New early land plant Capesporangites petrkraftii gen. et sp. nov. from the Silurian, Prague Basin, Czech Republic

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Review of Palaeobotany and Palynology 322 (2024) 105048

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New early land plant Capesporangites petrkraftii gen. et sp. nov. from the Silurian, Prague Basin, Czech Republic ´ a, b, c, *, Josef Pˇseniˇcka b, Jakub Sakala a Monika Uhlíˇrova a

Institute of Geology and Palaeontology, Faculty of Science, Charles University, Albertov 6, 128 43 Praha 2, Czech Republic Centre of Palaeobiodiversity, West Bohemian Museum in Plzeˇ n, Kopeck´eho sady 2, 301 00 Plzeˇ n, Czech Republic c Department of Paleobiology and Paleoecology, Institute of Geology of the Czech Academy of Sciences, Rozvojov´ a 269, 165 00 Praha 6, Czech Republic b

A R T I C L E I N F O

A B S T R A C T

Keywords: Silurian Pˇrídolí Prague Basin Embryophytes Capesporangites

Research on the earliest terrestrial plants often brings difficulties related to uncertain systematic classification. As plant macrofossils are usually poorly preserved, no internal anatomy is recorded thus enabling only morpho­ logical features to be used for plant description and classification. Most of these early land plants show dichotomous branching and terminal sporangia, placing them among the basal polysporangiophytes. However, some plants may exhibit unusual characteristics which raise further questions. The classification of early terrestrial plants has been debated, with discussions about whether they exhibit features that more closely resemble bryophytes or tracheophytes. These debates point to distinct evolutionary pathways. The recent emergence of a new group of plants known as eophytes could represent a link between bryophytes and tracheophytes. This manuscript focuses on a recently discovered Silurian fossil plant named Capesporangites petrkraftii gen. et sp. nov. The plant shows features reminiscent of bryophytes as elongated sporangium having a cap-like formation at the apex and a columella-like structure at the base but also fits into the classification of polysporangiophytes. The study highlights the difficulty of investigating the morphology of fossils with respect to preservation methods and emphasizes the importance of a thorough description of even minor aspects of early land plant fossils. This attention to detail contributes to a more comprehensive understanding of the evolutionary path these plants have followed.

1. Introduction The process of plants adapting to land can be indirectly observed in the fossil record, through both micro- and macrofossils. Estimates of molecular biologists on the origin of terrestrial plants differ consider­ ably, but the prevailing opinions suggest the origin in the CambrianOrdovician interval (Morris et al., 2018) or during Ordovician (Sand­ erson, 2003; Smith et al., 2010). An early indicator of the presence of terrestrial plants in the fossil record would most likely be cryptospores. Especially in terrestrial plants these are covered with a sporopollenin wall, having a higher potential for preservation. There are two different concepts of the term cryptospores. According to Strother and Beck (2000), cryptospores include all spore-like palynomorphs of non-marine origin, including non-embryophytes, from presumed algal ancestors containing sporopollenin in the spore wall. The other definition pro­ posed by Steemans (2000) specifies cryptospores as propagules of the

oldest embryophytes. In the following we will use the term cryptospores sensu Steemans (2000). Among the oldest spore-like palynomorphs are findings from the Cambrian with uncertain affinity (Strother and Beck, 2000; Taylor, 2009) and of probable algal producers (Strother and Foster, 2021). Cryptospores assignable to land plants appear in the fossil record since the Middle Ordovician (summarized in Wellman et al., 2023). However, true trilete spores, which are characteristic for tra­ cheophytes are dating back to the Upper Ordovician (Steemans et al., 1996). Plant macrofossils unequivocally assigned to terrestrial plants appear later in the fossil record since the lower Silurian (Libertín et al., 2018; summarized in Edwards and Wellman, 2001; Pˇseniˇcka et al., 2021). New older terrestrial plant fossils are being discovered over time, shifting the boundary of their earliest recorded occurrence downwards. Although earlier Ordovician meso- and macrofossils have also been published (Salamon et al., 2018; Naugolnykh, 2019), it has not yet been unequivocally proven to represent remains of terrestrial plants.

