Palaeobiodiversity and Palaeoenvironments

, Volume 97, Issue 3, pp 419–438 | Cite as

Palynology, microfacies and biostratigraphy across the Daleje Event (Lower Devonian, lower to upper Emsian): new insights from the offshore facies of the Prague Basin, Czech Republic

  • Petra Tonarová
  • Stanislava Vodrážková
  • Lenka Ferrová
  • G. Susana de la Puente
  • Olle Hints
  • Jiří Frýda
  • Michal Kubajko
Original Paper

Abstract

The Zlíchovian/Dalejan boundary interval (Emsian, Lower Devonian) of the Pekárek Mill section was studied employing biostratigraphy (dacryoconarid tentaculites, conodonts) and palynology (chitinozoans, prasinophytes, scolecodonts) and microfacies analysis in order to shed more light on the timing and characteristics of the Daleje Event. The results of our study stress the great importance of the base of the Nowakia elegans Zone for the substage level division of the Emsian. Onset of the Daleje transgression is linked with higher terrigenous input, and coinciding changes in the chitinozoan assemblages were recorded at this level. The transgression at the base of the N. elegans Zone preceded the main transgression taking place in the N. cancellata Zone; it can be correlated with the Upper Zlíchov Event. For the first time, Emsian chitinozoans and a jawed polychaete fauna are described in detail from the Prague Basin and can be correlated with other northern Gondwanan regions. The family-level composition of scolecodont assemblage confirms the dominance of paulinitids in the peri-Gondwanan realm.

Keywords

Daleje Event Prague Basin Biostratigraphy Chitinozoans Scolecodonts Microfacies N. elegans Zone 

Introduction

Elucidation of the timing, duration and nature of the lower to upper Emsian (Lower Devonian) transgressive-regressive cycles and decisions on the Emsian substages are among the key issues of the Subcomission on Devonian Stratigraphy (see Becker 2007). Two events linked with the lower and upper Emsian transgressions were identified by García-Alcalde (1997) in the Iberian Peninsula and adjoining areas, specifically the Upper Zlíchov Event (UZE) and the Daleje-Cancellata Event. The latter event was named as the Daleje Event by House (1985), who associated the level with the extinction of some goniatite groups (De Baets et al. 2012; Korn et al. 2015) and pointed out the coincidence of the observed bioevent and facies deepening seen in the base of the Daleje Shales of the Prague Basin. Chlupáč and Kukal (1986, 1988) described gradual faunal and lithologic changes and interpreted them as the result of a progressive sea-level rise that took place in the Nowakiabarrandei-Nowakia elegans and Nowakia cancellata biozones. The main deepening associated with this event is correlated with the base of the N. cancellata and laticostatus zones, thus with the onset of fully argillaceous sedimentation (Walliser 1985).

According to García-Alcalde (1997), the UZE corresponds to a transgression in the uppermost N. barrandei Zone and N. elegans and lower N. cancellata Zone, within the gronbergi Zone. It thus overlaps with the stratigraphic range given for the Daleje Event by Chlupáč and Kukal (1988).

Ferrová et al. (2012), who studied facies changes and biostratigraphy of the Daleje Event interval in the carbonate-dominated environment of the Prague Basin, recorded a shallowing in the upper N. barrandei Zone, followed by deepening at the base of the N. elegans Zone, which was correlated by Aboussalam et al. (2015) with the UZE. This is in contrast to Chlupáč and Kukal (1988), who recorded a single transgressive episode (gradual deepening) in this interval.

Within our study, we focus on the critical interval in the deeper water facies of the Prague Basin, the type area of the Daleje Event, exemplified by the Pekárek Mill section (Fig. 1). This section displays a gradual facies transition from the Zlíchov Limestone (Zlíchov Formation, lower Emsian) to the Daleje Shale (Daleje-Třebotov Formation, upper Emsian). In an attempt to shed light on the timing and nature of the environmental changes identified by previous authors, detailed palynological and microfacies analyses as well as tentaculite and conodont biostratigraphy were employed.
Fig. 1

Schematic map showing geographic distribution of the Zlíchov, Daleje-Třebotov and Choteč formations in the Prague Basin, with the position of the Pekárek Mill section marked (black dot). Modified after Ferrová et al. (2012)

Geological settings

The Prague Basin is situated between the cities of Prague and Pilsen (Fig. 1) and represents the central part of the Teplá-Barrandian unit of the Bohemian Massif. It is interpreted as part of a rift-system basin formed as a result of the Ordovician extension at the northern Gondwana margin, linked with the opening of the Rheic Ocean (Kraft et al. 2004). The basin infill is comprised of non-metamorphosed marine volcanosedimentary successions of Early Ordovician to Middle Devonian (Givetian) age, and it disconformably overlies the Neoproterozoic basement and partly the middle Cambrian sediments (summary in Chlupáč et al. 1998).

The Teplá-Barrandian unit, either as a separate microcontinent within the “Armorican terrane assemblage” (Perunica concept of Havlíček et al. 1994) or rather remaining closely attached to Gondwana (Drost 2008; Žák et al. 2013), drifted towards lower latitudes in the early Palaeozoic, reaching the peri-equatorial zone in the Devonian (Krs et al. 1986). This is also manifested by the takeover of carbonate sedimentation, which persisted in the Prague Basin from the Silurian (Ludlow) to the Middle Devonian (Givetian), when sedimentation in the Prague Basin terminated (Chlupáč et al. 1998).

The onset and manifestation of the Daleje Event in the Prague Basin, as described by Chlupáč and Kukal (1988), falls close to the boundary of the Zlíchov and Daleje-Třebotov formations (Fig. 2). A summary of facies development and palaeontological content of these units can be found in Havlíček and Vaněk (1996), Chlupáč et al. (1998) and Ferrová et al. (2012).
Fig. 2

Lithostratigraphic units and biostratigraphy of the Daleje Event interval as used by previous authors (on the left), and the recently suggested scheme by Ferrová et al. (2012). The character “+” means that the N. (N.) elegans and N. (N.) barrandei occur together, and this interval forms the barrandei-elegans Subzone within the N. (N.) elegans Zone. Modified after Ferrová et al. (2012)

The Zlíchov Formation is formed by the Zlíchov Limestone and its only member, the Chýnice Limestone, which is developed in the uppermost part of the Zlíchov Formation and occurs only in the NW part of the Prague Basin (Svoboda and Prantl 1948). In the SE part of the Prague Basin, the upper Zlíchov Limestone is developed as a grey, mostly micritic thin-bedded nodular limestones with intercalations of grey and grey-green calcareous shales with common traces of Chondrites. The upper Emsian Daleje Shale is represented by dark calcareous shales with sparse intercalation of thin micritic limestone beds or lenses in its lowermost part. The upper part of the Zlíchov Limestone was assigned by Chlupáč (1983a) to a deeper subtidal, quiet sedimentary environment as proven by the presence of thin-walled bivalves, articulated trilobite exoskeletons, ubiquitous Chondrites burrows and rich planktic and nektic faunas. Chlupáč (1983a) interpreted the sedimentary environment of the Daleje Shale similarly, with the main difference being the increased influx of clastic material in his conception.

