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;
[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
Nautiloid cephalopods of the Carnic Alps are well
known since the second half of the eighteenth century. In
1872, the Austrian geologist Guido Stache (1833-1921)
pointed out for the first time the occurrence of the
Silurian rocks in the Carnic Alps. Successively he
recorded “Orthoceras limestones” in many localities of
the Carnic area and collected a great number of
cephalopod specimens, but did not produce any
systematic work. In 1887, Fritz Frech (1861-1917), a
German geologist, in “Über das Devon der Ostalpen, nebst
Bemerkungen über das Silur und einem
paläontologischem Anhang”, described two species of
nautiloid cephalopods: Orthoceras potens and
Orthoceras alticola.
The first systematic work on cephalopod fauna, from
the Silurian outcrops, is dated to 1909: the Italian
geologists Michele Gortani (1883-1966) and Paolo
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
“Faunen aus dem Silur der Ostalpen”, a detailed
palaeontological systematic study on Silurian cephalopod
faunas with specimens collected by himself and with
material, stored in the Geological Survey of Vienna, and
belonging to collections of von Gaertner, Geyer and
Stache.
Palaeontological studies on cephalopod fauna stopped
after the Second World War. Only in 1968 Heinrich
Ristedt, from the Bonn University, published an essential
work on cephalopod early stages and protoconchs, with
specimens coming from the terranes forming the
northern margin of Gondwana during the Silurian.
Renewed interest for the research on cephalopods arose
ISSN 0375-7633
close to the end of the century thanks to Gnoli & Histon
(1998), Histon (1999), Serventi (1999), Serventi et al.
(1999), Gnoli et al. (2000), Serventi & Gnoli (2000),
Serventi (2002) and Gnoli & Serventi (2006).
The main investigated cephalopod faunas concern
members of the orders Orthocerida Kuhn, 1940,
Oncocerida Flower in Flower & Kummel, 1950,
Barrandeocerida Flower in Flower & Kummel, 1950,
Discosorida Flower in Flower & Kummel, 1950 and
Nautilida Agassiz, 1847.
Except few fragmentary specimens reported by
Serventi & Gnoli (2000) and left in open nomenclature,
members of the order Actinocerida Teichert 1933, have
not been studied. The goal of this paper is to describe a
new material of actinoceroids collected in the Italian side
of the Carnic Alps in the Mt. Cocco area (Fig. 1) and stored
in the Museo Friulano di Storia Naturale, Udine, and in
Palaeontological collections of the University of Modena
and Reggio Emilia.
GEOLOGICAL REMARKS
In the Friuli Venezia Giulia region (in the northeastern
part of Italy) three mountain chains are welded together:
the Palaeocarnic chain, the eastern section of the South
Alpine chain and the northwestern part of the Outer
Dinarides. The Palaeocarnic chain, about 180 km long
(from Comelico to Caravanche) and 10-20 km wide,
presents a metamorphic part, confined in the northwestern
sector, and a non-metamorphic part represented by an
almost continuous Palaeozoic sedimentary succession
(Carulli, 2006) (Fig. 1). The Palaeozoic outcrops, very
rich in palaeontological remains, range from the Upper
Ordovician to Permian.
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Bollettino della Società Paleontologica Italiana, 49 (1), 2010
The Silurian deposits show wide lateral facies
diversity, a limited total thickness (50-100 m) and are
irregularly distributed within the Carnic Alps. It is possible
to find bioclastic limestones testifying a shallow water
environment, cephalopod-bearing limestones,
intercalations of limestones with shales and finally black
graptolitic shales and cherts testifying deep water basinal
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
Fig. 1 - Location of the Carnic Alps (A), locality map of the Mt.
Cocco area outcrop (indicated by an asterisk) (B), lithology of Silurian
sediments of the four different lithofacies of the Carnic Alps (C)
(after Wenzel, 1997).
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
primary organic feature” (Flower, 1955; Teichert, 1964).