* Corresponding author. E-mail address: [email protected] (M. Uhlíˇrov´ a). https://doi.org/10.1016/j.revpalbo.2023.105048 Received 21 August 2023; Received in revised form 21 December 2023; Accepted 23 December 2023 Available online 28 December 2023 0034-6667/© 2023 Elsevier B.V. All rights reserved.

M. Uhlíˇrov´ a et al.

Review of Palaeobotany and Palynology 322 (2024) 105048

According to the general anatomy and morphology, it is supposed the earliest terrestrial plants possessed features at the bryophyte grade (Gray, 1985; Ligrone et al., 2012). The earliest land plants found in the macrofossil record exhibit a branched sporophyte bearing terminal sporangia and are thereby classified as polysporangiophytes (Kenrick and Crane, 1991). Although all vascular plants belong to the poly­ sporangiophytes, it is assumed that representatives of the basal poly­ sporangiophytes did not have developed true conducting tissues (Kenrick, 2000). In contrast, some of the early land plants have been shown to have vascularity indicating their assignment to tracheophytes (Edwards and Davies, 1976; Edwards et al., 1992). Despite the close similarities between bryophytes and tracheophytes, it should be noted that these are separate evolutionary lineages. The evolutionary rela­ tionship between these groups is still unresolved and several possible variants are considered (Morris et al., 2018), but according to Morris et al. (2018) monophyly of bryophytes is most likely. The recent dis­ covery of plant remains, which are placed between bryophytes and tracheophytes (branched sporophyte without any specialized WCCs, bryophyte-type FCCs, more see in Tomescu, 2022), has introduced a group of plants called eophytes (Edwards et al., 2022b) that is consistent with this concept. Thus, the description of new material exhibiting bryophyte-like characters could provide the key to continued research regarding the relationship of vascular plants with bryophytes, as these characters may often be overlooked. The present paper deals with a rhyniophytoid (sensu Edwards and Edwards, 1986) from the Pˇrídolí (Silurian) of the Prague Basin (Czech Republic) possessing morphological bryophyte-like features. The small slab shows a fragment of dichotomously divided branches with a terminally arranged sporangium lacking in situ spores. The specimen was precisely studied under SEM (low vacuum) and under a stereomi­ croscope immersed in ethanol. We focused primarily on the distribution and thickness of the coaly mass, which reflect the structure of an original organic matter. Based on unique features, we established a new genus and species Capesporangites petrkraftii gen. et sp. nov. and placed among polysporangiophytes. We also offer a suggested reconstruction the aerial part of this fossil plant.

2. Material and methods The specimen F21762 described in this paper comes from the collection of the Centre of Palaeobiodiversity (West Bohemian Museum in Pilsen, Czech Republic - hereafter WBM)). The plant occurs in decalcified tuffitic shale intercalated with micritic limestone layers and lenses. Morphological features were determined by combining information of both light microscopy and SEM observations. For a better result, we combined observations obtained immersed in ethanol with examination when dry. Most observations were performed using the Olympus SZX12 stereomicroscope. Drawings were created using a camera lucida optical device attached to a stereomicroscope or from detailed photographs taken with the Olympus DP73 digital microscope camera and the Canon EOS 5D Mark III digital single-lens reflex camera. Scanning electron microscopy in low vacuum mode was used to observe delicate structures inside of the sporangium (JEOL 6380LV, Institute of Geology and Palaeontology, Faculty of Science, Charles University). The maceration method was also used but did not yield any results. 3. Locality The plant was found by Dr. Petr Kraft (Faculty of Science, Charles University; West Bohemian Museum in Pilsen) in the Kosov quarry near Beroun (Fig. 1). The site is situated in the Prague Basin (comprising sediments from the Lower Ordovician to Middle Devonian), which is part of the Barradnian area. Like most of the plant fossils found there, this one was found as an isolated block below the quarry wall. The sequence is composed of alternating limestones and calcareous shales ´ry Formation. Within this sequence, the within the lower part of the Poˇza Neocolonograptus parultimus – Neocolonograptus ultimus Zone (biostra­ tigraphy according to Loydell, 2012) occurs, which corresponds with the base of the Pˇrídolí. The locality is situated approximately 100 m southeast of the profile No. 356 described by Kˇríˇz (1992). This locality is highly valued paleontologically, especially for the abundant records of Pˇrídolian plants described by Kˇríˇz (1992), Obrhel (1962), Schweitzer (1980), Kraft et al. (2019), and Uhlíˇrov´ a et al. (2022). This plant assemblage is allochthonous, with plants having been transported from

