Early Cretaceous Equisetites from Slovakia

doi:10.21203/rs.3.rs-3419325/v1

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Early Cretaceous Equisetites from Slovakia

https://doi.org/10.21203/rs.3.rs-3419325/v1

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published 11 Dec, 2023

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A new find of terrestrial plant Equisetites cf. lyellii is reported from the Early Cretaceous of Slovakia. It comes from the Mráznica Formation of the Rajec Basin in Fatricum, Zbyňov locality, Rajecké Teplice (Žilina district). The presence of a 53 mm long horsetail axis, which cannot float a long distance, provides good evidence of terrestrial environments during sedimentation of the studied strata. According to our interpretation such a plant strongly indicates a moist to wet habitat (even swampy environments) on the presumed dryland from where it was transported. This dryland could represent an isolated unknown small island(s) in the vicinity or the Vindelician-Bohemian Massif that was active as a dryland for the entire period of time from the Triassic through the Late Cretaceous. Note, however, that the exact paleogeographic position of Fatricum during Mesozoic in relation to Vindelician Land is not entirely clear and such an interpretations need to be met with a caution.

Equisetites

terrestrial fossil plants

Early Cretaceous

Slovakia

Finds of terrestrial flora in Cretaceous strata in Slovakia are very rare and and are virtually unknown from the Jurassic. One incomplete frond of Cladophlebis sp. was described from the Early Cretaceous of the Butkov Hill, from the same Mráznica Formation, by Michalík et al. (2012: Fig. 7; 2013). The rarity of such finds means even fragments of fossil plants are important. These finds document significant Cretaceous sea-level changes on a global scale (Haq 2014). Terrestrial plants provide clear proof of above-sea-level areas (at least islands or archipelagos), perhaps part of the Vindelician Landmass in close vicinity of sedimentation basin, as well as provide information about possible types of terrestrial environments during sedimentation of the studied strata. The plant found in the Mráznica Formation in Fatricum, its description and interpretation are the principal subjects of this short report.

The genus Equisetites Sternb. is known in the fossil record since the Triassic (e.g. Kelber and Van Konijnenbur-van Cittert 1998; Pott et al. 2008; Van Konijnenburg-van Cittert et al. 2020). Its occurrence in the European Jurassic is documented by Harris (1960), Hauptmann and Hauptmann (1994), Barbacka (2009). Equisetites was widespread in the Early Cretaceous, particularly in the English Wealden (Watson and Batten 1990; Pott 2021). Its occurrence in the Cretaceous is of interest, because in some cases in this stratigraphic position, Equisetites had already been replaced by the extant genus Equisetum. Differential characters of those two genera were discussed by Watson and Batten (1990).

The fossil plant described here was found by Mr. Portašík in 2021. The specimen is housed in the Slovak National Museum in Bratislava and inventoried as B 1868. Photo-documentation of the axes is based on direct observations, using an Olympus SZX 12 binocular microscope and an Olympus BX 51 fluorescence light microscope, both equipped with a DP-72 digital camera. The axes appeared to be well-preserved in fine-grained marl, showing the arrangement of epidermal cells and stomata. The epidermal micro-morphology was observed in low-vacuum using a Hitachi S-3700N scanning electron microscope.

