Alcheringa. Associated conchs and opercula of Triplicatella disdoma from the early Cambrian of South Australia

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Swedish Museum of Natural History, Department of Palaeozoology Topper, Timothy; Statens Naturhistoriske Museum, Geologisk Museum,

1 Associated conchs and opercula of Triplicatella disdoma from the early Cambrian of South Australia Journal: Manuscript ID: Draft Manuscript Type: Standard Research Article Date Submitted by the Author: n/a Complete List of Authors: Skovsted, Christian; Swedish Museum of Natural History, Department of Palaeozoology Topper, Timothy; Statens Naturhistoriske Museum, Geologisk Museum, Betts, Marissa; Macquarie University, Department of Biological Sciences Keywords: Early Cambrian, Hyolitha, Orthothecida, Shell Articulation

2 Page of Associated conchs and opercula of Triplicatella disdoma from the early Cambrian of South Australia CHRISTIAN B. SKOVSTED, TIMOTHY P. TOPPER, MARISSA J. BETTS & GLENN A. BROCK Skovsted, C.B., Topper, T.P., Betts, M.J. & Brock, G.A., xxxx. Associated conchs and opercula of Triplicatella disdoma from the early Cambrian of South Australia., xxx, p. xx-xx. Abstract Internal moulds of complete shells, including conch and associated opercula, of the hyolith Triplicatella disdoma Conway Morris in Bengtson et al., 0 from Cambrian Series, Stages - of South Australia are described. The conch of T. disdoma is shown to be narrow, coneshaped and with a rounded triangular cross-section. The conch shows a gentle dorsal curvature in lateral view. The fossils reveal no evidence supporting the presence of helens and the operculum was smaller than the apertural diameter of the conch and could be withdrawn a short distance into the conch. Triplicatella was probably closely related to orthothecid hyoliths. Christian B. Skovsted [christian.skovsted@nrm.se]; Department of Palaeobiology, Swedish Museum of Natural History, Box 000, SE- 0 Stockholm, Sweden

3 Page of Timothy P. Topper [timothy.topper@snm.ku.dk]; Geologisk Museum, Statens Naturhistoriske Museum, Øster Voldgade -, DK-0 København K, Denmark Marissa J. Betts & Glenn A. Brock [marissa.betts@mq.edu.au; glenn.brock@mq.edu.au]; Department of Biological Sciences, Macquarie University, Sydney, NSW, Australia Keywords: Early Cambrian, Hyolitha, Orthothecida, Shell Articulation SUB-TRIANGULAR shells of Triplicatella Conway Morris in Bengtson et al., 0 were first described from the Cambrian of South Australia (Cambrian Series, Stage -) but have since been reported from Cambrian strata in Greenland, Canada and Siberia (Skovsted et al. 0) and the presumably related genus Holoplicatella Clausen & Alvaro, 0 is known from the middle Cambrian of Spain. Originally, the shells were considered as possible opercula of an unidentified tubular shell (Conway Morris in Bengtson et al. 0) although the fossils have also been considered as potentially related to polyplacophoran shell plates (Yates et al. ) or sclerites of the problematic cambroclavids (Clausen& Alvaro 0). However, Demidenko (in Gravestock et al. 0) illustrated a partial internal mould of a specimen apparently combining the Triplicatella shell with a tubular conch and Triplicatella was eventually identified as the operculum of a poorly understood group of Cambrian hyoliths of uncertain affinity (Skovsted et al. 0). Unfortunately, the conch of Triplicatella has never been fully recovered, despite the relative abundance of phosphatized opercula at several different localities worldwide. Herein we describe internal moulds of complete specimens (combining conch and operculum) of the type species Triplicatella disdoma Conway Morris in Bengtson et al., 0 from the Ajax Limestone, Mt. Scott in the Flinders