ˇ Fig. 1. Schematic map of the central part of the Prague Basin (modified after Storch, 1994; area outline marked in the bottom right box) with the location of the Kosov quarry marked as a star. 2

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Review of Palaeobotany and Palynology 322 (2024) 105048

the nearby onshore (in the form of volcanic elevation) caused by changes in sea level and climatic conditions into a sedimentary shallowwater environment (see Kraft et al., 2019).

coaly matter. The sporangium is 3.5 mm long and 1.2 mm wide. Most of the sporangium is composed of black coaly matter enveloped by a 0.1 mm thin margin of light coaly matter (Plate I, 4 “C”). This margin widens in the apical part of the sporangium and extends above the dark elliptical filling forming a 0.5 mm high “cap” (Plate I, 4 “Ap”).

4. Systematics and description

5. Discussion

Clade: POLYSPORANGIOPHYTES Kenrick and Crane, 1991 ´, Pˇseniˇcka et Sakala, gen. nov. Genus: Capesporangites Uhlíˇrova Generic diagnosis: Plant with dichotomously branched narrow leafless axes. Sporangia terminal, vertically elongated, almost thrice longer than wide, termi­ nating in a cap-like structure at apex. Gradual transition between sporangium and subtending axis. Etymology: Generic name from the Latin words “cap “+ -e, and “sporangium” + -ites. Based on the characteristic sporangium of the plant, covered by a “cap-like” structure. Type species: Capesporangites petrkraftii Uhlíˇrov´ a, Pˇseniˇcka et Sakala, sp. nov. ´, Pˇseniˇcka et Sakala, sp. nov. Capesporangites petrkraftii Uhlíˇrova 2019 ?Fusiformitheca sp., Kraft et al., p. 150, fig. 4F. 2021 cf. Fusiformitheca sp., Pˇseniˇcka et al. p. 11, fig. 7d. Holotype: Specimen F21762 housed in WBM. Type locality: Kosov Quarry, Barrandian, Czech Republic. ´ry Formation, Pˇrídolí, Silurian. Stratigraphic horizon: Poˇza Etymology: Specific name after Dr. Petr Kraft who discovered the plant macrofossil, and who devoted his life to the study of marine fauna in the Barrandian area. Specific diagnosis: Axes narrow, leafless, 0.5–0.95 mm in width and up to 11.5 mm in length, at least three times dichotomously branched. Axes of higher orders shorter. Vertically elongated sporangium 1.2 mm wide and 3.5 mm long, forming a ‘cap-like’ structure at apex. Subtending axis grad­ ually widens into sporangial base. Description: The specimen represents a small three times dichotomously branched plant, which is at least 18 mm high (Plate I, 1). The plant possesses narrow, naked axes with dichotomous branching and terminal sporangia. A significant feature of this plant is the sporangium, which is almost thrice longer than wide with narrowing into a sharp tip. The base of the plant is not preserved. The main axis A (Plate I, 2), reaches a length of 11.5 mm and a diameter of 0.9–1 mm. It is slightly bent and gradually widens towards its distal end, where it dichoto­ mously divides into the branches B1 and B2 (Plate I, 2, 7). The branch B1 is completely preserved. On the contrary, the B2 axis reaching a diameter of 0.85–0.9 mm is probably broken and only a 6 mm long part is pre­ served and its distal end is formed by a noticeably lighter coaly matter. The B1 axis is slightly bent, 11.5 mm long and the 0.8–0.95 mm wide. It slightly tapers in the half until a significant narrowing in the distal part. It is followed by a widening of the axis below the branch point. The axis is dichotomously divided into the branches C1 and C2 (Plate I, 2) of which the axis C2 is preserved only as its ca. 2.7 mm long basal part. Indistinct dark line of coaly matter running into the branch C1 (Plate I, 9) is visible in the centre of the distal part of the axis B1. Axis C1 is 5 mm long and 0.7–0.8 mm wide. The axis is deformed in its half, showing a sharp bend in its right outline. In the distal part, the axis shows distinct widening due to branching. Axis D2 is not preserved. The terminal axis D1 is very short, only 1.4 mm long, with a gradual widening from 0.35 to 0.5 mm transitioning into the base of the elongated sporangium. Through the centre of the D1 axis, there is a 0.15 mm wide line of dark coaly matter (Plate I, 4 “CT”; pl. 8 – yellow arrow). The line is preserved intermittently approximately 0.4 mm below the sporangium. The course of the line is followed by the distinct elongated structure at the base of the sporangium (Plate I, 4 “BS”), which is visible in the length of only 0.5 mm. The structure is surrounded by spots without coaly matter (Plate I, 4 “AC1”, “AC2”). The elongated sporangium S1 is well distinguished from the terminal axis by its dark color caused by dark