Geological setting

The remains of the plant described here were discovered in a small outcrop located on a forest road close to the village of Zbyňov, 1.3 km north from the village in the Strážov Highlands. The Zbyňov locality (49°07'59.4"N 18°38'51.6"E; Fig. 1) is in the Rajec Basin, close to the city Rajecké Teplice, the district of Žilina (north-central Slovakia). The fossil comes from Early Cretaceous grey thin bedded marly limestones of the Mráznica Formation (Borza et al. 1987) of the Fatricum. The formation comprises clayey grey inconspicuously spotted limestones, marly limestones and claystones, distributed in marginal parts of the basin (e.g., Vašíček et al. 1994; Michalík 2007). The same formation outcrops at the Butkov Quarry, where its thickness is about 30 meters. Plant remains were also discovered there (Michalík et al. 2012, 2013). In its upper part, activities of currents transporting material are suggested (for details of the Mráznica Formation, see Borza et al. 1987; Michalík et al. 2005, 2013; Pelech and Olšavský 2018; Hraško et al. 2021). The age of the Mráznica Formation in general is dated to the Early Cretaceous (late Valangian – Barremian) based on abundant ammonite remnants (Late Valanginian Furcillata and Early Hauterivian Radiatus-, Loryi and Nodosoplicatum zones) and a rich association of Upper Valanginian to Lower Hauterivian non-calcareous dinoflagellates (see Vašíček and Michalík 1986; Vašíček 2010, Michalík et al. 2005, 2012). However, it should be noted that the exact position of boundaries of the Mráznica Formation is unclear and, in fact, varies (Early Hauterivian in regard to Butkov, but widens to Early Valanginian to Earliest Aptian in the Zliechov Basin of the Krížna Unit in the Fatricum; see Vašíček et al. 1994).

Systematic palaeontology

Equisetopsida

Equisetales

Equisetaceae

Equisetites Sternb. 1833

Type: Equisetites muensteri Sternb. Vers. Fl. Vorwelt, [2] (5–6): 43. 1833

The type was designated by Harris (1931). The lectotype of E. muensteri was designated by Kvaček and Straková (1997). For more details, see Kvaček et al. (2021).

Equisetites cf. lyellii (Mantell) Morris 1843

(Figs. 2 and 3)

Material

horsetail axis B 1868.

Locality and horizon

Zbyňov in the Strážov Highlands, Slovakia; Early Cretaceous.

Description

The axis fragment is 53 mm long and 3 mm in widthdiameter. It consists of three well-developed nodes and four internodes (Fig. 2a). Its basal part is partly buried in sediment. The nodes show six leaves forming sheath without tips, which are probably eroded (Fig. 1b). It means there were about 12–14 leaf segments per node. The internodes are smooth with longitudinal delicate grooving, reflecting perhaps a cellular structure, giving the stem a rather grooved look when magnified (Fig. 1c). Cuticle covering leaves and stems shows only partly preserved elongated ordinary cells. Their detail is shown in Fig. 2a. Stomata are arranged in rows oriented parallel to the axis (Fig. 1d). Stomata are surrounded by subsidiary cells bearing characteristic microscopic papillate ornamentation (Figs, 2a, b).

The studied specimen is most similar to material described as species Equisetites lyellii (Mantell) Morris from the English Wealden, published by Watson and Batten (1990). The species was revised and lectotype designated by Watson and Batten (1990). The present material is particularly similar in showing similar stomata that are surrounded by microscopic ornamentation, much like the beaded sculpture characteristic for E. lyellii (Watson and Batten 1990). The presently described material differs from E. lyellii in a smaller number (12–14) of leaf segments (scars) per node instead of 18 in younger axes of E. lyellii, and similar in this respect to the genus Equisetum. We did not observe any crater pits, as are described in Equisetum subg. Hippochaete (Watson and Batten 1990). Additionally, we follow the reasons stated by Watson and Batten (1990) in studying the better-preserved material of E. lyellii from the English Wealden, where they retained the species in the genus Equisetites.

The present specimen differs from Equisetites beanii (Bunbury) Seward from the Jurassic of Yorkshire (Seward 1898, Harris 1960) in having ornamented stomata. Equisetites columnaris (Brongn.) Sternb. and E. laterale (J. Phillips ex Lindl. et Hutton) Morris differ in having a higher number of leaf segments per sheath and node. Younger axes of both columnaris and laterale show about 25 segments per node.

The genus Neocalamites differs from the studied material in having leaves free, not forming a sheath, smooth stem, and in having typically larger stems (Harris 1960).