Pekárek Mill section

The section is located SW of Prague, in the Švarcava valley (GPS 49° 57′ 53.26″ N, 14° 16′ 35.35″ E; Fig. 1) near the village of Solopisky. The outcrop provides a great opportunity for high-resolution stratigraphical research through the middle and upper parts of the Zlíchov Limestone to the lower part of the Daleje Shale in one uninterrupted section, with a gradual transition between these two units. The section has been studied for its rich fossiliferous content since the pioneer studies of Joachim Barrande, who used the name “Pekarkowitz” for that section (Chlupáč 1983b). The abundant well-preserved dacryoconarids were studied by Alberti (1971) and Lukeš (in Chlupáč et al. 1979; Chlupáč and Lukeš 1999). Conodonts were studied by Zikmundová (in Chlupáč et al. 1979). The occurrence of early goniatites is described in Chlupáč and Turek (1983). In contrast to the intense study of biostratigraphically important groups, microfossils have been far less studied. The first more detailed data on palynomorphs, specifically spores, from this outcrop were provided by McGregor (1979), who also briefly summarised the previous data on this stratigraphic level from the Barrandian area. This author also mentioned the presence of other potentially important microfossils, such as acritarchs, chitinozoans and scolecodonts. However, these fossils have not been studied in detail since then.

Materials and methods

The middle and upper parts of the Zlíchov Limestone (Zlíchov Formation) as well as the lower Daleje Shale (37 m in total) were sampled for the purposes of biostratigraphy and palynological studies, microfacies analyses, as well as stable isotope geochemistry (δ13Ccarb).

Forty-two samples of 50 to 100 g were dissolved in the micropalaeontological laboratory of the Czech Geological Survey in order to obtain microfossils (Fig. 3; the sample number denotes meterage in the Pekárek Mill section; as a reference, bed numbers described by Chlupáč and Lukeš (1999) starting with “CH” are also used). Three palynological techniques were used to disintegrate the rock, according to the lithology of the samples: limestones were dissolved in either 6% acetic acid or 20% hydrochloric acid, shales were treated by the classical palynological HCl-HF-HCl method (Green 2001) and several shale samples were also dissolved using Rewoquat surfactant (methodology and results described in Jarochowska et al. 2013). The majority of samples of microfossils came from the shale layers. The insoluble residue was sieved into fractions (20–50, 50–80, >80 μm). The studied microfossils were hand-picked using the “wet technique” introduced by Kielan-Jaworowska (1966) from the two latter fractions. The specimens were stored in small plastic containers filled with glycerin. The 20–50-μm fraction was used to prepare micropalaeontological permanent slides, which turned out to be productive for spores (which are not discussed in this study). All samples yielded microfossils, but their abundance was variable (Fig. 4 and discussion below). Quantitative data were obtained by the following procedure: The residue after maceration was placed in a 10-ml test tube and whirled up, and 1 ml (=1/10) of the solution was completely picked for microfossils that were counted. This procedure was repeated 2–3 times to verify the counting result.
Fig. 3

Simplified section of the Pekárek Mill locality with the distribution of the chitinozoan, conodont and scolecodont species. The samples from 26.4 to 26.8 were taken in 10-cm intervals (labels are not included for all samples in the figure). The blank shapes in the range charts denote questionable identification of the species. Conodont ranges by Zikmundová are from Chlupáč et al. (1979). The following acronyms are used due to space limitations: R. mar. = Ramochitina marettensis, R. aff. mar. = Ramochitina aff. marettensis, Kett. spp. = Kettnerites spp., Oen spp. = Oenonites spp., Hind. sp. = Hindenites sp., Plac. indet. = Placognatha indeterminable, Moch. sp. = Mochtyella sp., Tetr. ind. = Tetraprionidae indeterminable

Fig. 4

Simplified profile of the Pekárek Mill section with quantitative charts of the abundances of chitinozoa and prasinophytes and maximum diameter values of prasinophytes

For purposes of conodont biostratigraphy, 25 samples of limestone (ca. 4 kg each) were collected through the section. Clean and crushed samples (ca. 4 × 5 cm) were placed in sieve cloths and hung on a plastic container with a lid. Acetic acid (6%) was used for the maceration; pH was regularly checked following the method of Jeppsson et al. (1999). Phosphatic microfossils were separated using Na-polytungstate and hand-picked under a stereomicroscope.

Samples for dacryoconarids were obtained from the Zlíchov Limestone-Daleje Shale transitional interval (26.6 to 27 m, beds CH16 to CH18). In the case of limestone beds, the rock was mechanically crushed into smaller pieces that were checked under a stereomicroscope for small shells of dacryoconarids. Rewoquat surfactant was used for disintegration of calcareous shale interbeds (the methodology is described in Jarochowska et al. 2013). The latter residues were checked for conodonts as well.

Microfacies analysis was carried out on 63 thin sections of the format 45 × 27, 45 × 55 and 75 × 100 mm, using both Carl Zeiss Stemi 2000C and Nikon Eclipse E600 microscopes; photos were taken with a Nikon DS-Fi1 camera mounted onto the latter. Both the carbonates and shales intercalated were examined. The Baccelle and Bosellini (1965) chart was used for estimations of dacryoconarid and radiolarian frequencies (other counting methods were not employed due to fragmentation of the bioclasts).

For the purpose of stable isotope analysis (δ13Ccarb, δ18Ocarb), 23 bulk-rock samples were extracted from the carbonate facies. Micritic matrix was preferably sampled from a fresh rock surface; the most appropriate samples were chosen under a binocular microscope, powdered and treated with phosphoric acid. The evolving CO2 gases were measured for carbon isotope ratios using a Finnigan MAT 251 mass spectrometer (Czech Geological Survey, Prague). The values are reported relative to Vienna Pee Dee Belemnite using the conventional notation. The accuracy was controlled by replicate measurements of laboratory standards, with the reproducibility being 0.15‰.

For imaging, the Hitachi S-3700N scanning electron microscope housed in the National Museum in Prague and Mira3 Tescan electron microscope housed in the Czech Geological Survey were used. The descriptive terminology for scolecodonts follows Kielan-Jaworowska (1966). The microfossil collection is housed in the Czech Geological Survey (under collection numbers PT40 to PT81 and SB 44–46).

Results

Carbon isotope record

As shown in Table 1, the δ13Ccarb values are shifted towards low values, notably in the uppermost Zlíchov Limestone. A similar trend was recorded by Ferrová et al. (2012) from the carbonate-dominated environment of the Prague Basin. Slightly lower δ13Ccarb values of carbonates from the Pekárek Mill section than those from the shallow-water Chýnice Limestone could reflect a deeper water environment recorded at the Pekárek Mill section. In addition, Buggisch and Mann (2004) reported comparable δ13Ccarb values ranking from 0 to 1‰ for some mid-Emsian limestones of Europe. The δ18O values of carbonates appear not to be as low as would be expected if 13C depletion was related to diagenetic alteration of carbonates, which is also supported by a lack of positive correlation between measured δ18Ocarb and δ13Ccarb values. Nevertheless, the δ13Ccarb values are rather low in some samples (Table 1). Therefore, we interpret them as not reflecting the primary isotope signature of marine carbonates. We assume that remineralisation of isotopically lighter organic carbon deriving from organic-rich intercalated shales (which occur through the sampled interval) was responsible for shifting the δ13Ccarb of some samples to lower values. For this reason, the carbon isotope record will be not discussed further here.
Table 1

Carbon and oxygen isotope composition of carbonates of the Zlíchov Limestone and Daleje Shale from the Pekárek Mill section

Sample

δ13C(‰ PDB)

δ18O(‰ PDB)

