Bollettino della Società Paleontologica Italiana, 49 (1), 2010, 75-81. Modena, 15 maggio 201075 Actinocerid cephalopods from the Silurian of the Carnic Alps (Italy) Paolo SERVENTI, Maurizio GNOLI & Luca SIMONETTO P. Serventi, Dipartimento di Scienze della Terra, Università degli Studi di Modena e Reggio Emilia, L.go S. Eufemia 19, I-41121 Modena, Italy; [email protected] M. Gnoli, Dipartimento di Scienze della Terra, Università degli Studi di Modena e Reggio Emilia, L.go S. Eufemia 19, I-41121 Modena, Italy; [email protected] L. Simonetto, Museo Friulano di Storia Naturale, Via Marangoni 39, I-33100 Udine, Italy; [email protected] KEY WORDS - Cephalopoda, Actinocerida, Armenoceratidae, Huroniidae, Ormoceratidae, Carnic Alps, Silurian, Italy. ABSTRACT - Silurian actinocerid cephalopods from the Italian side of the Carnic Alps are reported on the basis of newly collected material. Three actinocerid taxa, belonging to Armenoceratidae, Huroniidae, and Ormoceratidae families, are described and left in open nomenclature. RIASSUNTO - [Cefalopodi actinoceridi nel Siluriano delle Alpi Carniche (versante italiano)] - Materiale raccolto nel corso di recenti campagne di studio condotte nel versante italiano delle Alpi Carniche ha permesso di individuare alcuni esemplari di cefalopodi appartenenti all’ordine Actinocerida Teichert,1933. Si tratta di tre specie attribuibili ai generi Elrodoceras Foreste,1924, Huroniella Foreste,1024 e Ormoceras Stokes,1840 che, a causa del precario stato di conservazione, sono temporaneamente lasciate in nomenclatura aperta. La presenza nelle Alpi Carniche di cefalopodi actinoceridi permette di aumentare le conoscenze relative alla distribuzione paleogeografica della fauna a cefalopodi durante il Siluriano. INTRODUCTION close to the end of the century thanks to Gnoli & Histon (1998), Histon (1999), Serventi (1999), Serventi et al. Nautiloid cephalopods of the Carnic Alps are well (1999), Gnoli et al. (2000), Serventi & Gnoli (2000), known since the second half of the eighteenth century. In Serventi (2002) and Gnoli & Serventi (2006). 1872, the Austrian geologist Guido Stache (1833-1921) The main investigated cephalopod faunas concern pointed out for the first time the occurrence of the members of the orders Orthocerida Kuhn, 1940, Silurian rocks in the Carnic Alps. Successively he Oncocerida Flower in Flower & Kummel, 1950, recorded “Orthoceras limestones” in many localities of Barrandeocerida Flower in Flower & Kummel, 1950, the Carnic area and collected a great number of Discosorida Flower in Flower & Kummel, 1950 and cephalopod specimens, but did not produce any Nautilida Agassiz, 1847. systematic work. In 1887, Fritz Frech (1861-1917), a Except few fragmentary specimens reported by German geologist, in “Über das Devon der Ostalpen, nebst Serventi & Gnoli (2000) and left in open nomenclature, Bemerkungen über das Silur und einem members of the order Actinocerida Teichert 1933, have paläontologischem Anhang”, described two species of not been studied. The goal of this paper is to describe a nautiloid cephalopods: Orthoceras potens and new material of actinoceroids collected in the Italian side Orthoceras alticola. of the Carnic Alps in the Mt. Cocco area (Fig. 1) and stored The first systematic work on cephalopod fauna, from in the Museo Friulano di Storia Naturale, Udine, and in the Silurian outcrops, is dated to 1909: the Italian Palaeontological collections of the University of Modena geologists Michele Gortani (1883-1966) and Paolo and Reggio Emilia. Vinassa de Regny (1871-1957) described 18 species of cephalopods from Italian side of the of the Carnic Alps. In 1929, Franz Heritsch (1882-1945) published GEOLOGICAL REMARKS “Faunen aus dem Silur der Ostalpen”, a detailed palaeontological systematic study on Silurian cephalopod In the Friuli Venezia Giulia region (in the northeastern faunas with specimens collected by himself and with part of Italy) three mountain chains are welded together: material, stored in the Geological Survey of Vienna, and the Palaeocarnic chain, the eastern section of the South belonging to collections of von Gaertner, Geyer and Alpine chain and the northwestern part of the Outer Stache. Dinarides. The Palaeocarnic chain, about 180 km long Palaeontological studies on cephalopod fauna stopped (from Comelico to Caravanche) and 10-20 km wide, after the Second World War. Only in 1968 Heinrich presents a metamorphic part, confined in the northwestern Ristedt, from the Bonn University, published an essential sector, and a non-metamorphic part represented by an work on cephalopod early stages and protoconchs, with almost continuous Palaeozoic sedimentary succession specimens coming from the terranes forming the (Carulli, 2006) (Fig. 1). The Palaeozoic outcrops, very