Frequency patterns of chitinozoans, scolecodonts, and conodonts in the upper Llandovery and lower Wenlock of the Paatsalu core, western Estonia

Abstract

Frequency patterns of chitinozoans, scolecodonts (polychaete jaws), and conodonts in the upper Llandovery and lower Wenlock of the Paatsalu core, western Estonia, are described. Chitinozoans are represented by nearly 50 species and constitute the most abundant group, outnumbering scolecodonts and conodonts by 10–100 times. Jawed polychaetes with about 60 species and up to thousands of specimens per kilogram of rock are the most diverse but least varying group. Conodonts display marked variations in abundance and relative frequency, which are partly caused by global evolutionary patterns of conodont faunas.The Llandovery Rumba Formation is characterized by large numbers of scolecodonts, common and diverse chitinozoans, and very rare conodonts. Good correlation between the abundance of microfossils and lithology in the Rumba Formation most likely indicates variations in the deposition/compaction rate. Interestingly, the lithologically sharp Rumba–Velise boundary is indistinct in chitinozoan and polychaete faunas; marked changes occur above the boundary. The Velise Formation, except its basal part, is characterized by a decreased number of scolecodonts and abundant conodonts. The changes in chitinozoan and conodont faunas at the Llandovery–Wenlock boundary are sharp, partly on account of a stratigraphical gap. The abundance of chitinozoans increases 10 times, whilst that of conodonts decreases; both groups display a significant turnover in assemblages and decrease in diversity. Changes in jawed polychaete faunas are less conspicuous, although a major change in relative frequency of dominating forms can be observed. The Wenlock Jaani Formation is characterized by maximum abundance of chitinozoans and scolecodonts, and decreased abundance and diversity of conodonts. The last group was most affected by the Ireviken Event.

Full text

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Proc. Estonian Acad. Sci. Geol., 2006, 55, 2, 128–155







Frequency patterns of chitinozoans, scolecodonts,
and conodonts in the upper Llandovery and
lower Wenlock of the Paatsalu core,
western Estonia

Olle Hintsa, Mairy Killinga,b, Peep Männika, and Viiu Nestora

a Institute of Geology at Tallinn University of Technology, Estonia pst. 7, 10143 Tallinn, Estonia;
olle@gi.ee, mairy@gi.ee, mannik@gi.ee, vnestor@gi.ee
b Department of Mining, Tallinn University of Technology, Ehitajate tee 5, 19086 Tallinn, Estonia

Received 5 December 2005, in revised form 27 February 2006

Abstract. Frequency patterns of chitinozoans, scolecodonts (polychaete jaws),
and conodonts in the upper Llandovery and lower Wenlock of the Paatsalu
core, western Estonia, are described. Chitinozoans are represented by nearly
50 species and constitute the most abundant group, outnumbering scolecodonts
and conodonts by 10–100 times. Jawed polychaetes with about 60 species and
up to thousands of specimens per kilogram of rock are the most diverse but least
varying group. Conodonts display marked variations in abundance and relative
frequency, which are partly caused by global evolutionary patterns of conodont
faunas.
The Llandovery Rumba Formation is characterized by large numbers of
scolecodonts, common and diverse chitinozoans, and very rare conodonts. Good
correlation between the abundance of microfossils and lithology in the Rumba
Formation most likely indicates variations in the deposition/compaction rate.
Interestingly, the lithologically sharp Rumba–Velise boundary is indistinct in
chitinozoan and polychaete faunas; marked changes occur above the boundary.
The Velise Formation, except its basal part, is characterized by a decreased
number of scolecodonts and abundant conodonts. The changes in chitinozoan and conodont faunas
at the Llandovery–Wenlock boundary are sharp, partly on account of a stratigraphical gap. The
abundance of chitinozoans increases 10 times, whilst that of conodonts decreases; both groups
display a significant turnover in assemblages and decrease in diversity. Changes in jawed polychaete
faunas are less conspicuous, although a major change in relative frequency of dominating forms can
be observed. The Wenlock Jaani Formation is characterized by maximum abundance of chitinozoans
and scolecodonts, and decreased abundance and diversity of conodonts. The last group was most
affected by the Ireviken Event.

Key words: microfossil dynamics, frequency patterns, diversity, chitinozoans, scolecodonts, conodonts,
Silurian, Estonia.

