Taphonomy and sedimentology of an echinoderm obrution bed in the Lower Devonian Voorstehoek Formation (Bokkeveld Group, Cape Supergroup) of South Africa

Accepted Manuscript Taphonomy and sedimentology of an echinoderm obrution bed in the Lower Devonian Voorstehoek Formation (Bokkeveld Group, Cape Supergroup) of South Africa Mhairi Reid, Emese Bordy, Wendy Taylor PII: DOI: Reference:

S1464-343X(15)00083-7 http://dx.doi.org/10.1016/j.jafrearsci.2015.04.009 AES 2257

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African Earth Sciences

Received Date: Revised Date: Accepted Date:

2 January 2015 15 April 2015 16 April 2015

Please cite this article as: Reid, M., Bordy, E., Taylor, W., Taphonomy and sedimentology of an echinoderm obrution bed in the Lower Devonian Voorstehoek Formation (Bokkeveld Group, Cape Supergroup) of South Africa, African Earth Sciences (2015), doi: http://dx.doi.org/10.1016/j.jafrearsci.2015.04.009

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Taphonomy and sedimentology of an echinoderm obrution bed in the Lower Devonian Voorstehoek Formation (Bokkeveld Group, Cape Supergroup) of South Africa Mhairi Reid*, Emese Bordy & Wendy Taylor Department of Geological Sciences, University of Cape Town, Cape Town, South Africa * Student author Research highlights: • • •

Exquisite Lower Devonian Malvinokaffric benthic assemblage from SW Gondwana. MicroCT scans of a co-occurring, fully-articulated ophiuroid-stylophoran assemblage. First report of an exceptionally preserved obrution bed from the South Africa.

Graphical abstract Corresponding author:

Emese Bordy [email protected] Department of Geological Sciences, University of Cape Town Private Bag X3, Rondebosch, 7701 Cape Town, South Africa Phone: +27 21 650 2901 http://www.geology.uct.ac.za/emese/bordy

Abstract The Lower Devonian Voorstehoek Formation is a lithostratigraphic unit within the Ceres Subgroup of the Bokkeveld Group (Cape Supergroup) in South Africa comprised of essentially mudstones and siltstones. This fossiliferous unit contains typical cool to cold water benthic biota (e.g., brachiopods, trilobites, crinoids) from the Malvinokaffric Realm of SW Gondwana, however, to date, not only the taphonomy of Voorstehoek invertebrates is understudied, but in general those of the Early Devonian marine communities of this Realm. The palaeontological and sedimentological features of the Emsian Voorstehoek Formation suggest that deposition took place in a shallow marine environment within the storm-influenced, proximal part of an offshore transition zone. 3D microCT scanning of this obrution bed allows us to report, for the first time from South Africa, on the co-occurrence of fully-articulated remains of both ophiuroids and stylophorans within the same sedimentary layer. Taphonomic analyses of this ophiuroid-stylophoran assemblage suggest a marine obrution deposit, which formed due to the rapid burial of the benthic community during high-energy storms, smothered both autochthonous and allochthonous taxa. This uniquely preserved, mixed ophiuroidstylophoran assemblage provides a taphonomic window into the marine ecosystems of the Early Devonian, including the structure of a benthic community within the Malvinokaffric Realm of SW Gondwana.

Keywords: taphonomy; Lower Devonian obrution bed; Malvinokaffric; ophiuroidstylophoran assemblage; SW Gondwana; 3D microCT scanning

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1. Introduction The Devonian Bokkeveld Group of South Africa comprises invertebrate fossil assemblages that belong to the unique cool to cold water, high latitude Malvinokaffric Realm, a biogeographic term first introduced by Richter (1941) and originally used to denote the highly endemic, benthic marine, Devonian invertebrate faunas of the Southern Hemisphere. Initially defined based on the distribution of endemic Devonian trilobites and brachiopods (Clarke, 1913; Richter and Richter, 1942; Boucot et al., 1969; Eldredge and Ormiston, 1979), this polar latitude biogeographic unit now encompasses the Early Palaeozoic (Late Ordovician to Middle Devonian (Eifelian)) invertebrate fossil assemblages of south-western Gondwana (i.e., South America, southern Africa, Falkland Islands, Antarctica; see inset in Fig. 1) - Boucot, 1985, 1988; Melo, 1988). Generally, the Malvinokaffric Realm is characterised by a low-diversity fauna with abundant conulariids and hyolithids and the absence of certain major groups such as stromatoporoids, conodonts, nautiloids, and graptolites and almost no thermophilic reef-building corals or bryozoans (Oliver, 1980; Bigey, 1985; Boucot, 1985, 1988; Hiller and Theron, 1988; Meyerhoff et al., 1996).

The Malvinokaffric biota of the Bokkeveld Group comprises both highly endemic species (e.g., certain bivalves and brachiopods) as well as several shared taxon (e.g., Australospirifer sp., Australocoelia sp., Burmeisteria sp.) that are also found in the Devonian of the Falkland Islands and southern parts of South America (Ponta Grossa Formation in the Parana Basin - e.g., Reed, 1906; Clarke, 1913; Richter and Richter, 1942; Melo, 1988; Almond et al., 1996; Boucot, 1999). The fossiliferous nature of the Lower Devonian Bokkeveld Group was first recorded in 1830 and onwards (e.g., 3

Grisbrook, 1830; Thom, 1830; Bain, 1856; Salter, 1856) and in comprehensive reviews by Reed (1925), Theron (1972) and Oosthuizen (1984). Furthermore, the Lower Bokkeveld stylophorans were described by Rennie (1936) and revised in detail Ruta and Theron (1997). Additionally, Jell and Theron (1999) provided an extensive revision of the Bokkeveld crinoids, blastoids and asterozoans.

The unique preservation of Bokkeveld invertebrate assemblages has often been associated with obrution beds (Hiller and Theron, 1988) which are deposits attributed to sudden smothering of benthic communities by rapidly deposited sediments and provide useful “snapshots” of offshore marine communities before burial. The Bokkeveld obrution beds contain fully-articulated echinoderms, well-preserved crinoid calyces, arms and stems (Gydo Formation - Fig. 2), as well as exquisitely preserved bryozoans, complete trilobites and adult to immature ophiuroids (Waboomberg Formation - Hiller and Theron, 1988; Jell and Theron, 1999). Furthermore, wellpreserved ophiuroids (brittlestars) and stylophorans, a group of extinct free-living echinoderms, were also reported from the obrution beds of the Voorstehoek and Waboomberg Formations (Fig. 2 - Ruta and Theron, 1997; Jell and Theron, 1999), but abundant, fully-articulated remains of both ophiuroids and stylophorans within the same beds have not been formally described from the Devonian of South Africa.

