GEOLOGICA CARPATHICA
, FEBRUARY 2019, 70, 1, 3–13
doi: 10.2478/geoca-2019-0001
www.geologicacarpathica.com
Changes in the composition of trace fossil assemblages
across the Paleocene–Eocene transition in
the north-western Tethys (Untersberg section, Austria)
ALFRED UCHMAN
1,
, HANS EGGER
2
and FRANCISCO J. RODRÍGUEZ-TOVAR
3
1
Institute of Geological Sciences, Jagiellonian University, Gronostajowa 3a; 30387 Kraków, Poland;
alfred.uchman@uj.edu.pl
2
Geological Survey of Austria, Neulinggasse 38, 1030 Vienna, Austria
3
Department of Stratigraphy and Paleontology, University of Granada, 18002 Granada, Spain
(Manuscript received August 7, 2018; accepted in revised form January 11, 2019)
Abstract: The Untersberg section (Northern Calcareous Alps, Austria) provides an expanded and biostratigraphically
well constrained deep-sea record of the Paleocene–Eocene transition in the north-western Tethyan realm. At the base of
the Eocene, massive carbonate dissolution and a shoaling of the calcite compensation depth (CCD) by at least 1 km is
recorded by 5.5 m-thick red claystone, which is intercalated into a grey marlstone succession. Previous studies documented
the benthic foraminifera extinction event (BEE) in this claystone. Now biodeformational structures and trace fossils were
investigated in this interval to evaluate the impact of the extinction event on the macrobenthic tracemaker fauna. Using
the stratigraphic distribution pattern of trace fossils, the lowermost Eocene claystone can be subdivided into three parts:
(1) the lower part shows a trace fossil assemblage consisting of Chondrites isp., Planolites isp., Thalassinoides isp., and
Zoophycos isp., (2) the middle part is characterized by primary sedimentary lamination and exceedingly rare ichnofossils,
and (3) the upper part shows a less abundant and less diverse trace fossil assemblage than the lower part, indicating a slow
recovery of the macrobenthic tracemaker community. This pattern demonstrates that macrobenthic communities were
severely affected by the ecological perturbations in the earliest Eocene. The change in sediment colouration towards red
colour in the middle part of the Paleocene–Eocene transition at the Untersberg section, together with decrease in
bioturbation degree indicate that oxygen consumption was rather reduced during the PETM, and the loss in bioturbation
is thus unrelated to oxygen limitation. Trace fossils can be used to improve the resolution of the benthic extinction
interval and provide an excellent proxy for the precise determination of timing of the climax of this global event.
Keywords: North-western Tethys, Eastern Alps, Paleocene–Eocene boundary, benthic extinction event, trace fossils.
Introduction
At the Paleocene–Eocene boundary (P–E boundary), around
56 Ma ago, an increase in Earth’s atmospheric temperature
by 5–8 ºC occurred (Kennett & Stott 1991; Sluijs et al.
2007; McInerney & Wing 2011). This hyperthermal event
(The Paleo cene–Eocene Thermal Maximum; PETM) is asso-
ciated with a massive perturbation of the global carbon cycle,
as indicated by a negative carbon isotope excursion (CIE)
in marine and terrestrial settings with a magnitude between
2.4 ‰ and 7 ‰ (e.g., Handley et al. 2008; McCarren et al.
2008). This is interpreted as the result of a massive release of
isotopically light carbon into the carbon cycle sourced by
a release of methane hydrates (Dickens 1999, 2011) or by
a major emission of volcanic gases (Gutjahr et al. 2017).
Based on X-ray fluorescence core-scanning of precessional
cycles the duration of the whole event is estimated approxi-
mately as 170 ka (Röhl et al. 2007).
The CIE is associated with severe carbonate dissolution and
a shoaling of the calcite compensation depth (Egger et al.
2005; Zachos et al. 2006; Penman et al. 2014) as well as
an increase in terrigenous input into marginal ocean basins,
which led to enhanced surface productivity in marginal ocean
settings (e.g., Egger et al. 2003; Thomas 2007; Speijer et al.
