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INTRODUCTION
Trace fossils are important environmental indicators
and provide valuable knowledge of animal behaviour
in the geological past (Seilacher 2007). Tracefossil
assemblages of the Ordovician of the Baltica craton
are comparatively well studied (e.g. Dronov et al.
2002; Mikuláš & Dronov 2005; Knaust & Dronov
2013; Hanken et al. 2016), but only limited studies deal
with material from Estonia (Männil 1966b; Vinn et al.
2014, 2015; Vinn & Toom 2016; Toom et al. 2019a,
2019b).
Coprolites (fossil faeces) represent a distinct category
of trace fossils common since the early Palaeozoic –
different morphotypes are known already from the
Cambrian (e.g. Vizcaïno et al. 2004; Eriksson & Terfelt
2007; Shen et al. 2014; Kimming & Strotz 2017; Mángano
et al. 2019). The term ‘pellet’ denotes grains of faecal origin
according to Flügel (2004); however, some authors have
used it without reference to their origin. The term is
frequently used for small invertebrate excrements of
millimetre scale, with simple elliptical or rodshaped form.
Like all softbodied organisms, coprolites have generally a
low preservation potential. Their findings from Palaeozoic
Estonian Journal of Earth Sciences, 2020, 69, 1, 1–19 https://doi.org/10.3176/earth.2020.01
1
Small faecal pellets in Ordovician shelly fossils from Estonia,
Baltoscandia
Ursula Tooma, Olev Vinnb, Mare Isakarc, Anna Madisond and Olle Hintsa
aDepartment of Geology, School of Science, Tallinn University of Technology, Ehitajate tee 5, 19086 Tallinn, Estonia;
ursula.toom@taltech.ee, olle.hints@taltech.ee
bInstitute of Ecology and Earth Sciences, University of Tartu, Ravila 14A, 50411 Tartu, Estonia; olev.vinn@ut.ee
cGeological collections of Natural History Museum, University of Tartu, Vanemuise 46, 51014 Tartu, Estonia; mare.isakar@ut.ee
dBorissiak Paleontological Institute, Russian Academy of Sciences, Profsoyuznaya ul. 123, Moscow, 117647 Russia;
Sunnyannmad@yahoo.com
Received 6 August 2019, accepted 23 October 2019, available online 19 December 2019
Abstract. Coprolites (fossil faeces) constitute a group of soft sediment trace fossils that provide useful palaeoecological and
sedimentological information, but have generally low preservational potential. In this paper we report abundant occurrence and high
diversity of small faecal pellets preserved inside different shelly fossils from Middle and Upper Ordovician carbonates of the
Baltoscandian palaeobasin. The material contains ca 180 body fossils with faecal pellets from 40 localities, corresponding to a range
of shallowmarine environments from coolwater carbonate ramp to tropical open shelf settings. Stratigraphically the finds range
from the Volkhov to Pirgu regional stages (Dapingian to uppermost Katian). The pellets are elliptical or rodshaped, 0.1–1.8 mm
long and 0.08–0.75 mm in diameter, with the length/diameter ratio ranging from less than 2 to ca 6. They occur in shells of gastropods,
bivalves, cephalopods, brachiopods, echinoderms and trilobites and represent two ichnospecies, Coprulus oblongus and Coprulus
bacilliformis, and some intermediate forms belonging to the same ichnogenus. Additionally, two compound traces were identified:
Tubularina (pellets inside small burrows with circular cross section) and Alcyonidiopsis (pellets inside ribbonshaped burrows). The
pellets were produced when the empty shells were located on the seafloor, or possibly during shallow burial in the oxic zone. The
preservation of faecal pellets is due to an interaction of several factors, notably protection by the shells and rapid mineralization.
The origin of trace makers remains speculative, but polychaete worms having compatible size and body plan and living representatives
who produce similar faecal pellets are among the most likely groups. Possibly organisms with different feeding strategies were
involved in producing the faecal pellets. Systematic examination of shelly fossils from selected localities showed that up to about
half of the shells may contain pellets, which indicates great abundance and diversity of pelletproducing organisms in the Ordovician
Baltoscandian basin. Our material also shows that the trace maker of Arachnostega was not related to the faecal pellets inside the
shells.
Key words: microcoprolites, faecal pellets, Coprulus, Tubularina, Alcyonidiopsis, shallowmarine carbonates, Ordovician, Estonia.
© 2019 Authors. This is an Open Access article distributed under the terms and conditions of the Creative Commons Attribution
4.0 International Licence (http://creativecommons.org/licenses/by/4.0).
siliciclastic sediments are mostly related to deeperwater
settings with high sedimentation rates and specific
preservation conditions. The occurrence of pellets in
carbonate sediments is commonly related to tropical
shallowmarine environments (Folk & Robles 1964; Shinn
1968; Wanless et al. 1981) and is widely reported from
Mesozoic and Cenozoic strata. Rapid lithification has
played an important role in their preservation in carbonates
(Knaust et al. 2012). The majority of faecal pellet ichnotaxa
in carbonates have characteristic inner structure and belong
to the ichnofamily Favreinidae. Small faecal pellets with
an ornamented outer surface are assigned to a number of
different ichnogenera (e.g. Heer 1853; Elliott 1963;
Gramann 1966; Gaździcki 1974; Gaillard 1978; Agarwal
1988; Živković & Bogner 2006), whereas isolated faecal
pellets without internal structure and ornamentation are
representing the ichnofamily Coprulidae (Knaust 2008).
Faecal pellets are common in the Ordovician rocks of
Europe (Häntzschel 1962; Benton & Hiscock 1996). They
were first mentioned from Bohemia by Barrande (1872)
and are commonly related to the ichnogenus Tomaculum
Groom, 1902 (e.g. Pickerill & Forbes 1979; Eiserhardt et
al. 2001 and references therein; Mikuláš & Slavíčková
2001; Bruthansová & Kraft 2003; Neto de Carvalho &
Farinha 2006; Podhalańska 2007; Martin et al. 2016; Neto
de Carvalho et al. 2016; van Keulen & Rhebergen 2017).
Groom (1902) left open the question of the origin of small
pellets, but later authors have suggested a faecal origin
for these particles (e.g. Frič 1908; GutiérrezMarco 1984;
Mikuláš 1991; Eiserhardt et al. 2001; Bruthansová &
Kraft 2003; Neto de Carvalho & Farinha 2006).
Faecal pellets may be associated with other trace
fossils (e.g. Fürsich 1974; Seilacher 2007; Knaust 2008),
constitute composite traces (Gaillard et al. 1994) or fill
shells of molluscs and other invertebrates (e.g. Mayer
1955, 1958; Zhang et al. 2007; Mángano et al. 2019).
From the Ordovician, faecal pellets in burrows have been
described as Syncoprulus (=Tomaculum) (Richter &
Richter 1939; Pickerill et al. 1987), Alcyonidiopsis
(Chamberlain 1977; Pickerill 1980; Pickerill & Narbonne
1995; Orr 1996; Uchman et al. 2005), in branching
burrows as Quebecichnus (Hofmann 1972) and in
burrows with segmented fill as Compaginatichnus
(Pickerill 1989). GutiérrezMarco (1984) described a
cylindrical elongated cluster filled with small pellets
inside a gastropod as Cilindrotomaculum.