In this preliminary report we describe a well-preserved ophiuroid-stylophoran burial assemblage in a thin, fossiliferous obrution bed from the Lower Devonian (Emsian ~ 400ma) Voorstehoek Formation with the goal of understanding the biostratonomic (syndepositional) and diagenetic processes that acted on fossil preservation during and 4

after deposition. To study the taphonomy and palaeoecology of the bed, we used nondestructive microtomographic (microCT) scanning to visualize the fossils in 3D without affecting the integrity of the specimens. Conventional palaeontological analysis of this deposit posed problems due to deep chemical weathering that had dissolved the calcitic fossils, leaving only moulds. The use of computer-aided visualization and analysis is an extremely powerful tool that is enabling paleontologists to gain new insights into the anatomy, development, and preservation of extinct groups (Cunningham et al., 2014).

Mass accumulations or “ophiuroids meadows” have been recorded in modern oceans and can form extensive carpet-like accumulations of hundreds to thousands of individuals per m2 on or within the seafloor (e.g., Warner, 1971; Allain, 1974; Aronson, 1987; Fujita and Ohta, 1989, 1990; Fujita, 1992; Shin and Koh, 1993; Aronson and Blake, 1997; Stöhr et al., 2012). Normally dominated by a single species (Brun, 1969; Fujita and Ohta, 1989, 1990), ophiuroid meadows and can occur over a wide range of depths from nearshore environments (Warner, 1971; Wilson et al., 1977) to the deep sea (Ohta, 1983; Fujita and Ohta, 1989). The fossil record of ophiuroid meadows is fragmentary due to the low preservation potential of their internal skeletons which are composed of thousands of calcitic ossicles held together by muscles and ligaments and covered by a thin dermal layer over surface of the body. Taphonomic studies focusing on modern forms (Meyer, 1971; Schäfer, 1972; Brett et al., 1997; Ausich, 2001; Kerr and Twitchett, 2004; Nebelsick, 2004; Gorzelak and Salamon, 2013) indicate that complete disarticulation occurs very rapidly after death, typically within 10 to 14 days and is temperature dependent. The discovery of well-preserved ophiuroids in the fossil record is the result of catastrophic burial by obrution events (sensu Brett, 1990) where large 5

pulses of sediment rapidly smother benthic communities, permanently shielding them from decay and scavengers and preventing the escape of mobile taxa (Brett and Baird, 1986; Speyer and Brett, 1991; Brett et al., 1997). Fossil ophiuroid beds have been described from the Ordovician to Cenozoic (Spencer, 1950; Aronson, 1989; Mikulas et al., 1995; Donovan et al., 1996; Radwanski, 2002; Kutscher and Villier, 2003; Salamon et al., 2003; Twitchett et al., 2005; Williams et al., 2006; Hunter et al., 2007; Shroat-Lewis, 2007; Zatoń et al., 2008; Martínez et al., 2010; Thuy, 2011; Rousseau and Nakrem, 2012; Thuy et al., 2013; Jagt et al., 2014). Aronson (1989) and references therein, relates the decline in the distribution of dense ophiuroid assemblages in the late Mesozoic and Cenozoic to the rise in durophagous and to an abrupt increase in bioturbation (“biological bulldozing”) during the late Mesozoic.

Stylophorans have a fossil record that extends from the Middle Cambrian-Upper Carboniferous and have been documented to occur with ophiuroids in mixed ophiuroidstylophoran aggregations from the Middle Ordovician to the Lower Devonian (Spencer, 1950; Caster, 1954; Gill and Caster, 1960; Caster, 1983; Haude, 1995; Mikulas et al., 1995; Donovan et al., 1996; Ruta, 1997; Bartels et al., 1998; Hunter et al., 2007; Lefebvre, 2007; Lefebvre et al., 2007, 2008, 2010; Hunter et al., 2010).

Studying the taphonomy of this obrution deposit is important not only to understand the palaeobiology of the Early Devonian invertebrate palaeocommunities, but to gain critical insights into the palaeoecology and the dynamics of these ancient depositional environments in SW Gondwana. Furthermore, by integrating the taphonomic and sedimentologic findings, a more accurate understanding of the Early Devonian marine 6

communities as well as a detailed reconstruction of the palaeoenvironment of the Voorstehoek Formation will emerge. To date, the taphonomy of the invertebrate assemblages of the Voorstehoek Formation has received little attention as previous studies mainly focused on the systematic palaeontology (Thom, 1830; Bain, 1856; Salter, 1856; Reed, 1925; Ruta and Theron, 1997; Jell and Theron, 1999) and the biostratigraphy of these Emsian invertebrates (Theron, 1972, 2003; Oosthuizen, 1984; Hiller and Theron, 1988).

2. Geological context The study area is located within the southern part of the north-south trending Clanwilliam Sub-basin of the Cape Basin and the exposed rocks lithostratigraphically belong to the Voorstehoek Formation (Rust, 1973), one of the lowermost units of the Bokkeveld Group (Figs. 1 and 2). The studied outcrop is located ~80 km E of Ceres near the Hex River Pass in the Western Cape Province (GPS: 33° 24’ 03.6’’ S, 19° 52’ 42.7’’ E) (Fig. 1). The outcrop is an E-W orientated road cutting of ~80 m in length and ~2 m in height along the N1. Situated within the northern limb of an E-W striking syncline in the Cape Fold Belt, the beds of the outcrop dip 20° S and show evidence of E-W faulting and shearing.