2012). The ecological perturbations at the P–E boundary also
affected deep-sea microfaunal biota and caused the global
extinction of 35–50 % of benthic foraminifera species within
the first 10 kyr of the Eocene (Benthic Foraminiferal Extinction
Event, BEE; Thomas 1998, 2003; Alegret et al. 2009a, b).
The benthic foraminifera assemblages at the top of the BEE-
interval show the predominance of opportunistic species
(Arreguín-Rodríguez et al. 2018). Several causes have been
proposed to explain the mass extinction, including low oxy-
genation, carbonate corrosion, changes in oceanic produc tivity
and increasing temperatures of deep ocean waters (Giusberti
et al. 2016 for a review).
McInerney & Wing (2011) stressed that benthic macroin-
vertebrate assemblages were not affected by the perturbations
during the BEE. However, effects on macrofossil communities
are indicated by the composition of trace fossil assemblages,
which reflect very well the activities of macrofaunas, even if
they are not preserved as body fossils (Nicolo 2008; Nicolo et
al. 2010; Cumming & Hodgson 2011a, b; Rodríguez-Tovar et
al. 2011a). The PETM seems to have had minor long-lasting
evolutionary impacts on benthic macrofauna inhabiting shelf
environments (Ivany et al. 2018). However, the effects of
4
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GEOLOGICA CARPATHICA
, 2019, 70, 1, 3–13
the PETM event on microbenthic fauna in bathyal and deep-
sea environment remain poorly explored.
In the latest years, ichnological analysis has been revealed
as a tool for the interpretation of major bioevents, such as
those associated with the Frasnian–Famennian boundary
(Stachacz et al. 2017), the T-OAE (Rodríguez-Tovar & Uchman
2010; Rodríguez-Tovar & Reolid 2013; Reolid et al. 2014;
Miguez-Salas et al. 2017; Rodríguez-Tovar et at. 2017),
the Cenomanian–Turonian boundary (Uchman et al. 2008,
2013a, b; Rodríguez-Tovar et al. 2009a, b; Monaco et al. 2012,
2016b), the latest Hauterivian (Rodríguez-Tovar & Uchman
2017), the Paleocene–Eocene (Rodríguez-Tovar et al. 2011a),
and, especially the Cretaceous–Paleogene (K–Pg) boundary
(Rodríguez-Tovar & Uchman 2004a, b, 2006, 2008; Rodríguez-
Tovar et al. 2004, 2006, 2010, 2011b, 2016; Rodríguez-Tovar
2005; Kędzierski et al. 2011; Sosa-Montes de Oca et al. 2013,
2016, 2017; Alegret et al. 2015; Monaco et al. 2015; Labandeira
et al. 2016; Łaska et al. 2017; Lowery et al. 2018).
In this paper we present a high resolution trace fossil record
for the Paleocene–Eocene transition at a section in the north-
western Tethys and demonstrate that macrobenthic communi-
ties there were severely affected by the ecological perturbations
during the BEE.
Material and methods
The Untersberg section is located southwest of Salzburg
(Austria) in a forested stream gorge (47°44’21” N, 12°59’09” E)
in the Northern Calcareous Alps, which form a 500 km-long
fold and thrust belt on the north-eastern margin of the Alps
(Fig. 1). The P–E boundary lays within the Nierental Formation
(upper Santonian–Lutetian) of the Gosau Group and accumu-
lated in a lower bathyal slope environment at a paleodepth of
about 2000 m in the north-western Tethyan realm (Egger et al.
2005). General plate tectonic reconstructions by the ODSN
Plate Tectonic Reconstruction Service (http://www.odsn.de)
suggest a position of this section at a paleolatitude of c. 44° N
in the early Paleogene.
Thanetian and Ypresian deposits consist essentially of marl-
stone, displaying CaCO
3
contents between 40–50 wt. %. Within
the dominantly marlstone succession, a 5.5 m-thick inter-
calation of red and green claystone and marly claystone indi-
cates the CIE, which is used to recognize the P–E boundary.
At Untersberg, the CIE was associated with a shallowing of
the calcite compensation depth by at least one kilometre
(Egger et al. 2005).
