Non-technical Summary
Zosterophyllopsids have a rich fossil record and represent one of the dominant components in Early Devonian plant assemblages around the world. However, knowledge about the paleogeographic distribution of different groups of zosterophyllopsids remains incomplete. In this article, we describe new plant macrofossils assignable to the genus Gosslingia, which was previously only known from the Lower Devonian of Europe. Our fossils show considerable similarities with the type species of Gosslingia, Gosslingia breconensis, and thus represent the first report of Gosslingia in the South China Block and the most convincing occurrence of this genus outside of Europe. The discovery of Gosslingia and other cosmopolitan genera in South China indicates that the dispersal of early vascular plants across different paleocontinents was more common than previously appreciated.
Introduction
Early vascular plants diversified rapidly and evolved significantly in terms of morphological complexity during the Early Devonian, as evidenced by numerous macrofossil plants from the Lower Old Red Sandstone, the Rhynie Chert, the Lower Devonian siliciclastic sequence of North America (e.g., Gaspé), the Posongchong Formation of South China, and many others (Gensel and Andrews, Reference Gensel and Andrews1984; Knoll et al., Reference Knoll, Niklas, Gensel and Tiffney1984; Kenrick and Crane, Reference Kenrick and Crane1997; Hao and Xue, Reference Hao and Xue2013; Edwards et al., Reference Edwards, Morris, Richardson and Kenrick2014; Gensel and Berry, Reference Gensel and Berry2016; Xue et al., Reference Xue, Huang, Wang, Xiong, Liu and Basinger2018; Niklas and Crepet, Reference Niklas and Crepet2020; Toledo et al., Reference Toledo, Bippus, Atkinson, Bronson and Tomescu2021; Capel et al., Reference Capel, Cleal, Xue, Monnet, Servais and Cascales-Miñana2022). The zosterophyllopsids had a widespread distribution around the world and constituted a dominant component in many Early Devonian assemblages, in particular in the South Chinese floras (Hao and Xue, Reference Hao and Xue2013). Although a large number of species of zosterophyllopsids have been documented, the phylogenetic relationships among the known species remain to be resolved (Gensel, Reference Gensel1992; Kenrick and Crane, Reference Kenrick and Crane1997; Hao and Xue, Reference Hao and Xue2013; Gensel and Berry, Reference Gensel and Berry2016; Nibbelink and Tomescu, Reference Nibbelink and Tomescu2022) and knowledge about the paleogeographic distribution of different genera/species, to be documented by further fossil evidence (Cascales-Miñana and Meyer-Berthaud, Reference Cascales-Miñana and Meyer-Berthaud2015; Capel et al., Reference Capel, Cleal, Servais and Cascales-Miñana2023).
In this paper, we describe new material of a zosterophyllopsid plant from the Lower Devonian of Guizhou Province, China. The fossils show morphology and well-preserved anatomical structures, justifying their assignment to the genus Gosslingia Heard, Reference Heard1927, which was previously known only from the Lower Devonian of the Laurussian paleocontinent (Heard, Reference Heard1927; Croft and Lang, Reference Croft and Lang1942; Edwards, Reference Edwards1970; Kenrick and Edwards, Reference Kenrick and Edwards1988; Kenrick and Crane, Reference Kenrick and Crane1997). Our discovery expands the diversity of the Early Devonian floras of South China and presents an opportunity to consider the floristic exchanges between South China and Laurussia.
Geological setting and stratigraphy
The fossils were collected from a bed of gray muddy siltstone of the Mangshan Group at a locality near Dahe Village, Duyun City, Guizhou Province, China (Fig. 1, Dahe section). Recently, abundant plant fossils were obtained from the lower part of this group at the nearby Baoyang section (Fig. 1), including Adoketophyton subverticillatum (Li and Cai) Li and Edwards, Hedeia sp. (or Yarravia sp. based on the recent systematic treatment sensu McSweeney et al., Reference McSweeney, Shimeta and Buckeridge2021), Teyoua antrorsa Huang, Liu and Xue, and Zosterophyllum australianum Lang and Cookson (Huang et al., Reference Huang, Liu and Xue2022; Zhou et al., Reference Zhou, Zhao, Wang, Wu, Xiao and Huang2022).