PM0

0.44

−3.40

PM2

0.23

−4.74

PM3

0.17

−4.14

PM4

0.01

−3.99

PM5

−0.15

−6.46

PM6.15

0.17

−3.76

PM7

0.27

−4.19

PM11.2

0.003

−5.02

PM12

0.04

−3.80

PM13

−0.07

−4.01

PM14

−0.27

−3.89

PM15.3

−0.27

−3.66

PM16

−0.53

−2.87

PM18.1

−0.50

−2.44

PM20.1

−0.79

−7.29

PM21

−0.66

−6.46

PM21.5

−0.54

−2.75

PM22

−0.42

−2.80

PM23

−0.48

−3.09

PM25.15

0.30

−2.08

PM27.6

−1.42

−3.92

PM28.5

−1.45

−3.81

PM31

−1.30

−3.26

Sample numbers PM0–PM31 correspond to meterage. The base of N. elegans Zone corresponds to 27 m

XRD results

Results of semiquantitative X-ray diffraction analyses shown in Table 2 indicate a multiple increase in clay mineral content (illite, corresponding to mica-type minerals in the analysis results), potassium feldspar and plagioclase (albite) recorded from level 30 m onwards, which corresponds to the petrographic observations (increase in clastic material). The absence of kaolinite and smectite suggests a strong diagenetic overprint of the primary clay assemblages. The rather uniform SiO2 content corresponds either to clastic quartz or more probably to biologically derived quartz (radiolarians and sponge spicules). The CaCO3 record is linked to the content of shelly fauna.
Table 2

Results of the whole-rock (semiquantitative) XRD analyses (in weight %) of the Zlíchov Limestone and Daleje Shale from the Pekárek Mill section

Sample

Calcite

Chlorite

Micaa

Quartz

Plagioclaseb

K-Feldspar

Smectitec

3.6–3.7

9

2.5

2.5

83

2.5

0.5

 

5.7

9

5

7

71

6

1

 

8.1

62

1

0.5

35

1.5

  

9.9

43

2

3

49

3

  

13.9

6.5

6

10.5

67

8

1.5

0.5

25–25.15

87.5

1

2

7

2.5

  

30.1

16

5

15

49

11

2

2

32.1

17.5

7.5

17

46

11

1

 

33.1

21

7.5

15

42

11.5

1

2

34.1

16

9

18

43

11

1

2

36

21

7.5

16

42

10.5

1

2

36.8

72

2.5

5.5

15

4.5

0.5

 

37.1

20

9

15.5

43

10

1.5

1

Sample numbers correspond to meterage. The base of N. elegans Zone corresponds to 27 m

aStructure model of muscovite was used for the calculation. Dioctaedric/trioctaedric mica cannot be reliably distinguished

bMineral from the albite-anorthite series. Data suggest Na-rich end member (albit)

cMineral from the smectite group

Microfacies analysis

The studied section is characterised by the succession of thin- to medium-bedded limestones, irregularly intercalated by mostly thin-bedded calcareous shales. Sedimentary textures and fossil content are similarly developed in both the carbonates and the intercalated shales. Silicification (chert nodules) is common, especially in the lower part of the section.

Interval 0–23.6 m, Zlíchov Limestone, beds with Criteriognathus steinhornensis (Ziegler, 1956)

The recorded microfacies is rather monotonous. Bioturbated (Chondrites ichnofabric) radiolarian wackestones with rare fragments of benthic (trilobites, echinoderms, calcitised sponge spicules), nektobenthic (ostracods) and planktic fauna (dacryoconarids) are the characteristic microfacies type recorded in the lower part of the succession (Fig. 5a–c). The matrix is microsparitic, with recrystallisation mostly associated with burrows, typically with peloid clusters, most probably representing calcified radiolarians in origin (Fig. 5b). The Chondrites ichnofabric is mottled, displaying discrete burrows. Except for calcitised radiolarians, fossil remains are rare and sparsely scattered. Trilobite exoskeletons, crinoid ossicles, ostracods and dacryoconarids represent the main bioclast types. Dacryoconarids, which only form a subordinate component (5–10%) in the lowermost part of the section, increase in number in the overlying beds (from 6 m onwards). The increase in abundance of dacryoconarids takes place along with a decrease in the abundance of radiolarians, which were only recorded sporadically higher up (from 26 m onwards). Micritic sedimentation was interrupted by episodic deposition of graded crinoidal grainstones (calciturbidite bed) with ostracods, trilobites, brachiopods and dacryoconarids, typically with syntaxial rim cements. This microfacies type (MF 2, Fig. 5g) was recorded in the lower part of the section (7.25–7.33 m), in beds with C. steinhornensis).
Fig. 5

Microfacies association of the Pekárek Mill section. a MF 1, bioturbated radiolarian wackestone with calcitised radiolarians and hexactinellid sponge spicule (central part). Note the recrystallisation of micritic matrix linked to burrows. Middle part of the Zlíchov Lm., beds with Criteriognathus steinhornensis. b MF 1, peloids (arrows) representing calcified radiolarians in origin. Middle part of the Zlíchov Lm., beds with Criteriognathus steinhornensis. c MF 1, bioturbated skeletal wackestone with fragmentary preserved dacryoconarids, trilobite (central part) and unidentified small-size skeletal debris. Middle part of the Zlíchov Lm., beds with Criteriognathus steinhornensis. d MF 3, bioturbated dacryoconarid packstone with fragmentary preserved trilobites and thin-walled bivalves, upper Zlíchov Limestone. Bed with Polygnathus perbonus and Criteriognathus steinhornensis (upper gronbergi Zone), N. barrandei Zone. e MF 3, bioturbated dacryoconarid packstone with fragmentary preserved shells of probably thin-walled bivalves and nautiloids. Arrows point to telescoped tentaculite shells. Upper Zlíchov Limestone, bed with Polygnathus perbonus and Criteriognathus steinhornensis (upper gronbergi Zone), N. barrandei Zone. f MF 3, bioturbated dacryoconarid wackestone with nautiloid and gastropod shell remains. Note the concentric arrangement of tentaculite shells in burrows. Uppermost Zlíchov Limestone (uppermost N. barrandei Zone). g MF 2, well-sorted crinoidal grainstone, uppermost Zlíchov Limestone (uppermost N. barrandei Zone). h MF 4, argillaceous skeletal wackestone with dacryoconarid (d) and unidentified small-size skeletal debris. Zlíchov Limestone-Daleje Shale transition

Interpretation:

The relatively low rate of sediment reworking here is ascribed to low epifaunal and infaunal activity. Skeletal remains of benthic organisms are rare, not exceeding 3 individuals per thin section. The scattering of larger skeletal fragments (e.g. trilobite exoskeletons) points to a low horizontal redistribution and is evidence of a low-energy sedimentary environment. The minimum of bioerosion observed here is also interpreted as a result of the deficiency of bioeroders rather than of insufficient exposure time on the seafloor. The sedimentary environment here is not assumed to represent a favourable environment for benthic colonisation. The only proliferating groups were zooplankton (radiolarians and dacryoconarids), nektobenthic ostracods, polychaete worms (evidenced by scolecodonts) and Chondrites. Taken together, the environment is interpreted as deep, calm, non-agitated and oxygen-stressed.

Interval 24.5–26.5 m, Zlíchov Limestone, upper N. barrandei Zone, beds with Polygnathus perbonus (Philip, 1966) and C. steinhornensis

Bioturbated tentaculite wacke- to packstones with a high component content are the characteristic microfacies type (MF 3, Fig. 5d–f). Dacryoconarids, crinoid ossicles, ostracods and trilobite exoskeletons represent the most common bioclast types; these are followed by bivalves, brachiopods, gastropods, nautiloids, ammonoids and rare rugose corals. Larger shells are preserved as fragments. Only very few bored shell fragments were observed. No micritisation was recorded. The telescoping of tentaculite shells is common. Locally, a grainstone texture is developed within this microfacies. Burrows of Chondrites are still present, although a homogenous ichnofabric prevails. On the top of this unit, a graded crinoidal grainstone with ostracods, trilobite exoskeletons, ammonoids and nautiloids was deposited.