High-resolution planktonic biostratigraphy of the section
was presented in Egger et al. (2005). Calcareous nannoplank-
ton assemblages of the entire now studied section indicate
the Discoaster multiradiatus Zone (Zone NP9) in the zonation
scheme of Martini (1971). Beneath the claystone the zonal
marker fossil co-occurs with common fasciculithids (F. tym
paniformis, F. involutus, F. schaubii). Close to the base of
the claystone the first specimens of Rhomboaster cuspis were
found, a species, which is indicative for the upper part of Zone
NP9. Above the claystone, which is devoid of calcareous
plankton, R. cuspis co-occurs with Discoaster araneus and
D. mahmoudii. The stratigraphic ranges of both species are
restricted to the CIE interval within the Zone NP9 (see Egger
et al. 2005, 2017). Planktonic foraminifera assemblages from
the base of the section contain Morozovella subbotinae and
were assigned to the Morozovella velascoensis Zone (Zone P5
of Berggren & Pearson 2005 and Wade et al. 2011). The clay-
stone contains an agglutinated fauna dominated by Glomospira
spp. The marlstone overlying the claystone is also assigned to
Zone P5.
Bioturbation structures, consisting of biodeformational
structures and trace fossils (Uchman & Wetzel 2011), were
studied directly on parting surfaces in the field during “bed-
by-bed” inspection and sampling. In the laboratory, the sam-
ples were polished mechanically in different plains with use of
abrasive paper. Features of ichnofabrics have been observed
on the polished and wet surfaces of collected rock samples.
Their visibility on photo-images was intensified using Adobe®
Photoshop® software. Collected specimens are housed in
the Nature Education Centre of the Jagiellonian University
(CEP) — Museum of Geology, labelled as Ft.
Results
Trace fossil assemblage
In general, the trace fossil assemblage of the Untersberg
section is poorly diverse and consists only of four ichnotaxa
(Fig. 2): Chondrites isp., Planolites isp., Thalassinoides isp.,
and Zoophycos isp. They are mostly visible in cross section
which precludes determinations at the ichnospecies level for
most of them.
Chondrites isp. was observed as patches of circular to ellip-
tical cross sections and short segments, occasionally with
branches, 0.4–2 mm wide (Figs. 3A–F and 4B). They are cross
sections of root-like burrow systems. Smaller forms, with bur-
rows less than 1 mm in diameter (Fig. 3A), probably belong to
Ch. intricatus and the larger forms (Fig. 3B) to Ch. targionii
(for morphometric parameters of Chondrites see Uchman 1999;
Uchman et al. 2012). Chondrites is a deep-tier trace fossil that
was most likely produced in dysaerobic conditions by chemo-
symbiotic organisms (Seilacher 1990; Fu 1991).
Planolites isp. is represented by circular or elliptical spots or
bars, which 1.4–4 mm wide (Fig. 3C, E). They represent hori-
zontal/oblique, straight, simple flattened cylinders without
any wall. Planolites is a pascichnion produced by several
different soft-bodied invertebrates in marine and non-marine
environments (Pemberton & Frey 1982; Keighley & Pickerill
1995).
Thalassinoides isp. is visible as circular or oval spots and
bars, 4–21 mm wide (Fig. 3C–E), which represent straight,
horizontal to oblique, locally vertical, branched cylinders.
Thalassinoides is a domichnial (i.e. dwelling) and fodinich-
nial (i.e. feeding) structure produced by arthropods, mostly
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TRACE FOSSILS ACROSS THE PALEOCENE–EOCENE TRANSITION (UNTERSBERG SECTION, AUSTRIA)
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decapod crustaceans; it occurs in a great variety of different
marine environments (Frey et al. 1984; Ekdale 1992; Schlirf
2000).
Zoophycos isp. is represented by horizontal or subhorizontal
stripes, 1.5–3.5 mm, filled with arcuate structures, locally with
elliptical pellets (Fig. 3F). They are cross sections of planar
spreite structures with a marginal tunnel and represent a larger,
three-dimensional, probably helical trace fossil. Zoophycos
has been interpreted either as a feeding structure (fodinichnia;
Seilacher 1967; Werner & Wetzel 1982; Ekdale & Lewis 1991;
Olivero & Gaillard 1996), or a combination structure pro-
duced by deposit-feeding and chemosymbiotic (Bromley &
Fig. 1. Location of the Untersberg outcrop.