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INTRODUCTION

Quantitative data are essential for analysing palaeoecology and dynamics of
fossil faunas. Moreover, numerical data on fossil distributions may contribute to a
better understanding of palaeoenvironments and depositional regime, and improve
stratigraphical resolution. The numerical specimen-level data on Early Palaeozoic
fossil distribution are nevertheless limited. The dynamics of different fossil groups
has been analysed jointly and in a detailed temporal and spatial framework only
in a few studies. The first and perhaps still the most thorough study is that of
Jaanusson et al. (1979) on the lower Wenlock strata of the Vattenfallet section,
Gotland, Sweden.
The present study focuses broadly on the same interval, the upper Llandovery
to lower Wenlock, of the Paatsalu drill core, western mainland Estonia. This
interval was selected for quantitative study for a number of reasons. Firstly, much
detailed biostratigraphical, but also chemostratigraphical and sedimentological back-
ground information is available from other coeval Estonian sections and elsewhere
in the world. Secondly, this interval spans across an internationally recognized
series boundary and embraces the best known Silurian event – the Ireviken Event,
which is associated with oceanic changes, a stable isotope excursion, and extinction
in several fossil groups (for a review see Jeppsson 2005 and references therein).
Thirdly, the transition from the Rumba to Velise formations embraces a supposedly
rapid facies change that reflects the most extensive flooding of the Baltic Shelf
during the Silurian (Nestor & Einasto 1997). The primary aim of our study was to
document and analyse the frequency patterns of chitinozoans, scolecodonts, and
conodonts. These groups are very common and diverse in Silurian rocks and
represent different modes of life and ecological niches. It was of particular interest
to explore if the curves of absolute and relative frequency of the three groups
(and individual taxa) correlate with each other and with the palaeoenvironmental
settings, and if the frequency data might be useful for stratigraphy and biofacies
analysis.
The previous knowledge of frequency patterns of chitinozoans, scolecodonts,
and conodonts in the Baltic Silurian is uneven. Conodonts are best known in this
respect, but the published record on quantitative data is still limited (e.g. Jeppsson
1979). Some data on scolecodonts are available from Gotland (Bergman 1979,
1989; Eriksson 1997, 2006a; Eriksson et al. 2004). Little is known about the
dynamics of chitinozoans (Laufeld 1979), however, several studies provide semi-
quantitative data (Nestor 1994) or analyse the abundance and diversity on a larger
scale (Kaljo et al. 1986). Other acid-resistant microfossils encountered during this
study include acritarchs, foraminiferans, melanoscleritoids, chitinous hydroids, and
graptolite siculae, but these were not studied quantitatively.
The following measures are used to characterize the quantitative data set:
abundance (= absolute frequency, the number of specimens per gram or kilogram
of rock), relative frequency (percentage of a taxon within the group), taxic

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diversity (the number of species or genera in a sample), and diversity index
(calculated using the well-known Shannon formula

((
i
H n n

where n is the total number of individuals and
taxon ).
i Further methodological details are discussed below for each of the groups
studied.
Scolecodonts and conodonts were extracted by acid digestion from ca 300–
500 g samples, whilst 5–50 grams of each sample were treated separately for
chitinozoans. The residues were sieved through 53 µm (chitinozoans) or 63 µm
(scolecodonts and conodonts) screens. All microfossils were then picked from the
residues, identified, and counted; the chitinozoans by M. Killing and V. Nestor,
scolecodonts by O. Hints, and conodonts by P. Männik. Samples, residues, and
all specimens are deposited at the Institute of Geology at Tallinn University of
Technology (abbreviated as GIT); for figured specimens the collection number
493 is allocated.


LOCALITY AND STRATIGRAPHY

The Paatsalu (527) drilling site is located in western mainland Estonia, close
to the coast of the Baltic Sea, ca 10 km south of the village of Virtsu (Fig. 1). The
208.5 m deep borehole penetrates Wenlock, Llandovery, and Upper Ordovician
strata.
The upper Llandovery to lower Wenlock succession considered in the present
paper is composed of nodular limestones, mainly wacke- to packstones, of the
Rumba Formation and dolomitic marlstones of the Velise and Jaani formations
(see Fig. 2). Chitinozoans were studied also from the upper part of the Nurmekund
Formation. The lithology and biostratigraphy of these stratigraphical units have
been discussed in detail in several earlier papers (Kaljo 1970; Einasto et al. 1972;
Jeppsson & Männik 1993; Nestor 1997; Nestor & Einasto 1997; Nestor & Nestor
2002; Kiipli 2004; Harris et al. 2005). A generalized lithological log of the Paat-
salu section is given in Fig. 2.
In order to facilitate direct comparison of quantitative palaeontological data
with rock characteristics, and thereby with the palaeoenvironment and sedimentary
processes, all samples used for microfossil extraction were also studied by means
of carbonate analysis and thin sections. The carbonate analysis shows that the
rocks of the Nurmekund and Rumba formations contain less than 25% insoluble
residue and are in part strongly dolomitized. The levels of dolomitization, for
example the top of the Nurmekund Formation (samples OM4-5 and OM4-6), may
be related to hiatuses. The basal part of the Velise Formation is very clayey,
containing nearly 80% insoluble residue. In successive strata the clay content
decreases on account of dolomite, whereas calcite is almost missing.
)ln( )),
i
n n
= −∑