Interpretation:

The diverse benthic, nektonic and planktic communities that are present; the packing of the components; as well as the intensity of sediment reworking (telescoping of tentaculite shells, local development of grainstone texture, homogenous ichnofabric) are suggestive of improved conditions on the seafloor in an oxygenated and more agitated environment (also see the decrease in organic walled microfossil abundance, Fig. 4). The allochthonous crinoidal grainstone on top represents calciturbidite deposited at the end of the shallowing cycle.

Interval 26.8 –37 m, uppermost Zlíchov Limestone and lower part of the Daleje Shale, N. elegans Zone

Bioturbated (Chondrites ichnofabric), recrystallised skeletal wackestones, locally developed as lime-mudstones, contain only sporadically dacryoconarids, trilobites, ostracods, radiolarians and thin-walled bivalves (MF 4, Fig. 5h). Most bioclasts are preserved as fragments. The matrix is microsparitic. Bioturbation is distinctive; the degree of reworking is lower than in the underlying beds. A higher clastic content in both the carbonate and the intercalated pelitic facies was recorded from the bed 27 m onwards, which was also confirmed by XRD analysis (see above).

Interpretation:

Facies development points at a more distal depositional environment. The progressive deepening of the basin is associated with a greater terrigenous input.

Scolecodonts

In general, there are only a handful of papers dealing with Lower Devonian scolecodonts. The pioneering study was published by Lange (1949), who studied Emsian scolecodonts from the Ponta Grossa Formation of the Paraná Basin, Brazil (Malvinokaffric cold water realm). The assemblage reported by Lange (1949) is monospecific, with a single paulinitid species, Paulinites paranaensis Lange, 1947. Lange’s materials were later restudied by Eriksson et al. (2011) who resampled the localities using modern microfossil extraction methods, ending up with similar results, and reporting a single species in the collection. Following Lange’s work, the Lower Devonian scolecodonts were reported by Taugourdeau (1968), who studied Silurian to Carboniferous scolecodonts from the Sahara, and Ye (1994), who described Silurian to Devonian scolecodonts from the Qinling Mountains; however, both of these authors used an outdated taxonomical approach. Suttner and Hints (2010) described a small and poorly preserved collection of imprecisely dated Devonian scolecodonts from the Tyrnaueralm, Graz Palaeozoic, Austria. More recently, Szaniawski and Drygant (2014) published a well-preserved collection from the Lochkovian of Podolia, Ukraine, and Tonarová et al. (2016) studied a rich Eifelian scolecodont collection from the Eifel region, Germany. The latter two papers provided information on taxa that are comparable with specimens from the Pekárek Mill section. There is only a single brief study on Devonian scolecodonts from the Prague Basin by Šnajdr (1951), who described findings of two species (Kettnerites langei Šnajdr, 1951, and Staurocephalites sp. A) from the Zlíchov Limestone coming from the Zabitá rokle locality near Řeporyje.

The most common families in the Pekárek Mill section are Paulinitidae with an eulabidognath jaw apparatus and Polychaetaspidae with a labidognath apparatus; these are followed by ctenognath Tetraprionidae, prionognath Skalenoprionidae and placognath Mochtyellidae. Some of the maxillae may also belong to the placognath family Xanioprionidae. In general, the placognath and ctenognath jaws are mainly preserved as fragments, and thus, their determinations are only tentative. Several maxillae could not be identified even at the family level (Figs. 6ad, aq and 7d, g, h, l, n), and their affinity can only be resolved by additional sampling. The family-level composition of the assemblage is rather similar to the better studied Ordovician and Silurian faunas, except for the strong dominance of paulinitids in some samples. It seems that paulinitids are relatively more common in the Prague Basin assemblages than in the other studied regions, such as Baltica (Šnajdr 1951; Tonarová et al. 2012; Eriksson et al. 2013). Another explanation might be that they are relatively more abundant in the whole peri-Gondwanan realm, as already described for the Late Ordovician of Saudi Arabia (Hints et al. 2015). The monospecific paulinitid assemblage in the Emsian Ponta Grossa Formation, Brazil (Lange 1949; Eriksson et al. 2011), may also support this idea.
Fig. 6

Photomicrographs of selected scolecodonts from the Zlíchov Limestone and Daleje Shale; all specimens are in dorsal view. The scale bar corresponds to 100 μm. av Paulinitids. wz Paulinitid/kielanoprionid maxillae. aaac Paulinitid? maxillae. aeap Polychaetaspids. ad, aq Affinity unknown. akKettnerites sp. 1: a left MI, sample PM19.9, specimen PT53A.1; b left MI, sample PM19.9, specimen PT53A.2; c right MI, sample PM13.9, specimen PT48A.1; d right MI, sample PM19.9, specimen PT53A.3; e right MI, sample PM19.9, specimen PT53A.4; f right MI, sample PM26.6, specimen PT65A.1; g right MI, sample PM29.1, specimen PT74A.1; h right MII, sample PM27.9, specimen PT70A.1; i right MII, sample PM0, specimen PT40A.1; j right MII, sample PM26.6, specimen PT65A.2; k right MII, sample PM21.9, specimen PT55A.1. ltHindenites sp.: l left MI, sample PM15.3, specimen PT49A.1; m left MI, sample PM27.9, specimen PT70A.2; n right MI, sample PM13.9, specimen PT48A.2; o right MI, sample PM19.9, specimen PT53A.5; p right MI, sample PM33.1, specimen PT78A.1; q right MII, sample PM27.9, specimen PT70A.3; r right MII, sample PM29.1, specimen PT74A.2; s left MII, sample PM13.9, specimen PT48A.3; t left MII, sample PM17.9, specimen PT51A.1. u, vKettnerites sp. 2: u left MI, sample PM26.6, specimen PT65A.3; v right MI, sample PM3.7, specimen PT42A.1. wz Paulinitid/kielanoprionid maxillae: w left MI, sample PM9.9, specimen PT45A.1; x left MI, sample PM29.1, specimen PT74A.3; y right MI, sample PM15.3, specimen PT49A.2; z left MI, sample PM27.9, specimen PT70A.4. aaac Paulinitid? maxillae: aa right MI, sample PM33.1, specimen PT78A.2; ab left MI, sample PM19.9, specimen PT53B.1; ac right MI, sample PM0, specimen PT40A.2. ad Left MI of affinity unknown, sample PM13.9, specimen PT48A.4. aeap Polychaetaspid maxillae belonging to the genus Oenonites: ae left MI, sample PM3, specimen PT41A.1; af left MI, sample PM3.7, specimen PT42A.2; ag left MI, sample PM0, specimen PT40B.1; ah right MI, sample PM21.9, specimen PT55A.2; ai right MI, sample PM19.9, specimen PT53A.8; aj left MI, sample PM29.1, specimen PT74A.4; ak right MI, sample PM19.9, specimen PT53A.6; al right MI, sample PM0, specimen PT40A.3; am right MI, sample PM19.9, specimen PT53A.7; an right MI, sample PM21.9, specimen PT55A.3; ao right MI, sample PM17.9, specimen PT51A.3; ap right MI, sample PM26.8, specimen PT66A.1. aq Right MI of affinity unknown, resembling ramphoprionid maxilla, sample PM17.9, specimen PT51A.2