Teichert (1933) elevated this taxon to ordinal level
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
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Bollettino della Società Paleontologica Italiana, 49 (1), 2010
large siphuncles and long septal necks. According to
Kobayashi (1935) and Schindewolf (1935) actinocerids,
with small siphuncles (species belonging to genera
Ormoceras or Sactoceras), arose from Michelinoceratida. In 1941, Flower proposed that Actinocerida
originated from Bathmoceras inside the Ellesmeroceratida (Upper Canadian), through Polydesmia “as the
oldest and most primitive genus” (Flower, p. 12) starts in
middle Arenigian (Teichert, 1933; Flower, 1957, 1968).
Some authors discussed the systematic position of
actinocerids (see, e. g., Mutvei, 1997, 2002a, 2002b;
Evans, 2005; Kröger & Mapes, 2007), for example
Mutvei stressed out the importance of connecting ring
structure and of muscle scars.
In our opinion the distinctive peculiar features of
actinoceroid cephalopods among others seem those
concerning the wide siphuncle and in particular the shape
of long curved septal necks, the endosiphuncular canals
system and associated primary deposits.
Order ACTINOCERIDA Teichert, 1933
Family ARMENOCERATIDAE Troedsson, 1926
Genus Elrodoceras Foerste, 1924
Type species - Cyrtoceras indianense Miller, 1892
by original designation.
Remarks - The genus Elrodoceras Foerste, 1924 was
regarded by Dzik (1984: p. 147) as a junior subjective
synonym for Ormoceras Stokes, 1840 and later reestablished by Holland (1998: p. 189). In the opinion of
the writers the main difference between the reported
genera is the wider adnation area shown by the siphuncular
features of Elrodoceras.
Elrodoceras sp. ind. A
(Pl. 1, fig. 1)
2000 Ormoceras sp. A SERVENTI & GNOLI, p. 11, pl. 1, fig. 1a-b.
FOSSIL PRESERVATION
All the studied specimens, with few exceptions, are
fragmentary and affected by dissolution and/or strongly
recrystallized. This occasionally makes impossible to
study internal structures - connecting rings or siphuncular
and cameral deposits - important in cephalopod
taxonomy. Preparation of fossils, consequently, has been
quite difficult as the majority of specimens are embedded
in a hard matrix with high iron-manganese mineralization.
Institutional abbreviations - IPUM: Museo di
Paleontologia dell’Università di Modena e Reggio Emilia;
MFSNgp: Museo Friulano di Storia Naturale, collezione
geologica-paleontologica.
Material studied - One fragment of phragmocone
registered under the number IPUM 27969.
Description - The material consists of one short
fragment of phragmocone, 54 mm long, of orthoconic
(presumably slender) shell where only one chamber is
preserved. Chamber long being 2/5 its diameter; septa
depth corresponds to 0.45 diameter; wide central
siphuncle whose diameter is 4.6 mm at septal foramen.
Barrel-like connecting rings reaching a diameter of about
13 mm in its middle part. Very short actinocerid
recumbent septal necks being of length 0.4 mm with 1
mm long brim. Endosiphuncular deposit in form of annuli
mainly developed on the ventral side (Pl. 1, fig. 1b),
showing alternate light and dark growth layers. No outer
ornamentation is preserved.
SYSTEMATIC PALAEONTOLOGY
The taxonomic scheme adopted here is mainly that
reported in the Treatise Part K, Mollusca 3 (Teichert,
1964) with integrations from the Data Retrieval System
Nautiloidea by T. Engeser, available on CD ROM thanks
to the courtesy of the author. Proposed systematics by
Dzik (1984) is also taken into account.
The terminology employed is essentially that
advocated by Flower (1964) including the terms adapical
(towards the apex of the shell) and adoral (towards the
aperture).
Remarks - Due to impossibility to extract the
specimen from the recrystallized hard matrix, internal
features of the shell have been studied in a longitudinal
polished section. Outline trend of the proximal parts of
the preserved connecting ring allows us to assume with
goodness of fit the measurement of the max. middle
diameter of the siphuncular segments close to 13 mm.