in is the number of individuals of

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Latvia
Russia
Belarus
Estonia
Paatsalu 527
borehole
N
50 km
bioclastic calcareous
mud
green terrigenous mud
grey terrigenous mud
with graptolites
dark kerogenous
graptolitic mud
land
present
extension
of rocks
argillaceous-calcareous
mud

Page 5

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50
m
60
70
80
90
100
110
Formation
Depth
Samples
Regional stage
Global stage
Adavere
Raikküla
Nurmekund
Rumba
Velise
Jaani
Jaani
Telychian
Aeronian
Sheinwoodian
Global series
Llandovery
Wenlock
OM4-50
OM4-49
OM4-48
OM4-47
OM4-46
OM4-45
OM4-44
OM4-43
OM4-42
OM4-41
OM4-40
OM4-39
OM4-38
OM4-37
OM4-36
OM4-35
OM4-34
OM4-33
OM4-32
OM4-30
OM4-28
OM4-26
OM4-24
OM4-22
OM4-19
OM4-16
OM4-13
OM4-11
OM4-9
OM4-6
OM4-5
OM4-4
OM4-3
OM4-2
OM4-1
calcite
Rock composition
nodular limestone
(wacke- to packstone)
micritic
limestone
bioclastic
dolostone
clayey
dolostone
clayey dolomitic
marlstone
dolomitic
marlstone
discontinuity
surface
Composition of skeletal material
Environments
insoluble residue
(siliciclastics)
dolomite
050
20 5050%
100%0
40%
I-0IV III II
0
100%
50%50%
echinodermsbrachiopods
trilobites
I-O - lagoon, land
II - shoal
III - open shelf
IV - deep shelf, transition to basin
unidentified (recrystallized)
Bioclasts

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133
Rumba formations (but also between the Raikküla and Adavere stages and
probably between the Aeronian and Telychian). The stratigraphical and spatial
extent of this gap has been recognized and discussed earlier by, e.g., Nestor (1976)
and Nestor (1997).
The Eisenackitina dolioliformis Zone ranges through most of the Rumba
Formation and lower part of the Velise Formation, and is followed by the
Angochitina longicollis, Conochitina proboscifera, and Conochitina acuminata
zones. At the lithologically distinct Velise–Jaani transition, the Margachitina
margaritana Zone sensu stricto (i.e., where M. margaritana co-occurs with
A. longicollis) is missing. According to Nestor (2005), the Llandovery–Wenlock
boundary is located within the latter zone. Hence, it seems very likely that in the
Paatsalu core the gap spans across the topmost Llandovery as well as the basal
Wenlock (see also discussion in Nestor & Nestor 2002). The boundary is succeeded
by chitinozoan Interzone IV (Nestor 1994) and the Conochitina mamilla and
Conochitina tuba zones.
Conodonts are too rare in the Nurmekund and Rumba formations to allow
recognition of zones in this interval. The appearance of Pterospathodus eopennatus
ssp. n. 1 in sample OM4-30 indicates that the base of the P. eopennatus ssp. n. 1
Zone lies in the upper part of the Rumba Formation. Conodonts become abundant
in the basal Velise Formation, and successive zones, primarily based on the
Pterospathodus lineage, can be followed throughout the formation. At the Velise–
Jaani transition, the Upper Pseudooneotodus bicornis, and the Lower and Upper
Pterospathodus pennatus procerus zones are missing, confirming the conclusions
based on the chitinozoan succession (see above). The Ireviken Event interval
recognized in more complete successions in Estonia and Gotland (Jeppsson &
Männik 1993) is almost entirely missing in the Paatsalu section (only datum
points 1 and 8 can be identified). Above this gap, the Lower and Upper Kockelella
ranuliformis zones are recognized.
In a sequence stratigraphical framework, the Raikküla–Adavere transition
corresponds to a large-scale sequence boundary (according to Harris et al. 2005 to
the boundary between two “supersequences”), followed by a major flooding of the
shelf during Adavere time. That sea-level rise caused laterally extensive spreading
of lime muds (Rumba Formation) unconformably covering older deposits and was
followed by further deepening of the basin and extensive formation of terrigenous
muds (Velise Formation). These strata are overlain by a shallowing-upward package
which ends up in widespread shoal deposits (starting slightly above the sampled
interval in the Paatsalu section).
A succession of sediments formed in variable environmental settings ranging
from shoal to deep shelf was recognized in the studied interval (Fig. 2; Nestor &
Einasto 1997). In terms of benthic assemblage zones (BA, sensu Boucot 1975),
these environments correspond to the BA2–3 to BA4–5 zones.