The presented fossil is poorly preserved, so its comparison is limited. For comparison and discussion, the terminal sporangium, axial charac­ teristics and type of branching are particularly important. However, as we already mentioned in the introduction, the interpretation of some features may appear different under binocular microscope, photo­ graphed in ethanol, and from SEM observation. Therefore, we try to discuss here all the characteristics that are observable in both ways of observation. This can eliminate the misinterpretation of some structures. 5.1. Sporangial character of presented plant Specimen shows only one preserved terminal sporangium and therefore the comparison with other similar species is somewhat limited. Compared to early land plants with globose sporangia, plants showing elongated sporangia are less represented in the fossil record and include the following taxa Fusiformitheca Xue et Wang (incertae sedis), Salopella Edwards et Richardson (rhyniophytoid), Steganotheca Edwards (rhy­ niophytoid), Tortilicaulis Edwards (rhyniophytoid). Sporangium shows some significant structures (especially the dis­ tribution of coaly matter and some linear structures, see Plate I, 4) that may indicate its internal sporangial tissue organization. However, it is necessary to consider the possibility that formation of these structures may have been caused by taphonomical process. Above all, it is neces­ sary to evaluate what is the meaning of the black coaly matter that forms mostly the sporangium fill (Plate I, 3). Nevertheless, this central part does not show any structures including the possible spores/cryptospores according to SEM observation (Plate I, 6), except very slightly almost invisible longitudinal striations running through the centre of the sporangium from the proximal part of the sporangium towards the “cap” (Plate I, 4 brown dashed line). Around the internal black fill, a lighter rim is visible (Plate I, 4 “C”), which in the apical part forms a distal “cap” (Plate I, 4 “Ap”). It may be assumed that the rim represents sporangial wall. For now, we can only speculate about function of the distal “cap” that could have either a protective function or was related to some kind of opening mechanism, or a combination of both. Similarly, as the lenticular structure on the flat top of the sporangium of Steganotheca (Edwards, 1970), which may represent a probable opening mechanism. Also, the notch at the apex of the sporangium described in Sporogonites sp. by Gess & Prestianni (2021; fig. 3g) could serve a similar function. The structures at the base of the sporangium are noteworthy. These include an elongated structure consisting of a distinct black coaly matter (Plate I, 4 “BS”) and two surrounding areas without coaly matter (Plate I, 4 “AC1”, “AC2”). The structures do not appear to have been a result of a taphonomic process, especially in view of the symmetry of spots place­ ment. The question is whether these structures (Plate I, 4 “BS” and Plate I, 4 “AC1”, “AC2”) were originally present only at the base of the sporangium or running along its length. The possibility that structures may have been present superficially also needs to be considered. These structures are visible both under light microscopy and SEM observations (Plate I, 3,5 versus Plate I, 6). The almost invisible longitudinal striations running through the centre of the sporangium (Plate I, 4 brown dashed line) from the proximal part of the sporangium towards the “cap” fol­ lows the course of the BS (see Plate I, 4). 5.2. Justification of the new genus and species Due to the fact that axes can be strongly influenced (deformation) by taphonomic processes, the sporangial morphology is a key feature for 3

