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For Peer Review 19 Precise Position Within Total�Group Mollusca Remains Unclear View metadata, citation and similar papers at core.ac.uk brought to you by CORE Lethaia provided by Apollo Sclerites and possible mouthparts of Wiwaxia from the temperate palaeolatitudes of Colombia, South America Journal:For Lethaia Peer Review Manuscript ID: LET-OA-02-15-0605.R1 Manuscript Type: Original Article Date Submitted by the Author: n/a Complete List of Authors: Smith, Martin; Royal Ontario Museum, Palaeobiology Hughes, Gareth; Petrostrat Ltd., Vargas, María; Instituto Colombiano del Petróleo-Ecopetrol, Biostratigraphy team de la Parra, Felipe; Instituto Colombiano del Petróleo-Ecopetrol, Biostratigraphy team Small Carbonaceous Fossils, Wiwaxia, Cambrian, South America, Keywords: palynological processing, biogeography Lethaia - manuscript copy Page 1 of 17 Lethaia 1 2 3 4 Sclerites and possible mouthparts of Wiwaxia from the temperate 5 6 palaeolatitudes of Colombia, South America 7 8 9 10 11 12 13 Martin R. Smith* [[email protected]], Department of Earth Sciences, Downing Site, 14 15 University of Cambridge, Cambridge, CB2 3EQ, UK 16 17 18 Gareth M. G. Hughes*For [[email protected]] Peer Review, PetroStrat Ltd., Tan-Y-Graig, 19 20 Parc Caer Seion, Conwy, LL32 8FA, UK 21 22 23 M. Carolina Vargas & Felipe de la Parra, Biostratigraphy team, Instituto Colombiano 24 25 del Petróleo-Ecopetrol, AA 4185, Bucaramanga, Colombia 26 27 28 * These authors contributed equally to the current study 29 30 Abstract 31 32 33 The problematic mollusc Wiwaxia is perhaps the most widely distributed 34 35 non-mineralized Cambrian metazoan, but has only been reported from palaeotropical 36 37 38 latitudes. Here we describe mid-Cambrian (Drumian, c. 504 Ma) sclerites and possible 39 40 tooth arrays from the northern Llanos Basin, Colombia, recovered from drilled ditch 41 42 43 cuttings by palynological processing – demonstrating that pristine material and 44 45 low-manipulation processing are not essential to the recovery of Small Carbonaceous 46 47 48 Fossils. This, the first report of Wiwaxia from South America, substantially expands 49 50 Wiwaxia ’s geographic range into the high palaeolatitudes. 51 52 53 54 55 56 57 58 59 60 Lethaia - manuscript copy Lethaia Page 2 of 17 1 2 3 4 Cambrian lagerstätten such as the Burgess Shale are renowned for their exceptionally 5 6 preserved non-mineralized metazoans. Among the most iconic of these animals is 7 8 9 Wiwaxia , a problematic slug-like mollusc covered in imbricating scales and spines. 10 11 Originally interpreted as an annelid worm (Walcott 1911; Butterfield 1990), Wiwaxia is 12 13 14 now recognized as a mollusc based on its radula, creeping ventral foot, and 15 16 aculiferan-like scleritome (Conway Morris 1985; Smith 2012, 2014), although its 17 18 For Peer Review 19 precise position within total-group Mollusca remains unclear. 20 21 The anatomy of Wiwaxia has been reconstructed from articulated specimens, 22 23 24 known from four Burgess Shale-type deposits in North America and China (Conway 25 26 Morris 1985; Zhao et al. 1994; Yang et al. 2014; Conway Morris et al. 2015). The 27 28 29 organism bears four sclerite morphologies (Conway Morris 1985): depending on their 30 31 position, sclerites are asymmetric (dorsal), rounded and symmetrical (upper lateral and 32 33 34 anterior), elongate and symmetrical (lower lateral) or sickle-shaped (ventral). Mature 35 36 Wiwaxia specimens also exhibit elongate dorso-lateral spines. 37 38 39 Sclerites bear a variable number of ribs and express a thickened margin and a 40 41 microvillar construction (Butterfield 1990; Smith 2014). They are constructed from a 42 43 44 broad oval blade and narrow, hollow root; this distinctive outline means that isolated 45 46 Wiwaxia sclerites are readily identified (Conway Morris 1985; Butterfield 1990). 47 48 49 Indeed, lone Wiwaxia sclerites have been reported on bedding-surfaces (Ivantsov et 50 51 al. 2005; Fatka et al. 2011; Sun et al. 2014; Zhao et al. 2015), as phosphatic casts 52 53 54 (Porter 2004), and – most importantly – as Small Carbonaceous Fossils (SCFs) 55 56 (Butterfield & Harvey 2012; Harvey et al. 2012; Pedder 2012). 57 58 59 60 Lethaia - manuscript copy Page 3 of 17 Lethaia 1 2 3 4 SCFs are robust carbon films, typically extracted from pristine siliciclastic 5 6 mudstones and siltstones through the hand-picking of sieved macerates (Butterfield & 7 8 9 Harvey 2012). Owing to their small size and taphonomic recalcitrance, they are 10 11 widely distributed in space and time, and extend the record of Wiwaxia across the 12 13 14 modern globe (Fig. 1). Nevertheless, Wiwaxia has not been reported outside the low 15 16 palaeolatitudes – possibly reflecting the increased sampling effort applied to 17 18 For Peer Review 19 low-palaeolatitude continents. Our material from the Llanos Basin of Colombia, 20 21 South America, was deposited at a latitude between 50–90° S (McKerrow et al. 1992; 22 23 24 Torsvik & Cocks 2013), providing the first sampling of SCFs from high 25 26 palaeolatitudes. 