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GEOLOGICA CARPATHICA
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Fig. 2. Stratigraphic column of the Fürstenbrunn section with indication of sedimentary and biogenic structures, stratigraphically important
calcareous nannoplankton taxa and biostratigraphic zonations.
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TRACE FOSSILS ACROSS THE PALEOCENE–EOCENE TRANSITION (UNTERSBERG SECTION, AUSTRIA)
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Hanken 2003) invertebrates of unknown origin, which ingest
organic detritus and accumulate fecal pellets in a subsurface
structure (Löwemark et al. 2006). Recent studies can be found
in Löwemark (2012, 2015), Kotake (2014), Zhang et al.
(2015a, b), Dorador et al. (2016), and Monaco et al. (2016a).
A large unidentified burrow was observed in the field (level
of sample Ft10) as an oval spot, 20–30 mm wide. Its ichnota-
xonomic affinity remains unknown, but Scolicia (an irregular
echinoid burrow) or a larger Thalassinoides can be assumed.
Trace fossil abundance and distribution
In modern oceans, bioturbated pelagic sediments show
a subdivision into two parts: (1) the up to 10 cm thick, smooth
near-surface mixed layer, where the majority of burrowing
organisms occur (Boudreau 1998, 2004), and (2) the under-
lying more consolidated transitional layer, which is affected
only by deep burrowing organisms (Ekdale & Berger 1978;
Berger et al. 1979; Bromley 1990, 1996). The interpretation of
trace fossil assemblages is mainly based on the preservation of
burrows, the cross-cutting relationship and their tiering
arrangement in the sediment. As a general rule, traces with
sharp outlines were produced deeper, and later, in more com-
pacted sediment than less distinct traces. Deeper traces cross
cut more shallow traces.
The trace fossil distribution pattern at the Untersberg sec-
tion shows a clear three-fold subdivision (Fig. 2). The highest
abundance and diversity of trace fossils was recognized in
the lower part of the section (0–2.40 m), where Chondrites isp.,
Planolites isp., Thalassinoides isp., Zoophycos isp., and large
unidentified burrows occur. All ichnotaxa show a more or less
consistent record, with the exception of Zoophycos and
the undeterminable large burrows, which are restricted to a few
horizons. These horizons indicate an episodic increase in
nutrients on the sea-floor (Löwemark 2015). The major part of
the deposits consists of grey and greenish grey mudstone with
occasional presence of red coloured spots and layers. This
mottled mudstone is attributed to near-surface mixed layer
bioturbation. Local exclusive presence of disrupted primary
lamination suggests that the uppermost part of the mixed layer
was temporarily bioturbated.
The middle part of the section (2.40–4 m) consists of
non-calcareous red coloured mudstone, which predominantly
shows disrupted primary lamination indicating very shallow
burrowing. Chondrites isp., Planolites isp., and Thalassinoides
isp. are the only trace fossils, which are exceedingly rare in
this interval. Around 3.10 m from the base, a 0.3 m thick layer
does not possess any indicators of bioturbation and displays
undisturbed sedimentary lamination.
In the upper part of the section (4–7.50 m) the same trace
fossil genera occur as in the middle part but with a significant
increase in abundance and stratigraphic persistence. Chondrites
shows a more or less consistent record, and Thalassinoides is
common, while Planolites occurs only sporadically. A 5 cm
thick unbioturbated layer with primary lamination occurs at
4.60 m of the logged section and records another short hostile
episode negatively affecting bioturbation producing macro-
benthos. In some intervals trace fossils are not recognized but
unrecognizable bioturbation structures are there.
Discussion
Previous studies (Egger et al. 2005, 2017) revealed a num-
ber of environmental changes associated with the Paleocene–
Eocene transition at the Untersberg section: (1) a shoaling
of the calcite compensation depth by at least 1 km indicated
by a lithological shift from marlstone to clayey mudstone,
(2) an increase in bottom water oxygenation indicated by a shift
from grey to red sediment colours, (3) enhanced continental
run-off indicated by an almost 50 % increase in the input of
terrigenous quartz and feldspar into the basin, (4) an increase
in plankton productivity indicated by increased numbers of
radiolarian casts, (5) acidification of surface waters indicated
by the occurrences of irregularly shaped discoasterids (D. ara
neus Bukry, and D. mahmoudii Bown), and (6) a change to
unfavourable ecological conditions at the sea-floor indicated
by a depauperation of agglutinated foraminifera assemblages,
which consist essentially of Glomospira spp. As we demon-
strate above, hostile conditions on the sea-floor also affected
macrobenthic communities.