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MICROFOSSIL FREQUENCY PATTERNS
Chitinozoans

The collection of chitinozoans consists of nearly 50 species. One sample
usually contains 5–8, but occasionally 12 different species, which is slightly more
than reported from the lower Wenlock of Gotland (Laufeld 1979). The latter fact
may, however, result partly from the recent progress in chitinozoan studies and
systematics. The abundance of chitinozoans (Fig. 3) varies considerably and ranges
from less than one to about 20 specimens per gram of rock in the Llandovery
Nurmekund, Rumba, and Velise formations (i.e., in the Llandovery). The abundance
increases markedly at the Llandovery–Wenlock boundary and ranges from 30 to
170 vesicles per gram in the Wenlock. The maximum was recorded in sample
OM4-45. It remains to be tested if the chitinozoan abundance displays the same
trend in stratigraphically more complete sections.
The abundance of chitinozoans in the Jaani Formation slightly exceeds the
values reported by Laufeld (1979) for the lower Wenlock Upper Visby Formation
of the Vattenfallet section, Gotland, where the corresponding values range from
ca 10 to 100. However, the abundance varies rather widely in both sections.
The most common genera that occur throughout the studied interval are
Conochitina and Ancyrochitina (see Plate I for most common species). The
former generally dominates in the Raikküla Stage and the latter in the Adavere
and Jaani stages. At some levels also Eisenackitina, Margachitina, Angochitina,
and Bursachitina form a significant part of the assemblage (Fig. 3).
Commonly, the dominant species constitutes 50–70% of the assemblage,
but exceptionally one species can make up more than 95% of all specimens
(Euconochitina electa in sample OM4-1; see Fig. 3). Other species that may
strongly dominate the assemblage are Conochitina elongata and C. iklaensis in
the Nurmekund and basal Rumba formations and Ancyrochitina ancyrea and
A. primitiva in the remaining part of the sequence (except for the few levels rich
in Angochitina longicollis, Bursachitina spp. or M. margaritana).
Despite different sampling density and extent of the study interval, this pattern
is very similar to the early Wenlock chitinozoan dynamics. Alternating Conochitina
and Ancyrochitina are complemented by Angochitina, Margachitina, and few
others at certain levels (Laufeld 1979).


________________________________________________________________________________
Fig. 3. Distribution of selected chitinozoan species and genera. Relative frequency is only shown
for taxa that account for more than ca 10% of the assemblage. To the left of the absolute frequency
curve the corresponding numerical value and actual number of specimens counted in each sample
(in parentheses) are indicated. Chitinozoan biozones after Nestor (1994, 2005) and Nestor & Nestor
(2002). Diversity of chitinozoans is illustrated in Fig. 6. For lithological key see Fig. 2.

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???
?
50
m
6070 8090
100
110
Formation
Regional stage
Global series
Adavere
Llandovery
Raikküla
Nurmekund
RumbaVelise
Jaani
Jaani
Wenlock
34152
7
6
8
10 1112
Chitinozoan zone
OM4-50 OM4-49OM4-48OM4-47 OM4-46OM4-45OM4-44OM4-43 OM4-42OM4-41OM4-40OM4-39OM4-38 OM4-37OM4-36OM4-35 OM4-34OM4-33OM4-32OM4-30OM4-28OM4-26OM4-24OM4-22OM4-19OM4-16OM4-13OM4-11OM4-9OM4-6OM4-5 OM4-4OM4-3OM4-2 OM4-1
0
50%
Absolute frequency, n / g
Euconochitina electa
Conochitina
iklaensis
Ancyrochitina ancyrea s. lato
Conochitina
elongata
Conochitina edjelensis
Conochitina
alargada
Conochitina malleus
Eisenackitina causiata
Eisenackitina dolioliformis
Belonechitina spp.
Bursachitina spp.
Eisenackitina inanulifera
Angochitina longicollis
Conochitina proboscifera
Margachitina margaritana
Conochitina mamilla
Conochitina claviformis
Chitinozoan zones are as follows:
1 - Euconochitina electa
2 - Ancyrochitina convexa
3 - Conochitina alargada
4 - Conochitina malleus
5 - Eisenackitina dolioliformis
6 - Angochitina longicollis
7 - Conochitina proboscifera
8 - Conochitina acuminata
10 - Interzone IV 11 - Conochitina mamilla
12 - Conochitina tuba
Note that 9, the Margachitina
margaritana Zone s. str. is missing)
Ancyrochitina convexa
Ancyrochitina rumbaensis
Ramochitina sp. nov.
Plectochitina pachyderma
Ramochitina nestorae
Conochitina acuminata
Conochitina cf. flamma
Conochitina leptosoma
Conochitina aff. tuba
Margachitina banvyensis
Conochitina tuba
60 (180)43 (213)
102 (408)
59 (294)82 (412)
167 (668)
36 (182)44 (175)19 (761)
130 (780)
32 (706)
132 (793)
11 (131)
18 (313)
9 (305)2 (100)
18 (439)
5 (116)
6 (290)5 (135)
2 (36)
5 (247)
1 (51)
2 (113)
6 (98)
2 (296)6 (303)4 (235) 4 (553)
2 (92)0 (21)
4 (178)
4 (84)
9 (460)
4 (1922)
Relative frequencies of individual species and genera, %
0
20
60
40
80
100