Forming the middle unit of the clastic Cape Supergroup, the distinctly argillaceous Bokkeveld Group conformably overlies the Table Mountain Group and underlies the Witteberg Group (see legend in Fig. 1). The Bokkeveld Group is a ~0.7 - 3.5 km thick succession of Lower to Middle Devonian siliciclastic, mostly marine sedimentary rocks, ranging in age from the Lochkovian (~419 Ma) to the Givetian (~382 Ma; e.g., Theron, 7

1970; Boucot et al., 1983; Hiller and Theron, 1988; Theron and Johnson 1991; Fourie et al., 2011). The Bokkeveld Group outcrops within the Permo-Triassic Cape Fold Belt of the Western and Eastern Cape Provinces of South Africa (Fig. 1). The six lowermost Bokkeveld formations collectively referred to as the Ceres Subgroup (Fig. 2), extend across the entire Cape Fold Belt from east to west. The Ceres Subgroup consists of three laterally continuous, upward-coarsening successions of mudstones, siltstones and sandstones (Fig. 2) that have been interpreted as evidence for: (a) deposition in muddy offshore to sandy nearshore settings on a stable, storm-dominated marine shelf; and (b) repeated basinward shifts of an E-W striking shoreline after major transgressive events (Theron and Johnson, 1991),

Found in the middle part of the Ceres Subgroup, the mudstone-rich Lower Devonian (Emsian) Voorstehoek Formation is sandwiched between the sandstones of the underlying Gamka Formation and the overlying Hex River Formation. The thickness of the Formation varies widely from east to west, but in this study area, it is estimated to be ~100 m thick (Theron, 1972; 2003). The Voorstehoek Formation is dominated by dark grey to olivine grey siltstones (up to 65%), mudstones (up to 60%) and minor sandstones. When fresh, these rocks are dark grey to olive grey in colour, however in most outcrops, they have been heavily weathered to pale yellow brown, grey, orange and red, and are often covered by a red, ferruginised crusty layer. Generally, the rocks of the Bokkeveld Group have a well-developed cleavage that formed during the PermoTriassic deformation of the Cape Supergroup (Almond, 2011).

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The lower part of the Formation is made up of dark grey mudstones with thin calcareous lenses, which up-sequence, give way to siltstone and thin sandstone lenses. Common features found in the upper part of the Formation are storm deposits (tempestites or event beds) comprising wave ripple cross-lamination and hummocky cross-stratification that increase in frequency indicating a shallowing upwards succession (Theron, 2003; Almond, 2011). The sedimentary features in these heterolithic tempestite-dominated successions suggest that the Formation was deposited in a predominantly shallow marine environment, and this is confirmed by the presence of diagnostic invertebrate fossils. These fossil assemblages are dominated by trilobites, articulate brachiopods, crinoids, ophiuroids, bivalves, bellerophontid gastropods, orthocone nautiloids, and conical-shelled groups such as hyolithids and tentaculitids as well as richly diverse association of trace fossils of the Cruziana ichnofacies (Theron, 1972; Cooper, 1982; Oosthuizen, 1984; Theron and Loock, 1988; Gresse and Theron 1992; Almond et al., 1996; Theron 2003; Almond, 2008, 2011). Invertebrate assemblages of the Voorstehoek Formation are often disarticulated and concentrated in shell lenses, which further suggest strong current action, created by storm waves in nearshore sediments (Almond, 2011). Compared to the collections of marine invertebrate faunas from the underlying Gydo Formation, the collections from the Voorstehoek Formation are smaller with sparser, less diverse and poorly studied fossils (Hiller and Theron, 1988; Almond, 2011).

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3. Methodology 3.1 Sedimentology At the outcrop, field data were gathered in the form of detailed sedimentary facies descriptions, photographs, sketches, and a representative sedimentary log. Panoramic views of the outcrop were also constructed from photographs, and allowed a more quantitative assessment of the outcrop, including the lateral extent and variation of the geometry of the sedimentary layers. Petrographic analysis of selected samples was conducted in order to establish a more accurate compositional and taphonomic relationships and to look for further evidence that could help document the local palaeoenvironments.

3.2 Taphonomy

A total number of 14 samples comprising 160 fossil specimens were carefully excavated for further taphonomic and micro-tomographic analyses from a portion of a fossiliferous lens (Fig. 1). The sample sizes are small, varying in surface area from 60.5 mm X 38.4 mm to 109.4 mm X 81.6 mm. The stratigraphic position and characteristics of the obrution bed within the outcrop were measured on the photos and detailed sedimentary log. All sedimentological and palaeontological observations of the samples have been summarized in the Appendix. For taphonomic assessment, we performed taxonomic counts and observed: 1) the level of articulation (especially for multielement skeletons), 2) the orientation and alignment of skeletal elements, 3) the nature of fragmentation, 4) the degree of weathering, 5) the association of body fossils with trace fossils and 6) the geometry of the fossiliferous lens. In addition to CT scans, 10

photographs have been taken of individual fossils and assemblages to assist with the analysis. On each sample, specimens were counted and arm length to disc diameter ratios measured from the 3D digital reconstructions. The ophiuroids were categorized in accordance to the quality of preservation, following the procedure of Kerr and Twitchett (2004). The orientations (i.e., normal, oral-side up or oblique) of specimens and associated fauna, were also noted (Dornbos and Bottjer, 2001; Kerr and Twitchett, 2004; Twitchett et al., 2005; Zatoń et al., 2008). All specimens referred to in this paper are deposited in the collections at the Council for Geoscience, Bellville, Western Cape where previous collections for the Formation are also curated.

3.3 MicroCT Scanning Non-destructive analysis of the 3D internal structure of 14 ophiuroids slabs was achieved by using the Central Analytical Facility (CAF) at Stellenbosch University, Cape Town. The instrument is a microfocus X-ray CT scanner, model General Electric Phoenix V|Tome|X L24 with additional NF180 option. X ray-settings were 160Kv and 100 uA, 2500 images were acquired in a full rotation at image acquisition time of 500 ms per image. Upon acquiring CT images, data analysis was performed using Volume Graphics VGStudioMax 2.2 software to render a 3D model.