The primary sedimentary lamination in the middle part of
the Paleocene–Eocene transition at the Untersberg section
reveals the absence of a bioturbating benthic macrofauna.
A similar red laminated horizon is recorded at the Paleocene–
Eocene interval at the Zumaia section at the Bay of Biscay,
Spain (Rodríguez-Tovar et al. 2011a). In contrast to laminated
horizons at other PETM sections (Dee Marl in Nicolo 2008;
Nicolo et al. 2010) the Untersberg and Zumaia sections display
red rock colours, which suggest oxic conditions on the sea-
floor (Rodríguez-Tovar et al. 2011a; Wetzel & Uchman 2018
and references therein). Consequently, we assume that oxy-
genation of deep-sea waters was not the critical factor for
the collapse of the benthic ecosystem there.
The red sediment colour indicates that all decomposable
organic matter enclosed had been decomposed prior to burial
and/or the input of organic matter was very low, thus mini-
mizing the potential for oxygen consumption by decaying
organic matter (in contrast to the interval before and after
the PETM event). Such oligotrophic sediment will not attract
burrowing organisms. However, low organic matter content
in red mudstones does not necessarily reflect a low primary
content of organic matter as shown for some of the so-called
variegated shales in the Carpathians on the basis of ichno-
logical analysis, where red turbiditic mudstones are deeply
bioturbated and relatively higher primary organic matter
content by oxidation and consumption by organisms (Wetzel
& Uchman 2018). At many sites, variegated shales are inten-
sively bioturbated (Leszczyński & Uchman 1993). Therefore,
it is unlikely that oligotrophic conditions caused the disap-
pearance of the benthic community at the Untersberg and
Zumaia sections.
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Fig. 3. Trace fossils from the Fürstenbrunn section: A — Chondrites isp., sample Ft12b; B — Chondrites isp., sample Ft5, horizontal section;
C — Chondrites isp. (Ch), Planolites isp. (Pl), Thalassinoides isp. (Th), sample Ft9, vertical section; D — Chondrites isp. (Ch) in
Thalassinoides isp. (Th), sample Ft17, vertical section; E — Chondrites isp. (Ch), Planolites isp. (Pl), Thalassinoides isp. (Th), sample Ft11,
horizontal section; F — Chondrites isp. (Ch) and Zoophycos isp. (Zo), sample Ft1, horizontal section. Scale bar is 1 cm.
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In the Untersberg section, abundant radiolarian casts sug-
gest high surface water productivity, which was probably trig-
gered by the enhanced terrigenous input into the ocean during
the Paleocene–Eocene transition. This increased continental
run-off is documented in many sections on a global scale and
is attributed to hydrological changes and to a sea level fall at
the Benthic Foraminiferal Extinction Event (see Pujalte et al.
2016). This seems to be contradictory to the notion of oligo-
trophic conditions on the sea-floor during that time. However,
Ma et al. (2014) suggested that an increase in surface water
productivity may not necessarily result in high carbon burial
in the sediments if the sinking particulate matter is efficiently
regenerated by increased bacterial activity and converted to
dissolve inorganic and organic carbon. This process can be
stimulated by high seawater temperatures. Climate model
simu lations indicate that mid- and high-latitude sea-surface
temperatures (SST) met or exceeded modern tropical tempera-
tures at the P–E boundary, and predict tropical SSTs higher
than 35 °C (see Frieling et al. 2017, for a review).
