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The Slottsmøya Marine Reptile Lagerstätte: Depositional

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The Slottsmøya Marine Reptile Lagerstätte: Depositional Downloaded from http://sp.lyellcollection.org/ by guest on September 25, 2021 The Slottsmøya marine reptile Lagersta¨tte: depositional environments, taphonomy and diagenesis LENE L. DELSETT1*, LINN K. NOVIS2, AUBREY J. ROBERTS3, MAAYKE J. KOEVOETS1, ØYVIND HAMMER1, PATRICK S. DRUCKENMILLER4 & JØRN H. HURUM1 1Natural History Museum, University of Oslo, 0318 Oslo, Norway 2Tromsø University Museum, 9037 Tromsø, Norway 3Ocean and Earth Science, National Oceanography Centre Southampton, University of Southampton, Southampton SO14 3ZH, UK 4University of Alaska Museum and Department of Geosciences, University of Alaska Fairbanks, Fairbanks, Alaska 99775, USA *Corresponding author (e-mail: [email protected]) Abstract: The Late Jurassic Slottsmøya Member Lagersta¨tte on Spitsbergen offers a unique opportunity to study the relationships between vertebrate fossil preservation, invertebrate occur- rences and depositional environment. In this study, 21 plesiosaurian and 17 ichthyosaur specimens are described with respect to articulation, landing mode, preservation, and possible predation and scavenging. The stratigraphic distribution of marine reptiles in the Slottsmøya Member is analysed, and a correlation between high total organic content, low oxygen levels, few benthic invertebrates and optimal reptile preservation is observed. A new model for 3D preservation of vertebrates in highly compacted organic shales is explained. Supplementary material: A taphonomic description of each marine reptile specimen is available at https://doi.org/10.6084/m9.figshare.c.2133549 Gold Open Access: This article is published under the terms of the CC-BY 3.0 license. Mesozoic marine reptiles have been known from Jurassic units: the Oxford Clay and the Posidonien- the Svalbard archipelago for more than 150 years, schiefer Formation (Martill 1985, 1986, 1993). How- particularly from Triassic units (Maxwell & Kear ever, the material upon which these studies were 2013; Hurum et al. 2014). However, it was not based was collected decades ago, and the specimens until 1914 that Wiman described the first Jurassic were not adequately stratigraphically constrained marine reptile, a plesiosaur, from the island of Spits- or contextualized geologically: thus, taphonomical bergen (Wiman 1914; Kear & Maxwell 2013). interpretations presented in these studies are some- Beginning in 2004, an extensive new field survey what contentious. Efimov (2001) also analysed the for Jurassic marine reptiles was undertaken by the taphonomy of a large number of Upper Jurassic and Spitsbergen Jurassic Research Group (SJRG), an Lower Cretaceous ichthyosaurs from two parts of international team of palaeontologists and geolo- the Ulyanovsk Section in the Volga Region. gists. During eight field seasons (2004 and 2006– Here, for the first time, we address plesiosaur 12) on Spitsbergen, SJRG collected more than 40 and ichthyosaur taphonomy based on a large sam- marine reptile skeletons from the dark marine shales ple size (n ¼ 38) with many articulated specimens of the Upper Jurassic–Lower Cretaceous Slottsmøya from a site where stratigraphic, sedimentological Member of the Agardhfjellet Formation (Fig. 1). and palaeontological data were collected simulta- Given the sheer abundance of material and quality neously (Table 1; Figs 2 & 3). In this paper, we of preservation, we have characterized this unit as a describe the preservational modes of the skeletons, Lagersta¨tte (Hurum et al. 2012). and attempt to interpret the major physical and In the course of this work, detailed taphonomical biotic factors affecting skeletal preservation. We data have been collected, permitting a rare insight incorporate new surface and well-log data to docu- into plesiosaur and ichthyosaur taphonomy. Previ- ment the stratigraphic distribution of skeletons in ous studies were limited primarily to two other the unit, especially in relation to the total organic From:Kear, B. P., Lindgren, J., Hurum, J. H., Mila`n,J.