Actinocerid cephalopods from the Silurian of the Carnic Alps (Italy)
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Bollettino della Società Paleontologica Italiana, 49 (1), 2010, 75-81. Modena, 15 maggio 2010
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;
paolo.serventi@unimore.it
M. Gnoli, Dipartimento di Scienze della Terra, Università degli Studi di Modena e Reggio Emilia, L.go S. Eufemia 19, I-41121 Modena, Italy;
maurizio.gnoli@unimore.it
L. Simonetto, Museo Friulano di Storia Naturale, Via Marangoni 39, I-33100 Udine, Italy; mfsn@comune.udine.it
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 0375-763376 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
Fig. 1 - Location of the Carnic Alps (A), locality map of the Mt.
a typical endosiphuncular canal system, which is “… a
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.P. Serventi et al. - Silurian nautiloid cephalopods from the Carnic Alps Pl.771
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 and most primitive genus” (Flower, p. 12) starts in
middle Arenigian (Teichert, 1933; Flower, 1957, 1968). Remarks - The genus Elrodoceras Foerste, 1924 was
Some authors discussed the systematic position of regarded by Dzik (1984: p. 147) as a junior subjective
actinocerids (see, e. g., Mutvei, 1997, 2002a, 2002b; synonym for Ormoceras Stokes, 1840 and later re-
Evans, 2005; Kröger & Mapes, 2007), for example established by Holland (1998: p. 189). In the opinion of
Mutvei stressed out the importance of connecting ring the writers the main difference between the reported
structure and of muscle scars. genera is the wider adnation area shown by the siphuncular
In our opinion the distinctive peculiar features of features of Elrodoceras.
actinoceroid cephalopods among others seem those
concerning the wide siphuncle and in particular the shape
of long curved septal necks, the endosiphuncular canals Elrodoceras sp. ind. A
system and associated primary deposits. (Pl. 1, fig. 1)
2000 Ormoceras sp. A SERVENTI & GNOLI, p. 11, pl. 1, fig. 1a-b.
FOSSIL PRESERVATION
Material studied - One fragment of phragmocone
All the studied specimens, with few exceptions, are registered under the number IPUM 27969.
fragmentary and affected by dissolution and/or strongly
recrystallized. This occasionally makes impossible to Description - The material consists of one short
study internal structures - connecting rings or siphuncular fragment of phragmocone, 54 mm long, of orthoconic
and cameral deposits - important in cephalopod (presumably slender) shell where only one chamber is
taxonomy. Preparation of fossils, consequently, has been preserved. Chamber long being 2/5 its diameter; septa
quite difficult as the majority of specimens are embedded depth corresponds to 0.45 diameter; wide central
in a hard matrix with high iron-manganese mineralization. siphuncle whose diameter is 4.6 mm at septal foramen.
Barrel-like connecting rings reaching a diameter of about
Institutional abbreviations - IPUM: Museo di 13 mm in its middle part. Very short actinocerid
Paleontologia dell’Università di Modena e Reggio Emilia; recumbent septal necks being of length 0.4 mm with 1
MFSNgp: Museo Friulano di Storia Naturale, collezione mm long brim. Endosiphuncular deposit in form of annuli
geologica-paleontologica. mainly developed on the ventral side (Pl. 1, fig. 1b),
showing alternate light and dark growth layers. No outer
ornamentation is preserved.
SYSTEMATIC PALAEONTOLOGY
Remarks - Due to impossibility to extract the
The taxonomic scheme adopted here is mainly that specimen from the recrystallized hard matrix, internal
reported in the Treatise Part K, Mollusca 3 (Teichert, features of the shell have been studied in a longitudinal
1964) with integrations from the Data Retrieval System polished section. Outline trend of the proximal parts of
Nautiloidea by T. Engeser, available on CD ROM thanks the preserved connecting ring allows us to assume with
to the courtesy of the author. Proposed systematics by goodness of fit the measurement of the max. middle
Dzik (1984) is also taken into account. diameter of the siphuncular segments close to 13 mm.
