A large trace fossil in nodular limestones (Lower Devonian, Czech Republic):
sedimentological consequences, ethology and taphonomy
Radek Mikuláš, Jindřich Hladil
Institute of Geology, Academy of Sciences of the Czech Republic, Rozvojová 269, 16502
Praha 6, Czech Republic. <mikulas@gli.cas.cz; hladil@gli.cas.cz>
RUNNING HEAD: Trace fossils in nodular limestones
KEYWORDS: Bioturbation, Fodinichnia, Carbonate, Nodules, Devonian, Czech Republic
CORRESPONDING AUTHOR: Radek Mikuláš, Institute of Geology, Academy of Sciences
of the Czech Republic, Rozvojová 269, 16502 Praha 6, Czech Republic. Phone: ++420-2734637090.
1
ABSTRACT
Large star-like trace fossil was found on the upper bedding plane of nodular limestone of the
Praha Formation (Pragian, Devonian) at Praha. It was tentatively placed to the ichnogenus
Capodistria. It is interpreted as a feeding trace that originated on the seafloor surface. The
structure consists of 10-11 radial rays and its longer axis is 60 cm. The rays are preserved as
concave epireliefs, up to 10 mm deep and 25 mm wide. The trace fossil partly intersects
nodules that cover surfaces of most bedding planes of the Praha Formation, demonstrating
that the nodules formed during the earliest stages of diagenesis.
2
INTRODUCTION
In general, the size of individual marine invertebrate burrows rarely exceeds few
decimetres, except uninterrupted locomotion traces. Exceptions from the norm deserved a
special study (Bromley et al., 1975). Large size does not automatically mean that the trace
fossils are among the first recognized during field documentation: when working on small
outcrops, debris and drill cores, a complete specimen cannot be seen. A description of such a
large trace from the Lower Devonian of the Barrandian area (Czech Republic) and its
sedimentological consequences are the subject of the present contribution.
The Praha Formation is characterized by calciturbditic, nodular packstone beds. These
are deposited in lower ramp and slope settings (e.g., Hladil et al., 1996; Hladil et al., 2010;
Koptíková, 2011). The specific sedimentary environment and the mode of formation of the
nodules is still a matter of a debate in terms of the Barrandian (e.g., Fürsich, 1973; Kukal,
1975; Bednarczyk, 1991; Holcová, 2004; Flügel, 2010; Koptíková et al., 2010; Poul and
Melichar, 2010). This debate centres on the relative importance of bioturbation and/or early
diagenetic concretionary cementation prior to early burial pressure solution. The effects of
primary heterogeneity and self-organizing processes along with shear-stress are considered as
potential factors in the formation or enhancement of nodular limestone fabric. An assessment
of the relationships between the formation (and preservation) of a large trace and nodular
fabric of the limestone can, therefore, provide also an improved understanding of the
sedimentary and diagenetic processes acting in the Praha Formation.
Despite the long tradition of research, no complex ichnologic research has been done
for the facies; minor works include only the contributions by Wiessenbach (1930) and Prantl
(1943).
3
GEOLOGICAL SETTINGS
The Barrandian area lies in the central and western Bohemia, Czech Republic, and
extends in subsurface to eastern Bohemia. The Neoproterozoic and Lower Palaeozoic rocks
belong to the Teplá-Barrandian Unit (TBU) of the Bohemian Massif (Máška and Zoubek
1961) or alternatively, a similarly defined Bohemicum Unit (Malkovský, 1979). The
uppermost units of the TBU (Ordovician to Lower Devonian) form a complex elongated
synform. The Lower Devonian limestone outcrops are mainly in the inner parts of the Prague
Basin (Havlíček, 1981; Mikuláš, 1999), alternatively known as the Prague Synform or
Synclinorium (Jaroš, 1984; Melichar, 2004). The Devonian occurrences in the Bohemian
Massif are described by Chlupáč (1993) and Chlupáč et al. (1998). The regional stratigraphy
has been described recently by Cháb et al. (2010).