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PLATE I

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137
The taxic diversity is relatively consistent with abundance in the Llandovery.
In the Wenlock, however, the high abundance samples are not necessarily related
to the highest number of species (Fig. 3). The Shannon diversity index for
chitinozoan assemblages displays a notable drop in the Velise Formation, and in
the overlying strata remains below the average recorded in the Rumba and lower
Velise formations. Due to rather small average sample weight, very rare species
may be under-represented, resulting in lower diversity.
Laufeld (1974) concluded that the abundance of chitinozoans on Gotland is
almost directly proportional to the content of clay and silt in the rocks: the more
argillaceous the rocks, the more chitinozoans they contain. In the light of this, it is
somewhat surprising that the most argillaceous interval, the Velise Formation,
and its basal portion in particular, is only a little richer than the Rumba Formation
(see also Nestor 1994).

________________________________________________________________________________
Explanation of Plate I
Figs 1–24. Typical chitinozoans from the Llandovery–Wenlock boundary interval of the Paatsalu
core. Scale bars correspond to 50 µm (figs 1–5, 18–24) and 100 µm (figs 6–17). 1. Calpichitina
densa (Eisenack), GIT 493-1, sample OM4-42, depth 62.22 m, Jaani Formation, Wenlock.
2. Bursachitina nestorae Mullins & Loydell, GIT 493-2, sample OM4-33, depth 80.10 m, Velise
Formation, Llandovery. 3. Margachitina margaritana (Eisenack), GIT 493-3, sample OM4-40,
depth 66.25 m, Jaani Formation, Wenlock. 4. Eisenackitina inanulifera Nestor, GIT 493-4, sample
OM4-33, depth 80.10 m, Velise Formation, Llandovery. 5. Eisenackitina causiata Verniers, GIT 493-5,
sample OM4-34, depth 78.20 m, Velise Formation, Llandovery. 6. Euconochitina electa Nestor,
GIT 493-6, sample OM4-1, depth 113.67 m, Nurmekund Formation, Llandovery. 7. Conochitina
edjelensis Taugourdeau, GIT 493-7, sample OM4-2, depth 111.50 m, Nurmekund Formation,
Llandovery. 8. Conochitina cf. elongata Taugourdeau, GIT 493-8, sample OM4-2, depth 111.50 m,
Nurmekund Formation, Llandovery. 9. Conochitina iklaensis Nestor, GIT 493-9, sample OM4-9,
depth 101.30 m, Rumba Formation, Llandovery. 10. Conochitina alargada Cramer, GIT 493-10,
sample OM4-9, depth 101.30 m, Rumba Formation, Llandovery. 11. Conochitina malleus (nomen
nudum, Van Grootel), GIT 493-11, sample OM4-16, depth 95.76 m, Rumba Formation, Llandovery.
12. Conochitina cf. emmastensis Nestor, GIT 493-12, sample OM4-34, depth 78.20 m, Velise
Formation, Llandovery. 13. Conochitina proboscifera Eisenack, GIT 493-13, sample OM4-37,
depth 72.10 m, Velise Formation, Llandovery. 14. Conochitina claviformis Eisenack, GIT 493-14,
sample OM4-42, depth 62.22 m, Jaani Formation, Wenlock. 15. Conochitina mamilla Laufeld,
GIT 493-15, sample OM4-40, depth 66.25 m, Jaani Formation, Wenlock. 16. Conochitina
leptosoma Laufeld, GIT 493-16, sample OM4-42, depth 62.22 m, Jaani Formation, Wenlock.
17. Rhabdochitina sp., GIT 493-17, sample OM4-13, depth 97.60 m, Rumba Formation, Llandovery.
18. Ancyrochitina ancyrea Eisenack, GIT 493-18, sample OM4-28, depth 85.78 m, Rumba
Formation, Llandovery. 19. Ramochitina sp. (nov.), GIT 493-19, sample OM4-16, depth 95.76 m,
Rumba Formation, Llandovery. 20. Ancyrochitina porrectaspina Nestor, GIT 493-20, sample OM4-37,
depth 72.10 m, Velise Formation, Llandovery. 21. Ancyrochitina primitiva Eisenack, GIT 493-21,
sample OM4-22, depth 91.45 m, Nurmekund Formation, Llandovery. 22. Plectochitina pachyderma
Laufeld, GIT 493-22, sample OM4-41, depth 64.23 m, Jaani Formation, Wenlock. 23. Ancyrochitina
rumbaensis Nestor, GIT 493-23, sample OM4-19, depth 93.40 m, Rumba Formation, Llandovery.
24. Angochitina longicollis Eisenack, GIT 493-24, sample OM4-34, depth 78.20 m, Velise Formation,
Llandovery.

