Introduction

Finds of terrestrial flora in Cretaceous strata in Slovakia are very rare. 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 a 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 the Fatricum, its description and interpretation are the principal subjects of this short report. The Fatricum is a tectonic unit of the Western Carpathians, comprising both the Vysoká and Krížna partial nappes. It is characterised by a wide stratigraphic range; from a lithostratigraphic point of view, it is generally composed of Middle Triassic to lower Upper Cretaceous strata. The Fatricum originated from the area between the Veporic in the south (the Fatricum originally formed a sedimentary basin in the north of Veporic) and the Tatric in the north. It is therefore understood as a space between these areas that no longer exists today, but also as rock bodies that originate from this area. The Fatricum was one of palaeogeographic domains of the northern passive margin of the Western Tethys (e.g. Michalík et al. 1995, 2017; Plašienka 2003). Its Mesozoic sediments are dominated by carbonate lithology detached from their mostly disappeared original basement, and transported to form a far-reaching allochthonous body (Andrusov et al. 1973; Maheľ 1983; Prokešová et al. 2012). Fatric nappes clearly overlie the Tatric cover – it has been transported over the underlying Tatric Superunit to a distance of several tens of kilometers (40—60 km) as a relatively coherent body (Biely and Fusán 1967; Jaroš 1971; Plašienka 1983; Prokešová et al. 2012 and references therein). It is usually overlain by rocks of the “higher” (generally Hronic) nappes (Michalík et al. 2017). The Lower Cretaceous sequence of the Fatricum consists of pelagic limestones and marls, relatively poor in macrofossils (Borza et al. 1984).

The genus Equisetites Sternb. is known in the fossil record since the Triassic (e.g. Kelber and Van Konijnenburg-van Cittert 1998; Pott et al. 2008; Van Konijnenburg-van Cittert et al. 2020). Its occurrence in the European Jurassic is documented by Harris (1961), Hauptmann and Hauptmann (1994), and 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). Besides the Cretaceous finds, Mesozoic plant fossils from Slovakia are known from the Tomanová Formation of the Tatra Mountains (see Raciborski 1890; Michalík et al. 1988). Macrofloral fossils from these Triassic (Rhaetian) deposits were unfortunately not the subject of detailed research. Bennettitalean genus Otozamites was reported from Middle Jurassic deposits of the Halečková quarry (Sitár and Jablonský 1997).

Material and Methods

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 (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 (lower Hauterivian in regard to Butkov, but widens to lower Valanginian to lowermost Aptian in the Zliechov Basin of the Krížna Unit in the Fatricum; see Vašíček et al. 1994).

Fig. 1
figure 1

Localization of Zbyňov site in Slovakia with photographs of locality. CZ Czechia, PL Poland, SK Slovakia, A Austria, HU Hungary, UA Ukraine. Source: OpenStreetMap

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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)

Fig. 2
figure 2

Equisetites cf. lyellii. a complete specimen B 1868, scale bar 10 mm, b detail of nodal area, scale bar 5 mm, c SEM image of nodal area showing ribs (arrows) and delicate grooving, scale bar 1 mm, d detail of stem, arrows indicate stomata, fluorescence LM, scale bar 500 µm

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Fig. 3
figure 3

Equisetites cf. lyellii B 1868. a detail of stem, stoma, fluorescence LM, scale bar 50 µm, b detail, stoma in SEM, scale bar 25 µm

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Material: horsetail axis B 1868.

Locality and horizon: Zbyňov in the Strážov Highlands, Slovakia; Early Cretaceous.

Description: The axis fragment is over 53 mm long. Its lower end is covered by sediment and the original upper termination is missing as the rock is broken off. The axis is 3 mm in diameter, consisting of nodes and internodes: there are three well-developed nodes and four internodes (Fig. 2a). The nodes are in some cases rather expanded to swollen (Fig. 2c.) They show six leaves forming a poorly preserved sheath without tips, which are probably eroded (Fig. 2b). It means there were about 12-14 leaf segments per node. The internodes are smooth, but showing pronounced ribs in areas near to the nodes (Fig. 2c), with delicate longitudinal grooving, perhaps reflecting a cellular structure (Fig. 2c). Cuticle covering leaves and stems shows only partly preserved elongated ordinary cells. Their detail is shown in Fig. 3a. Stomata are arranged in rows oriented parallel to the axis (Fig. 2d), and are surrounded by subsidiary cells bearing characteristic microscopic papillate ornamentation (Fig. 3a, b).

Discussion

The studied specimen is assigned to the genus Equisetites, based on the following characters: 1. small size, 2. small number of leaves per node, 3. ribs alternating through the nodes. Alternation of ribs is a typical character of Equisetites (Gnaedinger et al. 2023), and this character is present, although rather blurred by secondary deformation of the stem (Fig. 2c).

The studied material is most similar to Equisetites lyellii (Mantell) Morris from the English Wealden, published by Watson and Batten (1990). The species was revised, including lectotype designation, by Watson and Batten (1990). The current material is particularly similar in showing similar stomata surrounded by microscopic ornamentation, much like the beaded sculpture characteristic for E. lyellii (Watson and Batten 1990). The currently 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 is 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. Watson and Batten (1990) state: “The evidence shows that the anatomical features (large amount of metaxylem, presence of the ring of small vascular traces between the vallecular canals) of E. lyellii are rather further removed from modern Equisetum than the external morphology suggests, and in the light of this we prefer to retain it within Equisetites Sternberg“.

The current specimen differs from Equisetites beanii (Bunbury) Seward from the Jurassic of Yorkshire (Seward 1894, Harris 1961) 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 E. columnaris and E. laterale show about 25 segments per node.

The genus Neocalamites differs from the studied material in several aspects: it occurs typically in Permian-Triassic-Jurassic strata (Harris 1961); it shows larger stems in diameter (Taylor et al. 2009); its stems are typically branched, showing a higher number of leaves per node. On the other hand, the studied material seemingly resembles Neocalamites in possessing no or poorly preserved leaf sheets. Some of its ribs seem to be continuous through the nodes (this is probably caused by preservation). In the nodal areas, the ribs are better pronounced and alternately arranged (Fig. 2c). Another similar character to Neocalamites is presence of swollen nodes (Bomfleur et al. 2013).

The horsetail was possibly 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 can float for longer distance. Experiments with axes of modern Equisetum showed it can float for several days up to weeks in fresh water as well as in sea water (Coffin 1971; Husby et al. 2011). There is abundant evidence that floating Equisetum fragments can act as means of propagation for this taxon along streams and seacoasts, as floating stem fragments or even mats of such fragments can develop adventive roots and easily colonise new localities (e.g. Wagner and Hammitt 1970). Due to the variable speed of water currents in coastal parts of the Tethys Sea, it is difficult to predict how far the fragment could get from its original dryland in one or even several weeks, but the rarity of plant fossils in the sediment and their comparatively small size indicate that dry land must have been a considerable distance away.

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 termed 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 the slightly younger, 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 palaeogeographic position of the Fatricum in the Western Carpathian area during the Mesozoic being relatively well constrained, its position in relation to Vindelician Land is not entirely clear and remains a matter of debate (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 considered cautiously. 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).

Conclusions

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 cannot be considered certain. 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.