The complex interaction of variable limiting factors
According to the above, other environmental changes than
changes in oxygen concentration could have a major relevance
during the Paleocene–Eocene transition at the Untersberg sec-
tion. Thus, other reasons for the benthic crisis can be taken
into consideration:
• As deep sea burrowing organisms are well adapted to a life
below the CCD, the carbonate solution cannot be a factor
influencing the distribution of trace fossils. However,
the presence of calcium carbonate in sediments influences
the shear strength and may positively influence the preser-
vation of trace fossils (Ekdale et al. 1984). Nevertheless,
any influence of this factor is not observed at the Untersberg
section.
• A significant increase in deep-sea water temperature during
the Paleocene–Eocene transition, even by 15 °C compared
to the present, was postulated by Brass et al. (1982) and
Shackleton (1986). Bowen et al. (2006) estimated an increase
in the temperature in the deep sea by c. 4–5 °C and the ocean
surface waters at all latitudes by c. 5–9 °C. Such warming
could be responsible for a collapse of benthic ecosystems
dominated by groups adapted to cold waters.
• Low sedimentation rates, which are important factors con-
trolling the deposition of the oceanic red beds (Wang et al.
2005) negatively impact the burial of organic matter.
Therefore, it is very probable that burial of particulate
organic matter was very low in the red sediments of
Fig. 4. Primary lamination and bioturbation structures from the Fürstenbrunn section: A — Bioturbation structures, sample Ft18, horizontal
section; B — Horizontal lamination in the lower part of bed and with Chondrites isp. (Ch) in the upper part, sample Ft4, vertical section;
C — Interrupted horizontal lamination, sample Ft25, vertical section; D — Horizontal lamination, sample Ft29, vertical section. Scale bar is 1 cm.
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the Untersberg section. For the Zumaia section, Rodríguez-
Tovar et al. (2011a) suggested that food for burrowing
organisms just concentrated in the shallowest sediment
depths, with deeper depths being food-depleted. However, it
is an open question whether this extreme situation, where
the food is completely absent in most of the sediment,
indeed leads to very limited bioturbation. There are no
recent equivalents of such conditions.
• Some authors assumed (e.g., Brass et al. 1982) that hypersa-
line bottom-water changed ecological conditions at the floor
of the deep-sea across the Paleocene–Eocene transition.
This would have a severe effect on macrobenthic life, even
if oxygen was available. However, no evidence of hyper-
salinity, such as evaporitic minerals, was found in the inves-
tigated section or elsewhere.
• Another scenario will be the presence of methane clathrates,
which will be dissociated with increasing bottom water tem-
perature. With increasing temperature, dissociation of
oceanic methane caused the enormous increase in the light
carbon isotopes characteristic for CIE and causing further
warming (e.g., Dickens 2000). The methane can saturate
pore waters and prevent functioning of burrowing
macroorganisms.
Thus, a single and conclusive interpretation of lacking bio-
turbation structures during the PETM at the Unterberg section
is difficult, and a complex scenario, with the interrelation of
several factors, is possible.
Conclusion
Ichnological analysis conducted at the Paleocene–Eocene
transition at the Untersberg section reveals a complex scenario
of environmental changes negatively affecting the macro-
benthic tracemaker community. The occurrence of laminated
oceanic red beds suggests that the extinction of the macro-
benthic tracemaker community in the earliest Eocene was not
generated by dysoxic or suboxic bottom waters. The distinct
change from grey to red sediment colours at the onset of
the extinction event, rather indicates that bottom-water oxy-
gen concentrations increased. This change is interpreted as
the result of an efficient regeneration of organic matter by
increased bacterial activity in the water column due to a signi-
ficant increase in water temperature and/or reduced export of
organic matter to sediment leading to low nutrient levels on
the sea floor.
Regardless of the scenario behind the severe crisis in
the burrowing activity, the occurrence of laminated red sedi-
ments confines the stratigraphic position of the PETM, with
unprecedented precision.
Acknowledgements: A.U. got additional support from
the Jagiellonian University (DS funds). Funding by RT was
provided by Project CGL2015-66835-P (Secretaría de Estado
de I+D+I, Spain), Research Group RNM-178 (Junta de
Andalucía), and Scientific Excellence Unit UCE-2016-05
(Universidad de Granada). The paper benefited from reviews
by Adam Tomašových (Bratislava, Slovakia), Vladimír
Šimo (Bratislava, Slovakia), and Michelle Zill (Riverside,
California, U.S.A.).
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