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Plate I. Capesporangites petrkraftii 1. Overall view of the plant. Immersed in ethanol. Scale bar 5 mm. 2. Line drawing based on previous photo with axis and sporangia designations. Scale bar 5 mm. 3–6. Detail of sporangium S1. Scale bars 0.5 mm. 3. View in the stereomicroscope. Immersed in ethanol. Photo by Petr Kraft. 4. Drawing of sporangium based on Plate I, 3. Black and gray areas show accumulation of coaly matter. Covering layer (C) projecting into apical part (Ap) as a tip. Arrows point to distinct areas at the base of the sporangium: areas without coaly matter (AC1, AC2), distinct elongated structure (BS). Coaly matter forming linear structures (CT, additional yellow lines). Longitudinal striation running through the centre of the sporangium (brown dashed line). 5. View in stereomicroscope; in a dry state. 6. SEM photograph of sporangium; low vacuum mode. 7–9. Detail of selected axial parts. 7. First point of branching where the main axis A divides into branches B1 and B2. Scale bar 1 mm. 8. Detail of the terminal axis. Yellow arrow marks the distinct line of organic matter. Photo by Petr Kraft. Scale bar 0.5 mm. 9. Branched point of the B1 axis into daughter axes C1 and C2. Yellow arrows show the distinct lines of coaly matter leading through the centre of axes. Immersed in ethanol. Photo by Petr Kraft. Scale bar 0.5 mm. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