4. Sedimentology 4.1. Observations The studied outcrop comprises successions of medium- to fine-grained sandstones interbedded with siltstones and sandy mudstones. The laterally continuous, tabular beds vary in thickness from 5 cm to 1 m. The great lateral continuity of the sandstones 11

(Fig. 3A) is especially well-exposed in the nearby Hex River Pass. Typical sedimentary features are summarised in Figure 4. In the eastern part of the outcrop, the sedimentary structures in basal sandstone layer vary laterally and vertically from massive to lowangle cross-bedding (Fig. 3). Westwards, this basal sandstone becomes a succession of thinner, erosionally based sandstone layers that incise into the underlying silty sandstone beds. In the middle part of the outcrop (Fig. 3B), these very fine to finegrained sandstones are hummocky cross-stratified (Facies Shc) and occur within the heterolithic, silty sandstone and partially eroded, scoured mudstones. The 5 to 20 cm thick sandstones show vertical and lateral variations in sedimentary structures from hummocky cross- stratification to wave ripple cross-lamination (Fig. 4). In the western part of the outcrop, the sandstone beds become fused again into a single layer of sandstone to which we refer to as ‘amalgamated’ in this study. While sandstones, especially the hummocky cross-stratified facies, is the most common rocks in this outcrop, locally partially eroded mudstones (Facies Fl) as well as mud pebble conglomerates (Facies Gm) with reworked brachiopod fragments (Fig. 5) are also present.

4.2. Interpretations 4.2.1 Depositional processes The very fine to fine-grained sandstones, with (1) low-angle bounding surfaces, (2) variations in thickness of individual laminae and dip angle and (3) internal laminae dipping in all directions, display characteristics of a distinctive type of cross-bedding with low-angled, gently undulating sets, known as hummocky cross-stratification (Fig. 6) - Harms et al., 1975; Walker, 1979; Cheel and Leckie, 1993). Our outcrop preserves 12

the ‘scour and drape’ form of hummocky cross-stratification that has been shown by Cheel and Leckie (1993) to result from scour events that generate an erosional hummocky topography on the sea floor, subsequently draped by finer sediments that settle out of suspension. Hummocky cross-stratification is generally produced in combined, oscillatory wave-generated and unidirectional storm-induced currents., and is considered to be diagnostic of shallow marine storm sedimentation in shoreface and shelf environments as well as in the offshore transition zone which occurs between the fair-weather and storm wave-base on storm-dominated shelves (Fig. 6 - Walker, 1979; Leckie and Walker, 1982; Walker et al., 1983; Duke et al., 1991; Cheel and Leckie, 1993). Although they may form in shallower water too, hummocky cross-stratification has low preservation potential in shoreface deposits above the fair-weather wave-base due to the reworking of the sediments by wave scour processes.

The sedimentary successions in the studied outcrop are identified as proximal storm deposits (event beds), because they display a characteristic facies succession ranging from massive conglomerate beds with reworked brachiopod fossil fragments (storm erosion) at the base (see Fig. 5C), to amalgamated beds of fine sandstone with hummocky cross-stratification (main storm deposition) (see Fig. 4C), overlain by minor wavy lamination, and wave ripple cross-lamination (waning storm deposition) (see Fig. 4D), which finally give way to fair weather mud deposits at the top (Figs. 5B and 6). Event beds or tempestites form when powerful offshore-directed storm-induced currents wane and deposit a basal layer of coarse clasts, consisting of mud pebbles and/or shell fragments, followed by hummocky cross-stratification and ripple crosslamination in finer sediments. These may be subsequently reworked by onshore13

directed, asymmetrical, oscillatory currents generated by the storm (Reineck and Singh, 1980; Aigner, 1985; Cheel and Leckie, 1991) (Fig. 6). A return to quiescence between storms allows a new layer of mud to accumulate, and if storms are not frequent, bioturbation may completely destroy the primary structures of the event beds. Generally, on storm-dominated shelves, the proportion of mud and the intensity of bioturbation increase whereas the sand content decreases offshore (Cheel and Leckie, 1991), even though, depending on the gradient of the seafloor, offshore-directed storminduced density currents can supply sand for tens kilometres (Walker, 1979; Johnson and Baldwin, 1996).

Erosionally based sandstone beds in our outcrop are each interpreted to represent a storm event that scoured into the underlying muddy-silty sandstone layer. The amalgamated sandstones in the eastern and western extreme of the outcrop that only contain localized mudstones lenses are reminiscent of proximal storm deposits, and are interpreted here as welded products of several storm events beds. In this case, the amalgamated sandstones may suggest either relatively frequent storm events or storm events so strong that all fair weather deposits were scoured away before the deposition of the next layer of hummocky cross-stratified sand (Dott and Bourgeois, 1982).

4.2.2 Deposition of the Voorstehoek Formation in the Clanwilliam Sub-basin The tabular and lateral continuity of the sandstone deposits in the Voorstehoek Formation and the occurrence of marine invertebrate fossil assemblages are all indicative of a shallow clastic marine environment (Fig. 3). The vertical increase in abundance of sandstones, their grain size and the prevalence of storm deposits are 14

indicative of an upward-shallowing (shoaling) succession. This has been interpreted as evidence for a large scale, basin-wide regressive episode in the southern part of the Clanwilliam Sub-basin in the Emsian (Theron, 2003; Almond, 2011). Furthermore, in the north-south trending Clanwilliam Sub-basin, the abundance of the sandstone facies increases from south to north, and this, together with other sedimentological evidence (Theron, 2003) suggests that in the north, the basin was shallower and deposition occurred above the fair-weather wave-base (Fig. 6). For that northern region, ancient delta-slope to inner shelf as well as tidal flats (based on some structures indicative of brief exposure) have also been suggested as potential depositional environments (Theron, 2003). All in all, based on sedimentological and palaeontological evidence, the Voorstehoek Formation was deposited in a shallow marine, inner shelf setting (with potential deltas and tidal flats) in the north, whereas in the south, near our outcrop, the sedimentation occurred in a deeper water setting closer to the outer shelf. In this southern region of the Clanwilliam Sub-basin, the older strata of the Voorstehoek Formation were initially deposited in the offshore zone, and then, as the overall shallowing of the sub-basin occurred, in the offshore transition zone (Fig. 6).

5. Taphonomy 5.1 Observations The fossils occur in a 5 cm thick, lens-shaped bed of medium-grained sandstone that pinches out laterally within 1.5 m (Fig. 4). This fossiliferous bed generally lacks any primary sedimentary structures, except for some very vague and locally developed horizontal laminations. The bed is located below fine-grained silty sandstones that are wave ripple cross-laminated and above horizontally laminated sandstones (Fig. 4). The 15

lower bounding surface of the bed appears to be an irregular erosional surface, while the upper surface appears to be a sharp, transitional surface into the overlying siltstone. Overall the size of the fossils decreases upward from ~10 cm to less than 1 cm. The skeletal remains occur as external moulds and the overall abundance of fossils is constant throughout the bed (Figure 7).