Fig. 7

Photomicrographs of selected scolecodonts (ao), prasinophytes (pw), conodonts (wy) and dacryoconarid tentaculites (z, aa) from the Zlíchov Limestone and Daleje Shale. All scolecodonts are in dorsal view, except for e, f, g, h and k that are in lateral view. The scale bars corresponds to 100 μm, except figures z1 and aa1 where it refers to 1 mm, and they refer to all subsequent specimens before the next scale bar appears. a, c, e, f, i Mochtyellids. d, g, h, k, l, n Maxillae of affinity unknown. m Skalenoprionids. b, j, o Tetraprionids?. a Partly fused mochtyellid? apparatus, sample PM36.1, specimen PT81A.1. b Partly fused tetraprionid? apparatus, sample PM36.1, specimen PT81A.2. c Partly fused mochtyellid apparatus, sample PM19.9, specimen PT53B.2. d Partly fused placognath apparatus, sample PM19.9, specimen PT53A.9. eMochtyella sp., left MI, sample PM26.6, specimen PT65B.1. fMochtyella sp., left MI, sample PM19.9, specimen PT53A.10. g Placognath maxilla, sample PM13.9, specimen PT48A.5. h ?Placognath maxilla, sample PM36.1, specimen PT81A.3. iPistoprion? sp., left MI, sample PM25.15, specimen PT59A.1. j Tetraprionid? maxilla, sample PM26.5, specimen PT64A.1. kLunoprionella sp., sample PM27, specimen PT67A.1. l Laeobasal plate? of affinity unknown, sample PM0, specimen PT40A.4. mSkalenoprion sp., left MI, sample PM16, specimen PT50A.1. n Second? maxilla of affinity unknown, sample PM13.9, specimen PT48A.6. o Tetraprionid? maxilla, sample PM19.9, specimen PT53A.11. pv Prasinophytes: p sample PM0, specimen PT40A.5; q sample PM3, specimen PT41A.2; r sample PM3.7, specimen PT42A.3; s sample PM24.55, specimen PT58A.1; t sample PM24.55, specimen PT58A.2; u sample PM26.4, specimen PT63A.1; v sample PM26.4, specimen PT63B.1. wy Conodonts from the uppermost Zlíchov Limestone, sample PM25.15: w1, w2Criteriognathus steinhornensis, upper and lower view of specimen SB45; x1, x2Polygnathus perbonus, upper and lower view of specimen SB44; yPseudooneotodus beckmanni, upper view of specimen SB46. z, aa Dacryoconarid tentaculites, the scale bar is different for each figure: z1, z2 Nowakia (N.) barrandei PM26.6 (=CH16); z1 whole shell, scale bar 1 mm; z2 detail of the sculpture, scale bar 100 μm; aa1, aa2 Nowakia (N.) elegans, sample PM27 (=CH18); aa1 whole shell, scale bar 1 mm; aa2 detail of the sculpture. Scale bar 100 μm

The most abundant family, Paulinitidae, is represented by two genera. The first one, Kettnerites Žebera, 1935 seems to be represented by at least two species Kettnerites sp. 1 (Fig. 6a–k) and Kettnerites sp. 2 (Fig. 6u, v). Kettnerites sp. 1 looks similar to “Kettneriteshuberti Bergman, 1987, documented from the Lochkovian of Podolia, Ukraine (Szaniawski and Drygant 2014), and K. aff. huberti from the Eifelian of the Eifel region (Tonarová et al. 2016). The Devonian species are almost indistinguishable from the Silurian K. huberti, described from the Wenlock to Ludlow of Gotland, Sweden (Bergman 1989). The latter species was described as an eurytopic long-ranging taxon, found especially in deep water marls and argillaceous limestone (Bergman 1989, p. 25). There is also a resemblance to the species Pa. paranaensis, but the first maxillae denticulation pattern of the latter species is very different. Kettnerites sp. 2 has more slender first maxillae than Kettnerites sp. 1, with dense, thin denticles. The maxillae of K. langei have a slightly similar appearance, but it was described based only on badly preserved rock surface material that does not allow for a definitive comparison. The second paulinitid genus is Hindenites Bergman, 1987, and the specimens are assigned to Hindenites sp. (Fig. 6l–t). This species may be similar to Hindenites sp. 1 from the Eifelian of the Eifel region (Tonarová et al. 2016). Other paulinitid maxillae (Fig. 6aa–ac) in the collection could not be determined due to insufficient numbers of specimens and/or their poor preservation.

The second most common family, Polychaetaspidae, is represented by several species belonging to the genus Oenonites Hinde, 1879; however, distinguishing them is complicated, and at this stage of our study, they remain identified only as Oenonites spp. (Fig. 6ae–ap) until more material is gathered.

The maxillae of the family Mochtyellidae are commonly broken, probably because of their more fragile constitution. One maxilla showing a striking resemblance to the posterior jaw of the genus Pistoprion Kielan-Jaworowska, 1966, common in the Ordovician and Silurian, was assigned to Pistoprion? sp. (Fig. 7i). This would mean that, up to now, it is the youngest occurrence of this genus. Other maxillae were identified as mochtyellids (Fig. 7a, c) or as Mochtyella sp. (Fig. 7e, f), even though the assignment is questionable because of missing secondary ridges that are typical for the compound maxillae of this genus.

Especially in the Zlíchov limestone, the majority of samples also contain representatives of the family Skalenoprionidae, Skalenoprion sp. (Fig. 7m), although in low numbers. The jaws are small (around 100 μm), and the number of denticles was only 3–4, which is lower than in the older strata (compare with Kielan-Jaworowska 1966) but similar to Skalenoprion sp. 1 from the Eifelian (Tonarová et al. 2016). Therefore, it could be an evolutionary trend. On the other hand, an atypical denticulation also occurs on the fang of the first maxillae, which has never been observed before.

Several specimens were assigned to the family Tetraprionidae (Fig. 7b, j, o), but their further identification was not possible. Three specimens belong to Lunoprionella sp. (Fig. 7k), which is a genus of unknown family affinity. This genus ranges from the Ordovician to the Devonian, with the first documented Devonian record from the Pekárek Mill section. Some maxillae reflect transitional characteristics between the families Paulinitidae and Kielanoprionidae (Fig. 6w–z). The indisputable record of the latter family is known from the Eifelian (Middle Devonian; Tonarová et al. 2016).

The abundance of scolecodonts is quite stable throughout the section (approximately 1 posterior maxilla per gram of rock) but lower than the abundance of chitinozoans (Fig. 4). On average, 4 species were found per sample (Fig. 3), and the total number of species identified in the section is at least 11. There are no obvious changes in either abundance or diversity that could be connected with the Daleje Event, which might confirm the opportunistic behaviour of jawed polychaetes (Tonarová et al. 2014). Nevertheless, the quantitative data may be biased due to a relatively small sample size when compared to other studies. The majority of scolecodonts are isolated maxillae, but five partly fused apparatuses were also recovered (Fig. 7a–d). The length of the maxillae generally varies from 100 to 300 μm, only occasionally reaching 500 to 600 μm (in samples PM0 and PM19.9 even to 1 mm). Based on a large collection of paulinitids from Gotland, Bergman (1989) suggested that a low diversity fauna dominated by small specimens and often coupled with low abundances indicates a relatively deeper water environment.