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comparison with other taxa. As we mentioned above, similar elongated sporangium is presented in the following Silurian plant genera: Steg­ anotheca, Tortilicaulis, Salopella and Fusiformitheca. The genus Steganotheca, reported from Ludlow and Pˇrídolí of the Anglo-Welsh Basin by Edwards and Rogerson (1979), differs mainly by the characteristic shape of sporangium (see Edwards, 1970). The apical part in Steganotheca sporangium is flat, whereas the sporangium of the studied plant has conical distal part. Steganotheca also exhibits a lenticular structure in the apex, but there is no structure of that kind present in our plant. The genus Tortilicaulis exhibits typical unbranched twisted axes with terminal elliptical sporangia (Edwards, 1979). In contrast, the axes of our studied plant show locally only slight deformations, which do not correspond to the twisted axes of Tortilicaulis. Species Tortilicaulis transwalliensis Edwards was described from the Pˇrídolian sediments of the Anglo-Welsh Basin by Edwards (1979) preserved as coalified com­ pressions. Compared to the studied plant, the body of sporangium T. transwalliensis Edwards is significantly wider and terminates with a very sharp tip. Another species, T. offaeus Edwards, Fanning et Richardson, was described on the basis of three-dimensional coalified fossils (Edwards et al., 1994). The sporangia of T. offaeus are not as sharp at the apex and are closer in morphology to our plant, but because of the three-dimensional preservation of the T. offaeus fossils, the features cannot be compared more closely with the coalified compression of presented plant. The genus Salopella occurs in the fossil record in sediments from the Silurian to the Devonian and it may resemble the studied plant in its appearance. The plant Salopella allenii Edwards et Richardson was established by Edwards and Richardson (1974) based on specimens from the Lower Devonian of Brown Clee Hill, Shropshire (England). Species shows a sharper sporangial tip that is comparable with studied plant and also the transition between sporangium and the subtending axis is imperceptible without any sign of widening into the sporangium base. Salopella also shows more robust axes than those of the studied plant with very small branching angles compared to the studied plant, which possesses very wide branching angles. It is worth noting the similarity in morphology of the three-dimensionally preserved sporangium of Salo­ pella cf. marcensis (fig. 1 in Edwards et al., 1994) described by Edwards et al. (1994) from the Lochkovian/Gedinnian strata of Shropshire, En­ gland. The sporangium of the studied plant resembles the one of the studied plants in shape, but due to the different types of preservation, it is not possible to compare them in detail. However, the type material of Salopella marcensis Fanning, Edwards et Richardson (Fanning et al., 1992) is preserved as coalified compressions and allows a better com­ parison of morphological characters with our plant. Based on the type material, S. marcensis possesses a sporangium with a more pointed apex and a sharp angle between the terminal axes compared to the studied plant, which shows an apex gently tapering into a tip and a wider branching angle of the terminal axes. The sporangial shape of the plant closely resembles that of the genus Fusiformitheca. The following comparison of our plant with the genus Fusiformitheca refers to the material of Fusiformitheca fanningiae (Well­ man, Edwards et Axe) Xue et Wang from the Lower Devonian of Shropshire originally described by Wellman et al. (1998) as Fusitheca fanningiae Wellman, Edwards et Axe. The sporangium of Fusiformitheca is three times as long as it is broader and so in presented plant, and it also gradually tapers to its apex. Fusiformitheca shows a short dichotomy bearing sporangia, but differs slightly from the studied plant in the characteristics of axes. Whereas in our plant the expansion of the axis width at the branch point is pronounced, in Fusiformitheca it is slight. There is also a difference in the transition between sporangium and subtending axis, with the axis gradually widening into the sporangial base in the studied plant, whereas in Fusiformitheca, the transition be­ tween sporangium and axis is apparent. The different types of preser­ vation should also be taken into account, with the studied plant being preserved as a coalified compression as opposed to Fusiformitheca, which