The obrution bed within the Voorstehoek Formation is dominated by an echinodermrich assemblage of articulated ophiuroids, two types of mitrate stylophorans and crinoid ossicles (Figs. 8, 9, 10). Furthermore, brachiopods (mainly Australospirifer sp.), bivalves, trilobite thorax fragments and numerous unidentifiable shell fragments are also present (Figs. 8, 9). Brachiopod shells ~4-5 cm in diameter are concentrated in the lower portion of the bed and are generally in a stable (convex downward) hydrodynamic position (Fig 8). The fragmentary arthropod and mollusc material is poorly sorted with shells showing evidence for little or no abrasion and both in convexup and convex-down positions. Crinoid remains are both disarticulated arms and disarticulated ossicles with moderate abrasion that are scattered throughout the deposit (Fig. 11),

5.1.1. Ophiuroidea Ophiuroids are found throughout the fossiliferous bed at varying levels (Fig. 7) and in varying orientations. Using non-destructive micro CT imaging, a total of 120 ophiuroid sp. indet, skeletons were recorded. Specimens range from intact arms right to the tips with a central disc to those with discs bearing fragmented arms, suggesting preservation stage 0 and 2 of Kerr and Twitchett (2004). In addition to the nearly 16

complete individuals, large numbers of arm fragments were present in all the samples (Figs. 8, 9, 10, 11). The fully-articulated ophiuroids have been found clustered together and are generally in dense aggregations in the samples (Table 1). Specimens are considered small with disc diameters between 5.6 mm and 10.7 mm and arm lengths averaging 15 mm. Determining the ratio of arm length (al) to disc diameter (dd) is a useful indicator of ophiuroid mode of life (Twitchett et al., 2005). The average ratio of 1.8 measured from complete specimens falls within the epifaunal category (al:dd < 5). They posses short arms in relation to disc diameter in contrast to burrowing brittle stars that display long arms in relation to disc diameter (al:dd >9). Detailed and systematic sampling of the bed is being conducted, but before more quantitative results are available, it is only possible to note the significant number of specimens in an overturned, oral-side up orientation along with in situ specimens. There are also what appears to be different age classes present, however this also remains to be quantified in the taxonomic study that is under way.

5.1.2. Mitrate stylophorans A large individual of the mitrate stylophoran Placocystella? (thecal length = 15.2 mm), as well as 40 smaller specimens of Paranacystidae gen. and sp. indet. (thecal length between 2 .5 and 5.8 mm) (Figs. 10, 11; Tables 1, 2) were also found in the obrution bed. Stylophorans occur throughout the bed and in varying positions, in some cases closely associated with intact ophiuroids (Fig. old 15). The thecae are complete, but also fragile elements such as the aulacophore and the paired posterior spines (when present) are preserved in connection with the theca (Fig. old 14 and 15). From a taphonomic view-

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point, the preservation of the stylophorans is excellent and suggests rapid in situ burial of the living organisms.

In Placocystella? and related infaunal taxa (anomalocystitid mitrates), the general body plan consists of a flattened theca, two posterior appendages and a articulated feeding appendage or auclacophore (Lefebvre, 2003). The absence of posterior spines in the smaller specimens (gen. and sp. indet.) and their slightly different thecal shape could correspond to a non-anomalocystitid epibenthic mitrate genus (Lefebvre, 2003). However, the quality of the current CT scans does not permit the clear identification of the stylophorans, and their further study will be needed.

5.2 Interpretations 5.2.1. Obrution by storm deposition Echinoderms possess fragile endoskeletons that are very sensitive to abrasion in depositional processes that affect them after death. Both ophiuroids and stylophorans have been classified into ‘type 1 echinoderms’ according to the Brett et al. (1997) system. Type 1 echinoderms are characterised by a skeleton of very weakly articulated plates (ossicles), which are bound together by ligaments, other soft tissues that have very limited preservation potential (Brett et al., 1997). This means that the ossicles are loosely connected, and after death, at the water-sediment interface rapidly decay and disarticulate in the gentlest of sedimentary currents. Experimental studies have shown that echinoderm disarticulation occurs within ~15 hours of death, and complete disarticulation into ossicles usually takes place within one to two weeks (Meyer, 1971; 18

Schäfer, 1972; Lewis, 1986, 1987; Kidwell and Baumiller, 1990; Kerr and Twitchett, 2004). Thus, the preservation of articulated ‘type 1 echinoderms’ can only occur if buried alive rapidly by a sufficiently thick sediment pile that inhibits biochemical degradation, and mechanical disarticulation as well as biogenic disturbances such as burrowing by scavengers (Brett and Baird, 1986, Speyer and Brett, 1991, Brett et al., 1997). While studies on modern echinoderms showed that burial by only 5 cm of sediment is sufficient to immobilise and kill certain ophiuroids (Donovan, 1991), however, if buried under insufficiently thick sediment pile, mobile taxa can escape by extracting themselves from the sediment (Schäfer, 1972).

Rapid burial or obrution events that supply sufficiently thick sediment piles capable of burying echinoderms alive are commonly generated in sudden sedimentological events either by storms (resulting in tempestites) or mass movement processes (resulting in sediment slides, slumps, debris flows, turbidites) that typically evolve into one another (Donovan, 1991; Brett et al., 1997). Preservation potential of obrution beds is especially high in depositional environments with otherwise calm background sedimentation, i.e., in offshore or offshore transition settings (Brett et al., 1997)

Stratigraphic, sedimentologic and taphonomic characteristics of siliciclastic deposits can be used to draw inferences about the palaeoenvironmental conditions that occurred prior to and after burial of benthic organisms. Based on primary sedimentary features, the fossiliferous bed is part of a succession that formed in a shallow marine environment, within the storm influenced, proximal part of an offshore transition zone (Fig. 6). The exceptional fossil preservation of the articulated stylophorans and 19

ophiuroids with intact arms right to the tip (Figs. 8, 9, 11) indicates that a rapid burial event occurred, preserving their very fragile and small skeletons. However, the fossiliferous bed also contains a large amount of fragmentary material, namely disarticulated crinoid ossicles, sections of ophiuroid arms, trilobite fragments, brachiopods and bivalves. The fragmentary material likely reflects the influence of mixed taphonomic processes, at least some of which are linked to high energy currents bringing fragmentary fossils from elsewhere and potentially incorporating into its sediment load time averaged material that decayed/accumulated on the seafloor over longer periods (cf. Speyer and Brett, 1991).