Figure 1. Maps showing the fossil locality (Dahe section) near Dahe Village, south of Duyun City, Guizhou Province, southwestern China. The location of the Baoyang section is also shown, where Adoketophyton subverticillatum (Li and Cai) Li and Edwards, Hedeia sp., Teyoua antrorsa Huang, Liu and Xue, and Zosterophyllum australianum Lang and Cookson were reported from the lower part of the Mangshan Group (Huang et al., Reference Huang, Liu and Xue2022).
The lithology and age of the Mangshan Group at the Baoyang section was recently reviewed by Huang et al. (Reference Huang, Liu and Xue2022). Based on our observations, the lithology of this group is consistent at the Dahe and Baoyang sections. The lower part of this group consists mainly of gray massive quartz sandstone, with a small amount of gray siltstone and mudstone, and the upper part consists of gray fine sandstone and muddy siltstone, with oolitic hematite beds (Dai et al., Reference Dai, Chen, Wang, Lu and Wang2017). A Pragian age has been suggested for the lower part of the Manshan Group, based on fossil plants including Adoketophyton subverticillatum and Zosterophyllum australianum (see Huang et al., Reference Huang, Liu and Xue2022), two species also occurring in the Pragian-aged Posongchong Formation (Hao and Xue, Reference Hao and Xue2013).
Material and methods
The specimens are preserved as impressions and compressions, as well as pyrite permineralizations. Morphologically preserved specimens were prepared with fine needles and photographed with a digital camera. Permineralized axes with internal cellular structures were prepared by the standard procedure of embedding, section, and polishing for examination by reflected light. Several pyritized axes without any acid treatment were observed with scanning electron microscopy (SEM) at the Key Laboratory of Orogenic Belts and Crustal Evolution, Peking University.
Repository and institutional abbreviation
All specimens and slides in this study were deposited at the Geological Museum, School of Earth and Space Sciences, Peking University (specimens PKUB18001 through PKUB18054; and slides 18001-24 through 18001-53).
Systematic paleobotany
Class Zosterophyllopsida Bierhorst, Reference Bierhorst1971
Order Gosslingiales sensu Hao and Xue, Reference Hao and Xue2013
Family Gosslingiaceae Banks, Reference Banks and Drake1968
Genus Gosslingia Heard, Reference Heard1927
Type species
Gosslingia breconensis Heard, Reference Heard1927, by original designation, from the Senni Formation at Brecon Beacons Quarry, South Wales, UK.
Diagnosis
See Edwards (Reference Edwards1970, p. 237).
Gosslingia cf. G. breconensis Heard, Reference Heard1927
Figures 2–7
Occurrence
Lower Devonian Mangshan Group, Dahe section, near Dahe Village, Duyun City, Guizhou Province, China (GPS data: 26°9′44″N, 107°28′15″E).
Figure 2. Gosslingia cf. G. breconensis Heard, Reference Heard1927: (1, 2) part and counterpart, main axis pseudomonopodially branched, producing two lateral branching systems (arrow lbs) and a possible third lateral branch (lowermost arrow); the positions of anatomy illustrated in later figures are indicated; PKUB18001a, b; (3) a first-order branch, producing a second-order branch to the right (arrow), which is dichotomously divided twice and terminates in circination; the anatomy of the other first-order branch is shown in Fig. 6.1; PKUB18003; (4–7) circinate apices of different development degrees: (4) PKUB18028; (5) PKUB18018; (6) PKUB18010; (7) PKUB18002. Scale bars = 20 mm (1, 2); 10 mm (3); 3 mm (4, 5); 1 mm (6, 7).