has been described from charcoal. Due to the difference in preservation, this makes it impossible to compare these fossils more consistently with each other. As a result of these comparisons, we decided to establish a new genus and species Capesporangites petrkraftii gen. et sp. nov. 5.3. Systematic position of a new finding Due to the type of preservation, where anatomy is absent, the plant may be classified according to morphological characteristics only. On the basis of the branched sporophyte bearing terminal sporangia, the plant is clearly classified among polysporangiophytes (sensu Kenrick and Crane, 1991). Although linear structures were observed in the axes, which could suggest the presence of conducting tissues, their presence was not clearly proved. In addition, the plant shows some unusual sporangial features including a ‘cap-like’ structure at the apex and enigmatic structures at the base of the sporangium. In part, these structures can show us certain analogies with mosses or hornworts. As an example, inside the capsule of mosses and hornworts, a sterile tissue called a columella is present (Goffinet and Buck, 2013). It should be noted that the structure at the base of the sporangium of the studied plant (Plate I, 4 – BS) closely resembles the columella both in character and position. Also, the observed structures without a coaly matter in our specimen (Plate I, 4 – AC1, AC2) may resemble the base of the spore sac or cavities between the spore sac and sporangium wall, which occur in some representatives of mosses (see Renzaglia et al., 2020). Of interest is Horneophyton Barghoorn et Darrah from Rhynie Chert, classified as a basal polysporangiophyte (Kenrick, 2000) while showing the central columella in sporangia (El-Saadawy and Lacey, 1979) as a bryophytelike feature, which is characteristic for recent mosses and hornworts (Goffinet and Buck, 2013). However, it must be stressed that Horneo­ phyton cannot be classified as related to bryophytes on the basis of the presence of columella only, as this feature also occurs in some Carbon­ iferous lycophytes, which would indicate homoplasy (Cascales-Mi˜ nana et al., 2019). The diagnosis of the polysporangiophyte clade implies it includes plants with a branching sporophyte. However, branching sporophyte does not occur in any group of the bryophytes. Recently, Edwards et al. (2022b) described Devonian plant remains from Ludford Lane showing characteristics of food-conducting cells of bryophytes, while possessing a branched sporophyte typical of polysporangiophytes. This gave rise to a new group of plants called eophytes (Edwards et al., 2022a,b) Thus, as already suggested, the systematic placement of the eophytes will be somewhere in between, with considerations about several scenarios (see Edwards et al., 2022b; Tomescu, 2022). Edwards suggests the placement of eophytes at the base of polysporangiophytes. If we compare the morphological characters of the eophytes with those of the studied plant, their common features include dichotomously branched small leafless axes and a terminal sporangium. The material, on which the eophytes were described (Edwards et al., 2022a; Edwards et al., 2022b), also preserves anatomical features (stomata, cryptospores, internal tis­ sues including the detail of the cells of the central bundle) that cannot be observed in our plant. The size of the plant is also noticeably larger than the mesofossils of eophytes described so far. Capesporangites possesses an isotomously branched sporophyte and a terminal sporangium, a list of characters that classify the plant among polysporangiophytes. In addition, the distinct dark lines of organic matter running through the centre of the axes (Plate I, 4 “CT”; pl. 9) suggest the presence of more resistant tissues, probably conducting, which would support its classification as a polysporangiophyte or perhaps at the base of the tracheophyte lineage. However, the sporan­ gium characteristics remain controversial, with some structures resem­ bling a bryophyte capsule. Given the features present, the placement of the plant within the phylogeny would then follow the discovery of both branching sporophyte and conducting elements. On the basis of morphological characters only, we would suggest the placement of 5

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Capesporangites among the basal polysporangiophytes, following a similar pattern as that proposed by Edwards et al. (2022b) for the eophytes. However, we should also consider that the branched sporo­ phyte does not occur only in polysporangiophytes, but rarely also in some mosses and liverworts (Edwards and Kenrick, 2015). If we also consider the schemes of phylogenetic placements proposed by Tomescu (2022, Fig. 1c), the plant would align into the first two schemes from the left. In the case of the first scheme (in Tomescu, 2022 – Fig. 1c – the first scheme from the left), where a branching sporophyte originated in the common ancestor and conductive elements arose independently in both the tracheophyte and bryophyte lineages, we assume a placement of Capensporangites at the base of the tracheophytes, but after the devel­ opment of the conductive elements. If we consider the second scheme (in Tomescu, 2022 – Fig. 1c – the second scheme from the left), where the origin of both the branching sporophyte and the conducting elements occurred in a common ancestor, we suggest placing the plant either immediately after these characters are obtained, or at the base of the bryophytes before the loss of the branching sporophyte, or at the base of the tracheophyte lineage. However, our estimates for the placement of the plant are strictly hypothetical unless other determining features, especially internal anatomy, are preserved.