The reports of pellets inside Ordovician shelly fossils are
few and mostly come from siliciclastic basins (Gutiérrez
Marco 1984; Mikuláš 1992; Bruthansová & Kraft 2003). The
occurrence of pellets in shells from carbonate settings has
only been mentioned by Põlma (1982), more recently by
Toom et al. (2017, 2019a, 2019b) and van Keulen &
Rhebergen (2017). All these papers refer to the material from
the Baltic region.
The aim of this study is to report the abundance of small
faecal pellets inside various shelly fossils in the Ordovician
shallowmarine carbonates of the Baltoscandian basin,
describe their morphology and discuss the taxonomic,
sedimentological and palaeobiological aspects.
GEOLOGICAL BACKGROUND
During the Ordovician, the study area was part of a
shallow sea, which covered the western part of the Baltica
craton. This epicontinental sea, the Baltoscandian basin,
extended from Norway to the Volga area in western
Russia, and from the Fennoscandian mainland in the north
to the Sarmatian mainland in the south (Fig. 1; Nestor &
Einasto 1997). The Ordovician outcrop area in northern
Estonia, where most of the material of the present study
derives from, was characterized by relatively shallow
water settings of the basin, whereas deeper shelf
environments, the socalled Livonian basin, were located
in the south (Fig. 1). Baltica drifted from high southern
latitudes to the tropical area (Torsvik & Cocks 2013 and
references therein), causing a gradual change in climate
and depositional conditions. In Estonia, carbonate
sedimentation commenced at the end of the Floian (latest
Early Ordovician) in a relatively cool, flatbottomed
epicontinental basin (Dronov & Rozhnov 1997). In the
Middle and early Late Ordovician, the basin was
characterized by extremely low sedimentation rates and
with little bathymetric differentiation (Jaanusson 1973).
In the Late Ordovician, the climatic change resulted in an
increase in carbonate production and sediment
accumulation rates on the platform. The basin started to
differentiate particularly in the early Katian (Nestor &
Einasto 1997). At that time the first tropical carbonate
buildups appeared in the region (Kröger et al. 2017 and
references therein).
The total thickness of the Ordovician succession in
Estonia reaches about 180 m (Nõlvak 1997). The
Ordovician carbonate rocks in Estonia are rich in shelly
fossils such as brachiopods, bryozoans, cephalopods,
gastropods, echinoderms, trilobites, corals, etc. Trace
fossils are also common and diverse (Toom et al. 2019a)
and the degree of bioturbation is generally high (Harris et
al. 2004). A significant feature of Palaeozoic rocks of
Estonia and the entire eastern Baltic region is a very low
burial temperature indicated by conodont colour alteration
index (CAI) values around 1 (Männik 2017).
The stratigraphic framework and timecorrelations in
the region are based on Baltic regional stages and high
resolution biostratigraphy, notably trilobite, conodont,
chitinozoan and graptolite zones (Nõlvak et al. 2006). We
refer to both regional as well as global stages and series
in the present study (Fig. 2).
Estonian Journal of Earth Sciences, 2020, 69, 1, 1–19
2
MATERIAL AND METHODS
Large palaeontological collections of Ordovician fossils
from Estonia, housed at the Department of Geology,
Tallinn University of Technology (indicated by the prefix
GIT) and the Natural History Museum, University of
Tartu (prefix TUG), were searched for shelly fossils
containing small coprolites. In addition, the extensive
lithological collection of eastern Baltic Ordovician rocks
by Lembit Põlma (prefix GIT) was examined in order to
reveal similar pellets dispersed in carbonate matrix.
The main method to identify the pellets was non
destructive observation of steinkerns, broken shelly fossils
and rock samples already cut during previous studies.
Betterpreserved specimens were studied on steinkern
surfaces only; the less valuable material was sectioned
and polished for the determination and measurements of
pellets. Only the specimens containing a large number of
individual pellets on sectioned shells or steinkerns were
used for measurements.
Additionally, a selection of shelly fossils from two
localities were sectioned and examined in order to assess the
relative abundance and distribution of pelletfilled shells. In
this way, 30 gastropods and 30 brachiopods from the Aluvere
quarry (Haljala Stage, Sandbian) and 30 gastropods from the
Mõnuste quarry (Vormsi Stage, Katian) were studied.
Specimens were photographed with a Canon EOS
5DS R digital camera and a Leica Z16 APO zoom
microscope system at the Department of Geology,
U. Toom et al.: Faecal pellets in Ordovician shelly fossils
3
1
1
1
2
3
3
4
Finland
Sweden
Norway
Russia
Latvia
Baltic Sea
Baltic Sea
Lithuania Belarus
Tallinn
Stockholm
St. Petersburg
Estonia
VÄR SK A
Riga
N
100 km0
Fig. 1. Locality map showing the outcrop area of Ordovician rocks in the Baltic region (green) and schematic configuration of the
Baltoscandian basin (after Männil 1966a and Nestor & Einasto 1997). 1, main land areas; 2, shallowwater Estonian shelf; 3, deeper
water Livonian basin and Central Baltoscandian facies belt; 4, deep shelf of the Scanian facies belt (modified after Toom et al. 2019a).
Facies (Estonia)
Aseri
Global stratigraphy Regional Stage
Ordovician
Early
Middle
Late
Floian
Dapingian
Darriwilian
Sandbian
Katian
Hirnantian
Porkuni
Pirgu
Vormsi
Nabala
Oandu
Keila
Haljala
Kukruse
Uhaku
Lasnamägi
Kunda
Volkhov
Billingen
Rakvere
Age (Ma)
458.4
470.0
443.8
Warm-water carbonates
Temperate carbonates
Cool-water
carb.
Siliciclastics
Fig. 2. Regional and international stratigraphy of the studied
interval, showing transition from coolwater to warmwater
carbonate deposits (modified after Toom et al. 2019a).
Tallinn University of Technology. Measurements were
taken from calibrated digital photos using Fiji image
analysis software (https://imagej.net/Fiji). Selected
specimens were additionally studied under a scanning
electron microscope and analysed for chemical
composition with EDS (TESCAN VEGA II XMU
SEM with an XRay Energy Dispersive INCA
ENERGY 450 microanalysis system at the A. A.
Borissiak Paleontological Institute, Moscow). Data of
individual specimens and related pellets (including
images and localities) are deposited in the database of
Estonian geocollections, which is accessible online at
https://geocollections.info.
RESULTS AND DISCUSSION
Characterization of faecal pellets and host shells
Small pellets have been identified inside about 180 shelly
fossils coming from 40 localities across the Ordovician
outcrop area in Estonia and representing normal shallow
marine settings of the Estonian shelf (Table 1). So far only
one specimen has been recovered from the deepershelf
Livonian basin (Fig. 1). Stratigraphically the pellets in
shells are known from the Volkhov (Dapingian) to Pirgu
regional stages (Upper Katian); however, the majority
comes from temperate and tropical carbonates (Table 1),
with only a single record from coolwater limestone of
the Volkhov Stage. The host rocks of shelly fossils with
pellets are varied, ranging from wackestones and packstones
to pure carbonate mudstones. Notably, so far, loose pellets
have not been found from the host rocks.