The terminology employed is essentially that The shape of the strongly recumbent septal necks, the
advocated by Flower (1964) including the terms adapical endosiphuncular annular deposits, and wide adnation area
(towards the apex of the shell) and adoral (towards the allow the assignment to the genus Elrodoceras. Due to
aperture). the lack of any information either on the outer
EXPLANATION OF PLATE 2
Fig.1 - Ormoceras sp. ind. A. Specimen MFSNgp 23221.
1a - Outer view, x 1.2;
1b - longitudinal polished section showing about six chambers and the relatively narrow central siphuncle, x 1.3;
1c - enlarged detail of the septal foramen area showing the recumbent septal neck and the trend of the connecting rings in their
proximal part, x 10;
1d - the same dorsal part enlarged, x 20;
1e - schematic draft showing the septal neck.P. Serventi et al. - Silurian nautiloid cephalopods from the Carnic Alps Pl.792
80 Bollettino della Società Paleontologica Italiana, 49 (1), 2010
ornamentation or on the most of the inner features, the diameter. Sub-central siphuncle with a mean diameter of
specimen is left in open nomenclature. 3.5 mm at septal foramina, but reaching its max. value in
the middle part of the connecting rings with a diameter
Stratigraphy and geographic distribution - Silurian of 10 mm representing about 1/3 the shell diameter.
of Mt. Cocco, Carnic Alps. Septal foramina are 2.8 mm across, corresponding to
about 1/10 the shell diameter.
Septal necks of actinocerid type are very short and
Family HURONIIDAE Foerste & Teichert, 1930 recumbent being in length 0.6 mm with a 1.8 mm long
brim (Pl. 2, fig. 1d).
Genus Huroniella Foerste, 1924
Remarks - This fragmentary specimen is assigned to
Type species - Huronia inflecta Parks, 1915 by the genus Ormoceras mainly because of the peculiar
original designation. siphuncular outlines. In particular the recumbent septal
necks and an adnation area are less developed than in the
specimen mentioned above and referred to Elrodoceras.
Huroniella? sp. ind. The brim shape is similar to that noted in members of
(Pl. 1, fig. 2) Elrodoceras, but the minor development of the adnation
area is closer to the genus Ormoceras. Incomplete
Material studied - One fragment of phragmocone preservation suggests to leave this form in open
registered under the number IPUM 27970. nomenclature.
Description - Short fragment of phragmocone of a Stratigraphy and geographic distribution - Silurian
large orthoconic shell 41.5 mm long preserving about of Mt. Cocco, Carnic Alps.
three chambers whose length corresponds medially to 14
mm. Sub-central wide actinocerid siphuncle whose
diameter reaching 28 mm. Septal necks short and strongly ACKNOWLEDGEMENTS
recumbent being 0.4 mm with a brim of 1.4 mm. Funnel-
shaped adnation area corresponds to 5.3 mm. Barrel-like Thanks are due to Dr. Carlo Morandini, director of MFSN for
connecting rings expanded in the chambers in its middle giving available for study fossil specimens stored in the Museum.
part about 7.2 mm from the line connecting two The authors are deeply indebted to Prof. Enrico Serpagli, Dr. V.
Turek and an anonymous referees for the critical reading of the
subsequent septal necks. No other features are preserved. MS, useful suggestions and advises. Dr. Theo Engeser give us
available his CD compilation Data Retrieval System Nautiloidea.
Remarks - Estimated diameter of the specimen is This research was funded by PRIN “Quo vadis, north
about 80 mm. The characteristic outline of the funnel- Gondwana? - Regional palaeogeography and migrational seaways
like adnation area and the strongly recumbent septal necks of pelagic organisms in the Early Palaeozoic” project grant
allow, even dubitatively, to assign this specimen to genus (responsible Prof. Annalisa Ferretti).
Huroniella.
Stratigraphy and geographic distribution - Silurian REFERENCES
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