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?
???
?
50
m
60708090
100
110
Formation
Regional stage
Global series
Adavere
Llandovery
Raikküla
Nurmekund
RumbaVelise
Jaani
Jaani
Wenlock
1044 (590)
572 (123)
1141 (331)
601 (137)823 (247)
1514 (215) 1213 (364)
1127 (338)
394 (248)628 (182)
327 (98)290 (84) 293 (79) 126 (29)170 (85)
90 (27)
132 (56)
479 (242)
745 (35)
488 (245)376 (109) 640 (228)358 (233)616 (175) 764 (278)472 (135)641 (346)454 (108)476 (199)
OM4-50OM4-49OM4-48OM4-47OM4-46OM4-45OM4-44 OM4-43OM4-42OM4-41OM4-40OM4-39OM4-38OM4-37OM4-36OM4-35OM4-34OM4-33OM4-32OM4-30OM4-28OM4-26OM4-24OM4-22OM4-19OM4-16OM4-13OM4-11OM4-9
Oenonites jennyensis
Tetraprion sp. A
Pistoprion sp. A
Oenonites spp.
Mochtyella sp. A
Kettnerites spp.
“Mochtyella” cf. trapezoidea
“Mochtyella” sp. D
Rhytiprion magnus
Kettnerites cf. sisyphi
Oenonites sp. A
0
800
400
Absolute frequency, n / kg
Relative frequencies of individual species and genera, %
0
50%
Mochtyella sp. B
Protarabellites staufferi
Tretoprion astae
Xanioprion cf. borealis
Vistulella kozlowskii
Leptoprion spp.
Atraktoprion spp.
Kozl. longicavernosus
Kalloprion spp.
Mochtyella sp. C
Protarabellites triangularis
Euryprion sp.
Skalenoprion spp.
Leptoprion sp. A
Lanceolatites gracilis
Ramphoprion gotlandensis
Hadoprion cf. cervicornis
Conjungaspis sp.
Xanioprion sp. A
Pistoprion cf. transitans

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Scolecodonts

Scolecodonts were found in all samples, mostly in abundance. They make up
the second most common acid-resistant microfossil group in most samples, next
to chitinozoans (and likely acritarchs that have been overlooked in this study).
Although the large numbers may partly result from shedding (see discussion in
Eriksson 2006b), jaw-bearing polychaetes undoubtedly constituted a common
faunal component of the benthic assemblages that inhabited the study area during
the late Llandovery and early Wenlock.
The entire jawed polychaete fauna comprises approximately 60 apparatus-
based species (see Plate II for selected forms). Up to 27 species were recorded in
one sample. Nearly half of the species recovered are not yet formally described
but are treated currently under open nomenclature, since systematics was beyond
the scope of this study. A rigorous taxonomic analysis will nevertheless follow,
utilizing material also from other Estonian sections and elsewhere, particularly
Gotland.
The counts of jawed polychaete “specimens” referred to below and used in all
calculations were obtained by the most frequent diagnostic element of each species
(or genus) in a sample (Hints 1998, 2000). Usually this means counting left or
right posterior maxillae. Other authors (e.g. Bergman 1989; Eriksson 1997, 2001,
2006a) have counted left and right posterior maxillae, resulting in counts that are
up to two times higher for a given sample. Taken that polychaete jaw apparatuses
usually contain many elements, some of which are not species- or genus-diagnostic,
the plain count of all scolecodonts can be several times higher. Altogether some
5500 specimens were counted. In addition to the common detached elements, this
number includes also more than 80 well-preserved jaw apparatuses.
The abundance of scolecodonts varies greatly, ranging from about 100 to 1500
specimens per kilogram of rock (Fig. 4). The Rumba and the lowermost Velise
formations are characterized by a rich assemblage with approximately 500
specimens per kilogram. The abundance drops abruptly in the lower part of the
Velise Formation and displays low but slightly increasing values throughout the
rest of the formation. In the Jaani Formation, abundance continues to increase and
reaches maximum values between depths of 55 and 60 m (Fig. 4).
The jawed polychaete fauna is dominated by polychaetaspids, mochtyellids,
and paulitinids as would be expected from a Silurian assemblage (Eriksson et al.
2004, fig. 6). Polychaetaspids, particularly the genus Oenonites, account for a
maximum of 70%, mochtyellids reach 60%, and paulinitids about 45% of all