27 28 29 30 Materials and methods 31 32 33 Mudstone samples were obtained from washed and dried ditch cuttings from the 34 35 Chigüiro-1 well, which was drilled in 1985 in the northern Llanos Basin, north-east 36 37 38 Colombia. Ditch cuttings were produced by the rotational grinding action of the drill 39 40 bit and brought to the surface by the conveyor-belt action of circulating drilling fluid. 41 42 43 Samples were collected and bagged at regular intervals; each sample thus represents the 44 45 depth of rock drilled since the last sample was taken. Over the Wiwaxia -yielding 46 47 48 interval (8750–9710 ft), each sample typically corresponds to an interval of 20–30 ft 49 50 (6–10 m) of rock; each quoted sample depth denotes the bottom of the sampled interval. 51 52 53 5–20 g of each sample was prepared either by standard palynological procedures – 54 55 maceration by successive applications of HCl and HF, followed by heavy mineral 56 57 58 separation in LST FastFloat™, sieving through a 10 µm mesh, and strew mounting (56 59 60 Lethaia - manuscript copy Lethaia Page 4 of 17 1 2 3 4 samples in total; 20 from the Wiwaxia -bearing interval, of which 12 yielded Wiwaxia ) – 5 6 or a low-manipulation procedure incorporating gentle dissolution in HF, sieving 7 8 9 through a 63 µm gauze, and manual mounting onto glass slides by pipette (21 samples 10 11 from the Wiwaxia -bearing interval, 4 yielding Wiwaxia ). 12 13 14 Light micrographs from multiple focal planes were combined using TuFuse 15 16 (tawbaware.com); backgrounds and extraneous objects have been removed. 17 18 For Peer Review 19 Measurements were recorded digitally from micrographs. SCFs are deposited at the 20 21 Sedgwick Museum of Earth Sciences, Cambridge (CAMSM); comparative macrofossil 22 23 24 material is accessioned at the Smithsonian Institute National Museum of Natural 25 26 History (NMNH) and the Royal Ontario Museum (ROM). 27 28 29 30 Geological setting 31 32 33 Co-occurring acritarchs (in particular Retisphaeridium dichamerum, Eliasum llaniscum, 34 35 Adara matutina and Cristallinium cambriense ), together with the first downhole 36 37 38 occurrence of the acritarch Vulcanisphaera lanugo in the interval below the last 39 40 persistent downhole occurrence of Wiwaxia sclerites, confine the range of Wiwaxia 41 42 43 sclerites to within the Rugasphaera terranovana acritarch zone (Martin & Dean 1988), 44 45 corresponding to the Ptychagnostus atavus / Tomagnostus fissus trilobite zone and an 46 47 48 approximate age of 504 Ma. 49 50 The frequent-to-superabundant occurrences of Siphonophycus spp. (stromatolite 51 52 53 filaments) throughout the Wiwaxia -bearing interval indicate a shallow water 54 55 depositional environment (Butterfield & Chandler 1992), whereas the regional geology 56 57 58 indicates a distal shelf setting (Torsvik & Cocks 2013). 59 60 Lethaia - manuscript copy Page 5 of 17 Lethaia 1 2 3 4 Sclerites 5 6 Twenty-five sclerites were recovered: eighteen through palynological preparations and 7 8 9 seven through manual extraction, in approximate proportion to the amount of sample 10 11 processed by each method. Sclerites ranged from 100–500 µm in maximum 12 13 14 dimension; neither the typical size nor the sclerite integrity exhibited meaningful 15 16 variation between the two extraction methods. 17 18 For Peer Review 19 Sclerites could be identified as Wiwaxia scales based on their construction from a 20 21 broad, ribbed blade and hollow root, and by the presence of microvillar chambers (Fig. 22 23 24 2). Sclerites can further be designated as: ventro-lateral (two, Fig. 2a, e), based on a 25 26 siculate habit; lower lateral (five, Fig. 2b, f), based on an elongate aspect and 27 28 29 symmetrical shape; upper lateral (four, Fig. 2c, g), based on their rounded profile and 30 31 1:1 height:width ratio; and dorsal (four, Fig. 2d, h), based on their marked asymmetry. 32 33 34 Ten incomplete sclerites are insufficiently preserved for a confident assignation. 35 36 Overall, the sclerites’ shape and dimensions correspond to those of articulated Wiwaxia 37 38 39 corrugata juveniles (Fig. 2k), though the sample size is insufficient to attempt 40 41 species-level taxonomy. Three specimens displayed superficial pustules with an 42 43 44 irregular appearance and distribution that conceivably correspond to surface ornament 45 46 (Fig. 2i–j). 47 48 49 50 Mouthparts 51 52 53 Sclerites are not the only tough component of Wiwaxia : its mouthparts also have a 54 55 robust carbonaceous construction and could in principle be preserved as SCFs (Smith 56 57 58 2012). The mouthparts were originally understood to represent a series of two to three 59 60 Lethaia - manuscript copy Lethaia Page 6 of 17 1 2 3 4 denticulate bars (Conway Morris 1985), and on this basis various SCFs from the Mount 5 6 Cap formation (mid-Cambrian of Canada) were compared with the rows of teeth in 7 8 9 Wiwaxia (Harvey & Butterfield 2011).
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