Host shells containing faecal pellets belong to
common Palaeozoic fossil groups: gastropods, bivalves,
cephalopods, brachiopods, echinoderms and trilobites
(Table 1). In general, the host dimensions range from
ca 10 to 45 mm; for cephalopods the diameter of the
body chamber is less than 25 mm. The size of pellets is
very variable, ranging from 0.1 to 1.8 mm in length, and
0.08 to 0.5 mm in diameter. Within a single host shell,
however, pellets are principally similar in their
dimensions. The pellets are elongated, mostly elliptical
or rodshaped and always with circular cross section.
Their length/diameter ratio is mostly less than 2
(Figs 3A, D, 4F–H, 5J, K) or 2–3 (Figs 3I, 4A, B, D, I,
5C). Only few specimens show more elongated pellets,
with the length/diameter ratio between 4 and 6 (Figs 3F,
5B). All pellets are devoid of the internal structure, and
no constructional wall or lining has been observed. The
EDS chemical analysis suggested a similar carbonate
composition for pellets and rock matrix, while only a
single specimen from the Haljala Stage (Sandbian)
showed traces of silicification.
Pellets inside shells are organized in two different
modes: the majority is represented by massive
accumulations (Figs 3A–I, 4A–I, 5A–F), and in fewer
cases (15 specimens), pellets are associated with small
burrows (Fig. 5F–K). Massive accumulations consist of
randomly oriented pellets which are not mixed with the
sediment. The boundary between the sediment and pellets
may be distinct (Fig. 3A) or transitional (Fig. 3G). The
degree of preservation of pellets is variable, especially for
the sets of massive accumulations. The decomposition
process may affect an entire set (Fig. 3C) or only the
periphery of an accumulation (Fig. 3G–I). The sediment
inside the shells probably consists of decomposed pellets,
especially if it does not contain bioclastic material. Some
specimens contain pellets of two different sizes, located
in separate sets (Figs 3E, 4I). The number of pellets in
accumulations varies widely: for large pellets, less than
hundred pieces make up an accumulation, but for the
smallest pellets a set may contain more than a thousand
pellets (Fig. 3A).
Sinuous, branching small burrows (diameter about 1–
3 mm, traceable length less than 10 mm) may contain
micritic, randomly oriented isolated elliptical pellets with
the length/diameter ratio around 2 (Fig. 5F–J). Such
pellets are well preserved, randomly oriented and loose.
The burrows have sharp outline, circular cross section,
which is mostly constant in diameter, and lack
ornamentation or lining. In most cases the burrows with
pellets are found within gastropods, but few are known
inside brachiopods and bivalves. These burrows are
oriented in different directions, but their total length and
general configuration are difficult to establish as the
pellets are mostly visible in cross section (Fig. 5G, H). A
massive accumulation of pellets and burrows with pellets
may occur together inside the same shell (Fig. 5I, J).
However, the pellets inside the burrows are not coming
from massive accumulations as they are different in size
and preservation, and are thus likely made by the
producers of the burrows. Additionally, a few trilobite
specimens demonstrate specific ribbonshaped, pellet
filled burrows with varying diameter (Fig. 5K).
The majority of pelletfilled fossils (a total of 112
specimens) are molluscs, especially gastropods (Figs 3E, F–
I, 4A–D, 5D, E, I, J). All common Ordovician gastro pod
genera are represented, but most finds are related to oblong
and conical forms such as Subulites, Murchisonia,
Hormotoma, Lophospira, Holopea, Liospira and
Deaechospira. Fewer specimens come from de pressed gas
tro pod shells (the diameter is much larger than the height)
like Pachystrophia, Lesueurilla, Cymbularia, Bucanella,
Megalomphala and Salpingostoma. The gastro pods vary in
size, with the smallest measured specimen being 10.1 mm
high and 16.4 mm wide, and the larg est 102.1 mm high and
53.8 mm wide. Massive clusters of pellets are typic al ly
Estonian Journal of Earth Sciences, 2020, 69, 1, 1–19
4
U. Toom et al.: Faecal pellets in Ordovician shelly fossils
5
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Table 1. Stratigraphic distribution of shelly fossils with pellets inside the Ordovician carbonate succession of Estonia. The basin margin and type of sediments after Dronov & Rozhnov
(1997). The numbers after the plus sign denote finds from 30 randomly selected sliced shells. Note that pellets have not been recorded in the topmost Ordovician Porkuni Stage so far
N/A, not analysed; – no finds.
Width (mm)
Length
(mm)
Total
Trilobita
Gastropoda
Cephalopoda
Bivalvia
Brachiopoda
–
–
– no finds.
Estonian Journal of Earth Sciences, 2020, 69, 1, 1–19
6
located in the apical parts of gastro pod shells (Fig. 3H, I).
For smaller shells, the entire whorl may be filled with pellets.
In larger specimens, like Hormotoma insignis from the
Rakvere Stage, the pellets may be accumu lated in one side
of the shell and the occurrence of pellets may be observed
also in body whorl (Fig. 4C). Clusters, clusters together with
burrows (Fig. 5I, J) and only burrows filled with pellets are
observed in gastropod shells, however, the last case is rather
exceptional. Stratigraphically the pelletfilled gastropods
have been recorded from the Uhaku (late Darriwilian) to
Pirgu stages (late Katian), with a higher num ber of finds
from two levels: the Haljala (Sandbian) and Rakvere stages
(Katian). The pellets inside gastropods show the highest
variability in size and shape, including the largest pel lets
recovered during this study (Fig. 4A). Elliptical and rod
shaped pellets with a length of 0.23–1.8 mm, diameter
0.09–0.75 mm and various length/diameter ratios (≤2, 2–3
and 4–5) are known. Notably, there is no correlation
between the size of the shells and the size of the pellets they
contain. The large open aperture of gastropods probably
made them easy to inhabit and may have controlled the
occurrence and variability of pellets. However, the higher
variability of pellets in gastropod shells may also result
from the largest number of specimens studied. The trace
fossil Arachnostega is observed on the surface of some
gastropod steinkerns (Fig. 4C, D).
Sixteen specimens of cephalopods contain pellets.
Infills have been found in the body chamber,
phragmocone between septae (Fig. 4E), as well as
siphuncle (Fig. 4F). Frequently only one chamber
between septae is filled (Fig. 4E) and bioturbation is
common. Finds of pellets in cephalopods are mostly
related to coiled and mediumsize specimens with a
diameter of aperture less than 25 mm. Stratigraphically
the occurrences range from the Kunda to Lasnamägi
stages (Darriwilian), the Haljala Stage (Sandbian) and
from the Vormsi to Nabala stages (Upper Katian). Most
of the pellets are poorly preserved and relatively small in
size (length 0.23–0.5 mm, diameter 0.09–0.2 mm), with
the length/diameter ratio below 2 or 2–3. Most finds come
from temperatewater carbonates, which may be the cause
of a relatively small size of pellets in cephalopod shells.
In addition, due to small holes in damaged phragmocones
the shells could be inhabited by only small animals with
a slender body plan. The small number of cephalopod
specimens with pellets may be biased due to insufficient
study, although, a recent thorough examination of a large
number of lower Katian cephalopods by Kröger &
Aubrechtová (2018) did not reveal any finds with pellets.