Conodonts

Conodonts are extremely rare in both abundance and diversity. Only 14 specimens of C. steinhornensis (Ziegler, 1956) (Fig. 7w), 51 specimens of Pseudooneotodus beckmanni (Bischoff and Sannemann, 1958) (Fig. 7y), a few fragments of Belodella sp. and two Pa elements of Po. perbonus (Philip, 1966) (Fig. 7x) were recorded. Most of the conodonts were recovered from sample 25.15, thus from the shallowest facies. No conodonts were extracted from calcareous shales. The occurrences of C. steinhornensis and Po. perbonus may be regarded as a confirmation of the gronbergi conodont Zone due to the association of the latter with Polygnathus gronbergi in the Prague Basin as reported by Klapper et al. (1978) as well as in other regions (e.g. Klapper and Johnson 1980). No conodonts were recorded from the N. elegans tentaculite Zone in this study, which hampers any allocation of the environmental changes recorded along with the conodont zonation.

Tentaculites

Research on dacryoconarid tentaculites has been focused on a very short interval from 26.6 to 27 m (corresponding to CH16 to CH18 sensu Chlupáč and Lukeš 1999) and its shale interbeds (i.e. an interval about 0.5 m thick). The range of this interval comes out of the previous work done by Pavel Lukeš (Chlupáč and Lukeš 1999), who recorded the disappearance of Nowakia (N.) barrandei Bouček and Prantl, 1959 (Fig. 7z(1, 2)) and the appearance of N. (N.) elegans Barrande, 1867 (Fig. 7aa(1, 2)) within this level. His work was, however, constrained to only the material from limestone beds and lenses within this level. Introduction of a new method using the surfactant Rewoquat (Jarochowska et al. 2013) allowed obtaining new data from the shale interbeds of this narrow interval.

The occurrence of N. (N.) barrandei in the limestone bed at 26.6 m (CH16) comprises dozens of excellently preserved specimens of this taxon in approximately 1 kg of material. The last occurrence of this taxon was recorded in the shale layer at 26.7 m, with more than 20 easily ascertainable specimens per 0.5 kg of the rock.

Three kilograms of rock of the subsequent limestone bed at 26.8 m (CH17) yielded only one specimen N. cf. elegans without any other determinable specimens of the subgenus Nowakia (Nowakia).

The first appearance of N. (N.) elegans falls into the shaly interval, which lies just above the limestone bed at 26.8 m (CH17). Ten easily determinable fragments of the shells of N. (N.) elegans were obtained from 0.5 kg of shale. The presence of a limestone bulge on the top of CH17 with a well-preserved specimen of N. (N.) elegans showed that the first appearance is not only restricted to shaly layers. The presence of N. (N.) elegans is accompanied by a large number (more than 100) of fragments of proximal shell parts, which clearly belong to the subgenus Nowakia (Nowakia); however, due to their fragmentation and poor preservation, it is impossible to assign them either to N. (N.) elegans or to N. (N.) barrandei, nor to treat them as a new species. More samples from different localities are needed to resolve this problem.

Ferrová et al. (2012) documented co-occurrences of adjacent index tentaculite biozones (e.g. N. (N.) barrandei + N. (N.) elegans; N. (N.) elegans + N. (N.) cancellata) in the Daleje Event interval. However, previously published tentaculite biozonal charts were used as the taxon-range zones. To solve this problem, Ferrová et al. (2012) revised the tentacutite biostratigraphy of the Daleje Event interval and emended all of the above-mentioned biozones (i.e. the N. (N.) barrandei, N. (N.) elegans and N. (N.) cancellata biozones) as interval zones (in accordance with the International Stratigraphic Guide, Chapter 7, Section 3; Salvador 2013). In addition, Ferrová et al. (2012) established a new barrandei-elegans Subzone within the N. (N.) elegans Interval Zone. The lower boundary of the barrandei-elegans Subzone was defined by the first occurrence of N. (N.) elegans, and its upper boundary by the last occurrence of N. (N.) barrandei. Therefore, the Subzone represents a typical concurrent-range biozone (in accordance with the International Stratigraphic Guide, Chapter 7, Section D2b; Salvador 2013). The common occurrence of N. (N.) barrandei and N. (N.) elegans was recognised not only in the Prague Basin (Chlupáč et al. 1979; Ferrová et al. 2012) but also at South Tien Shan (Kim et al. 1978; Kim 2011; Frýda, unpublished data) as well as in Spain (Montesinos and Truyols-Massoni 1987; García-Alcalde et al. 1988). Ferrová et al. (2012) noted that the lack of densely sampled sections within the Daleje Event interval and the low number of tentaculite specialists are probably the main reasons for the missing records of the co-occurrences of N. (N.) elegans and N. (N.) barrandei from other areas around the world. Nevertheless, at present, the barrandei-elegans Subzone, representing a rather short time interval (see Ferrová et al. 2012), is a more precise biostratigraphic tool within the Daleje Event interval, not only within the Prague Basin but also among some palaeocontinents.

However, the presence of this subzone was not confirmed at the Pekárek Mill section studied herein. Therefore, there might be a short stratigraphical gap or period with extremely low sediment accumulation rates concerning at least the barrandei-elegans Subzone on the Pekárek Mill section. This gap might be placed on the boundary between the shale interval at 26.7 m and the limestone bed at 26.8 m (CH17). However, two problems have to be taken into account. The first is the absence of data from the CH17 layer and the second is the presence of numerous proximal parts of shells certainly belonging to the subgenera Nowakia (Nowakia) from the shale interval at 26.9 m. An evaluation of this new material consisting of proximal parts would best be accomplished with a comparative study of the material from other localities.

Prasinophytes

Prasinophytes (Fig. 7p–v) are abundant, especially in the transition interval from the Zlíchov Limestone to the Daleje Shale and in the lower part of the Daleje Shale (sample 24.5 m, interval 28–29 m, and above 35 m; Fig. 4). Their abundance in the most productive samples reached over 200 specimens per gram of rock. On the other hand, other microfossils, such as chitinozoans and scolecodonts, are very rare in these samples. The diameters of the prasinophytes also increase towards the Zlíchov Limestone/Daleje Shale boundary (reaching over 1 mm in sample PM24.5). These phenomena could be associated with changes in the oceanic circulation resulting from a transgression and eutrophication of the basin due to increased terrigenous input, which was herein also recorded close to the Zlíchov Limestone/Daleje Shale boundary (for comparison, see Vodrážková et al. 2013). Moreover, Tappan (1980, 1986) already regarded prasinophytes as a “disaster species” because they appear to prefer specific environmental conditions and are generally more abundant when other phytoplankton is absent.

Chitinozoans

The assemblage of chitinozoans is not very diverse with only one to seven species per sample (Fig. 3). The abundance of chitinozoans is variable throughout the section (Fig. 4). It is higher in the calcareous shale intercalations than in the limestone. The well-known higher abundance of chitinozoans in low-energy siliciclastic environments (e.g. Paris 1981a, b) was also confirmed by the Lower Devonian associations of eastern Canada (Achab et al. 1997). The differential compaction of the sediments could also have had an influence on the relative abundance of the species in the different levels. Thus, the shale beds contain a higher number of chitinozoans than in limestone (Paris 1981a, b). The approximate abundance value is lower than 5 chitinozoans per gram of rock, although especially in samples with Bursachitina Taugourdeau, 1966, it increases to over 40 specimens per gram. The latter values may be less precise because of problematic counting. Similarly, the abundance values are only approximations in samples with prevailing prasinophytes, as they could have covered some of the vesicles of the chitinozoans. The preservation state of chitinozoans is, as in other microfossils, moderate to well, with the best preservation in the limestone layers. Ornamented specimens such as Ancyrochitina spp. are commonly laterally compressed and appendices are broken off, which hampers their determination (Fig. 4). On the other hand, Bursachitina riclonensis (Paris, 1981b), a simple chitinozoan, is easier to identify even when the preservation of the specimens is poor. Therefore, the difficulty in the identification of these species can have an influence upon the determination of their diversity (Paris 1981a, b).