Fig. 2. Presumed reconstruction of the plant. Scale bar 5 mm.

remains from the Devonian showing characteristics of both poly­ sporangiophytes and bryophytes. The specimens studied by Edwards et al. (2022a, 2022b) are preserved as charcoals, in which the internal anatomy is preserved in detail. However, most Silurian plant macro­ fossils are coalified compressions preserving none or very little of the original internal anatomy and any anatomical structures can rarely be isolated from them. When the maceration method fails, the only option left is to focus on plant morphology and striking structures that can be observed on the fossil surface. It is possible, therefore, that the macro­ fossil findings of Silurian plants reported so far could conceal even more than expected. If the research usually did not reveal any conducting tissues in the rhyniophytoid plant, the plant conventionally fell among polysporangiophytes. Thus, any specific bryophyte-like features may have been consequently overlooked or deliberately not published, because the branching sporophyte (as a major feature of poly­ sporangiophytes) may have been considered to be contrary to the occurrence of bryophyte-like features. It is highly unlikely that the observed sporangial structures are of the same nature as those in bryo­ phytes, but given the apparent similarities, it is worth looking to bryo­ phytes for inspiration to explain the origin and function of these structures. Then the similarities of these features could be explained as homoplasy. We would hereby like to encourage the description of the fossils of early land plants, including small details that, although they may seem irrelevant, could later fit into the puzzle.

5.4. Growing character Although we do not know how much of the original plant it is pre­ served on specimen, based on some indicia we can partially discuss its original appearance. The daughter axes are bent in a similar way to the ´ previously studied plant Tichavekia grandis Pˇseniˇcka et al. (in Uhlíˇrova et al., 2022; Bek et al., in press), whose bending can be assumed to be due to upright growth on land and the associated loading of the axes by branches. As the proximal part of the fossil is unknown, the overall height of the plant cannot be determined. Numerous dark lines formed by coaly matter running through the centres of some axes (Plate I, 2 – gray lines, Plate I, 9 arrows) could also indicate a course of possible conductive tissues. On the basis of the observed characteristics, we have created a pre­ sumed reconstruction of the plant shown in Fig. 2. This specimen pos­ sesses three branch points, with the daughter axes forming a wide angle between them. The sporangia are vertically elongated and taper at the apex. We can only hypothesize about the coloring of the axes. We incline to the opinion that the axes were not green. Thus, one possibility would be that the fragment of the studied plant represents a sporophyte, which is photosynthetically dependent on a green gametophyte, similar to that of bryophytes (supposed in eophytes and the common ancestor of land plants as well). This would also be supported by the hypothesis of Boyce (2008), who suggests that tiny axes of less than 1 mm in diameter might not have sufficiently developed internal anatomy to carry out the photosynthetic process, and hence could likely be nutritionally depen­ dent on asexual gametophytic generation to supply the necessary assimilates.

Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Data availability

6. Conclusions

No data was used for the research described in the article.

This manuscript presents morphological features of the plant macrofossil from the upper Silurian (Pˇrídolí) of the Prague Basin. Ac­ cording to comparison of the features with previously published plant taxa, it was appropriate to establish a new genus and species Capespor­ angites petrkraftii gen. et sp. nov. On the basis of the branching sporo­ phyte bearing terminal sporangia, we place the plant among the polysporangiophytes. Unusual characteristics related to the sporangium were also described, namely a “cap-like” structure in the apical part and enigmatic basal structures. We discuss these characteristics here and provide analogies from bryophytes that may shed more light to explain origin and possible purpose of these structures. No in situ spores were observed. Edwards et al. (2022b) reported an exceptional assemblage of plant

Acknowledgements Funding was provided by the Grant Agency of the Czech Republic (grant project 21-10799S) and co-financed by institutional support RVO 67985831 of the Institute of Geology of the Czech Academy of Sciences. We thank P. Kraft (Centre of Palaeobiodiversity, West Bohemian Museum in Pilsen) for kindly providing us the specimen to study and some figures to publish in this manuscript. We are also grateful to M. Mazuch (Institute of Geology and Palaeontology, Faculty of Science, Charles University) for his assistance with the scanning electron microscope. 6

M. Uhlíˇrov´ a et al.

Review of Palaeobotany and Palynology 322 (2024) 105048

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