TRACE FOSSIL SARONICHNUS ABELI IGEN. ET ISP. NOV. FROM DEPOSITS OF LOWER AUSTRIA 111
GEOLOGICA CARPATHICA, 55, 2, BRATISLAVA, APRIL 2004
A LUCINOID BIVALVE TRACE FOSSIL SARONICHNUS ABELI IGEN.
ET ISP. NOV. FROM THE MIOCENE MOLASSE DEPOSITS OF
LOWER AUSTRIA, AND ITS ENVIRONMENTAL SIGNIFICANCE
PETER PERVESLER and MARTIN ZUSCHIN
Department of Paleontology, University of Vienna, Althanstrasse 14, A-1090 Vienna, Austria;
(Manuscript received June 5, 2003; accepted in revised form December 16, 2003)
Abstract: A new ichnogenus and ichnospecies Saronichnus abeli is described for a protrusive trace fossil produced by
the lucinoid bivalve Thyasira michelottii from storm-dominated siliciclastics of the Miocene (Lower Badenian) Grund
Formation. A chemosymbiotic life strategy under dysaerobic conditions is proposed for this bivalve. The trace fossil
expresses an adaptation to oxygen-poor habitats.
Key words: Miocene, Austria, chemosymbiosis, molasse, trace fossils, Thyasira.
The availability of oxygen is an important factor for the distri-
bution of benthic organisms (Savrda & Bottjer 1991). The dis-
tribution of marine trace fossils is frequently reported to indi-
cate oxygen levels in the water column and sediment (e.g.
Savrda et al. 1991). Even if their producers are unknown, trace
fossil associations and sequences are used to determine depo-
sitional history and environmental parameters (e.g. energy,
nutrients, oxygen). Sediment-ingesting polychaetes, sipuncu-
lids, or even detritus-feeding arthropods have been regarded
as potential tracemakers, providing evidence for chemosym-
biotic life strategies under dysaerobic conditions (Bromley &
Ekdale 1984; Savrda 1992). Chemosymbiosis in fossils is very
difficult to prove and the only reliable method is probably the
study of ichnofossils, especially if associated with specimens
preserved in life position (Savazzi 2001).
Dwelling below the substrate surface is a very common life
habit for bivalves. Depending on the physical properties of the
environment and substrates, specialized life strategies have
been developed by such molluscs. Lithophaga, Gastrochaena,
Gregariella and Teredo produce borings by corrosion or me-
chanical abrasion in hard or firm substrates such as limestones
or wood. The corresponding trace fossils are known as Gas-
trochaenolites o r Teredolites (Kelly & Bromley 1984; Bro-
mley et al. 1984; Tewari et al. 1998). Almost all such borings
are clearly defined permanent structures not obscured by shift-
ing of the valve position, as commonly occurs in soft sub-
strates. More or less stable bivalve burrows are known from
the recent Solemya velum, which produces Y-shaped struc-
tures that shift their position only every few days (compare
Frey 1968; Stanley 1970; Levinton & Bambach 1975), and
from the fossil state (ichnogenus Solemyotuba Seilacher
1990). Clearly defined shapes of bivalve burrows in soft sub-
strates were considered to be relatively rare in the fossil
record. Vagile deposit-feeding bivalves such as Nucula and
Yoldia (Bender & Davis 1984), which belong to the Paleotax-
odonta, were recognized as producers of the trace fossil Pro-
tovirgularia (Han & Pickerill 1994; Seilacher & Seilacher
1994; Ekdale & Bromley 2001). Detailed morphological stud-
ies enabled the feathered-serpent-shaped Hillichnus lobosen-
sis to be identified as a tellinacean trace fossil (Bromley et al.
The thyasirid bivalve Thyasira michelottii (Hörnes, 1875)
produced a shaft-like trace fossil in the Miocene of Austria
(Zuschin et al. 2001). Here, we present an additional distinct
bivalve trace fossil produced by the same species. The unique
in situ preservation of the burrowing bivalve Thyasira miche-
lottii in the siliciclastics of the Grund Formation (Middle Mi-
ocene, Lower Badenian), clearly associated with trace fossil
Saronichnus abeli Pervesler et Zuschin, allows the life habits
of these bivalves and the characteristics of their marine envi-
ronment to be reconstructed.
Material and methods
Thyasira shells associated with the discussed trace fossil
were excavated during the field campaigns in 1998 and 1999
(Zuschin et al. 2001; Zuschin et al. 2004; Roetzel & Pervesler
2004). The slightly cemented trace fossil could be exposed by
the use of an air jet, which allowed careful removal of the sur-
rounding sediment. A pistol-shaped valve was used to control
the direction and volume of the airflow (Pervesler & Uchman
2004). Clear varnish helped to preserve the trace fossil. This
method, mainly developed for loose sands, provides an oppor-
tunity to observe three-dimensional aspects of even very deli-
cate structures. Thyasira shells in life-position were studied in
the sections D, E, G and H (Fig. 1).