Description
Morphology.—The largest specimen shows a main axis that is ~3.5 mm wide at the basal position and up to 19.8 cm long (Fig. 2.1, 2.2). This main axis is pseudomonopodially branched to produce at least two lateral branching systems (Fig. 2.1, arrows lbs), within which first-order branches further divide pseudomonopodially and bear second-order branches. A third lateral branching system is probably present at a lower position (extending into the rock matrix), indicated by the twist of the main axis (Fig. 2.1, lower arrow). First-order branches, ~1.8 mm wide at the proximal position, depart from the main axis at angles of ~35°, with an interval of ~50 mm. Other specimens are preserved as small fragments. The specimen in Figure 2.3, interpreted as a lateral branching system, shows a first-order branch that appears to pseudomonopodially branch to produce a second-order branch (Fig. 2.3, arrow), which further divides twice and terminates in circinate apices. All axes are naked, with no spines or appendages on the surface. Unfortunately, no fertile axes with sporangia attached were found.
More than 23 circinate apices were found (Fig. 2.3–2.7), but only a few are attached to larger axes (Fig. 2.3). Some apices are oppositely recurved and paired, indicating that they resulted from the dichotomy of a parent axis (Fig. 2.5); many others are singly preserved and it is deduced that the other apex of each pair has been broken during preservation. From the branching point to the tapering tip, these apices are ~5–8 mm in length. Two apices are unique in showing two or more loops, reaching > 15 mm length and ~2.9 mm width in loop diameter (Fig. 2.6, 2.7), i.e., these looped apices are approximately twice as long as the above paired, recurved apices. Similar looped apices were also found occurring on fertile axes of Gosslingia (Edwards, Reference Edwards1970). Nevertheless, it is difficult to determine the distribution of these different types of circinate apices on the plant body, due to their fragmentary preservation.
Subaxillary tubercle branches can be seen through careful dégagement in some specimens (Fig. 3, arrows), although their complete morphology is difficult to determine. In our examples, such branches are located below or quite close to branching points of the main axes, extending into the rock matrix, and thus are oriented at right angles to the plane of the main branching system. The orientation of such subaxillary tubercles makes them difficult to observe in many compression specimens.
Figure 3. Gosslingia cf. G. breconensis Heard, Reference Heard1927: (1–4) subaxillary tubercle branches (arrows) below or close to branching points of main axes: (1, 3) subaxillary branch extending into rock matrix; (2) subaxillary branch with only its base visible (as coalified compressions); (4) subaxillary branch with its base visible; dotted line indicates the base of a lateral branch, which has been partially removed through dégagement. (1, 2) PKUB18001; (3) PKUB18003; (4) PKUB18030. Scale bars = 2 mm.
Anatomy.—The xylem strand is elliptical in transverse sections of pyritized axes and shows a consistent shape at different levels of the main axes (Fig. 4.1–4.9) and lateral branches (Fig. 4.10–4.12). Xylem strands of the main axes are ~1.6 mm wide along the long axis, 0.5 mm wide along the short axis, with a mean area of 0.6 mm2 (n = 9); those of the lateral branches are 0.9 mm by 0.3 mm wide with a mean area of 0.2 mm2 (n = 3). It should be noted that these axes are apparently compressed, so that the ratio between the measured long axis and short axis is larger than its actual value, whereas the area of xylem strand can little change.
Figure 4. Gosslingia cf. G. breconensis Heard, Reference Heard1927, successive transverse sections of the main axis and its branch shown in Figure 2.1 and 2.2, examined by light microscope with reflected light, showing variations in size and shape of xylem strand. Slides (1) 18001-24a; (2) 18001-26b; (3) 18001-28b; (4) 18001-30a; (5) 18001-31a; (6) 18001-36a; (7) 18001-40b; (8) 18001-44a; (9) 18001-44b; (10) 18001-46a; (11) 18001-51a; (12) 18001-48a. Scale bars = 500 μm; that in Figure 4.5 is for 1–10.