________________________________________________________________________________
Fig. 4. Distribution of selected polychaete taxa. Relative frequency is only shown for taxa that
account for more than ca 10% of the assemblage. Note that the scolecodont counts are based on
the most abundant element of the taxon in the sample. To the left of the absolute frequency curve
the corresponding numerical value and actual number of specimens counted in each sample (in
parentheses) are indicated. Diversity of jawed polychaetes is illustrated in Fig. 6. For lithological
key see Fig. 2.

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PLATE II

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specimens. Oenonites is also the most species-rich genus, represented by more
than 10 species. It is, however, usually not possible to identify them all and thus,
except for O. jennyensis and Oenonites sp. A (Pl. II, fig. 7), they are grouped into
Oenonites spp. in Fig. 4. Among the mochtyellids, Pistoprion and Mochtyella
sensu lato are the most frequent forms. The latter group, representing at least three
different genera that are yet to be formally described, contains ca 10 species, some
of which are shown in Fig. 4 and Plate II. The paulinitids are represented by
Kettnerites and Lanceolatites. The former genus varies greatly in frequency but
occurs in all samples, whereas the latter is very rare.
Occasionally rhytiprionids, tetraprionids, and xanioprionids make up a
considerable part of the assemblages. Other families like Ramphoprionidae,
Atraktoprionidae, Kalloprionidae, Tretoprionidae, Conjungaspidae, and
Hadoprionidae may be common in certain parts of the succession but far less
abundant than the above mentioned taxa.
The stratigraphical ranges of individual species and genera are usually
restricted to certain intervals of the succession studied (Fig. 4). For instance,
O. jennyensis, Mochtyella sp. B, and Protarabellites triangularis have been

________________________________________________________________________________
Explanation of Plate II
Figs 1–21. Selected scolecodonts. All in dorsal view, except 1 and 2 that are in left lateral view.
Scale bars correspond to 100 µm. 1. “Mochtyella” cf. trapezoidea Kielan-Jaworowska, left MI, GIT
493-25, sample OM4-50, depth 46.60 m, Jaani Formation, Wenlock. 2. Mochtyella sp. A, right MI,
GIT 493-26, sample OM4-48, depth 50.56 m, Jaani Formation, Wenlock. 3. “Mochtyella” sp. D,
right MI, GIT 493-27, sample OM4-36, depth 74.23 m, Velise Formation, Llandovery.
4. Pistoprion sp. A, left MI, GIT 493-28, sample OM4-13, depth 97.76 m, Rumba Formation,
Llandovery. 5. Tetraprion sp. A, left MI, GIT 493-29, sample OM4-13, depth 97.76 m, Rumba
Formation, Llandovery. 6. Rhytiprion magnus Kielan-Jaworowska, left MI, GIT 493-30, sample
OM4-43, depth 60.36 m, Jaani Formation, Wenlock. 7. Oenonites sp. A, right MI, GIT 493-31,
sample OM4-43, depth 60.36 m, Jaani Formation, Wenlock. 8. Oenonites sp., right MI, GIT 493-32,
sample OM4-50, depth 46.60 m, Jaani Formation, Wenlock. 9. Kozlowskiprion longicavernosus
Kielan-Jaworowska, right MI, GIT 493-33, sample OM4-50, depth 46.60 m, Jaani Formation,
Wenlock. 10. Oenonites jennyensis Eriksson, left MI, GIT 493-34, sample OM4-13, depth 97.76 m,
Rumba Formation, Llandovery. 11. Oenonites jennyensis Eriksson, right MI, GIT 493-35, sample
OM4-13, depth 97.76 m, Rumba Formation, Llandovery. 12. Hadoprion cf. cervicornis (Hinde), left
MIII, GIT 493-36, sample OM4-48, depth 50.56 m, Jaani Formation, Wenlock. 13. Kalloprion sp.,
right MI, GIT 493-37, sample OM4-48, depth 50.56 m, Jaani Formation, Wenlock. 14. Ramphoprion
gotlandensis Eriksson, right MI, GIT 493-38, sample OM4-48, depth 50.56 m, Jaani Formation,
Wenlock. 15. Leptoprion sp. A, right MI, GIT 493-39, sample OM4-48, depth 50.56 m, Jaani
Formation, Wenlock. 16. Xanioprion sp. A, right MI, GIT 493-40, sample OM4-50, depth 46.60 m,
Jaani Formation, Wenlock. 17. Lanceolatites gracilis Bergman, left MI, GIT 493-41, sample OM4-40,
depth 66.38 m, Jaani Formation, Wenlock. 18. Kettnerites cf. sisyphi Bergman, left MI, GIT 493-42,
sample OM4-43, depth 60.36 m, Jaani Formation, Wenlock. 19. Kettnerites cf. sisyphi Bergman,
right MII, GIT 493-43, sample OM4-50, depth 46.60 m, Jaani Formation, Wenlock. 20. Tretoprion
astae Hints, left MI, GIT 493-44, sample OM4-13, depth 97.76 m, Rumba Formation, Llandovery.
21. Skalenoprion sp., right MI, GIT 493-45, sample OM4-40, depth 66.38 m, Jaani Formation,
Wenlock.