4 Page of Ranges of South Australia. The new fossils reveal, for the first time, the morphology of the conch and demonstrate that the operculum could be withdrawn completely into the conch. Material and Methods The specimens illustrated herein are derived from limestone samples collected from a measured section (AJX-M) through the lower Cambrian (Cambrian Series, Stages -) Ajax Limestone at Mt. Scott in the central Flinders Ranges, South Australia. The section was originally described by Gravestock (, section M) and is the type section for Triplicatella disdoma (Conway Morris in Bengtson et al. 0). For a full description of the lithologies associated with the AJX-M section and the stratigraphical position of sampled horizons see Skovsted et al. () and Topper et al. (). Samples were digested with dilute % acetic acid and acid resistant residues were screened for phosphatized shells and phosphatic internal moulds of T. disdoma. Phosphatized opercula of T. disdoma (Fig. ) were recorded from an interval spanning the A. huoi and P. tatei trilobite Zones (from.0 m to. m above the base of the section) and internal moulds combining conch and operculum were found at several horizons throughout this interval (Fig. ; Table ). Specimens selected for Scanning Electron Microscopy (SEM) are housed in the collections of the South Australian Museum, Adelaide (acronym SAMP). Internal moulds of Triplicatella disdoma The preserved internal moulds are composed of an elongate cone-shaped and gently curved conch with rounded to sub-triangular cross-section (Fig. ). In cross section the dorsum is

5 Page of rounded triangular and the ventral surface is gently convex. The conch is gently curved dorsally and in lateral view the apical portion of the conch is clearly elevated relative to the main part of the conch (Fig. B, C). The apex is unfortunately not well preserved in the available specimens. The antapical termination of the fossils reveals a strongly depressed pyramidal structure representing the internal surface of an operculum (Fig. G-K). The apex of the operculum is represented by an elevated sub-central structure (Fig. I, K). The dorsal part of the operculum is strongly concave and medially developed into a prominent dorsal fold which is slightly overhanging the apex of the operculum (Fig. G-K). The ventral edge of the operculum reveals two or three low folds (Fig. G, J, K). One specimen exhibits phosphatized parts of the operculum attached to the internal mould revealing faint concentric growth-lines similar to those evident on isolated, phosphatized opercula (Fig. J). The conical terminations of all other specimens otherwise represent moulds of the internal surface of the opercula and each consistently exhibits a marginal zone of low, pustules and a central, more or less, smooth zone (Fig. G, H). Low dorso-lateral folds are present on both sides of the central dorsal fold but these do not appear to reach the margin of the operculum (Fig. H). Phosphatized operculum and conch A single secondarily phosphatized specimen from AJX-M/ at Mt. Scott appears to reveal evidence for the direct association of conch and operculum in Triplicatella disdoma. The specimen is fragmentary and poorly preserved, but the apertural part of a hyolith conch

6 Page of appears to be preserved with a transversely oriented operculum (Fig. ). The external surface of the conch bears fine, straight transverse growth-lines (Fig. C). Although the apertural margins are probably not preserved, the straight growth-lines indicate that the aperture was planar, without a projecting ventral ligula. The possible operculum forms a transverse wall situated about 0 µm from the supposed apertural margin (Fig. B, C). The operculum is pyramidal with a blunt apex and a central fold on the dorsal side, a morphology strongly reminiscent of the operculum in associated internal moulds. Discussion The shape of the operculum in internal moulds is closely comparable to the morphology of phosphatized opercula of Triplicatella disdoma (Fig ; see also Bengtson et al. 0, figs, ) and the specimens described above are considered to represent the conjoined operculum and conch of the same organism. This find demonstrates unequivocally that T. disdoma as originally described is the operculum of a hyolith. Skovsted et al. (0) suggested that the operculum of Triplicatella could be withdrawn some distance inside the conch and the phosphatized partial conch and operculum of T. disdoma described above (Fig. ) appear to confirm this hypothesis. Further evidence that the diameter of the operculum was smaller than the diameter of the conch at the aperture, and that the operculum of T. disdoma was naturally recessed inside the conch comes from the relatively large number of phosphatic internal moulds combining conch and operculum in T. disdoma at Mt. Scott. In hyoliths where the operculum tightly seals the aperture of the conch, the two skeletal elements are likely to be separated post-mortem while the chance of

