on their findings, this affinity
seems to be clear for the hyolithids but such high taxonomic placement for orthothecids is not at the moment possible as the organization of the internal organs remains unknown.
Marek et al. (1997) used the rare preservation of the intes-tines in orthothecids for a palaeoecological interpretation. They naturally reasoned that in order for the orthothecid digestive
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 with external side down. C. MGUH 31857. Ventral side of nearly complete conch. D. MGUH 31854. Detail of latex cast of conch showing two shell layers. See Fig. 9A for entire specimen. Scale bars in A, C = 0.5 cm. Scale bars in B, D = 0.1 cm.
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.
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the Ministry of Culture of the Czech Republic (DKRVO 2018/06, National Museum, 00023272 – M. Valent). The manuscript was improved consider-ably by comments from two reviewers and robust feedback from the editor.
Disclosure statement
No potential conflict of interest was reported by the authors.
Funding
This work was supported by Ministerstvo Kultury [grant number DKRVO 2018/06].
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