Fifteen specimens of bivalves representing the genera
Modiolopsis, Aristerella and Cypricardinia contain pellets
(Fig. 5A–C, F–H). Shells with a length of 28–61 mm,
height 16–44 mm and width 23–44 mm are partially filled
with irregular clusters. Only the largest Cypricardinia
contain several small burrows filled with pellets (Fig. 5F).
Bivalves filled with pellets are recorded from the Kukruse
to Rakvere stages (Sandbian to Katian). The pellets are
elliptical (length 0.26–0.76 mm, diameter 0.18–0.43 mm),
elongated elliptical (length 0.61 mm, diameter 0.16 mm)
or rodshaped (length 1.19 mm, diameter 0.22 mm), with
the length/diameter ratio of ≤2, 2–3 or 4–6. A relatively
small number of finds exhibit surprisingly high variability
of the shape of pellets. Few bivalve steinkerns show the
presence of Arachnostega.
Thirtytwo specimens of brachiopods contain pellets
(examples in Figs 3C, D, 4G–I). Shell sections may be
filled only with pellets (Fig. 3C), or more commonly, the
irregular accumulations of pellets occur in some parts of
the shell sections examined, like the spondylium
(Fig. 4G). The smallest brachiopod with pellets is
Platystrophia with a height of 9.4 mm and a width of
21.6 mm, the largest is Porambonites wesenbergensis
with a height of 45.4 mm and a width of 33.9 mm.
Bioturbation and multiple accumulations with different
sizes of pellets are related to larger specimens (Fig. 4I). A
single occurrence has been recorded from the Volkhov
Stage, other finds come from the Haljala to Rakvere
stages (Sandbian to Katian). Only elliptical pellets are
observed, with a length of 0.2–1.1 mm, diameter 0.1–
0.5 mm and the length/diameter ratio ≤2 or 2–3.
Commonly large pellets occur inside large brachiopods,
which may be related to the size of the pedicle opening.
Few specimens of brachiopods also show the presence of
Arachnostega.
Two echinoderm specimens belonging to
Echinosphaerites and Sphaeronites contain irregu lar
accu mulations of small, elliptical pellets with a length of
0.1–0.4 mm, diameter 0.08–0.24 and the length/diameter
U. Toom et al.: Faecal pellets in Ordovician shelly fossils
7
Fig. 3. Faecal pellets inside Ordovician shelly fossils from Estonia. Scale bars: 1 cm for A, C, H; 1 mm for B, E, D, F, G, I. A,
echinoderm Sphaeronites, massive accumulation of pellets, vertical section, GIT 1561066, Värska 6 drill core, 381.8 m, Haljala Stage,
Sandbian. B, detail of GIT 1561066, C. oblongus. C, brachiopod Porambonites, two sets of pellets inside, boundary between sets is
distinct, one of the sets is almost completely decomposed, GIT 3991961, Aluvere quarry, Haljala Stage, Sandbian. D, detail of GIT
3991961, C. oblongus. E, gastropod Hormotoma insignis, surface of steinkern, two different sizes of Coprulus (L/W ratio 2.7), TUG
80484, Piilse, Rakvere Stage, Katian. F, bivalve Aristerella, detail of surface, Coprulus (L/W ratio 4), GIT 69493, Kullaaru ditch,
Oandu Stage, Katian. G, gastropod Subulites wesenbergensis, horizontal section through the apical part of steinkern, boundary between
the accumulation of pellets and sediment is transitional, decomposition of pellets is almost finished, GIT 404639, Mõnuste quarry
Harjumaa, Vormsi Stage, Katian. H, gastropod Lesueurilla, horizontal section through the apical part, Coprulus (L/W ratio 2.3), GIT
7201, Aluvere quarry, Haljala Stage, Sandbian. I, detail of GIT 7201, on the left side of the image are decomposed pellets.
Estonian Journal of Earth Sciences, 2020, 69, 1, 1–19
8
ratio below 2. The Sphaeronites shell is so far the only
specimen characterizing the deeper shelf environments. It
is noteworthy that the pellets it contains are the smallest
among those recorded during the present study (Fig. 3A).
The two specimens come from the Kukruse and Haljala
stages (Sandbian).
Two trilobites have ribbonshaped burrows on the
dorsal surface of sediment infill of the cephalon of
Oculichasmops and the pygidium of an asaphid trilobite
(Fig. 5K). Stratigraphically the finds come from the
Haljala and Keila stages (Sandbian and lower Katian).
The pellets inside trilobites are elliptical in shape, with a
length of 0.13–0.19 mm, diameter 0.32–0.36 mm and the
length/diameter ratio below 2 or 2–3.
To some extent the data on pellet occurrences
presented in Table 1 are biased by the different detection
and study methods and the number of previous studies
on different fossil groups. Pellets can rarely be observed
on natural break surfaces due to the similar colour of
pellets and matrix. However, the sectioning and
polishing of surfaces reveals the pellets inside shells.
The number of finds also depends on the degree of
preservation of the shelly fossils. For instance,
brachiopods have commonly wellpreserved shells and
pellet finds are related only to damaged specimens or
material which was sectioned for other purposes. The
prevalence of molluscs, and especially gastropods, can
be explained by the conditions of ‘calcite sea’, where
aragonitic shells dissolved rapidly (Palmer et al. 1988;
Palmer & Wilson 2004) and therefore the internal
moulds with pellets became visible. Pellets were
particularly well observable on the steinkerns of
gastropods and bivalves derived from pure tropical lime
mudstones of the Rakvere and Nabala stages (Katian).
These specimens were redeposited on the sediment
surface before complete lithification of steinkerns but
after the dissolution of shells. Steinkerns are deformed,
with crush marks (Fig. 5D), containing small cracks
(Fig. 5E), overgrown by trepostome bryozoans (Fig. 5D,
E) and covered by pyrite threads (Fig. 5A). It can easily
be identified on which side a gastropod or bivalve was
lying on the sea floor after redeposition, as selective
erosion by dissolution has made the pellets more distinct
(Fig. 5D, E). The dominance of gastropods among pellet
substrates can also be explained by a more favourable
microenvironment inside empty gastropod shells for
pellet producers. An appropriate shape and size of shells
may have contributed to better preservation. In addition,
the open apertures of gastropod shells made them easily
habitable, which is supported by a high variability in the
size and shape of pellets.
The present collection is probably too small and
taxonomically biased to fully assess the distribution of
pelletcontaining shells, including differences between
fossil groups. However, the 90 randomly examined
gastropod and brachiopod shells from two localities
provide useful insights into these questions. Shelly
fossils with pelletoidal infill were especially common in
the Haljala and Rakvere stages (middle Sandbian and
middle Katian; Table 1). Thirty random gastropods and
30 brachiopods from the fossil collection of the Aluvere
quarry (Haljala Stage, Sandbian), without external
indications of pellet occurrence, were sectioned and
examined. The previous data from the same locality had
shown high abundance of pellets inside gastropods and
their absence inside brachiopods. From the selected 60
shells, 15 gastropods and 12 brachiopods yielded pellets,
suggesting that nearly half of the shells contained pellets
in both fossil groups. For comparison, 30 gastropods
from the Mõnuste quarry (Vormsi Stage, upper Katian)
were sectioned. Only few previous finds of shelly fossils
with pellets were known from this stratigraphical
interval. However, the study of 30 gastropod steinkerns
revealed pellets in 11 cases, that is, in every third shell.
