Biogeosciences, 12, 6945–6954, 2015 www.biogeosciences.net/12/6945/2015/ doi:10.5194/bg-12-6945-2015 © Author(s) 2015. CC Attribution 3.0 License. Stable isotope study of a new chondrichthyan fauna (Kimmeridgian, Porrentruy, Swiss Jura): an unusual freshwater-influenced isotopic composition for the hybodont shark Asteracanthus L. Leuzinger1,2,a, L. Kocsis3,4, J.-P. Billon-Bruyat2, S. Spezzaferri1, and T. Vennemann3 1Département des Géosciences, Université de Fribourg, Chemin du Musée 6, 1700 Fribourg, Switzerland 2Section d’archéologie et paléontologie, Office de la culture, République et Canton du Jura, Hôtel des Halles, 2900 Porrentruy, Switzerland 3Institut des Dynamiques de la Surface Terrestre, Université de Lausanne, Quartier UNIL-Mouline, Bâtiment Géopolis, 1015 Lausanne, Switzerland 4Universiti Brunei Darussalam, Faculty of Science, Geology Group, Jalan Tungku Link, BE 1410, Brunei Darussalam anow at: CRILAR, 5301 Anillaco, La Rioja, Argentina Correspondence to: L. Leuzinger ([email protected]) Received: 12 June 2015 – Published in Biogeosciences Discuss.: 12 August 2015 Revised: 30 October 2015 – Accepted: 25 November 2015 – Published: 7 December 2015 Abstract. Chondrichthyan teeth (sharks, rays, and chi- competition of the hybodont shark Asteracanthus with the maeras) are mineralized in isotopic equilibrium with the rapidly diversifying neoselachians (modern sharks). surrounding water, and parameters such as water tempera- ture and salinity can be inferred from the oxygen isotopic 18 composition (δ Op) of their bioapatite. We analysed a new chondrichthyan assemblage, as well as teeth from bony fish 1 Introduction (Pycnodontiformes). All specimens are from Kimmeridgian coastal marine deposits of the Swiss Jura (vicinity of Porren- Chondrichthyan remains are common in the Mesozoic fossil truy, Ajoie district, NW Switzerland). While the overall fau- record of Western Europe, and in many different palaeoen- nal composition and the isotopic composition of bony fish are vironmental settings (e.g. lagoonal, open marine, reduced generally consistent with marine conditions, unusually low salinity) (Duffin and Thies, 1997; Müller, 2011; Underwood, 18 δ Op values were measured for the hybodont shark Aster- 2002). Their teeth are predominantly composed of fluor- acanthus. These values are also lower compared to previ- apatite, the most resistant variety of apatite (Vennemann et ously published data from older European Jurassic localities. al., 2001) and are continuously shed and replaced, except Additional analyses on material from Solothurn (Kimmerid- in chimaeras (Cappetta, 2012; Stahl, 1999). In addition to gian, NW Switzerland) also have comparable, low-18O iso- their abundance, their mechanical and chemical resistance topic compositions for Asteracanthus. The data are hence in- make them an ideal material for stable isotope analyses. terpreted to represent a so far unique, freshwater-influenced They mineralize in isotopic equilibrium with the surround- isotopic composition for this shark that is classically con- ing water, hence their primary oxygen isotopic composition 18 sidered a marine genus. While reproduction in freshwater or (δ Op) reflects that of the ambient water at a given tempera- brackish realms is established for other hybodonts, a similar ture when they formed (Kolodny et al., 1983; Longinelli and behaviour for Asteracanthus is proposed here. Regular ex- Nuti, 1973). This makes them a valuable palaeoenvironmen- cursions into lower salinity waters can be linked to the age of tal proxy, used in numerous studies (e.g. Kocsis et al., 2007; the deposits and correspond to an ecological adaptation, most Lécuyer et al., 2003; Vennemann et al., 2001). likely driven by the Kimmeridgian transgression and by the This research is based on fossil material – mainly chondrichthyans – found between 2000 and 2011 during Published by Copernicus Publications on behalf of the European Geosciences Union. 6946 L. Leuzinger et al.: Stable isotope study of a new chondrichthyan fauna (Kimmeridgian, Porrentruy, Swiss Jura) 0° 5°E Based on phosphate oxygen isotope analyses obtained London-Brabant Massif UK N from this Late Jurassic chondrichthyan fauna, this study pro- Germany poses answers to the following questions: (1) is there any 50°N unexpected isotopic composition for the associated marine fauna recorded in Porrentruy? (2) Are the Porrentruy isotopic Frankfurt data unique so far, or comparable to other European locali- CH ties? (3) What do we learn about the palaeoecology of the 45°N France Italy hybodont shark Asteracanthus based on the isotopic compo- sition? Paris Basin 2 Material and methods BOREAL REALM The chondrichthyan dental material of the PAL A16 collec- tion is rich and diverse, comprising more than 2000 