The well exposed limestones are rich in fossils, and are lying concordantly on the
Silurian; several stratotypes and parastratotypes (namely Silurian/Devonian boundary and
base of the Pragian Stage) were defined herein (Chlupáč et al., 1998). Most of the Lower
Devonian in the study area consists of open marine limestones with rare argillaceous and
submarine tuffitic interbeds. The stratigraphic succession and palaeontology is similar to that
of the Carnic Alps, Sardinia, and other peri-Gondwanan areas (e.g., the Ibero-Maghrebian
domain; Chlupáč et al., 1998; Plusquellec and Hladil, 2001; Slavík, 2004).
One of the prominent facies of the Praha Formation is the Dvorce–Prokop facies
(Chlupáč, 1962; Chlupáč et al., 1998), and is of Pragian–Emsian age (cf. Hladil et al., 2010;
Hladil et al., 2011). It consists of grey, rhythmically bedded packstones, mostly with knobbly
bedding surfaces. These nodular limestones are considered to have been deposited in
hemipelagic and calciturbiditic regimes with periods of sediment starvation and submarine
4
lithification evidenced by submarine corrosion/erosion marks (e.g., Hladil et al., 1996; Suchý
et al., 1996; Hladil and Kalvoda 1997; Hladil et al., 2010). The nodular and wavy limestone
beds of the Dvorce sub-type are typically developed at the locality Branická skála,
geographically on the periphery of Prague, between the settlements of Dvorce and Braník (No
1 on Fig. 1). This site provided the stellate trace documented herein. The Loděnice facies of
the Praha Formation is composed of variegated platy limestones with rare to absent
nodulation and local concentrations of coarse-grained organic detritus. This facies is poorly
represented on the Cikánka site (No 2 on Fig. 1), where it also yielded a find of large stellate
trace fossil (Mikuláš, 1998).
The localities
Branická skála (The Braník Rock; No 1 on Figure 1): Large abandoned quarry
between co-ordinates 50°2'26.12"N, 14°24'37.90"E and 50°2'34.68"N, 14°24'53.38"E, in the
southern part of the city of Prague. The quarry contains medium- to thick-bedded limestones
(packstones) of the Praha Formation (Pragian Stage). Neither the lower nor upper boundary of
the Praha Formation is exposed in the quarry, and the estimated thickness of the exposed
portion is 140 m (Kříž, 1999). On the opposite bank of the Vltava River, on better exposed
sites at distance of ca 1 km to NW, the whole thickness of the Praha Formation is 180 m.
The profile studied in detail (Fig. 2) belongs to the middle part of the Praha Formation
and is adjacent to a planar outcrop (50°2'26.30"N, 14°24'39.85"E) with strata association with
the horizon that yielded the star-like trace.
The Braník Rock is lithologically monotonous, consisting of a well-bedded greyish
packstone in (5)10-100 cm thick layers. Beds are nodular or undulose. The average/typical
size and shape of nodules are laterally uniform and characteristic of each bed; typical size of
nodules is 5-15 cm in length/diameter. Some limestone layers are separated by thin
5
intercalations of dark-grey calcareous shale with Chondrites isp. The limestones contain
rather rare but diverse macrofauna, including trilobites Odonotchile sp. and Phacops sp.,
bivalve Kralovna sp., minute brachiopods, columnals of crinoids, fragments of corals. During
the present fieldwork, probable hardgrounds are recognized by clusters of Trypanites isp.
(Fig. 2).
Cikánka (No 2 on Fig. 1; between 49°59'58.08"N, 14°19'17.52"E and 49°59'56.17"N,
14°19'45.88"E) is a quarry in operation on the SW margin of Prague. The quarry and its close
vicinity are important as a source of the Slivenec Marble that was used in numerous historical
buildings, chiefly in Prague (Chlupáč, 1993). The Praha Formation (Pragian) is there
composed mainly of the thick-bedded, coarse-grained, biodetritic to sparitic, pink Slivenec
Limestone (Chlupáč et al., 1998). The Slivenec facies is overlain by a thin layer of variegated
limestones with poorly developed nodules on bedding planes; these are similar to the facies of
the Loděnice Limestone (Chlupáč et al., 1998; Mikuláš, 1998) which is otherwise not
developed at Cikánka. In these layers, a giant star-like trace fossil similar to the find from the
Braník Rock was found and collected in the past but subsequently the specimen was lost
(Mikuláš 1998). These layers are overlain by reddish-brown, fine-grained, nodular limestones
of the Řeporyje Facies (4-5 m thick) and the remaining portion of the Praha Formation is
composed of the Dvorce-Prokop limestones as described on the previous locality.