In the Zlíchov Limestone, the prevailing genera are Ramochitina Sommer and van Boekel, 1964, Angochitina Eisenack, 1931 and Bursachitina (Fig. 3). The most common species are Ramochitina marettensis (Paris, 1981b) (Fig. 8d, f, s, z, ai), Angochitina milanensis Collinson and Scott, 1958 (Fig. 8a–c, j, l), Angochitina devonica Eisenack, 1955 (Fig. 8n, q, al), Bursachitina sp. A sensu Paris et al. 2000 (Fig. 8o, p) and Bur. riclonensis (Fig. 8h, u, w–y, ab–ad, af, an). Ang. milanensis and Ang. devonica were considered synonyms by Urban (1972) and Urban and Newport (1973) in the Givetian of Laurussia (USA).
Fig. 8

Photomicrographs of selected chitinozoans from the Pekárek Mill section. The scale bar corresponds to 100 μm. aAngochitina milanensis, sample PM0, specimen PT40A.6. bAngochitina milanensis, sample PM0, specimen PT40A.7. cAngochitina milanensis, sample PM0, specimen PT40A.8. dRamochitina marettensis, sample PM3, PT41A.3. eAngochitina aff. globosa, sample PM3, PT41A.4. fRamochitina marettensis, sample PM3, PT41A.5. gAngochitina aff. pilosa, sample PM13.9, specimen PT48A.7. hBursachitina aff. riclonensis, sample PM16, specimen PT50A.2. i ?Bulbochitina sp., sample PM17.9, specimen PT51A.4. jAngochitina milanensis, sample PM17.9, specimen PT51A.5. kAncyrochitina aff. tumida, sample PM19.9, specimen PT53B.3. lAngochitina milanensis, sample PM19.9, specimen PT53B.4. mAncyrochitina aff. tumida, sample PM19.9, specimen PT53A.12. nAngochitina devonica, sample PM21.5, specimen PT54A.1. oBursachitina sp. A sensu Paris et al. (2000), sample PM21.5, specimen PT54A.2. pBursachitina sp. A sensu Paris et. al. (2000), sample PM21.9, specimen PT55A.4. qAngochitina devonica?, sample PM23, specimen PT56A.1. rBursachitina bursa, sample PM26.4, specimen PT63A.3. sRamochitina marettensis?, sample PM26.4, specimen PT63A.4. tAncyrochitina sp., sample PM26.4, specimen PT63A.5. uBursachitina riclonensis?, sample PM26.5, specimen PT64A.2. vBursachitina bursa?, sample PM26.5, specimen PT64A.3. wBursachitina riclonensis, sample PM26.6, specimen PT65A.4. xBursachitina riclonensis, sample PM26.6, specimen PT65A.5. yBursachitina riclonensis, sample PM26.6, specimen PT65A.6. zRamochitina marettensis?, sample PM26.6, specimen PT65A.7. aaRamochitina sp., sample PM26.6, specimen PT65A.8. abBursachitina riclonensis, sample PM26.6, specimen PT65C.1. acBursachitina riclonensis, sample PM26.8, specimen PT66A.2. adBursachitina riclonensis, sample PM26.8, specimen PT66A.3. aeBulbochitina bulbosa?, sample PM26.8, specimen PT66A.4. afBursachitina riclonensis, sample PM27, specimen PT67B.1. agRamochitina sp., sample PM27, specimen PT67A.2. ah ?Angochitina sp., sample PM27, specimen PT67B.2. aiRamochitina marettensis?, sample PM27, specimen PT67A.3. ajAncyrochitina parisi?, sample PM27.9, specimen PT70A.5. akAncyrochitina aff. parisi, sample PM27.9, specimen PT70A.6. alAngochitina devonica, sample PM29.1, specimen PT74A.5. amDesmochitina sp., sample PM33.1, specimen PT78A.3. anBursachitina riclonensis, sample PM33.1, specimen PT78A.4. aoBursachitina sp., sample PM35.1, specimen PT80A.1. apEisenackitina parkerae, sample PM35.1, specimen PT80A.2. aqAngochitina sp., sample PM36.1, specimen PT81B.1. arAngochitina sp., sample PM36.1, specimen PT81B.2. asAncyrochitina sp, sample PM36.1, specimen PT81B.3. atRamochitina magnifica?, sample PM36.1, specimen PT81A.4. auRamochitina magnifica?, sample PM36.1, specimen PT81B.4

R. marettensis was described from the upper Emsian of the Armorican Massif, France (Paris 1981a, b), and was also recorded from the Emsian of eastern Canada (Achab et al. 1997). Bursachitina sp. A was recorded from the Emsian (Paris et al. 2000) and Bur. riclonensis has a long record from the lower Emsian to the Eifelian (Paris et al. 2000). These Bursachitina species and Ang. devonica are common components of the global Bursachitina bursa Biozone, mostly recorded from northern Gondwana, and tentatively assigned to the lower Emsian (Paris et al. 2000). Bur. bursa (Taugourdeau and de Jekhowsky, 1960) (Fig. 8r, v), originally described from the Lower Devonian of Algeria (Taugourdeau and de Jekhowsky, 1960), occurs near the Zlíchovian/Dalejan boundary and disappears at the boundary.

Starting with the Daleje Shale, the number of representatives of the genus Ancyrochitina Eisenack, 1955 is found to be increasing (in some samples, this genus represents the prevailing component of the assemblage; e.g. sample PM29.1). Ancyrochitina parisi Volkheimer et al., 1986 (Fig. 8aj, ak) appears in this unit. It was recorded from the upper Emsian of southwestern Gondwana (Argentina, Bolivia and Brazil) (Volkheimer et al. 1986; Grahn 2005; Grahn et al. 2013). Ramochitina magnifica? (Fig. 8at, au) and Bulbochitina bulbosa Paris, 1981a (Fig. 8ae) are present in the uppermost studied level of the Daleje Shale. The species R. magnifica Lange, 1967 is indicative of the Ramochitina magnifica Biozone of the Pragian of southwestern Gondwana (Grahn 2005; Grahn et al. 2013), but the documented stratigraphic range is until the upper Emsian (Paris et al. 2000); therefore, our finding probably does not represent the FAD of the species in the region. Bul. bulbosa is a Pragian-basal Emsian species, which indicates the upper Pragian Bulbochitina bulbosa Biozone in northern and eastern Gondwana as well as in Laurussia (Paris et al. 2000). Its presence also needs to be confirmed from other sections before a conclusion on its longer stratigraphical range can be made. There is no evidence for reworked material in the Daleje Shale.