northern margin of Gondwana during the Silurian. rich in palaeontological remains, range from the Upper Renewed interest for the research on cephalopods arose Ordovician to Permian. ISSN 0375-7633 07 S ti t l P65 75 09/06/10 13 40 76 Bollettino della Società Paleontologica Italiana, 49 (1), 2010 The Silurian deposits show wide lateral facies environment, cephalopod-bearing limestones, diversity, a limited total thickness (50-100 m) and are intercalations of limestones with shales and finally black irregularly distributed within the Carnic Alps. It is possible graptolitic shales and cherts testifying deep water basinal to find bioclastic limestones testifying a shallow water environment (Corradini & Simonetto, 2006). This justifes the subdivision of the Silurian rocks in four major facies (Selli, 1963; Schönlaub, 1997, 1998; Wenzel, 1997), reflecting the depths and the different hydrodynamic conditions of the environments (Fig. 1). Proceeding from North-West towards South-East, it is possible to find: the Wolayer facies, characterized by shallow-water sediments, the Plocken and the Findenig facies, with intermediate conditions, and finally the Bischofalm facies with deep-water sediments. From Llandovery to Ludlow, Silurian sedimentation shows generally a transgressive trend, whereas a diffuse Pridoli carbonate sedimentation shows a more steady conditions (Schönlaub, 1997). Most of Llandovery and sometimes also the lower Wenlock are lacking due to a significant gap between Ordovician and Silurian sediments (Histon & Schönlaub, 1999). SOME REMARKS ON ACTINOCERIDA The order Actinocerida ranging from Middle Ordovician to late Carboniferous is represented by cephalopods having mainly slender and straight or in some cases slightly curved orthoconic shell. The specimens of this order are medium to large in size, ranging from 90 cm to 6 m (genus Rayonnoceras Teichert, 1964). The cross section is generally circular to subcircular, though some specialized genera have the conch strongly compressed and flattened. The inner features are distinctive and diagnostic: siphuncle is large, subcentral to marginal in position with broadly expanded segments. The septal necks in actinocerids are cyrtochoanitic. In many families (e.g. Armenoceratidae) the septal neck is strongly recumbent and the brim is near to or in contact with the inferior part of the septa. Epi/hyposeptal calcareous cameral deposits are well developed, as well as endosiphuncular deposits. In mature stage of the conch the endosiphuncular deposits may fill the space, producing a typical endosiphuncular canal system, which is “… a Fig. 1 - Location of the Carnic Alps (A), locality map of the Mt. Cocco area outcrop (indicated by an asterisk) (B), lithology of Silurian primary organic feature” (Flower, 1955; Teichert, 1964). sediments of the four different lithofacies of the Carnic Alps (C) Teichert (1933) elevated this taxon to ordinal level (after Wenzel, 1997). and attempted to link actinocerids to endocerids because EXPLANATION OF PLATE 1 Fig. 1 - Elrodoceras sp. ind. A. Specimen IPUM 27969. 1a - Polished oblique section preserving the last chamber of phragmocone, x 1.5; 1b - detail showing the central siphuncle with annular deposits more developed on the ventral side, x 3; 1c - enlarged detail showing the dorsal annular deposit, the recumbent septal neck and the trend of the connecting ring in its proximal part, x 8.5; 1d - enlarged detail of the above reported features in the ventral side of the septal foramen, x 7; 1e - schematic draft showing the septal neck. Fig. 2 - Huroniella? sp. ind. Specimen IPUM 27970. 2a - Longitudinal polished section showing three chambers, the wide sub-central siphuncle and the preserved inner features, x 2; 2b - enlarged detail of the strongly recumbent actinocerid septal neck with funnel-like huronionid adnation area, x 10; 2c - schematic draft showing the transversal section with the shaded area indicates available material; 2d - schematic draft showing the septal neck. 07 S ti t l P65 76 19/05/10 12 53 P. Serventi et al. - Silurian nautiloid cephalopods from the Carnic Alps Pl.77 1 07 S ti t l P65 77 19/05/10 12 53 78 Bollettino della Società Paleontologica Italiana, 49 (1), 2010 large siphuncles and long septal necks. According to Order ACTINOCERIDA Teichert, 1933 Kobayashi (1935) and Schindewolf (1935) actinocerids, Family ARMENOCERATIDAE Troedsson, 1926 with small siphuncles (species belonging to genera Ormoceras or Sactoceras), arose from Michelino- Genus Elrodoceras Foerste, 1924 ceratida. In 1941, Flower proposed that Actinocerida originated from Bathmoceras inside the Ellesme- Type species - Cyrtoceras indianense Miller, 1892 roceratida (Upper Canadian), through Polydesmia “as the by original designation. oldest