Tracheids show an obvious decrease in size toward the periphery (Figs. 4, 5), and thus maturation of the xylem is exarch. Tracheids at the periphery, interpreted as protoxylem, range from 9–13 μm wide (n = 13); and the other tracheids, interpreted as metaxylem, range from 34–65 μm wide (n = 123) in transverse sections. It appears that protoxylem is more prominent at the two ends of the elliptical strand (Fig. 4).
Figure 5. Gosslingia cf. G. breconensis Heard, Reference Heard1927, enlarged views of transverse sections of main axis, examined by light microscope with reflected light: (1) elliptical xylem strand; slide 18001-24a; (2) enlarged view of xylem strand showing protoxylem tracheids; poorly preserved inner cortex cells are shown; slide 18001-24a; (3–6) enlarged views of protoxylem and metaxylem tracheids; slides (3) 18001-25a; (4) 18001-26b; (5) 18001-27a; (6) 18001-46b. Scale bars = 200 μm (1); 100 μm (2–6).
The cortex can be divided into two zones. Where preserved, the outer cortex is composed of two to four layers of thick-walled cells (Fig. 6.5). In transverse sections, the outer cortical cells are polygonal and measure 19–38 μm in width (n = 23; Fig. 6.5). Cells are rarely preserved in the region of the inner cortex. There are some poorly preserved cells surrounding the xylem strands (Fig. 5.2, 5.4, 5.6), probably representing the vestige of inner cortex tissues; these cells measure ~14–28 μm wide (n = 36), and their cell walls appear thinner than those of the outer cortical cells.
Figure 6. Gosslingia cf. G. breconensis Heard, Reference Heard1927, transverse sections of main axes and lateral branches (SEM): (1, 2, 5–7) permineralized fragments from the upper first-order branch shown in Figure 2.3; PKUB18003-02: (1) first-order branch and its elliptical xylem strand; (2) enlarged view of xylem tracheids in (1); (5) epidermis and outer cortex tissue; (6, 7) detail of metaxylem tracheids, showing C-shaped coalified cell wall (arrow wc), middle lamella (arrow ml), and lumen cast (lc); (3, 4) permineralized fragments from the main axis shown in Figure 2.1; PKUB18001-23: (3) marginal region of xylem, showing pyritized tracheids; (4) detail of metaxylem tracheids. Scale bars = 200 μm (1); 150 μm (2); 100 μm (3, 5); 25 μm (4, 6); 30 μm (7).
In transverse sections, tracheids show a pyrite lumen cast and coalified remains of cell walls (Fig. 6.6, lc, wc; Fig. 6.7). Outside the coalified cell wall, the layer that corresponds in position to the middle lamella is heavily permineralized, showing as irregular crystals (Fig. 6.6, arrow ml). In transverse sections, coalified remains of cell walls are shown as thick, dark bands, often C-shaped and protruding into the lumen (Fig. 6.6, arrow wc), and these remains represent annular or helical thickenings of the cell walls (compare Kenrick and Edwards, Reference Kenrick and Edwards1988, fig. 5).
Lumen casts of tracheids are commonly seen in longitudinal sections, and protoxylem tracheids are located at the periphery of the xylem strand, confirming the exarch maturation pattern observed in transverse sections (Fig. 7.1). These lumen casts show grooves that are separated by a regular distance, indicating the position of annular and helical thickenings of cell walls. Coalified walls are also visible in longitudinal sections and show annular and helical thickenings (Fig. 7.2–7.8), corresponding well to the grooves in lumen casts (e.g., Fig. 7.7). In some longitudinal sections, coalified wall thickenings can be seen forming fold-like structures, protruding into the grooves of lumen cast (i.e., protruding into the cell lumen; Fig. 7.7, arrows); the bar thickenings are uniform in thickness and there is a break inside of such fold-like structures. The mean height of the thickening bars is ~3.7 μm and the mean distance between two annular bars is ~6.6 μm.