&Vajda, V. (eds) 2016. Mesozoic Biotas of Scandinavia and its Arctic Territories. Geological Society, London, Special Publications, 434, 165–188. First published online December 16, 2015, updated December 17, 2015, http://doi.org/10.1144/SP434.2 # 2016 The Author(s). Published by The Geological Society of London. Publishing disclaimer: www.geolsoc.org.uk/pub_ethics Downloaded from http://sp.lyellcollection.org/ by guest on September 25, 2021 166 L. L. DELSETT ET AL. Fig. 1. Geological map of the study area in central Spitsbergen, with the main marine reptile locations marked with red, brown and yellow circles. Redrawn and adapted from Dallmann et al. (2001). content (TOC). The causes for the exceptional in an open-marine shelf (Hammer et al. 2012). abundance of marine reptile skeletons found in the The thickness ranges from 70 to 100 m, and consists Slottsmøya Member compared to other members of black to grey shales and siltstones with siderite is beyond the scope of this paper. interbeds (Dypvik et al. 1991). Shelf conditions were slightly dysoxic with periodical oxygenation of the bottom water, which might have been a result Geological setting of influx of clastic sediments (Collignon & Hammer 2012). The shales of the mid-section show little The Svalbard archipelago is located between alternation in mineralogy, suggesting a stable depo- latitudes 74–818 N and longitudes 10–358 E, in sitional environment, while the sediments in the the northwestern corner of the Barents Sea shelf. silty intervals were transported into the basin by The Middle Jurassic–Lower Cretaceous succession turbidity currents (Collignon & Hammer 2012). forming the Janusfjellet Subgroup comprises the This interpretation is supported by taphonomical Agardhfjellet and Rurikfjellet formations. The and ecological evidence. Some crinoids and echi- Agardhfjellet Formation consists of black shales noids were situated in the sediment as if they were and siltstones deposited in an open-marine, oxygen- transported, while other echinoderms like asteroids deficient shelf setting (Dypvik et al. 1991; Col- and ophiuroids were found in situ (Rousseau & lignon & Hammer 2012). The Myklagardfjellet Nakrem 2012). TOC values of the Slottsmøya Mem- bed, a distinct thin marker horizon of weathering ber show considerable fluctuation, with the largest clays, marks the boundary between the two forma- excursion reaching 9.7% (Hammer et al. 2012). tions (Birkenmajer 1980). In the uppermost part (39–49 m) of the Slotts- The marine reptiles and invertebrates discussed møya Member, several cold-seep communities have in this study occur in the Slottsmøya Member, the been discovered. From these, a diverse and low- uppermost member of the Agardhfjellet Formation dominance invertebrate fauna has been described (Figs 1 & 2). The Slottsmøya Member rests on the (Hammer et al. 2011, 2013; Wierzbowski et al. Oppdalsa˚ta Member and is overlain by the Wimanf- 2011; Hryniewicz et al. 2012, 2014). The non-seep jellet Member of the Rurikfjellet Formation (Mørk fauna invertebrate diversity was probably also quite et al. 1999). The Slottsmøya Member was deposited high. This assumption is based on a study of the Downloaded from http://sp.lyellcollection.org/ by guest on September 25, 2021 THE SLOTTSMØYA LAGERSTA¨ TTE 167 Slottsmøya Member from East Spitsbergen by Bir- bones were disarticulated but associated, and the kenmajer et al. (1982), the echinoderm fauna in disarticulation was caused by gravitational collapse the section (Rousseau & Nakrem 2012) and field of the skeleton. There were no scattered skeletal observations in the area. elements. In the second type, bones were scattered over a considerable distance, and the disarticulation Abbreviations could be a consequence of scavenging, current activity or both (Martill 1993). The five preservatio- PMO, Natural History Museum, University of Oslo nal modes of Martill (1985) form the basis for the (Palaeontological collection); SJRG, Spitsbergen categorization of the Slottsmøya Member marine Jurassic Research Group; SVB, Svalbard Museum, reptile skeletons in this study, which we segregate Longyearbyen. into three different categories: † Articulated skeleton: this definition (Martill Materials and methods 1985, p. 159) is used to describe a specimen with a true bone to bone relationship. However, parts The 38 marine reptile skeletons used in this study of the skeleton can be missing due to surface were excavated during eight field seasons (2004 erosion. and 2006–12) by the SJRG. For each skeleton, † Partly articulated skeleton: this is a new cate- the locality information, stratigraphic position and gory based on the definition of the first type of taphonomical data (e.g. orientation, nature and disarticulated skeletons used by Martill (1985), degree of articulation, and associated invertebrates) which recognizes that some skeletons found were recorded in the field at the time of collection in the Slottsmøya Member are intermediate (Table 1; Figs 2 & 3). During preparation, additional between articulated and disarticulated (also information was obtained concerning the articu- observed by Martill 1993). A partly articulated lation and association of skeletal elements, quality specimen consists of two or more skeletal ele- of preservation, completeness and bone modifi- ments in articulation: for example, sections of cation. Field drawings and photographs before and associated vertebrae, ribs still articulated to during preparation were utilized in constructing vertebrae or partly articulated fins, along with skeletal maps of 28 of the specimens
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