The autochthonous or allochthonous nature of obrution deposits is determined by criteria that indicate whether the organisms have been buried in place or been caught up in currents and subsequently transported for some distance prior to deposition (Brett et al., 1997). However, care needs to be taken when interpreting echinoderm preservation as fully-articulated echinoderms may not be exclusively autochthonous (in situ buried), but allochthonous (transported). This was demonstrated in experiments by Kerr and Twitchett (2004) who showed that the multielement skeletons of echinoderms can be transported large distances without any disarticulation even after death, provided that the transport occurs within a couple of hours (see above). Consequently, fully-articulated echinoderms may indeed be allochthonous or para-autochthonous (somewhat transported before burial) as opposed to autochthonous (in situ buried).

The fully-articulated ophiuroids examined in the study display both a normal (life 20

position) or an inverted orientation (oral-side up) within the bed (Fig. 11). Therefore, the inverted specimens strongly indicate the displaced nature of the fossils that were likely transported by currents prior to deposition (Zatoń et al., 2008). On the other hand, the fully-articulated nature of the specimens also indicates that the transport may have happened while the ophiuroids were alive or very soon after their death and the transport had to be followed by immediate sediment burial during a single obrution event (Rousseau and Nakrem, 2012).

5.2.2. Gregarious behaviour of ophiuroids

Dense beds of ophiuroids have been recorded in the fossil record since the late Middle Ordovician (Fujita and Ohta, 1989, 1990; Hunter et al., 2007). Present day dense epifaunal populations of ophiuroids exhibiting gregarious behaviour can generally persist only in low predation refuges both in shallow and deeper water environments (Aronson and Blake, 1997). Considering that ophiuroids can sometimes autotomize their arms (self-mutilation) during stressful environmental changes or as a defence mechanism against predators (Donovan, 1991), sub-lethal arm damage followed by regeneration provide evidence of predation pressure on extant and fossil ophiuroids (Aronson, 1987, 1989; Fujita and Ohta, 1989, 1990). A lack of evidence for partially amputated arms or regrowth in the specimens suggests that predation pressure was possibly low in the Voorstehoek ophiuroid population. Therefore it is possible that the large number of ophiuroids preserved in the samples (Appendix-Table 1) is due to the movement of the storm induced current over an in situ gregarious ophiuroid population, the elements of which were picked up and transported for some distance. 21

Tentatively, we presume that the stylophorans co-existed with the gregarious ophiuroid population, and were also displaced by the current in the same manner as the ophiuroids.

5.2.3. Differences in preservation of the echinoderm taxa The fossiliferous bed shows different states of preservation of the echinoderm taxa. The large number of ophiuroid arms preserved with fully-articulated taxa (Figs. 9, 11) is challenging to interpret because of self-mutilating behaviour in ophiuroids. However, as mentioned earlier, the lack of evidence for partially amputated arms or regrowth of the specimens implies that the large number of ophiuroid arms is not due to casting off, but rather mechanical fracturing of the arms either when the ophiuroids tried to escape from the sediment influx caused by the storm or when they tumbled a short distance in the currents. In such events, the arms could remain intact as the ligaments are strong enough to hold the individual arms together for a short while before being entombed in the sediment (Donovan, 1991; Kerr and Twitchett, 2004). Fracturing due to tumbling in currents is more plausible due the presence of the allochthonous, oral-side up ophiuroid specimens (Fig. 11).

Brett et al. (1997) have classified crinoid remains as ‘type 2 echinoderms’, because crinoids can resist disarticulation better than ‘type 1 echinoderms’. This is confirmed by experimental studies that demonstrated that crinoid stalk fragments can survive on the seafloor without disarticulating for extended periods (Oji and Amemiya 1998). The disarticulated and moderately abraded crinoid ossicles scattered throughout the deposit may indicate that the crinoid specimens spent a large amount of time decaying 22

on the seafloor and form part of the substrate that the ophiuroids were inhabiting (Hunter and Underwood, 2009). Alternatively, semi-articulated crinoid stalk fragments might have been picked up from nearby settings by the abrasive currents in which winnowing and reworking broke them apart into individual and isolated ossicles. The co-occurrence of crinoid ossicles with fully-articulated ophiuroids and stylophorans is seen here as a result of mechanical mixing after the current, loaded with transported crinoid remains and other fragmentary taxa, washed over the in situ stylophoran and, potentially gregarious, ophiuroid community (Rousseau and Nakrem, 2012).

Finally, the preservation of articulated and fragmentary echinoderms as well as fragmentary trilobite, brachiopods and bivalves within the same bed might be explained by the evolution of the transporting medium. More specifically, the initial highly abrasive current, by incorporating increasingly more solid particles (i.e., sediment, various benthic organisms, dead or alive) into itself, could have evolved into a higher sediment concentration current. The mode of transportation of the (organic and inorganic) fragments in the dense current was no longer in traction (where the moving fragments rubbed against one another and the sea floor), but "en masse". Consequently, those echinoderms that were picked up in the final stages of current evolution could remain better to fully-articulated, because they were moved in isolation (i.e., without abrasion) in these sediment laden currents. It is also plausible that, at least some of the poorly sorted debris of trilobite moults and mollusc shells in the fossiliferous bed was not exclusively brought in by high energy currents, but was part of a shell pavement. This time averaged, concentrated debris, which shows limited or no abrasion, had accumulated in situ on the sea floor near or among the elements of the stylophoran and 23

ophiuroid community.

In summary, taphonomic evidence shows that the fossiliferous bed in this study preserves both allochthonous and para-autochthonous specimens. The storm current may have transported disarticulated crinoid ossicles and other taxa downslope to a setting inhabited by stylophorans and potentially gregarious ophiuroids. Some of the live echinoderms were also picked up from the community and transported for some distance by the sediment laden current, which entrapped and suffocated them, leading to the preservation in the fossiliferous deposit of fully-articulated stylophorans and ophiuroids both in oral-side up and life positions.