SYSTEMATIC ICHNOLOGY
Ichnogenus Capodistria Vialov, 1964
6
Diagnosis: A stellate structure preserved in hypichnial semi-relief, composed of
relatively few narrow radial ridges and a central knob or knobs (modified after Uchman,
1995).
? Capodistria isp.
Figures 3, 4, 5
Material: Two specimens, one lost, the second one collected.
Description: Large ? Capodistria composed of approx. 10 radiating surface grooves
(convex hyporelief or concave epirelief). The rays are moderately bent such that the whole
structure is bilaterally symmetrical. The first specimen (the collected one, coming from the
Braník Rock) is a star-shaped structure, roughly circular in plan view. Its long axis is 60 cm,
the short one is 45 cm. The trace consists of 10 pronounced rays plus one poorly preserved
ray. Neighbouring rays diverge mostly at angle of 30°, in two cases 60°. Well preserved rays
are moderately bent, 18-45 cm in length and 22-25 mm in width. The rays are preserved as
concave epireliefs, up to 10 mm deep. Bending of the rays varies from negligible to distinct;
the bending is systematic. Opposing rays show equal curvature. An irregular, diagenetically
deformed depression, oval in outline, ca 3 x 6 cm in size, occurs where the rays would
intersect if extrapolated. The rays terminate abruptly, the terminations are rounded and the
depth of the ray usually decreases slightly near the termination.
The trace fossil is preserved in the bedding surface of nodular limestone (packstone).
The nodules have preferred orientation, and in some cases the rays intersect the nodules;
hence, the biogenous activity occurred after the appearance of the basic ground plan of
nodules. The rays are partly filled with softer calcareous mud without observable internal
structure.
7
The second, lost specimen was described as ? Phoebichnus isp. (Mikuláš, 1998). It
was a star-like trace fossil, approx. 50 cm in diameter, consisting of 8–10 straight or
moderately bent radial rays, about 3 cm wide. The trace was preserved as a convex hyporelief.
As in the previously described specimen, the nodularity of limestone was partly influenced by
the shape of rays and vice versa. The fill of the rays was homogeneous and probably passive.
The description was recorded by R. Horný, I. Chlupáč and A. Galle (personal
communications 1996–1997) and it was subsequently supplemented by I. Chlupáč (pers.
comm. 2000).
Remarks and relations: The first (lost) specimen of the described ichnofossil was
determined by Mikuláš (1998) as ? Phoebichnus isp., basically because of similar size.
According to criteria complied by Bertling et al. (2006), size should not be ordinarily used as
ichnotaxobase; therefore, the argumentation is not admissible under the present state of
knowledge.
The find of another specimen at Branická skála allows full characterization of the trace
fossil. Most importantly, it is clear from that specimen that the rays originally formed as
surface grooves, not full-relief tunnels as originally presumed. We can therefore exclude the
affiliation of the trace to some clearly subsurface radiating ichnogenera, namely: Phoebichnus
Bromley and Asgaard, Dactyloidites Hall, and Stelloglyphus Vialov. Phoebichnus is a
complex trace fossil containing a wide vertical shaft, from which radiate smaller-diameter
horizontal tunnels containing backfilled meniscate structures (Gregory and Campbell, 2003).
Dactyloidites is a vertical radial spreiten-structure developed deep in the substrate at the
bottom of a central shaft (Uchman and Pervesler, 2007). Somewhat similar is Stelloglyphus
that consists of multi-level tunnels normally radiating and curving downward from a central
shaft (Le Roux et al., 2008). Also the possible resemblance of the trace to the “deep-tier”
8
ichnogenus Skolichnus Uchman can be questioned for the same reason, i.e., the deep-substrate
nature (Uchman, 2010).