This approach clearly shows that the pelletoidal infill is
a very common phenomenon related to different shelly
fossil groups and stratigraphical intervals within the
Middle and Upper Ordovician carbonate succession of
Estonia.
Ichnogenus Coprulus – pellets in massive
accumulations and inside burrows
Richter & Richter (1939) proposed the term Coprulus as
an informal name. The formal ichnogenus Coprulus was
erected by Mayer (1952) for isolated small pellets. Two
U. Toom et al.: Faecal pellets in Ordovician shelly fossils
9
Fig. 4. Faecal pellets inside Ordovician shelly fossils from Estonia. Scale bars: 1 mm for A, B, D, F–I; 1 cm for C, E. A, gastropod
Liospira wesenbergense, weathered steinkern, largest Coprulus (L/W ratio 2.5), TUG 1780380, Rägavere quarry, Rakvere Stage,
Katian. B, gastropod Hormotoma insignis, polished surface of steinkern, Coprulus (L/W ratio 2.4), TUG 7663, Rägavere quarry,
Rakvere Stage, Katian. C, gastropod Hormotoma insignis, steinkern surface with pellets and trace fossil Arachnostega, TUG 2393,
Rakvere, Rakvere Stage, Katian. D, detail of TUG 2393, Coprulus (L/W ratio 3), Arachnostega and openings of small burrows filled
with sparry calcite. E, section through the cephalopod phragmocone, only one chamber is filled with pellets, GIT 362740, Viki drill
core, 360.9 m, Kunda Stage, Darriwilian. F, section of cephalopod Estonioceras, detail from siphuncle filled with C. oblongus, GIT
1467, Valkla outcrop, Kunda Stage, Darriwilian. G, section of brachiopod Clinambon anomalus, detail of spondylium filled with C.
oblongus, GIT 5431367, Saku 1098 drill core, 10.3 m, Keila Stage, Katian. H, brachiopod Porambonites, polished surface, C. oblongus
(L/W ratio 2), GIT 61985, Oandu River outcrops, Rakvere Stage, Katian. I, brachiopod Porambonites wesenbergensis, detail of
sectioned surface, bioturbated pellets different in shape and size, TUG 1766141, Oandu, Oandu Stage, Katian.
ichnospecies, C. oblongus and C. sphaeroideus, were
described and later C. bacilliformis was added (Mayer
1955). Knaust (2008) revised the diagnoses for the
ichnogenus Coprulus and ichnospecies C. oblongus and
brought out an important diagnostic feature for small
pellets – the length/diameter ratio. Coprulus sphaeroideus
Estonian Journal of Earth Sciences, 2020, 69, 1, 1–19
10
and the ichnogenus Tibiokia Hatai et al., 1970 were
regarded as junior synonyms of C. oblongus; thus, the
ichnogenus Coprulus includes two ichnospecies, C.
oblongus and C. bacilliformis (Knaust 2008). The pellets
of C. oblongus are isolated pills with a smooth surface,
cylindrical to oval in shape and having a length/diameter
ratio commonly between 1.5 and 2.0 (Knaust 2008). The
pellets of C. bacilliformis have the length/diameter ratio
around 6 and are rodshaped (Mayer 1955).
It should be noted that diagnoses of C. oblongus by
Mayer (1952) and Knaust (2008) are based on material
where pellets are partly or completely washed in different
burrows. According to Mayer (1952), C. oblongus has
‘longoval’ shape, but Knaust (2008) refers to ‘cylindrical
to oval’ shape. The pellets that were possibly washed in,
instead of being formed in situ, may represent composite
traces (structures made by combined activity of two or
more species) and, according to Bertling et al. (2006), have
no ichnotaxonomic standing. Arakawa (1970) introduced
a detailed morphological classification of bivalve faeces.
The faeces were divided into several types according to the
structure and form. Three basic types were distinguished:
oval, rodshaped and ribbonlike pellets, and a large
number of subtypes (pellettypes by Arakawa). According
to Knaust (2008), C. oblongus includes pellets from oval
to cylindrical shape, thus from two different basic types of
Arakawa (1970). It has to be noted that for poorly preserved
very small pellets with small length/diameter ratio values,
differentiating between elliptical and rodshaped forms may
be complicated (see Fig. 3B).
Estonian faecal pellets in accumulations are mostly
elliptical in shape, with the length/diameter ratio below 2
or 2–3 (Table 1). In few specimens with two different
outlines, elliptical (Fig. 4F) and rodshaped (Fig. 5B), this
value is between 4 and 6. Only half of shelly fossils in the
Estonian material contained faecal pellets with an average
length/diameter ratio around 2, and these can be
confidently identified as C. oblongus. A small number of
rodshaped faecal pellets have the length/diameter ratio
about 4–6, and these can be named as C. bacilliformis.
However, a large number of pellets are elliptical with the
length/diameter ratio over 2; these cannot be named at
present without erecting a new species or emending the
diagnoses of the existing species (Figs 3E, F, H, 4A, B,
D, 5C). Péneau (1941) described this type of pellets as
Tomaculum. The Estonian material suggests that it is
reasonable to include the elliptical pellets with the
length/diameter ratio of 2–3 in the ichnospecies C.
oblongus. However, it is questionable that there is a
good basis to assign the oblong elliptical pellets with
length/diameter values over 4 to C. bacilliformis. As
far as the original material of C. oblongus and C.
bacilliformis described and figured by Mayer (1952,
1955) is not available for reexamination, we identify the
elongated elliptical pellets with the length/diameter ratio
over 2 only at the genus level.
Compound trace Tubularina – pellets in small
burrows
Compound traces consist of combined individual traces
with different morphologies that would be named
differently if they were preserved in isolation (Miller
2003). Compound traces can only be named if all
structures have been produced simultaneously (Bertling
et al. 2006). For simple burrows filled with pellets, two
compound tracefossil genera Alcyonidiopsis Massalongo,
1856 and Tomaculum Groom, 1902 are most commonly
identified and discussed from Palaeozoic strata. From
Jurassic lagoonal limestones, Tubularina Gaillard et al.
1994, a small (diameter up to 2 mm) firmground burrow
filled with sparry calcite and loose pellets has been
described. The walls of Tubularina are smooth, without
ornamentation, and branching is observed. Tubularina is
penetrating the sediment sinuously in very different
directions. Burrows inside shelly fossils from the Haljala
to Oandu stages (Sandbian and lower Katian) are very
similar to the Jurassic material. Skeletal debris displaced
concentrically around the burrows, and also the sharp
contours and circular cross section of the burrow indicate
that the traces were made into a coherent substrate.