6. Conclusion This study on sedimentary and taphonomic context of the Lower Devonian Voorstehoek Formation suggests that the deposition of the sedimentary layers in the study area took place in a shallow marine environment, within the storm-influenced, proximal part of an offshore transition zone. The tempestites in the Voorstehoek Formation formed during storm events and resulted in a characteristic facies succession that comprises an erosional basal surface, an amalgamated sandy unit with hummocky cross-stratification and wave-rippled cross-lamination as well as massive mudstones.

Taphonomic and sedimentological analyses also suggest that the abundant cooccurrence of fully-articulated remains of both ophiuroids and stylophorans within the same bed were preserved due to sudden burial in an obrution deposit. The large number of intact ophiuroids and stylophorans, with some in life position, indicates 24

rapid sedimentation during a single storm event. The differences in preservation of the echinoderm taxa imply the presence of both allochthonous and para-autochthonous specimens. Disarticulated crinoid ossicles (allochthonous taxa) were most likely transported by storm currents and deposited in a setting inhabited by stylophorans and potentially gregarious ophiuroids. Possibly, some of the live echinoderms were also picked up from the community and transported for some distance by the sediment laden current, which entrapped and suffocated them, leading to their preservation as fully-articulated stylophorans and ophiuroids both in oral-side up and life positions.

Finally, by describing this Voorstehoek ophiuroid-stylophoran assemblage, that comprises fragmentary and fully-articulated taxa and is preserved in an obrution bed in the Lower Devonian of South Africa, our study helps refine the palaeoecology of the Malvinokaffric echinoderm communities in the Early Devonian of SW Gondwana.

25

Acknowledgements We would like thank Cameron Penn-Clark for all the helpful discussions, support and never-ending enthusiasm, and to the field assistance of Adrian Bunge, Antónia Reis De Carvalho and Tirelo Mputle.. Research funds received by MR from the South African DST/NRF Centre of Excellence in Palaeosciences (CoE-Pal) and by EB from Incentive Funding programme of the National Research Foundation are gratefully acknowledged. The funding sources had no other involvement in this research. We are also very grateful to the Editor Pat Eriksson as well as reviewers Bertrand Lefebvre and Aaron W. Hunter for their very detailed and constructive comments that assisted in improving this manuscript.

The authors contributed to this study in the following manner: MR, EB, WT conceived and designed the project; MR, EB, WT conducted the fieldwork; MR, EB analysed the sedimentological data; MR, WT analysed the taphonomical data in the field and via CT scanning; MR, EB, WT wrote the paper and designed the illustrations.

26

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 Theron, J.N., 1970. A stratigraphical study of the Bokkeveld Group (series). Proceedings 2nd IUGS Symposium Stratigraphy Paleontology, Gondwana System, Cape Town, South Africa, pp. 197–204. Theron, J.N., 1972. The stratigraphy and sedimentation of the Bokkeveld Group. DSc thesis, University Stellenbosch, unpublished. 34

Theron, J.N., 2003. Lithostratigraphy of the Voorstehoek Formation (Bokkeveld Group). Lithostratigraphic Series 38, SACS, 11 pp. Theron, J.N., Johnson, M.R., 1991. Bokkeveld Group (including the Ceres, Bidouw and Traka Subgroups). Catalogue of South African Lithostratigraphic Units 3, 3-5. Theron, J.N., Loock, J.C., 1988. Devonian deltas of the Cape Supergroup, South Africa. In: McMillian N.J., Embry, AF., Glass, D.J. (Eds.), Devonian of the World, Volume 1: Regional Syntheses. Canadian Society of Petroleum Geologists, Memoir No. 14, 729740. Thom, G., 1830. Remarks on the geology of South Africa. Quarterly Journal of South Africa, 1, 269-271. Thuy B., 2011. Exceptionally well-preserved brittle stars from the Pliensbachian (Early Jurassic) of the French Ardennes. Palaeontology 54, 215-233. Thuy B., Marty D., Comment, G., 2013. A remarkable example of a Late Jurassic shallowwater ophiuroid assemblage from the Swiss Jura Mountains. Swiss Journal of Geosciences 106: 409-426. Twitchett, R.J., Feinberg, J.M., O'Connor, D.D., Alvarez, W., McCollum, L.B., 2005. Early Triassic ophiuroids: their paleoecology, taphonomy, and distribution. Palaios 20, 213223. Walker, R.G., 1979. Facies Models 7. Shallow Marine Sands. Geoscience Canada 3, 75-89. Walker, R.G., 1982. Hummocky and swaley cross-stratification. 11th International Congress on Sedimentology, Hamilton, Ontario, Canada, McMaster University, Field guide book, Excursion 21A, 22–30. Walker, R.G., Duke, W.L., Leckie, D.A., 1983. Hummocky stratification: Significance of its variable bedding sequences: Discussion and reply. Geological Society of America Bulletin 94, 1245–1251. Warner, G.F., 1971. On the ecology of a dense bed of the brittle-star Ophiotrix fragilis. Journal of the Marine Biological Association U.K. 51, 267-282.
 Williams, M., Smellie, J.L., Johnson, J.S., Blake, D.B., 2006. Late Miocene Asterozoans (Echinodermata) from the James Ross Island Volcanic Group. Antarctic Science 18, 117–122. Wilson, J.B., Holme, N.A., Barrett, R.L., 1977. Population dispersal in the brittle-star Ophiocomina nigra (Abildgaard) (Echinodermata: Ophiuroidea). J. Mar. Biol. Assoc. 35

U.K. 57, 405–439. Zatoń, M., Salamon, M.A., Boczarowski, A., Sitek, S., 2008. Taphonomy of dense ophiuroid accumulations from the Middle Triassic of Poland. Lethaia 41, 47–58.