7 Page of intact preservation would increase if the operculum was recessed some distance inside the conch. The AJX-M section at Mt. Scott has produced large numbers of isolated phosphatized conchs and opercula of several different hyolith taxa, including Hyptiotheca karraculum Bengtson in Bengtson et al., 0, Parkula bounites Bengtson in Bengtson et al., 0, Conotheca australiensis Bengtson in Bengtson et al., 0, several species of Microcornus and Hyolithes conularoides Tate, (see Bengtson et al. 0), but only T. disdoma is preserved as complete internal moulds combining conch and operculum. The distinction between the two main hyolith groups, the orders Hyolithida Syssoiev, and Orthothecida Marek,, is mainly based on Ordovician and younger taxa (Marek, ) and although some Cambrian hyoliths fall within these two groups, the earliest hyolith faunas are dominated by taxa that are difficult to place in either group (Dzik ; Skovsted et al. 0; Malinky & Skovsted 0). As discussed by Skovsted et al. (0), the morphology of the operculum of Triplicatella combines characters commonly associated with both hyolithid and orthothecid hyoliths. This situation is not unique to Triplicatella but is rather common among hyoliths from the first half of the Cambrian Period (Dzik ; Malinky & Skovsted 0). The new material of Triplicatella disdoma described herein confirms that the operculum was able to move within the apertural part of the conch, a feature which is common in orthothecids (Marek ). This observation, in combination with the absence of characteristic hyolithid features such as ligula, helens and clavicles suggest that Triplicatella is more closely related to orthothecids than to hyolithids, despite the superficial similarity (triangular cross-section; dorsal curvature) of the conch to typical hyolithid conchs. However, Triplicatella exhibits a non-planar operculum without cardinal processes (although strongly developed marginal folds in at least one species, T. sinuosa Skovsted et al., 0, from Greenland and Holoplicatella margarita Clausen & Alvaro, 0

8 Page of from Spain mimic the morphology and possibly function of cardinal processes), features considered to be characteristic of younger orthothecids (Marek ). Consequently, Triplicatella is probably best considered to represent an ancestral group of orthothecids that evolved before the establishment of the typical orthothecid bodyplan as defined by Marek (). Acknowledgements The authors thank Graham Ragless of Beltana Station for access to localities in the Mt Scott Range. Jim Jago (University of South Australia, Adelaide), John Paterson (University of New England, Armidale), Peter Cockle and Rebecca Smart (both Macquarie University, Sydney) are thanked for assistance in the field. Ida S. Skovsted assisted in the construction of figures. Financial support from Australian Research Council Discovery Project #(to GAB) and from the Swedish Museum of Natural History to CBS are gratefully acknowledged. References BENGTSON, S., CONWAY MORRIS, S., COOPER, B.J., JELL, P.A. & RUNNEGAR, B.N., 0. Early Cambrian fossils from South Australia. Association of Australasian Palaeontologists, Memoir,. CLAUSEN, S. AND ÁLVARO, J.J., 0. Skeletonized microfossils from the Lower Middle Cambrian transition of the Cantabrian Mountains, northern Spain. Acta Palaeontologica Polonica,.