Figure 7. Gosslingia cf. G. breconensis Heard, Reference Heard1927, longitudinal sections of tracheids of the main axis shown in Figure 2.1 (SEM): (1) lumen cast, showing interpreted protoxylem (px) and metaxylem (mx) tracheids, with annular thickenings (represented by grooves); note the changes in tracheid diameter; outside of the xylem strand is area of inner contex (ic); PKUB18001-22; (2) two trachieds of different preservation status; right tracheid shown as lumen cast, preserved by pyrite permineralization; left tracheid showing coalified cell walls with perforations; PKUB18001-02; (3, 5, 7, 8) PKUB18001-05: (3) coalified cell walls with perforations; middle lamella (ml) poorly preserved and featureless; (5) perforations on cell walls between thickening bars; (7) tracheids of different preservation status; left tracheid shows coalified cell walls with thickening bars and perforations; right tracheid shows both coalified cell walls and pyrite lumen cast; arrows indicate fold-like structures of coalified cell walls, which protrude into grooves of lumen cast; (8) smooth surfaces, probably of middle lamella remains; (4) smooth surfaces, probably of middle lamella remains; PKUB18001-02; (6) coalified cell walls with thickening bars (broken) and perforations; PKUB18001. Scale bars = 40 μm (1); 20 μm (2, 3, 6, 8); 30 μm (4); 10 μm (5, 7).
The coalified walls between annular and helical thickenings are perforated. The perforations are circular and range from 1.0–2.0 μm in diameter. On lumen casts, there are some projections on the surface that might be formed during permineralization and cannot be considered to directly correspond to the perforations on coalified walls. Except for the perforated secondary walls, some coalified remains of cell walls show smooth surfaces in longitudinal section, and these remains are interpreted as possibly representing a part of the middle lamella without perforations (Fig. 7.4, 7.8).
The features of tracheids in our plant are consistent with the G-type tracheids, defined as having annular and helical thickenings, fine perforations between thickening bars, and two-layered cell walls comprised of a coalified inner layer and an outer layer that is usually heavily permineralized in fossils (Kenrick and Edwards, Reference Kenrick and Edwards1988; Kenrick et al., Reference Kenrick, Edwards and Dales1991; Kenrick and Crane, Reference Kenrick and Crane1997; Wang et al., Reference Wang, Hao and Wang2003).
Materials
Fifty-four specimens with main axes and lateral branches, some of which are preserved by pyrite permineralization, and 30 slides produced from them.
Remarks
Compared to other known early vascular plants, our plant is more similar to the genus Gosslingia (Edwards, Reference Edwards1970; Kenrick and Edwards, Reference Kenrick and Edwards1988; Kenrick and Crane, Reference Kenrick and Crane1991), with the shared characters including pseudomonopodial branching of main axes, pseudomonopodial lateral branching systems, branches developed from subaxillary tubercles, circinate apices, elliptical xylem strand, exarch maturation of xylem, and G-type tracheids (Table 1). Deheubarthia Edwards, Kenrick, and Carluccio also exhibits the above characters, but its axes are spiny (Edwards et al., Reference Edwards, Kenrick and Carluccio1989). In the type and only species of Gosslingia, Gosslingia breconensis, there are protuberances on the surface of axes (Edwards, Reference Edwards1970), whereas such structures are not present in our plant. The lack of fertile specimens in our material hinders further comparisons with Gosslingia, but the similarities demonstrated in both branching pattern and anatomical details of sterile axes strongly support the assignment of our plant to Gosslingia.
Table 1. Comparisons of our plant (Gosslingia cf. G. breconensis Heard, Reference Heard1927) and the type species of Gosslingia (Gosslingia breconensis). Data for Gosslingia breconensis based on Edwards (Reference Edwards1970) and Kenrick and Edwards (Reference Kenrick and Edwards1988).
We further compare our plant with Gosslingia breconensis in terms of tracheid dimensions (Table 2; see Fig. 8 for compared parameters). Four parameters of tracheids are quite comparable between our plant and Gosslingia breconensis, including the maximum tracheid lumen diameter (W), thickening aperture diameter (D), thickness of annular bars (δ), and diameter of perforations (Dp). However, both the distance between annular bars (ɛ) and the distance of annular bars protruding into cell lumen (t) in our plant are significantly greater than in Gosslingia breconensis. Nevertheless, such differences in tracheid features are to be expected as part of natural variability among plants of the same species, and at present it is appropriate to name our plant as Gosslingia cf. G. breconensis. An exact determination at the specific level needs further discovery of fertile specimens.