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41

Figure 1: (A) Simplified geological map of the Cape Supergroup in South Africa showing the approximate location (red square) of the study area within the Cape Fold Belt, ~145 km NE of Cape Town (figure redrawn and modified from Theron and Loock, 1988). Inset shows Africa’s position within Gondwana. (B) Extract from 1:250 000 geological map sheet of Worcester 3319 (Council for Geoscience, Pretoria) showing the approximate location (red star) of the study area, along the N1 national road (yellow line) in the Western Cape Province (South Africa). Dv refers to the outcrop area of the Devonian Voorstehoek Formation. Figure 2: Stratigraphy of the Devonian Bokkeveld Group. Triangles indicate large-scale, upward-coarsening cycles, which represent shallowing upwards successions. Note that the age of the Voorstehoek Formation is Emsian (~400 Ma old; figure redrawn and modified from Thamm and Johnson, 2006). Figure 3: (A) E-W photographic panorama of the outcrop showing the lateral persistence of the tabular sandstone beds and associated sedimentary features. Red dashed line indicates the erosional surface that is laterally traceable over the entire length of the outcrop. Yellow rectangle shows the location of the log and fossil-rich bed (for details see Figure 4). (B) Outcrop portion illustrates a succession of erosionally based storm events beds that scoured into the underlying silty sandstone layers. Each sandstone bed represents a storm event; they appear to become amalgamated in the easternmost and westernmost parts of the outcrop. Figure 4 : (A) Location of fossiliferous bed within the outcrop (indicated by the yellow arrows) and the sedimentary log. For overall context, refer to Figures 3 and 6. (B) Sedimentary log with features indicative of an offshore transition zone in a storm-influenced shelf. (C) Fine-grained sandstones with hummocky cross-stratification which is diagnostic of shallow marine storm deposits. (D) Minor wavy lamination and wave ripple cross-lamination. (E) Fossiliferous bed (5 cm X 2 m; note that the orange colouring is due to weathering). Figure 5: (A) Conglomerate beds (Gm) overlain massive (Sm) and hummocky crossstratified (Shc) sandstones. Top of the sandstone bed shows horizontal laminations (Sr). Height of outcrop ~2 m. (B) Close-up (red square in A) showing alternating layers of ripple cross-laminated sandstones (Sr), massive conglomerates (Gm), hummocky cross-stratified sandstones (Shc) and another conglomerate bed (Gm) in which brachiopod fossil are found. (C) Close-up of the rounded and subrounded, intraformational clasts consisting of mostly mudstones (some sandstones) and ranging in size from 1 to 5 cm. For interpretation of the features, refer the schematic illustration of a proximal storm deposit in Figure 6. For facies codes, refer to the legend in Figure 4. Figure 6: (A) Generalized depositional model of a storm-influenced shelf setting within the southern part of the Clanwilliam Sub-basin during the deposition of the Voorstehoek Formation (modified from Walker, 1979; Duke, 1990). Not to scale. (B) Schematic illustration of a proximal storm deposit (tempestite) showing basal conglomerate beds with fossils and mudstone pebbles (see Fig. 5C); amalgamated beds (see Fig. 4A) of hummocky 37

cross-stratified sandstones (see Fig. 4C) and minor wavy lamination and wave ripple crosslamination (see Fig. 4D). Not to scale. Modified after Cheel and Leckie (1992). Figure 7: Polished slabs of the fossiliferous bed show that all fossils are orange in colour. Figure 8: (A) Photograph of Australospifer sp. and trilobite debris at the base of the obrution bed. (B) Photograph of fully-articulated ophiuroid, oral-side up and disarticulated arms. (C) Photograph of disarticulated bivalve valve positioned convex down (top right). Also note the impression of a valve on the top left in convex up position. Figure 9: MicroCT scanning images of selected samples. (A-B) Sample 1: both fullyarticulated and disarticulated ophiuroids seen throughout the samples at different levels (indicated by A’ and B’) and abundant shell fragments. (C) CT image of sample 10: two fully-articulated ophiuroids in the same level (indicated by C’). Figure 10: MicroCT scanning images of selected samples. (A-B) Sample 2: the mitrate stylophoran Placocystella? with articulated theca, posterior spines and proximal aulacophore. A large, fully-articulated ophiuroid also present (on the right). (C) Green line marks the position of the figured scanned area within Sample 2. (D & E) CT image of Sample 1 and 5: small mitrate stylophorans Paranacystidae gen. and sp. indet. showing theca and proximal aulacophore. Note that numerous individuals have been found throughout the samples but not figured here. Figure 11: MicroCT scans of sample 1 from the obrution bed shows fully-articulated ophiuroids (pink) and small, intact mitrate stylophorans (green). The dark pink ophiuroid is upside down (mouth facing up) and the light pink individual is in life position (right side up). A number of disarticulated ophiuroid arms (yellow) and crinoid ossicles (blue) are scattered throughout the sample.

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Table 1: Surface area of the samples versus the number of ophiuroids and stylophorans in the Lower Devonian Voorstehoek Formation (HR- Hex River N1 locality).

Sample No. Surface area (mm)

Ophiuroid count (No. of discs)

Stylophoran count (No. of thecae)

HR-01

not measured

9

12

HR-02

85.00 X 85.00

10

5

HR-03

124.04 X 45.97

28

3

HR-04

not measured

11

1

HR-05

93.83 X 63.75

9

10

HR-06

102.09 X 61.47

6

0

HR-07

81.71 X 65.05

8

2

HR-08

109.36 X 81.63

6

0

HR-09

79.25 X 76.48

7

1

HR-10

109.94 X 74.57

10

0

HR-11

92.20 X 41.11

2

3

HR-13

60.48 X 38.43

8

2

HR-14

91.19 X 50.82

6

1

120

40

Total

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Table 2: Measured thecal width versus length of the stylophorans in each of the samples from the Lower Devonian Voorstehoek Formation (HR- Hex River N1 locality).

Sample No.

Thecal width (mm)

Thecal length (mm)

HR-01

2.32 2.29 2.71 1.53 2.20 3.09 2.43 2.36 1.93 1.83 11.91 2.53 2.00 2.11 2.01 2.00 2.11 2.51 2.41 1.87 2.34 2.91 2.64 2.61 2.36 2.08 2.72 2.16 1.90 2.10

4.20 3.96 4.51 2.89 3.05 4.76 4.79 4.18 3.62 3.36 15.20 4.82 4.20 4.47 2.67 3.46 4.31 4.86 4.90 3.33 4.76 5.87 5.13 4.56 4.81 3.19 3.93 3.91 3.40 4.20

HR-02

HR-03 HR-04 HR-05

HR-07 HR-09 HR-11 HR-14

40

43

44

45

46

47

48

49

50

51

52

53