Resemblance to Glockerichnus Pickerill is doubted by the simple character of the rays;
in Glockerichnus, the rays are bifurcated (Uchman, 1998). Further, the ichnogenus
Asterichnus Bandel is represented by irregular rosette structure preserved in full relief on
bedding planes (e.g., Głuszek 1998); consequently, it originated deeper in the substrate,
preferably on a lithologic boundary.
The ichnogenus Capodistria was also discussed by Uchman and Wetzel (2001) who,
in the same paper, erected a new ichnogenus of a stellate trace, Estrellichnus. This large (up to
1 m in diameter) trace fossil is composed of straight, long narrow ridges or grooves
radiatingfrom a central mound or depression. The radial rays turn up at their end, which
suggests that they were originally formed below the substrate surface. Uchman and Wetzel
(2001) presume the analogous subsurface nature also for Capodistria; nevertheless, the is no
direct evidence for this. In addition, firmground to hardground grooves radiating from a
central burrow aperture have been described from the Middle Triassic of Germany (e.g.,
Knaust, 2008) being associated with a complex trace fossil Balanoglossites ramosus.
However, no complex burrowing to boring structures comparable to Balanoglossites have so
far been discovered in the Praha Formation.
Resemblance of the described trace fossil to Capodistria is questioned by the length
and shape of the rays. As stated by Uchman (1998), Capodistria is composed of “short,
simple hypichnial ridges”. It can be argued how far the adjective “short” refers to the size of
the trace fossil (which is generally not a valid ichnotaxobase; Bertling et al., 2006) and how
much it characterizes the width/length ratio. If we regard the second possibility, then the find
form the Braník Rock does not fit well to the diagnosis by Uchman (1998). Moreover, the
present-day diagnosis of Capodistria allows us to involve both surface and subsurface traces.
9
As the unambiguous identifying with any of the existing ichnogenera is not possible and the
erection of a new ichnotaxon based of the very limited material is not advisable, the find is
kept in the open nomenclature.
INTERPRETATION OF THE STRUCTURE, SUBSTRATE CONSISTENCY,
SEDIMENTOLOGICAL CONSEQUENCES
The find of ? Capodistria isp. may provide a unique key for the relative dating of the
formation of nodularity in the limestones. The large and deep radial rays partly intersect
individual “half-relief” nodules of the carbonate, and some nodules partly intersect or deform
margins of the trace fossil, that are otherwise of constant width. It is, therefore, evident that
the biogenic activity was contemporaneous with the active formation of the undulatory
surfaces on the sea floor, not during deeper burial (e.g., Kukal, 1975; Bathurst, 1987) or
tectonic deformation (e.g., Gründel and Rösler, 1963; Poul and Melichar, 2010). According to
the material from the Braník Rock, the late diagenetic processes (and those due to regional
shear and cleavage) modified the geometry of nodules very slightly, rather with a few
degrees, by tilting of nodules, or millimetre to centimetre lateral shifts of them. The branches
of the large stellate trace in the upper part of the bed remained almost unchanged with the
further development of nodular objects and provide an excellent piece of evidence about the
earliest lithification and nodule formation of a thin calciturbidite bed (and its very thin
hemipelagite coating). Nodules were formed during a period of sedimentary starvation, before
the sedimentation of the overlying bed, i.e. practically in contact with precipitationdissolution processes on the sea floor.
The oriented slabs of the Braník trace-fossil hosting bed rarely show lamination or
imbrications of minute clasts, usually at the base of the bed or in its thin cover. These fabrics
are indicative of intermittent deposition from low-density turbulent flows. However, the
10
majority of the bed is inferred to be deposited from a low-velocity settling of a high-density
limestone mud suspension. In its lower part, imbricated clasts of trilobite carapaces and
cephalopod shells occur, but in the upper part, an inclined, double-cone orientation pattern of
dacryoconarid shells predominate and the rock structure is massive (see Hladil et al., 1996). A
few isolated, large but low density bioclasts were rotated to subvertical positions. The
laminated (or gently rippled) turbidite to hemipelagite in the uppermost part of the beds are
poorly preserved as being altered during the period of sediment starvation. Infills of
depressions in relief are a few millimetres thick. Bioturbate structures in millimetric sizes are
present (Fig. 6).