The high predominance of Ancyrochitina spp., observed in some levels of Emsian carbonate deposits of the Armorican Massif, has been suggested as being related to the presence of superficial currents, with the input of a sudden and ephemeral influx of plankton, or of modifications in the marine biochemistry (Paris 1981a, b). In the Armorican section, ornamented species such as Ancyrochitina spp. are the only component in some beds, whilst Bur. riclonensis, a species without long processes, shows minor dependence to the lithology throughout the section. This observation was in agreement with the planktic nature of the chitinozoans (Paris 1981a, b). In the Lower Devonian assemblages of eastern Canada, species adapted to carbonate-dominated sedimentation were susceptible to facies variations with the preservation of large and bulbous forms in the shallower lithofacies (Achab et al. 1997). The varied distribution of the Bursachitina and Ancyrochitina throughout the Pekárek Mill section can respond to some of these facies changes, considering abundance zones of 99% of the assemblage for Bursachitina in the upper part of the Zlíchov Limestone unit, from PM21.9 to PM27, and for Ancyrochitina in the uppermost part of the Zlíchov Limestone (shale beds) and the Daleje Shale units, from PM27.8 to PM29.1 (Fig. 3).

Discussion

In the studied section, the Zlíchov Limestone displays a rather uniform microfacies (except for the uppermost part), being mostly skeletal wackestones with radiolarians and dacryoconarids; only rarely, they contain the remains of benthos and ubiquitous Chondrites ichnofabric. The abundance of dacryoconarids and radiolarians varies throughout the section; starting from the interval around 24 m (upper N. barrandei Zone), tentaculites prevail and radiolarians are either absent or occur in small abundances. At the same time, current activity is increasing as indicated by the presence of skeletal packstones with a locally developed grainstone fabric; the telescoping of shells; and diverse planktic, nektic and benthic assemblages. Also, abundant conodonts (mainly Ps. beckmanni) were recovered from this interval, which is in contrast to the underlying beds where conodonts are very rare. Based on these findings, we interpret it as a regressive phase in the upper Zlíchov Limestone, culminating in the uppermost N. barrandei Zone with deposition of well-sorted crinoidal graded grainstones (calciturbidites) at 26.5 m. Wackestones, locally developed as lime-mudstones, with only rare occurrences of skeletal remains, were deposited at the base of the N. elegans Zone, which is interpreted herein as a deepening phase of the sea level. Higher amounts of silt were recorded during petrographical observations and proven by XRD analysis in both carbonate and pelitic facies in this interval. Interestingly, large and abundant prasinophytes were recovered, and the predominance of Ancyrochitina spp. was recorded, which might be connected to a higher nutrient load due to increased terrigenous input.

The interplay of bathymetric changes due to a sea-level rise at the base of the N. elegans Zone, together with increasing terrigenous input, supposedly connected with a higher nutrient load, is interpreted as the main cause of the changes in the chitinozoan assemblages recorded herein. These changes also coincide with faunal changes summarised by Chlupáč and Kukal (1988). Siliciclastic input also took place during the sedimentation of the Zlíchov Limestone (irregular alternation of limestone and argillaceous beds); it increased in the Zlíchov Limestone-Daleje Shale transition and took over in the Daleje Shale. The cyclic nature of the Zlíchov Limestone and Daleje-Třebotov Formation was interpreted by Chlupáč (2000) as a result of “climatic variations, which periodically affected the regime of water masses, oxygenation, production of CaCO3, clay supply…” (Chlupáč 2000, p. 120). Nevertheless, the character of terrigenous input and the source area is not specified herein; this is the aim of an ongoing study.

The transgressive-regressive phases recorded herein correlate well with the findings of Ferrová et al. (2012), who studied the uppermost Zlíchov Limestone and Chýnice Limestone and the lowest part of the Daleje Shale in the carbonate-dominated environment near Bubovice (located approximately 6 km to the west of the Pekárek Mill section). These authors recorded shallowing in the uppermost N. barrandei Zone and deepening at the base of the N. elegans Zone. The sea-level changes recorded herein and by Ferrová et al. (2012) differ from the interpretation of Chlupáč and Kukal (1988), who considered a gradual deepening from the Zlíchov Limestone onwards. On the other hand, it is in agreement with the results of García-Alcalde (1997) and Aboussalam et al. (2015) from Spain and Morocco, respectively (Upper Zlíchov Event). It is highly probable that a transgression at the base of the N. elegans Zone also took place elsewhere, but was lumped into a single event with a younger transgressive episode in the N. cancellata Zone (Daleje Event sensu stricto) due to problematic biostratigraphic correlations: the taxonomy of N. (N.) barrandei Bouček and Prantl, 1959 and N. (N.) elegans Barrande, 1867 was settled only recently (Ferrová et al. 2012) and conodonts from shaly facies are mostly absent.

Summary

The most important results of our study can be summarised as follows:
  • We recorded a regressive trend in the N. barrandei Zone and in the gronbergi conodont Zone, culminating in the uppermost N. barrandei Zone, with onset of the transgression from the base of the N. elegans Zone, representing the initial transgressive phase of the main flooding taking place in the N. cancellata Zone (Daleje Event sensu stricto). Our interpretation of sea-level change confirms the findings of Ferrová et al. (2012) from a shallow water area. It can be correlated with the “Upper Zlíchov Event” of García-Alcalde (1997) and Aboussalam et al. (2015).

  • The report presented here for the first time describes in greater detail Emsian chitinozoans and scolecodonts from the Prague Basin.

  • Scolecodonts show both a low abundance and diversity of jawed polychaetes throughout the section, without significant quantitative or qualitative changes across the event interval, thus confirming their reputation as opportunistic taxa. The low abundance and diversity, together with the small size of the scolecodonts, likely indicates a deeper water environment. The family-level composition of scolecodont assemblage confirms the dominance of paulinitids in the peri-Gondwanan realm.

  • The reported chitinozoan species can be correlated with, e.g. the Lower Devonian of the Armorican Massif, Libya, or other northern Gondwanan regions.

  • The results of our study suggest the greater importance of the base of the N. elegans Zone, marking the onset of the transgression linked with higher terrigenous input, changes in chitinozoan assemblages (extinction of Bur. bursa followed by a predominance of Anc. parisi in the lower N. elegans Zone) and changes in the size and abundances of prasinophytes. Similarly, the base of the N. elegans Zone seems to be at the level of distinct fauna turnover (end of Zlíchovian fauna; see Ferrová et al. 2012 for details) in the carbonate-dominated environment of the Prague Basin.

  • Taking all the above data into account, we follow Ferrová et al. (2012) and regard the base of N. elegans as a suitable candidate for the definition of the Emsian substage boundary.

Notes

Acknowledgements

This research was supported by the Czech Science Foundation through the Project No. P210/12/2018. This study is a contribution to IGCP 596 (Climate change and biodiversity patterns in the Mid-Palaeozoic – Early Devonian to Early Carboniferous). One of the authors (SV) wishes to acknowledge the support of Alexander von Humboldt Foundation as part of the study was carried out during her AvH Research Fellowship in Universität Erlangen-Nürnberg. The authors gratefully acknowledge the thorough reviews by C. Klug (Paläontologisches Institut und Museum, Universität Zürich), L. Slavík (Institute of Geology AS CR, Prague) and an anonymous reviewer.

Compliance with ethical standards

Conflict of interest

The authors declare that they have no conflict of interest.

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Copyright information

© Senckenberg Gesellschaft für Naturforschung and Springer-Verlag Berlin Heidelberg 2017

Authors and Affiliations

  1. 1.Czech Geological SurveyPrague 5Czech Republic
  2. 2.Institute of Geology and Paleontology, Faculty of ScienceCharles UniversityPragueCzech Republic
  3. 3.CONICET, Dpto. de Geología y Petróleo, Facultad de IngenieríaUniversidad Nacional del ComahueNeuquénArgentina
  4. 4.Institute of GeologyTallinn University of TechnologyTallinnEstonia

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