Table 2. Comparisons of tracheid dimensions of our plant (Gosslingia cf. G. breconensis Heard, Reference Heard1927) and the type species of Gosslingia (Gosslingia breconensis). See Figure 8 for the definition of compared parameters. All measurements in μm and shown as minimum value—mean—maximum value (n = number of measurements). Data for Gosslingia breconensis based on Kenrick and Edwards (Reference Kenrick and Edwards1988, table 1), except for its Dp, which was obtained by measuring the illustrations of Kenrick and Edwards (Reference Kenrick and Edwards1988, figs. 15, 16). - = unknown.
Figure 8. Diagrammatic longitudinal sections of two adjacent tracheids of Gosslingia cf. G. breconensis Heard, Reference Heard1927, showing parameters for comparison of tracheid features in Table 2. Annular thickening bars protrude into the cell lumen. Black region = secondary cell walls. Diagram and parameter definition based on Kenrick and Edwards (Reference Kenrick and Edwards1988, fig. 26). Measured parameters include: δ = thickness of annular bars; ɛ = distance between annular bars; t = distance of annular bars protruding into cell lumen; D = thickening aperture diameter; Dp = diameter of perforation; Twc = thickness of coalified wall; W = maximum tracheid lumen diameter.
Discussion
Zosterophyllopsids were a prominent component of floras all around the world from the Silurian (Ludfordian) through the Devonian (Frasnian), including > 37 genera (Hueber and Banks, Reference Hueber and Banks1979; Gensel and Andrews, Reference Gensel and Andrews1984; Gensel, Reference Gensel1992; Kenrick and Crane, Reference Kenrick and Crane1997; Kotyk et al., Reference Kotyk, Basinger, Gensel and de Freitas2002; Hao and Xue, Reference Hao and Xue2013; Gensel and Berry, Reference Gensel and Berry2016; Nibbelink and Tomescu, Reference Nibbelink and Tomescu2022). However, the phylogenetic relationships among these genera are controversial, and thus a consensus on the taxonomic treatment at the ordinal and family levels has not been reached. Niklas and Banks (Reference Niklas and Banks1990) divided zosterophyllopsids into one group with ‘terminate’ (i.e., determinate) fertile axes (e.g., Zosterophyllum Penhallow and Gumuia Hao) and the other group with ‘nonterminated’ (i.e., indeterminate) fertile axes (e.g., Gosslingia and Sawdonia Hueber). In addition, sporangium orientation, sporangial arrangement, circinate apices, etc., have also been used for classification and phylogenetic inference (Gensel, Reference Gensel1992). The phylogenetic schemes of Kenrick and Crane (Reference Kenrick and Crane1997) and Nibbelink and Tomescu (Reference Nibbelink and Tomescu2022) similarly recognized a ‘large zosterophyll clade’ (designated as the Sawdoniales by the former authors), which includes Deheubarthia, Thrinkophyton Kenrick and Edwards, Gosslingia, Serrulacaulis Hueber and Banks, Sawdonia, and Crenaticaulis Banks and Davis, among others. Hao and Xue (Reference Hao and Xue2013) divided the Zosterophyllopsida into two orders, the Zosterophyllales and the Gosslingiales. The latter order includes the Gosslingiaceae that is represented by Gosslingia and Oricilla Gensel (Hao and Xue, Reference Hao and Xue2013).