The burrowing bivalve Thyasira michelottii from the Grund
Formation was found together with associated trace fossils
(Fig. 3). An inhalent tube from the anterior shell margin pro-
112 PERVESLER and ZUSCHIN
vided the connection to the former sea floor (Fig. 3.4), and a
system of probes below the life position of the bivalves protrud-
ed more than 10 cm into the surrounding sediment. The probes
show a variety of shapes from broom- to star-like structures
(Figs. 3.24). In the proximal portion of the trace fossil the
probes converge and form a bundle starting from the ventral
shell margin (Fig. 2). Distally, the probes are tubular to blade-
shaped, up to 3 mm wide, with rounded tips. Their orientation
was perpendicular or slightly inclined with respect to the bed-
ding. Horizontal protrusions were never observed. The shifting
of these probes started close to the shell margin. Rootlike
branchings along the distal portions, as observed in the burrows
of the recent Thyasira flexuosa (Dando & Southward 1986),
were mentioned as being morphologically reminiscent of Chon-
drites (Savrda & Bottjer 1991; Savrda 1992). These features
were not observed in the Thyasira michelottii burrows.
Saronichnus igen. nov.
Derivation of name: Saronichnus (Greek
Type ichnospecies: Saronichnus abeli Pervesler et
Zuschin (this paper).
Diagnosis: A system of vertical to steeply inclined tubu-
lar to blade-shaped simple unbranched probes diverging from
broom- or star-like bunches. The probes are up to 100 mm
long and about 3 mm wide. The bunch structures start from
the posterior ventral edge of bivalve shell. The probes overlap
in the upper part of the bunched and form a spreite structure
visible in cross-section. The structures are typically cemented.
Fig. 2. Thyasira michelottii and Saronichnus abeli from the Grund
Formation. 1 Horizontal cross-section of Saronichnus abeli
(paratype IPUW No. 2003-0001-0002). The rootlike structures are
composed of falcate backfill-like structures produced during the
penetration of the sediment by the extendable foot of the bivalve Thya-
sira. Excavation 1999, section G. Scale bar = 1 cm. 2 Relation
of cross-section shown in figure 2.1 with reconstructed probes and
position of the bivalve.
Discussion: Although the chemosymbiotic trace fossils
Chondrites and Saronichnus may have functional similarities
as sulphide wells, and although both are typical elements of
the Zoophycos ichnofacies, they can clearly be distinguished
Fig. 1. Type area of the Grund Formation. Sections A, B1, B2, C, D, E excavated in 1998, sections F, G and H in 1999. Thyasira michelottii
shells in life position together with their trace fossils were found in the sections D, E, G and H.
TRACE FOSSIL SARONICHNUS ABELI IGEN. ET ISP. NOV. FROM DEPOSITS OF LOWER AUSTRIA 113
Fig. 3. Thyasira michelottii and Saronichnus abeli from the Grund Formation. 1 Overview of an artificial outcrop in the type area of the
Grund Formation during the excavation in 1999 (section G). Several individuals of Thyasira michelottii in life position, connected to their
broom-shaped trace fossils (Saronichnus abeli), are associated with a broad variety of other trace fossils typically start from pelitic layers
that cover each fining upward sequence. Trace fossils were exposed by using compressed air. Unidentifiable fragments of molluscs are com-
ponents of the sediment. 2 In situ Thyasira michelottii with Saronichnus abeli (detail of Fig. 3.1). 3 Two in situ individuals of Thyasira
michelottii (T) with their trace fossil Saronichnus abeli (S). Left specimen was defined as the holotype (IPUW No. 2003-0001-0001). Note
the large variability of these trace fossils (excavation 1999, section G). 4 Close-up view of in situ Thyasira michelottii with inhalent tube
and Saronichnus abeli attached to the shell. The single shafts have diameters up to 3 mm and protrude several centimeters into the sediment
below the ventral shell margin (detail of Fig. 3.2).
114 PERVESLER and ZUSCHIN
on the basis of their morphology. We therefore do not apply
the ichnogenus Chondrites to these Thyasira burrows, but
postulate the new ichnogenus Saronichnus. Saronichnus is
similar to Pragichnus fascis Chlupáè (1987), which was de-
scribed from the Ordovician of Czech Republic. However,
Pragichnus displays rather only club-shaped, not blade-
shaped, branched probes, which are actively filled. The lower
part of the Pragichnus probes displays dichotomous branches.
The probes were interpreted as sulphide wells produced by an
animal using chemosymbiotic bacteria (Mikulá 1997). Sa-
ronichnus is also similar to Syringomorpha Nathorst, 1886,
which is a vertical fan-like structure composed of steep, tight-
ly spaced cylinders (Bartholomäus 1993; Jensen 1997). The
upper and lower termination of the latter is not clear. It is not
excluded that these two ichnogenera may be synonymized in
the future, but Syringomorpha requires further studies to rec-
ognize its full morphology. Heimdallia Bradshaw, 1981 is
composed of vertical, superimposed cylinders, but it is a wall-
like, long structure in which the terminations of the cylinders
Saronichnus abeli isp. nov.