Ichnogenus Alcyonidiopsis – ribbonshaped
burrows with pellets inside trilobites
Two specimens of trilobites from the Haljala and Keila
stages contain curved ribbonshaped burrows inside, filled
with sparry calcite and pellets (Fig. 5K). No
U. Toom et al.: Faecal pellets in Ordovician shelly fossils
11
Fig. 5. Faecal pellets inside Ordovician shelly fossils from Estonia. Scale bars: 1 cm for A, D, F, I; 1 mm for B, C, E, J, G, H, K. A,
bivalve Modiolopsis, weathered steinkern covered by pyrite threads, TUG 1779477, Rakvere, Rakvere Stage, Katian. B, detail of
TUG 1779477, rodshaped pellets C. bacilliformis. C, bivalve Modiolopsis, detail of strongly weathered steinkern surface, Coprulus
(L/W ratio 3), TUG 1779439, Estonia, Rakvere Stage, Katian. D, gastropod Hormotoma insignis, redeposited steinkern with crush
mark, cracks, overgrown by trepostome bryozoans, TUG 7662, Rägavere quarry, Rakvere Stage, Katian. E, detail of TUG 7662,
steinkern surface with pellets and small bryozoan colony. F, bivalve Cypricardinia, vertical section, bioturbated sediments and
composite trace fossil Tubularina, GIT 1561079, Oandu, Keila Stage, Katian. G, H, detail of GIT 1561079, Tubularina filled with
C. oblongus, cross sections at different angles. I, gastropod Subulites amphora, vertical section, bioturbated accumulations of pellets
and trace fossil Tubularina filled with C. oblongus, GIT 399999, Aluvere quarry, Haljala Stage, Sandbian. J, detail of GIT 399
999. K, pygidium of asaphid trilobite, Alcyonidiopsis filled with C. oblongus, GIT 362726, Vasalemma quarry, Keila Stage, Katian.
constructional wall or lining is observed; the boundary of
the structure is marked by calcite. The burrows are located
on the dorsal surface of the sediment infill, one on the
cephalon and the other on the pygidium. The burrows are
1.7–2.9 mm and 1.1 mm wide, of somewhat variable
width. The pellets are elliptical in shape and represent the
ichnogenus Coprulus. These burrows are different from
Tubularina, which has a circular cross section (Gaillard
et al. 1994), and from Phymatoderma, which is a
subhorizontally branching burrow system filled with
pellets (Izumi 2012). The location of burrows on
steinkerns is similar to that of Arachnostega, but the traces
do not demonstrate the network characteristic of the latter;
besides, the pelletoidal infill is unknown in Arachnostega.
Bruthansová & Kraft (2003) described pellets arranged in
rows inside Ordovician trilobites as Tomaculum.
However, a number of authors have considered that
Alcyonidiopsis is the proper name for tubular burrows
filled with faecal pellets (Chamberlain 1977; Pickerill
1980; Pickerill & Narbonne 1995; Uchman 1995, 1999;
Orr 1996; Uchman et al. 2005, 2013; Buatois at al. 2017;
Mángano et al. 2019). Tomaculum consists of tightly
packed pellets on bedding planes and is indicative of
deepwater settings (e.g. Benton & Trewin 1978; Zagora
1997; Podhalańska 2007). The trace is rare in Palaeozoic
carbonates (Chamberlain 1977) but is also known from
nonmarine settings (Metz 2015). Mángano et al. (2019)
described small burrows with pellets inside Cambrian
bivalved arthropods as Alcyonidiopsis. The Estonian
ribbonshaped small burrows inside trilobites are most
similar to Alcyonidiopsis. Additionally, a few shelly
fossils with pellets inside demonstrate poorly preserved
burrows with unclear shape and cross section (Table 1),
which cannot be named at present.
Notes on the preservation of pellets
For all sectioned specimens, the taphonomic evidence
suggests that the large accumulations of pellets were formed
in situ, inside the shells. This is also proved by the
specimens containing only pellets inside (Fig. 3C). Massive
accumulations of pellets are not evenly mixed with the
sediment, which also indicates that the pellets were not
washed into the shells together with the sediment. Random
placement of skeletal grains around the sets of pellets
suggests that the sediment was not affected by the formation
of pellets and proves that the pellets were not made in shells
filled with sediment. In the case of shells with large
apertures, like in gastropods, it is more likely that pellets
were produced when the empty shell was lying on the sea
floor and was afterwards filled with sediment. However, it
is also possible that the dead body closed the aperture of the
gastropods and pellets were formed when the shell was
shallowly buried. More likely, shells with small openings,
like brachiopods or echinoderms, could also be colonized
by meiofauna and small macrofauna after shallow burial and
the pellets were produced by infauna. According to Wilson
& Palmer (1992), the cementation is fastest just below the
water–sediment interface. Rapid lithification is important
for the formation and preservation of trace fossils in
carbonates (Knaust et al. 2012). Eriksson et al. (2011)
expressed the same opinion for the preservation of
coprolites. Rapid lithification was likely favoured by small
dimensions of the pellets examined in our study.
Cementation by calcite resulting from aragonite dissolution
may be confined to the areas immediately adjacent to
dissolving bioclasts (Wilson & Palmer 1992). This may be
the explanation for the large number of finds related to the
different mollusc shells.
In the Ordovician sediments of Estonia, pellets are
found in coolwater to tropical carbonates (Table 1). Two
stratigraphical levels, tropical carbonates of the Rakvere
Stage (Katian) and temperatewater carbonates of the
Haljala Stage (Sandbian), demonstrate a higher occurrence
and better preservation of pellets. According to Flügel
(2004), carbonate pellets can be preserved in warmwater
environments with low energy and reduced sedimentation
rates. The fossilization of the originally soft particles
requires bacterial decomposition of organic mucus and
intragranular cementation by Mgcalcite. The higher
number of findings and large scale in the shape and size
of pellets are related to the Upper Ordovician, possibly due
to the better preservation conditions in warm waters.
The increased number of shells with pellets in the
Haljala Stage could be caused by the beginning of
climate warming and supported by silicification of
sediments. Extremely wellpreserved pellets inside shelly
fossils from Bohemia also showed a high degree of
silicification (Bruthansová & Kraft 2003). Tarhan et al.
(2016) and Liu et al. (2019) have discussed rapid
silicification for the preservation of softbodied fossils.
Slightly silicified rocks have a wide distribution in
Estonian shallowwater carbonate rocks, especially in the
Haljala Stage. The source of silica is supposed to be
organic, in particular siliceous sponges, and/or volcanic
(Jürgenson 1958; Siir et al. 2015).
Environmental conditions, such as low oxygen levels
in the sediment and/or fast burial, related to increase in
sedimentation rates, may have contributed to the
preservation of pellets. Anoxic conditions slowed down
or stopped the decomposition of pellets. The processes
occurring at the oxic/anoxic boundaries are controlled by
the temperature, supply of organic matter, light, water
currents and bioturbation (Kristensen 2000). The
bioturbation inside a large number of steinkerns and on
their surfaces indicates the existence of oxic
environmental conditions inside the shells after the burial
and before lithification. In addition, the activities of
Estonian Journal of Earth Sciences, 2020, 69, 1, 1–19
12
infaunal benthos stimulate microbial activity (Aller &
Aller 1982) and may in turn accelerate the decomposition
of pellets. The physical mixing of sediments lowered the
carbonate cementation (Wright & Cherns 2016) and may
have supported the degradation of pellets. The best
preserved pellets were found inside the shells where
bioturbation was not recorded. The tropical pure
carbonates of the Rakvere Stage are, in general,
characterized by a relatively low bioturbation rate and a
small number of softsediment traces (Toom et al. 2019a).
Highenergy environments with enhanced oxygen
exposure are generally characterized by a very low burial
efficiency of organic matter (Arndt et al. 2013).