9 Page of DZIK, J.,. Evolution of small shelly fossils assemblages of the Early Paleozoic. Acta Palaeontologica Polonica,. GRAVESTOCK, D.I.,. Archaeocyatha from lower parts of the Lower Cambrian carbonate sequence in South Australia. Memoirs of the Association of Australasian Palaeontologists,. GRAVESTOCK, D.I., ALEXANDER, E.M., DEMIDENKO, YU.E., ESAKOVA, N.V., HOLMER, L.E., JAGO, J.B., LIN TIANRUI, MELNIKOVA, L.M., PARKHAEV, P.YU., ROZANOV, A. YU., USHATINSKAYA, G.T., ZANG WENLONG, ZHEGALLO, E.A. & ZHURAVLEV, A.YU., 0. The Cambrian biostratigraphy of the Stansbury Basin, South Australia. Transaction of the Palaeontological Institute,. MALINKY, J.M. & SKOVSTED, C.B., 0. Hyoliths and small shelly fossils from the Lower Cambrian of North-East Greenland. Acta Paleontologica Polonica,. MAREK, L.,. New Hyolithid Genera from the Ordovician of Bohemia. Casopis Narodniho Muzea,. MAREK, L.,. The class Hyolitha in the Caradoc of Bohemia. Sborník Geologikych ved Paleontologie,. SKOVSTED, C.B., PEEL, J.S. & ATKINS, C.J., 0. The problematic fossil Triplicatella from the Early Cambrian of Greenland, Canada and Siberia. Canadian Journal of Earth Science,. SKOVSTED, C.B., BROCK, G.A., TOPPER, T.P., PATERSON, J.R. & HOLMER, L.E.,. Scleritome construction, biofacies, biostratigraphy and systematics of the tommotiid

10 Page of Eccentrotheca helenia sp. nov. from the early Cambrian of South Australia. Palaeontology,. SYSSOIEV, V.A.,. Xxxxx [To the morphology, systematics and systematic position of the hyoliths]. Akademiâ Nauk SSSR, Doklady, 0 0 (Russian). TOPPER, T.P., SKOVSTED, C.B., BROCK, G.A., PATERSON, J.P.. The oldest bivalved arthropods from the early Cambrian of East Gondwana: systematics and biogeography. Gondwana Research,. YATES, A.M., GOWLETT-HOLMES, K.L. & MCHENRY, B.J.,. Triplicatella disdoma reinterpreted as the earliest known polyplacophoran. Journal of the Malacological Society of Australia,.

11 Page of Figure Captions Figure. A; Generalized map of the study-area showing the location of section AJX-M in the Mt Scott Range, Flinders Ranges, South Australia. B; stratigraphical column through the Ajax Limestone at AJX-M showing the distribution of phosphatized opercula and phosphatic internal moulds of Triplicatella disdoma Conway Morris in Bengtson et al., 0. Figure. Phosphatized operculum of Triplicatella disdoma Conway Morris in Bengtson et al., 0; SAMPxxx0; from sample AJX-M., Mt. Scott, Flinders Ranges, South Australia.; A, dorsal view with strongly developed dorsal fold; B, external view with dorsal side upwards; C, Oblique lateral view showing elevated apex and ventral and dorsal folds. Scale bar equals 0 µm. Figure. Phosphatic internal moulds of Triplicatella disdoma Conway Morris in Bengtson et al., 0 from sample AJX-M., Mt. Scott, Flinders Ranges, South Australia. A, B, G, H, SAMPxxx0; A, dorsal view of almost complete conch; B, oblique anterolateral view of conch and operculum; G, oblique view of operculum, scale bar equals 00µm; H, detail of operculum with accessory radial folds lateral to central dorsal fold, scale bar equals 0µm. C, I, SAMPxxx0; C, lateral view of conch and operculum; I, operculum in lateral view, scale

12 Page of bar equals 00µm. D, SAMPxxx0, dorsal view. E, J, SAMPxxx0; E, dorsal view; J, Oblique lateral view of apertural end showing partly phosphatized operculum, scale bar equals 00µm. F, K, SAMPxxx0; F, dorsal view; K, Oblique dorsal view of operculum, scale bar equals 00µm. Unless otherwise stated scale bars equal mm. Figure. Phosphatized operculum and partial conch of Triplicatella disdoma Conway Morris in Bengtson et al., 0; SAMPxxx0; from sample AJX-M, Mt. Scott, Flinders Ranges, South Australia. A, dorsal view; B, oblique view of aperture and possible transverse operculum; C, oblique lateral view of apertural region showing co-marginal external ornament. Scale bar equals 00 µm. Table. List of samples (with sample numbers defined by measured height above base of formation as well as true thickness data for each sample) from the AJX-M stratigraphic section yielding opercula and conchs of Triplicatella disdoma Conway Morris in Bengtson et al., 0.