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recovered only from the Rumba Formation, Pistoprion sp. A, Tetraprion sp. A,
and Mochtyella sp. C are restricted to the Rumba and lowermost Velise formations.
Oenonites sp. A, Conjungaspis sp., Xanioprion sp. A, Hadoprion cervicormis,
and Ramphoprion gotlandensis occur in the Jaani Formation, and Rhytiprion
magnus is found in the Velise and Jaani formations.
However, for some of the mentioned species longer ranges have been reported
from other localities. Eriksson (1997) described O. jennyensis from the lower
Wenlock Högklint Formation of Gotland. Protarabellites triangularis ranges
from the Slite Group (upper Sheinwoodian) to the Hamra Formation (Ludfordian)
and R. gotlandensis throughout the Silurian of Gotland (Eriksson 2001, 2002;
Eriksson et al. 2004). Pistoprion sp. A (= Eunicites serrula sensu Bergman 1979)
has been reported previously from Gotland (Bergman 1979), Severnaya Zemlya
(Männil & Zaslavskaya 1985), and the Canadian Arctic (Hints et al. 2000). At least
on Gotland, its distribution extends well into the Wenlock (Högklint Formation),
where it is one of the most common species (Bergman 1979). Thus in many cases
the stratigraphical ranges recorded in the Paatsalu section seem not to be true time
constrained ranges, but represent local distribution intervals that result from
environmental settings and facies changes.
The relative frequency patterns of individual taxa may also show distinct and
regular changes. Among the dominant species, Pistoprion sp. A provides the best
example. In the lower part of the Rumba Formation it makes up 10–20% of
specimens, but between samples OM4-22 and OM4-24, its relative frequency
increases abruptly several times, mainly on account of Oenonites spp. Although
in terms of absolute frequency this growth is less prominent, displaying only
twofold increase, it is unlikely to be a random change. Upsection, this species
continues with a rather stable share until abrupt disappearance between samples
OM4-33 and OM4-34. Notably its disappearance does not coincide with the sharp
lithological change at the Rumba–Velise formation boundary. The genus reappears
in the topmost sample, represented by P. cf. transitans. Studies on the Baltic
Ordovician have shown that the distribution of Pistoprion is strongly facies-
controlled (Hints 2000, 2001) and events like volcanic ash-falls may affect this
genus severely (Hints et al. 2003). Presumably, the same applied to Silurian
representatives of Pistoprion. Mochtyella sp. A, “Mochtyella” sp. D, “Mochtyella”
cf. trapezoidea, Oenonites sp. A, and R. magnus constitute other examples with
rather well-formed peaks in frequency curves. The last species is known to be
also facies-controlled, preferring distal muddy environments (Bergman et al. 2003).
The abundance and diversity of scolecodonts change very little at the
Llandovery–Wenlock boundary. The assemblage structure, however, displays a
notable turnover expressed in the disappearance of “Mochtyella” sp. D, decrease
in polychaetaspids, and increase in the abundance of Kettnerites cf. sisyphi and
Mochtyella sp. A. Rhytiprion magnus and Oenonites sp. A increase in abundance
shortly above the boundary, where also L. gracilis shows up temporarily.