Microenvironment inside the shells was probably less
affected, the oxygen diffusion into the sediment was
decreased and the degradation of pellets was slowed
down. The shells also shielded pellets and pelletfilled
burrows with pellets from compaction and allowed an
early cementation to occur as suggested by Mángano et
al. (2019). This may explain the absence of pellets in the
host rocks. However, water circulation in the uppermost
sediment column favoured the lithification (Coimbra et
al. 2009), and the shells may have acted as traps for
calcium ions and promoted early cementation of small
pellets. Supposedly, the microenvironment inside the
protective shell was an important factor for the
preservation of pellets in cool and temperatewater
environments of the Ordovician of Estonia.
Microbial communities are important in the
decomposition of organic matter (Solan & Wigham 2005;
Morata & Seuthe 2014). Microbes on pellets could
originate from the water or sediment, or the gut of pellet
producers being ingested with the food. The accumulations
of fresh pellets, which contained less microorganisms, had
a better preservation potential (Hargrave 1976). The
decomposition process, which started from the outer edge
of the set, did not reach the end of the set inside an
elongated and narrow whorl (Fig. 3H). This may be the
reason for a large number of finds of pellets in the apical
parts of gastropod steinkerns. The organic matter may have
played a major role in inhibiting precipitation of
sedimentary carbonates (Morse et al. 2007).
In conclusion, it is difficult to identify one main reason
for the observed preservation of pellets. Most likely it is
due to the interaction of several factors including sea water
and pore water chemistry, temperature, rapid lithification
of small particles, favourable microenvironment inside the
shells and the composition of pellets and microbial
communities.
The trace makers
Flügel (2004) summarized a modern view on trace makers
of small pellets: the assignment of pellets to specific pellet
producing animals is difficult because the pellets of most
invertebrates lack specific morphological features.
However, there are many publications dealing with
excrements of different recent aquatic invertebrate groups
(e.g. Moore 1931a, 1931b, 1939; Moore & Kruse 1956;
Manning & Kumpf 1959; Kornicker 1962; Arakawa 1970,
1971; Kraeuter & Haven 1970; Pryor 1975; Martens 1978;
Ladle & Griffiths 1980; Wotton & Malmqvist 2001;
Kulkarni & Panchang 2015). In addition to detailed
descriptions and classifications of pellets, a wide range of
problems have been discussed. Moore (1939) concluded
that in general carnivorous animals tend to produce faeces
of loose consistency, and faeces of deposit feeders are the
most resistant of all. Arakawa (1970) noted that variations
in pellets are related to feeding habits and mode of life,
and that the pellets of carnivores are very soft and irregular
in shape. Excrements of loose consistency cannot be
preserved as fossils (Kornicker 1962). The nature and form
of faecal pellets are related to the structure and function of
the digestive organs (Stamhuis et al. 1998). Unsculptured
faecal rods are formed as midgut is very simple and
circular (Arakawa 1970). The exact shape of pellets may
vary; for instance, the excrements of a stressed or starved
animal may be thinner and irregular in shape (Arakawa
1971).
Similar pellets inside different shells are described and
discussed by Bruthansová & Kraft (2003). These authors
demonstrated that the pellets were not related to the
animals that originally inhabited the shells. Our
observations on the Ordovician material from Estonia
confirm this conclusion – the pellets located on the ventral
muscle scar of a brachiopod shell and in cephalopod
phragmocone could not be produced by a brachiopod and
cephalopod, respectively.
It is possible that some pellets were made by
scavengers feeding on soft parts after the death of the
animal with the shell (Bruthansová & Kraft 2003) or on
microbial halo formed around the decaying soft body.
Deposit feeders appear to be limited largely to worms
longer than 1 cm. In general, juveniles and small worms
are restricted to ingest highly digestible organic matter and
rich food items (Jumars et al. 2015) and may be the trace
makers, especially in the case of complete brachiopod
shells, where a slender body shape was needed to enter
the shell. Priapulids and arthropods, trilobites included,
with mobile epifaunal lifestyle, are discussed as crayon
feeders (Hu 2005). In addition, nematodes could feed
on bacteria, fungi, algae and protozoans (Sun et al.
2014). Predators and scavengers have high assimilation
efficiencies and produce small numbers of faecal pellets
(Wotton & Malmqvist 2001). The accumulations with a
relatively small number of pellets (much less than 100
pellets per cm2 according to Wotton & Malmqvist 2001)
can be made by scavengers. However, in some cases the
U. Toom et al.: Faecal pellets in Ordovician shelly fossils
13
pellets were located in parts of the shell which did not
contain a soft body, like between septae in the phragmocone.
Other animals than scavengers should have been
producing these clusters. Estonian pellets, composed of the
same material as the host sediment, suggest that deposit
feeders or suspension feeders were trace makers rather
than predators and scavengers.
In shallowmarine environments, a cryptic en vir on
ment inside shells provides shelter and food to different
encrusters (Vinn et al. 2018). Our unpub lished material
of steinkerns brought up new data showing that the
encrusting community is widespread and consists of dif
ferent bryozoans, cornulitids and inarticulate brachiopods.
It confirms that the empty shells provided suitable living
environments in shallowmarine conditions. However, co
occurrence of pellets and different encrusters has not been
observed. Considering the large number of pellets that the
suspension feeders produce (Pryor 1975; Wotton &
Malmqvist 2001, and references therein), different
cavities inside the shells were used as temporary hiding
places rather than permanent domicle. Mobile trilobites
used empty cephalopod shells for hiding (Davis et al.
2001). Bruthansová & Kraft (2003) suggested that
producers selected the shells according to their body
dimensions. Our material supports this idea – pellets have
not been observed inside the largest shells. In addition,
the apertures/holes in shells that controlled the inhabiting
community of shells were important.
GarcíaRamos et al. (2014) discussed a situation
where pellets were stored to be used as a bacteria
enriched resource. Gardeners have low turbative activity,
inhabiting relatively simple and almost permanent
burrows with storage rooms (Stamhuis et al. 1998), and
pellets may be stored inside the shells by gardeners.
Invertebrates may live in a mixture of faecal pellets and
fine particles (Levinton 2017).
Mayer (1952, 1958) originally interpreted Coprulus as
faecal pellets of annelids. Recent excrements similar to
Coprulus are commonly produced by polychaetes (Bałuk
& Radwański 1979; Gaillard et al. 1994; Knaust 2008;
Kulkarni & Panchang 2015). Kraeuter & Haven (1970)
investigated pellets of six modern shallowmarine phyla
represented by 70 species and described thoroughly
several characteristics. The elliptical or rodlike shape was
common, but faecal pellets of polychaetes were of rodlike
or ellipsoidal shape, with circular cross section, lacking
sculpture and having mostly solid consistency. Various
muddwelling and suspensionfeeding polychaetes form
solid and resistant pellets of very constant shape (Moore
1931b; Pryor 1975). Alcyonidiopsis and Tubularina,
burrows actively filled with pellets, were interpreted as a
feeding structure of polychaetes (Chamberlain 1977;
Gaillard et al. 1994; Uchman et al. 2005; Mángano et al.
