Decapod crustaceans from the Middle Jurassic Opalinus Clay of northern Switzerland, with comments on crustacean taphonomy

Abstract

Four species of decapod crustaceans from the Middle Jurassic Opalinus Clay (Aalenian) of Northern Switzerland are described. Of these, Mecochirus cf. eckerti is the most common one, while Eryma cf. bedelta, Glyphea sp. and Aeger sp. were present as individuals, or only a few specimens. The preservation of these crustaceans ranges from moderate to excellent, reflecting the favourable taphonomic conditions of the depositional environment. An interesting aspect of the taphocoenosis in the Opalinus Clay is that the decapod crustaceans are by far outnumbered by small peracarid crustaceans (isopods and tanaids). This is interpreted as reflecting the original differences in abundance. Yet this distribution is not frequently encountered in sedimentary sequences where decapods (although rare) are far more common than isopods and tanaids. In rare instances, this reflects the original predominance of decapods, more often it is a consequence of the differential taphonomic behaviour of these two groups. A new model relating the ecology to the taphonomic behaviour of decapod and peracarid crustaceans is proposed. According to this model, decapods dominate in settings that were deposited under extremely dysoxic (peracarids wiped out by seasonal anoxia) as well as under fully oxic conditions (peracarids destroyed by taphonomic processes). Only in muddy dysoxic depositional environments are peracarid crustaceans frequently preserved. In these settings with equal preservation potential of decapods and peracarids, the original composition of the crustacean fauna would show a predominance of peracarid crustaceans. Examples from some well known fossiliferous settings are provided to illustrate the use of the new model. ZUSAMMENFASSUNGAus dem Opalinuston (Mittlerer Jura, Aalenian) der Nordschweiz werden vier Arten von decapoden Krebsen beschrieben. Von Aeger sp., Eryma cf. bedelta und Glyphaea sp. wurden nur ganz wenige Exemplaren gefunden, während Mecochirus cf. eckerti etwas häufiger ist. Die Erhaltungsbedingungen waren während der Ablagerung des Opalinustones günstig, was sich in einer geringen Disartikulations-und Fragmentationsrate der Krebse widerspiegelt. Ein interessanter Aspekt der Taphocoenose ist die deutliche Dominanz der Kleinkrebse (Peracarida: Isopoden und Tanaidaceen). Dies dürfte die Zahlenverhältnisse der ehemaligen Lebensgemeinschaft widerspiegeln. In den meisten Ablagerungen dominieren jedoch die decapoden Krebse, wogegen Peracarida äusserst selten sind. Nur in den wenigsten Fällen entspricht dies einem ursprünglichen Überwiegen der Decapoda, viel öfter ist es eine Folge des unterschiedlichen Fossilisationspotentials der beiden Gruppen. Hier wird ein neues Modell vorgeschlagen, welches Ökologie und taphonomisches Verhalten der decapoden und peracariden Krebse miteinander verknüpft. Nach diesem Modell dominieren die Decapoda in Ablagerungen, welche in stark dysoxischem (Peracarida werden durch anoxische Ereignisse ausgelöscht) oder in vollkommen oxischem Milieu (Peracarida werden durch taphonomische Prozesse zerstör...

Jaws of a large belemnite and an ammonite from the Aalenian (Middle Jurassic) of Switzerland

“…Serious oxygen-depletion of the bottom-water and high sediment accumulation rates were responsible for the sometimes exceptional preservation of fossils including articulated arthropods, articulated echinoderms and vertebrates and preservation of organic materials like the ligament of bivalves and the periostracum of ammonites as well as other molluscs (e.g., Etter 1988Etter , 1990Etter , 1995Etter , 2004a.…”

“…In southern Germany and northern Switzerland, the Opalinus Clay is exposed in several clay pits and a few natural exposures. This formation is characterized by a rather low benthic diversity, well-preserved arthropods and the rare preservation of articulated echinoderms and vertebrate skeletons (e.g., Etter 1988Etter , 1990Etter , 1995Etter , 1996Etter , 2004a. The name of this lithostratigraphic unit derives from the locally abundant ammonite Leioceras opalinum, whose species name refers to the opal-like nacre preservation widely found in central Europe (Quenstedt 1843).…”

New early Jurassic decapod crustacean from Patagonia (Chubut province), Argentina

“…This is the oldest record of a mecochirid from South America, and the first one for Argentina, and considerably extends the palaeogeographic distribution of the genus Beurlen 1928;Carter 1886Carter , 1898Collins and Rasmussen 1992;Etter 2004;Feldmann and Keyes 1992;Feldmann et al 2007;Förster 1971;Förster and von Hillebrandt 1984;Frentzen 1937;Garassino 1996;Garassino and Schweigert 2006;Glaessner 1929Glaessner , 1960Hée 1924;Herrick and Schram 1978;Krause 1891;Oppel 1862;Quenstedt 1854Quenstedt , 1856Quenstedt -1857Secretan 1964Secretan , 1968van Straelen 1924van Straelen , 1936Taylor 1979;da Veiga Ferreira 1955;Woodward 1876 New early Jurassic decapod crustacean from Patagonia (Chubut province), Argentina 151 during the Early Jurassic, previously known only from Europe by that time.…”

“…5). By contrast, very little is known of the geographic distribution of the genus during the Early and Middle Jurassic, when up to now it was only known from Europe (Quenstedt 1854, 1856-1857, Oppel 1862Carter 1886;Krause 1891;van Straelen 1924;Beurlen 1928;Glaessner 1929;Frentzen 1937;Förster 1971;Etter 2004;Garassino 1996). Nevertheless, Feldmann and Schweitzer (2006) indicated that Mecochirus ''had its earliest occurrence in the North Atlantic in Sinemurian time but radiated into the Southern Hemisphere by the Toarcian,'' but they did not provide any evidence to back this statement.…”

The genusErymaMeyer, 1840 (Crustacea: Decapoda: Erymidae): new synonyms, systematic and stratigraphic implications

“…It is known by many fossils supporting the greatest number of described species among Erymidae Van Straelen, 1925(Schweitzer et al, 2010Hyžn y et al, 2015). Eryma is well known in European Jurassic and Early Cretaceous deposits (Carter, 1886;Étallon, 1859Oppel, 1861Oppel, , 1862Ferry, 1865;Krause, 1891;Lahusen, 1894;Harbort, 1905;Van Straelen, 1920, 1925Hée, 1924;Beurlen, 1928;Woods, 1925Woods, -1931Birshtein, 1956;Förster, 1965Förster, , 1966Martill, 1991;Garassino, 1996;Schweigert et al, 2000;Fischer, 2003;Etter, 2004;Carpentier et al, 2006;Garassino and Schweigert, 2006;Charbonnier, 2009;Schweitzer et al, 2009;Charbonnier et al, 2010Charbonnier et al, , 2012Charbonnier et al, , 2014Bravi et al, 2014;Devillez et al, 2016), and some occurrences are also reported throughout the world: in Middle East (Roger, 1946;Förster and Seyed-Emani, 1982;Garassino, 1994), in Africa (Beurlen, 1933;Secrétan, 1984), in North America (Rathbun, 1923(Rathbun, , 1926Feldm...…”