Figs. 2, 3.14
Derivation of name: abeli [Abel, Othenio * 20. 6.
1875 Vienna, 4. 7. 1946 Mondsee (Upper Austria), was one
of the founders of paleontology as an independent science. He
worked on understanding how extinct animals interacted with
Types: Holotype IPUW No. 2003-0001-0001 (Fig. 3.3),
paratype IPUW No. 2003-0001-0002 (Fig. 2). Together with
5 further specimens of Saronichnus abeli they are housed in
the Department of Paleontology of the University of Vienna,
Diagnosis: As for the ichnogenus.
Description: As for the ichnogenus.
Life habit of Thyasira michelottii
The posterio-ventral probes are thought to record the search
of the burrowing, worm-shaped foot for pockets of sulphidic
material in an otherwise low-sulphide environment, similarly
to the recent bivalve Lucinoma aequizonata (Cary et al.
1989). Chemosymbiotic strategies may be responsible for the
similarity between recent Thyasira burrows and Saronichnus
abeli trace fossils. In chemoautotrophic symbioses, bacteria
that use sulphides for their metabolism serve as food for the
bivalve, which in turn creates a protected environment for the
bacteria (Felbeck 1985). Among the superfamily Lucinoidea,
special anatomical and morphological features such as thick
gills, reduced palps, short simple gut, and an elongated bur-
rowing foot are interpreted as an adaptation to oxygen- and
nutrient-poor habitats (Allen 1958). The broom-like burrow
systems are formed when the clams use their extendable foot
to obtain hydrogen sulphide from interstitial water in the un-
derlying sediments (Turner 1985). The length and number of
burrows are related to the concentration of hydrogen sulphide
in the sediment (Dufour & Felbeck 2003). For a detailed dis-
cussion of the life habit see Zuschin et al. (2001).
The Grund Formation is characterized by a rapid alternation
of allochthonous psammitic and autochthonous pelitic sedi-
mentation, with a distinct decrease in water energy from the
base to the top. The sandy layers, especially in the lower part
of the section, commonly show channel structures. They con-
sist predominantly of thick skeletal concentrations; common
features include sharp erosive bases, graded bedding and a
densely packed bioclast-supported fabric (Roetzel & Per-
vesler 2004; Zuschin et al. 2004). They are therefore interpret-
ed as high-energy, short-term events and most likely represent
proximal tempestites, similar to the Jurassic deposits of India
(Fürsich & Oschmann 1993). Thyasira michelottii occurs in
life position in close association with the pelitic layers. The
life position is confirmed by valve articulation and preserva-
tion of the inhalant and posterio-ventral Saronichnus probes.
The abundance of T. michelottii, as well as its monospecific
occurrence, indicate that the chemical and physical conditions
of the sediment preferred by this thyasirid species were un-
suitable for other molluscs.
The post-event pelitic layers are background sediments that
accumulated mostly between storm events. These poorly bio-
turbated pelitic layers represent periods of colonization (colo-
nization window sensu Pollard et al. 1993). The trace fossils
occurring in sandy layers are related to colonization surfaces
in the overlying pelitic layers. All the burrows represented by
the trace fossils had connections to the sea floor. Primary sedi-
mentary structures are well preserved, and the trace fossils
disturb them only to a limited extent. The connection of the
burrows to the sea floor and the limited bioturbation suggest
poor oxygen conditions within the sediments, but not in the
water column above the sea floor. The impermeable pelitic
layers sealed the coarse event layers rich in organic matter and
prevented the exchange of oxygen between the sediment and
the water column. This resulted in a low-oxygen content in
the coarse sediments and the colonization by a low-diversity
post-event climax community. The community produced
long-occupied burrows that document complex living strate-
gies (e.g. chemosymbiosis, gardening).
Acknowledgments: Project 13743-BIO (Temporal and spa-
tial changes of microfossil associations and ichnofacies in the
Austrian marine Miocene) and Project P 13745-BIO (Evolu-
tion Versus Migration: Changes in Austrian Marine Miocene
Molluscan Paleocommunities) of the Austrian Science Fund,
and the Department of Paleontology at the University of Vien-
na supported this study. We extend our special thanks to Re-
inhard Roetzel for his support in sedimentology and to all
helpers during fieldwork. Special thanks are due to Alfred
Uchman (Kraków) and Richard Bromley (Copenhagen), who
were the official reviewers and Michael Stachowitsch (Vien-
na) for critical reading.
TRACE FOSSIL SARONICHNUS ABELI IGEN. ET ISP. NOV. FROM DEPOSITS OF LOWER AUSTRIA 115
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