13 Cambrian Series (undefined) B Cambrian Stage (undefined) FAD Trilobites Abadiella huoi Trilobite Zone Pararaia tatei Trilobite Zone Parachilna Fmn m m Sample No. (AJX-M) AJAX LIMESTONE Mt Scott Range, Page S.A of Section AJX-M Red and grey dolomitised stromatolitic limestones Red quartz - rich calcarenite and minor brecciated limestone interbeds Grey and red massive bioclastic limestones (minor nodular interbeds) Grey to buff bioclastic (commonly silicified) and nodular mottled limestones Red archaeocyathid - rich bioclastic limestones (wackstones and packstones) Hyolithid calcareous mudstone Grey dolomitised bioclastic & cryptalgal limestones (nodule interbeds common) Stromatolitic boundstone (chert nodules common) Triplicatella disdoma opercula Triplicatella disdoma conch + opercula Base of section coorinates : 0 ' "S / '."E Top of section coorinates : 0 ' "S / '."E A

14 Page of Figure. Phosphatized operculum of Triplicatella disdoma Conway Morris in Bengtson et al., 0; SAMPxxx0; from sample AJX-M., Mt. Scott, Flinders Ranges, South Australia.; A, dorsal view with strongly developed dorsal fold; B, external view with dorsal side upwards; C, Oblique lateral view showing elevated apex and ventral and dorsal folds. Scale bar equals 0 µm. xmm (00 x 00 DPI)

15 w ie ev rr ee rp Fo On Figure. Phosphatic internal moulds of Triplicatella disdoma Conway Morris in Bengtson et al., 0 from sample AJX-M., Mt. Scott, Flinders Ranges, South Australia. A, B, G, H, SAMPxxx0; A, dorsal view of almost complete conch; B, oblique anterolateral view of conch and operculum; G, oblique view of operculum, scale bar equals 00µm; H, detail of operculum with accessory radial folds lateral to central dorsal fold, scale bar equals 0µm. C, I, SAMPxxx0; C, lateral view of conch and operculum; I, operculum in lateral view, scale bar equals 00µm. D, SAMPxxx0, dorsal view. E, J, SAMPxxx0; E, dorsal view; J, Oblique lateral view of apertural end showing partly phosphatized operculum, scale bar equals 00µm. F, K, SAMPxxx0; F, dorsal view; K, Oblique dorsal view of operculum, scale bar equals 00µm. Unless otherwise stated scale bars equal mm. ly Page of xmm (00 x 00 DPI)

16 Page of w ie ev rr ee rp Fo ly On Figure. Phosphatized operculum and partial conch of Triplicatella disdoma Conway Morris in Bengtson et al., 0; SAMPxxx0; from sample AJX-M, Mt. Scott, Flinders Ranges, South Australia. A, dorsal view; B, oblique view of aperture and possible transverse operculum; C, oblique lateral view of apertural region showing comarginal external ornament. Scale bar equals 00 µm. xmm (00 x 00 DPI)

17 Page of Table. Stratigraphic positions of Triplicatella disdoma opercula, and conchs with opercula in the AJX-M section. Tape Measure True Thickness Triplicatella disdoma opercula Triplicatella disdoma conch + opercula.. x. x.. x. 0.0 x 0. x 0 0. x.. x.. x. x.0 x x.. x x. x x. x.. x.. x. x..0 x.. x. x. x..0 x

The operculum and mode of life of the lower Cambrian hyolith. Cupitheca from South Australia and North China

1 1 2 The operculum and mode of life of the lower Cambrian hyolith Cupitheca from South Australia and North China 3 4 5 By Christian B. Skovsted 1, Bing Pan 2, 3, Timothy P. Topper 1, Marissa J. Betts