The horsetail was most likely transported from nearby land to a submarine environment. The plant represents a rare find, and beside the fern Cladophlebis sp. previously described (Michalík et al. 2012: Fig. 7; Michalík et al. 2013), there are no more plant fossils recorded from the Mráznica Formation. Equisetites axis cannot float a long distance (in contrast to some seeds which are, in some cases, transported by sea currents thousands of kilometres). The rarity of plant fossils in the sediment and its comparatively large size indicate the land was in distance of tenth or first hundreds of kilometres. Palaeoecological requirements of Equisetites indicate moist, even swampy environments (Pott et al. 2008) in this unknown dryland. A transport from an isolated unknown small island(s) in the vicinity (emerged parts of the Fatricum or other nearby tectonic units as Tatricum or Veporicum) could be a plausible candidate. Another probable candidate (of an unknown distance) appears to be the Vindelician-Bohemian Massif (Dill and Klosa 2011, also called the Bohemian Massif with the Vindelician Land, see, e.g., Galasso et al. 2022). The Vindelician-Bohemian Massif was active as a dryland in Central Europe for the entire period of time from the Triassic through the Late Cretaceous (Dill and Klosa 2011). Interestingly, the genus Cladophlebis is also known from slightly younger, the mid-Cretaceous Peruc-Korycany Formation in the Czech Republic (the Bohemian Massif remained as one of the islands in a shallow epicontinental sea that flooded middle Europe at that time; see e.g. Kvaček and Dilcher 2000). Despite the paleogeographic position of the Fatricum in the Western Carpathian area during the Mesozoic is relatively well constrained, its position in relation to Vindelician Land is not entirely clear and remains debated (e.g., Rakús et al. 1988; Vašíček et al. 1994; Plašienka 2000; Chlupáč et al. 2002; Michalík 2007, 2011). Therefore, an exact area of the origin of the plants from the Mráznica Formation is only hypothetical and all these interpretations need to be met with caution. In any case, the Mráznica Formation indicates a more humid climate on land (associated with climate instability) at the time of its deposition – a conclusion supported mainly by the presence of a fine clastic admixture (silty quartz grains and muscovite leaflets), and by occasional terrestrial plant remains (Michalík et al. 2012).

Fossil plant Equisetites cf. lyellii representing a terrestrial plant provides evidence that Early Cretaceous marly limestones of the Mráznica Formation were formed in an area that was adjacent to some terrestrial environment (a dryland). An area of the origin of the plant could represent an unknown small island(s) in the vicinity or the Vindelician Landmass, although these interpretations need to be met with caution. In any case, the palaeoenvironment of this dryland was probably a moist or even swampy habitat, supporting the previous suggestions about the presence of humid (although instable) climate conditions during the deposition of the Mráznica Formation.

Acknowledgements

We thank Ing. Juraj Portašík (Rajecké Teplice) for his donation of the fossil studied here, and for providing photography of the locality. We are indebted to Prof. J. Michalík (the Slovak Academy of Sciences), Dr. M. Olšavský and Dr. O. Pelech (both State Geological Institute of Dionýz Štúr) for useful advice. We are grateful to Lenka Váchová (National Museum, Prague) for macro-photography of the studied specimen. Additionally, we are thankful to Alexei Herman (Geological Institute of Russian Academy of Science, Moscow, and Mihai E. Popa from the University of Bucharest for their valuable advice. This work was supported by the Scientific Grant Agency of the Ministry of Education of Slovak Republic and Slovak Academy of Sciences, Grant Nr. 1/0191/21 (to A. Č) and by the Czech Ministry of Culture (IP DKRVO 2019–2023 2.II.e, National Museum, 00023272).

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Early Cretaceous Equisetites from Slovakia

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Abstract

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Introduction

Material and Methods

Geological setting

Systematic palaeontology

Discussion

Conclusions

Declarations

Acknowledgements

References

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