To resist erosion by turbidite current, it is inferred that the surface was lithified to
hardground after creation of the soft-sediment trace fossil ? Capodistria. No large star-shaped
trace trace fossils similar to Capodistria have so far been interpreted as borings. Moreover,
deformation of walls of trace fossils through continuing growth of nodules would be excluded
in the case of borings, which would rather cross-cut nodules. When examined in further detail,
the upper part of the relevant calciturbitite beds bear many signs of sediment starvation and
gradual surface hardening; e.g., local recrystallization of the surface, truncation of grains, and
both fissures and burrows filled by a different sediment than either the surrounding or the
overlying rocks have. In addition, some holes and fissures are filled with iron-oxide rich
sediment. Diagenesis enhanced nodular relief, but formation of other additional patterns in
sculpted rock surfaces was possible (cf. Jamtveit and Hammer, 2012). Similar carbonate
hardgrounds are known on recent slopes, in the depths of several hundreds to thousands of
meters (e.g., Coniglio and Dix, 1992; Wilber and Neumann, 1993; Messing, 2004; Schroeder,
2007; JAMSTEC, 2013).
In the Branická skála locality, hardground trace fossils Trypanites isp. were found in
two layers occurring few decimetres below the surface that yielded the find of ? Capodistria.
11
Trypanites colonization surfaces are much less undulose than other limestone bedding planes.
The details of hardgrounds have not been described so far either from the Branická skála
locality or from the Praha Formation, though indications of their occurrence exist (Mikuláš,
1995 – ichnology, Cikánka site; Hladil et al., 2010 – sedimentary petrology).
To summarize, both firmgrounds and hardgrounds (which variety was developed
depending probably on the time to be at the disposal for the process) were subsequently
exposed to slow mud sedimentation; calcareous mud filled preferably depressions in the
substrate, both those resulting from geochemical self-organized processes and biogenic
activity of the tracemaker. This is the present material of the trace fossil fill. Further
sedimentation event was most probably an episodic gravity (? turbidity) current that created
the next limestone bed; surface of the gravity-transported material then again became selforganized by precipitation processes (cf. Hammer, 2008) and colonized by benthos. The
development only occasionally attained the hardground consistency of the substrate as
evidenced by the low number of layers with Trypanites.
Ethologic sense of the stellate trace
In general, radial/stellate trace fossils, despite the coincidence of the basic ground
plan, are considered to have several quite different ethologic purposes (e.g., Häntzschel, 1970;
Seilacher, 2007, p. 136). Long-time known, well studied and relatively easy to interpret are
resting traces (cubichnia) of ophiuroids and asteroids, which reflect substantially body shape
of the tracemaker (the ichnogenus Asteriacites von Schlotheim). Radial traces adjacent to a
vertical structure can be interpreted mostly as combined feeding-dwelling structures; e.g., an
informal group of asterosomids as introduced by Seilacher (2007). Yet another important
ethologic variant of a star-like trace is the structure that resulted from a utilization of a
superficial film of algae or a detritus-rich lamina on the sediment surface (gyrophyllitids
12
sensu Seilacher, 2007). Last but not least, stellate forms can be found among typical “fucoids”
(in-faunal feeding traces or chemichnial structures, e.g., Phymatoderma; Fu 1991) and among
graphoglyptids (Lorenzinia da Gabelli and Sublorenzinia Książkiewicz).
Therefore, there is a considerably wide scope of possible analogues of ? Capodistria
isp. We can propose two possibilities: 1, a feeding trace (rather than a cubichnion or
domichnion) that is inferred to collect detritus trapped to the grooves/rays; 2, trace of active
surface deposit feeding/predation for in-faunal prey, analogously with the assumption of
Geister (1998) expressed for groove-like traces. For star-like combined dwelling and feeding
structures, Seilacher (2007) characterized an informal group of gyrophyllitids, which poses
radial probings around a vertical shaft. To compare the presently described find with
graphoglyptids, there is no evidence of the vertical shaft in the specimen presented herein,
except a weak central hollow. Moreover, no vertical shafts except thin Trypanites isp. have so
far been observed in the Praha Formation.