2019). Recently, palaeoscolecidans were discussed as
potential trace makers (see Martin et al. 2016 and
references therein). Based on evidence of scolecodonts,
polychaetes were abundant and diverse in the Ordovician
of Baltoscandia, especially starting from the Darriwilian
(Hints 2000; Hints & Eriksson 2007; Eriksson et al. 2013),
and might have been responsible for various trace fossils,
including the pellets reported herein. However, in
ichnology the identity of trace makers mostly remains
speculative. We may have the case of unknown feeding
strategies and the animals that made the pellets may have
no close living representatives.
Bruthansová & Kraft (2003) described several
Coprulus (as Tomaculum) specimens inside shells
associated with Arachnostegalike traces. They suggested
that pellets could be the faeces of the cryptic producers of
Arachnostega which is a feeding trace inside the body
fossils found at the contact of sediment filling and the inner
surface of body fossils. Arachnostega is common also in
the Ordovician of Estonia (Vinn et al. 2014). We recorded
the cooccurrence of faecal pellets and Arachnostega in
only few specimens, indicating that the trace maker of
Arachnostega was probably not the producer of pellets
inside the shells. Moreover, Arachnostega and Tubularina
were made within coherent sediment, whereas faecal
pellets were put into empty shells. Thus, the two traces
could not be created simultaneously.
Bruthansová & Kraft (2003) concluded that the
producers of pellets inside shells come from different
growth stages of a limited number of nondepositfeeding
taxa. The Estonian material inside shells is similar to the
Bohemian pellets; they were made by trace makers with
similar feeding strategies. Most likely filter feeders and
scavengers looking for shelter were the producers of the
pellets described herein. However, the large variation in
size, shape and the number of pellets in accumulations and
burrows suggest a wide spectrum of potential trace makers.
CONCLUSIONS
● Small faecal pellets are common inside shelly fossils
(gastropods, bivalves, cephalopods, brachiopods,
echinoderms and trilobites) in the Ordovician
carbonate succession of Estonia. They may fill up half
of the fossil shells.
● The faecal pellets are representing the ichnogenus
Coprulus Mayer, 1952 and ichnospecies C. oblongus
Mayer, 1952 sensu Knaust 2008, C. bacilliformis
Mayer, 1955 and transitional unidentified forms.
● Compound traces consisting of small burrows with
circular cross section and filled with faecal pellets
inside various shelly fossils are representing the
ichnogenus Tubularina Gaillard et al., 1994. Ribbon
shaped burrows with faecal pellets inside trilobites are
Estonian Journal of Earth Sciences, 2020, 69, 1, 1–19
14
representing the ichnogenus Alcyonidiopsis
Massalongo, 1856.
● Stratigraphically, faecal pellets occur in the Volkhov to
Pirgu regional stages (Dapingian to upper Katian), thus
they can be preserved in cool, temperate and warm
water shallowmarine conditions. The highest number
of finds and largest morphological diversity, however,
are encountered in the Upper Ordovician sediments.
● The size and shape of host shells control the occurrence
of pellets in two ways: shells with large apertures were
easier to inhabit by animals with different size and
body plan, while in narrow and elongated shells, the
process of decomposition was slowed down.
● The preservation of pellets has been due to interaction
of several factors: chemical and physical processes
that affect rapid lithification, factors that disturb
mechanical and biological decomposition and the
composition of pellets. In some cases, silicification
may have supported the preservation of pellets in
carbonates.
● Fauna inhabiting various empty shells was diverse
through the Middle and Late Ordovician in Estonia. It
consisted of mobile trace makers with different feeding
strategies.
● The trace maker of Arachnostega was most probably
not the producer of the faecal pellets inside shells.
Acknowledgements. We are grateful to the referees Dirk Knaust
and Diego Kietzmann for their constructive comments that helped
to improve the manuscript. We thank Dirk Knaust, Diego
Kietzmann and Mike Reich for their help in acquiring research
papers, Eberhard Schindler and Rainer Brocke for the fresh images
of specimens from Péneau’s collection, Toomas Post and Nordkalk
for providing access to the Vasalemma quarry and Gennadi Baranov
for excellent photographs of the specimens of faecal pellets. U. T.
acknowledges support from the Doctoral School of Earth Sciences
and Ecology (TalTech ASTRA development programme 2016–
2022). Financial support to O. V. and O. H. was provided by the
Estonian Research Council (grants IUT2034, PRG836). This paper
is a contribution to the IGCP project 653 ‘The Onset of the Great
Ordovician Biodiversification Event’. The publication costs of this
article were partially covered by the Estonian Academy of Sciences.
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Mikrokoproliidid Eesti Ordoviitsiumi makrofossiilides
Ursula Toom, Olev Vinn, Mare Isakar, Anna Madison ja Olle Hints
Fossiilsed ekskremendid ehk koproliidid on halvasti säilivad jäljekivistised, mis kannavad olulist paleoökoloogilist ja
sedimentoloogilist teavet. Artiklis on kirjeldatud Eesti Kesk ja ÜlemOrdoviitsiumist (Baltoskandia paleobassein)
makrofossiilide kodade sisemuses säilinud mikrokoproliite ning nende levikut ja mitmekesisust. Uuritud materjal sisaldab
üle 180 koproliite sisaldava makrofossiili, mis on kogutud enam kui 40 leiukohast ja esindavad mitmesuguseid madalaveelisi
keskkondi alates jahedaveelisest karbonaatsest platvormist troopilise avašelfini. Stratigraafiliselt pärinevad mikrokoproliitide
leiud Volhovi lademest kuni Pirgu lademeni (Dapingi kuni Kati globaallade). Mikrokoproliidid on ovaalsed või pulkjad,
0,1–1,8 mm pikkused ja 0,08–0,75 mm läbimõõduga; pikkuse/läbimõõdu suhe ulatub 6ni. Mikrokoproliidid esinevad
tigude, karpide, peajalgsete, käsijalgsete, okasnahksete ja trilobiitide kodades ning kuuluvad valdavalt liikidesse Coprulus
oblongus ja Coprulus bacilliformis. Lisaks tuvastati kaks kompleksjäljekivistist: Tubularina (koproliidid ümmarguse
ristlõikega käikudes) ja Alcyonidiopsis (koproliidid lintjates käikudes). Mikrokoproliidid tekitati siis, kui organismide tühjad
kojad asusid merepõhjas või olid veidi settesse mattunud. Koproliidid säilisid tõenäoliselt tänu kodade mehaanilisele kaitsele
ja kiirele mineraliseerumisele. Mikrokoproliitide tekitajate bioloogiline päritolu jääb spekulatiivseks, kuid tõenäolisemate
rühmade hulka kuuluvad polüheedid, kellel on sobiv suurus ja kehakuju ning kelle tänapäevased esindajad tekitavad
ühesuguseid fekaale. Võimalik, et mikrokoproliitide tekitamisel osalesid erineva toitumisstrateegiaga organismid. Valitud
leiukohtade makrofossiilide süstemaatiline uuring näitas, et kuni pooltes kodades esineb mikrokoproliite. See näitab
koproliite tootvate organismide suurt arvukust ja mitmekesisust Baltoskandia Ordoviitsiumi basseinis. Meie materjal näitas
ka, et Arachnostega jäljekivististe tekitaja ei olnud tõenäoliselt mikrokoproliitidega seotud.