The genus Gosslingia is monotypic and the type species, Gosslingia breconensis, has been extensively studied based on materials from the Lower Old Red Sandstone of Wales, UK (Heard, Reference Heard1927; Croft and Lang, Reference Croft and Lang1942; Edwards, Reference Edwards1970; Kenrick and Edwards, Reference Kenrick and Edwards1988; Kenrick and Crane, Reference Kenrick and Crane1991). Thus Gosslingia breconensis represents one of the best-characterized plants among zosterophyllopsids. However, the geographic distribution of this genus outside of Wales has been questionable until now. There have been some other records of Gosslingia, e.g., from Germany (Schweitzer, Reference Schweitzer1979, Reference Schweitzer1983, Reference Schweitzer1987), Belgium (Gerrienne, Reference Gerrienne1983, Reference Gerrienne1993), and the United States (Tanner, Reference Tanner1982), but these occurrences have been questioned due to the lack of clear diagnostic characters (e.g., Kenrick and Crane, Reference Kenrick and Crane1997). Our material shows convincing evidence for the occurrence of Gosslingia in South China, although further efforts are still needed for collecting fertile specimens.
Capel et al. (Reference Capel, Cleal, Servais and Cascales-Miñana2023) recognized two phytochoria units during the Pragian–Emsian periods based on paleogeographical distribution of plant genera, i.e., Equatorial to mid-latitudes and Northeast Gondwana. The Early Devonian floras of the South China Block, as a part of the Northeast Gondwana unit, show high endemism at the generic level, and contain only a limited number of cosmopolitan genera. The latter belong to distinct lineages, including zosterophyllopsids (Distichophytum Mägdefrau, Oricilla, and Zosterophyllum), lycopsids (Baragwanathia Lang and Cookson and Drepanophycus Göppert) and euphyllophytes (Estinnophyton Fairon-Demaret, Hedeia Cookson, and Psilophyton Dawson) (Hao and Xue, Reference Hao and Xue2013; Xue et al., Reference Xue, Huang, Wang, Xiong, Liu and Basinger2018, and references therein). Gosslingia is a new addition to the list of cosmopolitan zosterophyllopsids. Distichophytum and Oricilla show rowed sporangia, like Gosslingia, indicating that zosterophyllopsids with rowed sporangia are not uncommon in South China. Zosterophyllum was widespread in western Europe, North America, and South China during the Early Devonian (Kenrick and Crane, Reference Kenrick and Crane1997; Capel et al., Reference Capel, Cleal, Xue, Monnet, Servais and Cascales-Miñana2022; Nibbelink and Tomescu, Reference Nibbelink and Tomescu2022), and the South Chinese species within this genus are predominantly those with helically arranged sporangia (Hao and Xue, Reference Hao and Xue2013; Xue et al., Reference Xue, Huang, Wang, Xiong, Liu and Basinger2018). The distribution of these cosmopolitan genera indicates that the dispersal of early vascular plants, which were relatively small and morphologically simple, among different paleocontinents was more common than previously appreciated.
It remains an open question about how early vascular plants migrated across areas that were separated by long distances and, usually, by geographic barriers, e.g., oceans. They might rely on spread by means of wind, ocean currents, or storms. Capel et al. (Reference Capel, Cleal, Servais and Cascales-Miñana2023) proposed that spatial differentiation of early vegetation was controlled by both climatic conditions and geographical barriers. During the Pragian, the Equatorial to mid-latitudes phytochoria unit and the Northeast Gondwana unit were separated by apparently wide oceans in low to middle latitudes, but were probably connected in high latitudes, e.g., from west to middle Gondwana (Boucot et al., Reference Boucot, Chen and Scotese2013; Huang et al., Reference Huang, Yan, Piper, Zhang, Yi, Yu and Zhou2018). Thus, dispersal routes that connected the Equatorial to mid-latitudes and Northeast Gondwana might have been possible in high latitudes under warmer climatic conditions, but the paucity of paleobotanic data from west-middle Gondwana hinders a better understanding.
Acknowledgments
This work was supported by National Key R&D Program of China (2022YFF0800200) and the National Natural Science Foundation of China (No. 41722201). We thank M. Tomescu and E.P. Coturel, and the editors, for their valuable comments.
Declaration of competing interests
The authors declare none.