fos- SUB-BOREAL 100 km REALM sils. Sharks and rays (subclass Elasmobranchii) are repre- Porrentruy sented by the hybodont sharks – the extinct sister group Jura Platform of modern sharks (Maisey et al., 2004) (order Hybodon- Massif TETHYSIAN REALM Bern Tethys tiformes: “Hybodus”, Planohybodus, Asteracanthus) – the Central modern sharks (subcohort Neoselachii, order Carcharhini- formes: Palaeoscyllium, Corysodon; order Heterodontif- Figure 1. Palaeogeographical map of the study site and sur- roundings (Late Kimmeridgian, modified from Comment et al., ormes: Heterodontus, Paracestracion; order Protospinaci- 2011). CH D Switzerland, palaeolatitude of Porrentruy D ∼ 30◦ N. formes: Protospinax; order Squatiniformes: Pseudorhina) Emerged land is outlined, darker grey corresponds to deeper water. and rays (superorder Batomorphii, order Rajiformes: Belem- Upper left corner: present-day geographical position of Porrentruy nobatis, Spathobatis). Chimaeras (superorder Holocephali (?) and other European sites (•) of previously published studies and (sensu Stahl, 1999), order Chimaeriformes: Ischyodus) are providing geochemical data compared in Fig. 5. The shaded square also present. The investigated material comes from the Kim- delimits the area detailed in the palaeogeographical map. meridgian Reuchenette Formation and more precisely from the latest Early Kimmeridgian (Cymodoce ammonite zone, Banné Marls) and up to the Late Kimmeridgian (Mutabilis controlled palaeontological excavations conducted by the ammonite zone, Corbis Limestones and Eudoxus ammonite Paléontologie A16 team (PAL A16, canton of Jura, NW zone, lower Virgula Marls) (Fig. 2). Except for Asteracan- Switzerland). All fossiliferous sites are located in the vicinity thus and Ischyodus remains that are of a considerable size of Porrentruy (Ajoie district) and are related to the building and were collected directly on the field, the material consists of the Trans-Jura highway (A16). The Ajoie region is part predominantly of microfossils resulting from sediment siev- of the Tabular Jura (Marty et al., 2007), mainly consisting ing. of subhorizontal Mesozoic (Oxfordian and Kimmeridgian) The oxygen isotopic composition of phosphate from bio- strata. genic apatite was measured on rays, the chimaeroid Ischyo- During the Kimmeridgian, the Ajoie region was a shallow- dus and the hybodonts Asteracanthus and Hybodus. Bioap- marine carbonate platform at a palaeolatitude of about 30◦ N atite of bony fish Pycnodontiformes was also analysed for (Marty, 2008) and surrounded by the Central and London comparison. Stratigraphically, samples were selected from Brabant massifs, the Tethys, and the Paris Basin (Fig. 1). The different beds in order to cover all units of the studied sec- palaeoclimate was semi-arid with high seasonality (Philippe tion (Fig. 2). Additionally, Kimmeridgian material from the et al., 2010; Waite et al., 2013). The platform had a very neighbouring Natural History Museum of Solothurn was complex morphology due to the basement structure and sea- analysed for comparison. level changes occurred during its depositional history. These The best mineralized part of the teeth, the enameloid, was processes induced several episodes of emersion suggested isolated in Pycnodontiformes and Asteracanthus (Fig. 3). by numerous dinosaur footprints (Marty, 2008; Marty et al., From eleven of the Asteracanthus teeth, dentine was also 2007) and hardgrounds, followed by erosion and rework- analysed in parallel to examine any isotopic differences be- ing. Lateral changes in water depth potentially occurred at tween the tissues. In the case of chimaeroid dental plates the a very local scale (Jank et al., 2006; Waite et al., 2013). densest parts were selected. For the very small material (1– The record of ammonites typical of the boreal and Tethyan 5 mm) – as in rays and Hybodus – several isolated teeth were domains show that the study area was influenced by water analysed together as bulk samples of enameloid and dentine. masses from both the Tethys and Paris Basin (Colombié and Due to the small size, only the outer aspect of this material Rameil, 2007; Comment et al., 2011). was sampled as it was visibly the best preserved, i.e. not Biogeosciences, 12, 6945–6954, 2015 www.biogeosciences.net/12/6945/2015/ L. Leuzinger et al.: Stable isotope study of a new chondrichthyan fauna (Kimmeridgian, Porrentruy, Swiss Jura) 6947 rd order Stage Member Bed Ammonite biozoneLithology and 3 age (Comment et al. 2011) orbital cycles (Jank et al. 2006) Formation Coral limestones Eudoxus Lower mfs Chevenez Virgula Marls 152.1 ts SB Mutabilis Upper Corbis limestones Courtedoux Upper pterocerian limestones Cymodoce 152.6 mfs Banné marls Kimmeridgian Reuchenette Banné Lower Lower ts Marly Limestone limestone