The shape of the strongly recumbent septal necks, the
endosiphuncular annular deposits, and wide adnation area
allow the assignment to the genus Elrodoceras. Due to
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
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Bollettino della Società Paleontologica Italiana, 49 (1), 2010
ornamentation or on the most of the inner features, the
specimen is left in open nomenclature.
Stratigraphy and geographic distribution - Silurian
of Mt. Cocco, Carnic Alps.
Family HURONIIDAE Foerste & Teichert, 1930
diameter. Sub-central siphuncle with a mean diameter of
3.5 mm at septal foramina, but reaching its max. value in
the middle part of the connecting rings with a diameter
of 10 mm representing about 1/3 the shell diameter.
Septal foramina are 2.8 mm across, corresponding to
about 1/10 the shell diameter.
Septal necks of actinocerid type are very short and
recumbent being in length 0.6 mm with a 1.8 mm long
brim (Pl. 2, fig. 1d).
Genus Huroniella Foerste, 1924
Type species - Huronia inflecta Parks, 1915 by
original designation.
Huroniella? sp. ind.
(Pl. 1, fig. 2)
Material studied - One fragment of phragmocone
registered under the number IPUM 27970.
Description - Short fragment of phragmocone of a
large orthoconic shell 41.5 mm long preserving about
three chambers whose length corresponds medially to 14
mm. Sub-central wide actinocerid siphuncle whose
diameter reaching 28 mm. Septal necks short and strongly
recumbent being 0.4 mm with a brim of 1.4 mm. Funnelshaped adnation area corresponds to 5.3 mm. Barrel-like
connecting rings expanded in the chambers in its middle
part about 7.2 mm from the line connecting two
subsequent septal necks. No other features are preserved.
Remarks - Estimated diameter of the specimen is
about 80 mm. The characteristic outline of the funnellike adnation area and the strongly recumbent septal necks
allow, even dubitatively, to assign this specimen to genus
Huroniella.
Stratigraphy and geographic distribution - Silurian
of Mt. Cocco, Carnic Alps.
Family ORMOCERATIDAE Saemann, 1853
Genus Ormoceras Stokes, 1840
Type species - Ormoceras bayfieldi Stokes, 1840 by
subsequent designation (Bassler, 1915).
Ormoceras sp. ind. A
(Pl. 2, fig. 1)
2000 Ormoceras? sp. B SERVENTI & GNOLI, p. 11, pl. 1, figs. 3a-b.
Material studied - One fragment of phragmocone
under the number MFSNgp 23221.
Description - Available material consists of one
fragment of phragmocone of an orthoconic shell, 85 mm
long, gradually expanding with an angle close to 4 degrees.
Six chambers are preserved.
Chambers depth about 2/5 their diameter. Septa
moderately concave corresponding to about 1/4 their
Remarks - This fragmentary specimen is assigned to
the genus Ormoceras mainly because of the peculiar
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.
The brim shape is similar to that noted in members of
Elrodoceras, but the minor development of the adnation
area is closer to the genus Ormoceras. Incomplete
preservation suggests to leave this form in open
nomenclature.
Stratigraphy and geographic distribution - Silurian
of Mt. Cocco, Carnic Alps.
ACKNOWLEDGEMENTS
Thanks are due to Dr. Carlo Morandini, director of MFSN for
giving available for study fossil specimens stored in the Museum.
The authors are deeply indebted to Prof. Enrico Serpagli, Dr. V.
Turek and an anonymous referees for the critical reading of the
MS, useful suggestions and advises. Dr. Theo Engeser give us
available his CD compilation Data Retrieval System Nautiloidea.
This research was funded by PRIN “Quo vadis, north
Gondwana? - Regional palaeogeography and migrational seaways
of pelagic organisms in the Early Palaeozoic” project grant
(responsible Prof. Annalisa Ferretti).
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Manuscript received 17 October 2009
Revised manuscript accepted 10 February 2010