CONCLUSIONS
1) The large, star-shaped ichnofossil found on the upper bedding plane of nodular
limestone of the Praha Formation (Pragian, Devonian) can be best compared to the
ichnogenus Capodistria Vialov, 1964. An older find of a similar structure, tentatively placed
by Mikuláš (1998) to the ichnogenus Phoebichnus, is synonymised.
2) ? Capodistria isp. from the Praha Formation is probably a feeding trace; it could
function as a trap for detritus particles, or it is the record of active searching for in-faunal
prey.
3) Relatively large and deep radial rays of the ichnofossil partly intersect individual
semirelief carbonate nodules widespread on the bedding planes of the Praha Formation.
Conversely, some nodules intersect or deform margins of the otherwise constant-width
13
groove-like rays. It is, therefore, evident that the biogenic activity was contemporaneous with
the active formation of undulated surfaces, indicating that the nodules formed during the
earliest diagenesis, almost on the seafloor.
ACKNOWLEDGEMENTS
The authors gratefully acknowledge the financial support provided by Czech Science
Foundation (GA14-18183S) and Academy of Sciences of the Czech Republic (RVO
67985831). Thanks are due to the official reviewers of the Ichnos journal and to the Associate
Editor Dirk Knaust for valuable comments.
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Figure captions
Fig. 1. Location map; localities: 1 – Branická skála (50°2'26.12"N, 14°24'37.90"E); 2 –
Cikánka (49°59'58.08"N, 14°19'17.52"E).
Fig. 2. Branická skála locality, the section through the nodular packstone bed succession, the
oldest bed of this interval at the bottom; coordinates: 50°2'26.184"N, 14°24'38.656"E. Scale =
50 cm. C – the layer with ? Capodistria isp.; T – layer with the smooth upper surfaces in
which numerous specimens of Trypanites isp. were found.
Fig. 3. ? Capodistria isp. Branická skála locality. A photograph of a fallen block prior its
collection; viewed perpendicularly to the upper surface of the bed. Scale bar = 30 cm.
Fig. 4. ? Capodistria isp., figured after the sampling. Compare with outlines of the trace, as
shown in Fig. 5. Scale bar = 30 cm.
Fig. 5. ? Capodistria isp., an interpretative drawing done prior the breaking and collection of
the limestone slab.
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Fig. 6. Thin-section images of a limestone bed with ? Capodistria isp. at Branická skála. A
and B – vertical sections, details, scale valid for both the pictures: A – Mixture of variously
sized, calcisiltitic but also larger angular bioclasts; the primary porosity of settled slurry bed
was low and lithification was achieved rather due to recrystallization than cementation. A
crinoid brachial fragment is in the lower left corner. B – A finer variety of this calcisiltite;
dacryoconarid shell shows a typical apex-obliquely-upward position and contains a geopetal
infill surface; a small bright object left of the shell is a sponge spicule replaced by calcite. C to
F – More complex fabrics in the otherwise monotonous alodapic slurry bed; vertical sections,
only D is horizontally oriented; equal enlargement for these four images. C – Inhomogeneities
from settling of the sediment, bioturbation and early compaction; the oval section left of the
centre is possibly a Chondrites burrow. D – Upper third of the bed, bioturbated during the
gradual stiffening of the substrate – Trichichnus (Tr) and diminutive Rosselia (Ro). E and F –
Examples of vertical differences in the nodular parts of the bed: E – Small shell fragments and
a sub-vertically rotated crinoid pluricolumnal at the base; very fine particles in the middle
(recrystallized, containing cephalopod shells); heterogenous material with microbored
trilobite carapaces is above (the uppermost lying ones near subvertical and often protruding
from the upper surface). F – Cephalopod and trilobite shell fragments in limestone mud, in the
middle part; uneven, truncated-corroded upper surface of the bed.
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