A New Fern-like Plant Xinhangia spina Gen. et sp. Nov. from the Upper Devonian of China

Simple Summary In 2019, Late Devonian Xinhang forest was reported from Anhui Province, China. It represents the earliest forest in Asia and China, and was regarded as monospecific with numerous small trees of the lycopsid. Recently, we found some other plants from the forest and now erect a new fern-like plant named Xinhangia. It is small with height of tens of centimeters, and usually has two orders of branches in alternate and sometimes triseriate pattern. Spines densely occur on the main axes and primary branches. Its leaf-like organs are very simple and dichotomize into recurved tips. Fertile organs are similar to the “leaves” but terminated in elongate and paired sporangia. Both the “leaves” and fertile organs are alternately arranged on secondary branches. In anatomy, the primary xylem is “8”-shaped and is surrounded by secondary xylem. With simple characters, Xinhangia represents a morphologically primitive plant and is of uncertain affinity at class or order level. As a component of Xinhang forest, Xinhangia will help understand the growth habit and habitat of fern-like plants, as well as the structure and ecology of ancient forests in the future. Abstract Palaeozoic fern-like plants show great diversity in their morphology and/or anatomy. Within this group, a novel taxon, Xinhangia spina gen. et sp. nov., is now reported from the Upper Devonian (Famennian) Wutong Formation of Anhui Province, China. The primary and secondary branches are borne alternately and sometimes in a triseriate pattern. Spines are evident on the main axes or stems and on the primary branches. Vegetative ultimate appendages with recurved tips are alternate, usually dichotomous 1–2 times, and sometimes as an aphlebia located at the base of primary or secondary branches. Fertile ultimate appendages are alternate, usually dichotomous 1–2 times, and terminate in elongated and paired sporangia. The stele has a clepsydroid-like primary xylem with each end bearing a protoxylem strand. The secondary xylem surrounding the primary xylem illustrates uniseriate rays. With rare divisions in both the vegetative and fertile ultimate appendages, Xinhangia represents a morphologically primitive plant. It is of uncertain affinity at the class or order level. The stelar architecture suggests that the clepsydroid stele may not be emphasized in discussing the relationship among fern-like plants such as rhacophytaleans.


Description
The description of Xinhangia involves its morphology (Figures 1-6) and anatomy (Figures 7 and 8). The measurements of the morphology and anatomy are given in Table 1.

Main Axes and Primary Branches
The main axes or stems have an upright habit and are covered by dense spines of 1-3 mm in length ( Figures 1A-F and 2a). One main axis is dichotomous, up to 6 mm in diameter and 11 cm in length ( Figure 1A,B). The main axes sometimes present a slightly zigzag shape, bending at the position where the primary branches occur ( Figure 1F). No roots are found on the main axes.
The primary branches are straight in most cases ( Figures 1F-H and 3B,D) and slightly zigzag in some instances (Figures 3E and 4A). They are arranged on the main axes in alternate ( Figure 1E) or triseriate (Figures 1F and 2a) branching patterns at 45-70 • . The paired primary branches ( Figure 1F, arrows 2, 3) and a single primary branch ( Figure 1F, arrow 6) constitute a triseriate pattern. A presumed aphlebia is dichotomous and inserted at the base of a primary branch ( Figure 1J). The primary branches are 0.7-3.0 mm in diameter and up to 6.5 cm long. Compared to those on the main axes, the spines on the primary branches are sparse and short. No ultimate appendages are visible on the primary branches.

Vegetative Secondary Branches and Vegetative Ultimate Appendages
The secondary branches are alternately arranged on the primary ones at 50-90 • (Figures 1F and 3D,F). They lack spines and measure 0.4-0.7 mm in diameter and up to 5 cm long. A single dichotomous aphlebia is located at the base of some of the secondary branches ( Figure 3B, arrow 2; Figure 3C, arrow 2; Figure 3G, arrow 1). The aphlebiae are similar in shape to the vegetative ultimate appendages but appear a little larger. The upper portion of a secondary branch is preserved, tapered and appears distally recurved ( Figure 3I).

Fertile Branches
Most of the fertile secondary branches occur alternately on the primary branches at 50-90 • (Figures 1H and 4A). However, as exemplified by one specimen (Figures 1G,H,K and 2b), three fertile secondary branches alternate on the lower part of the primary branch; a pair of fertile secondary branches and a single branch occur oppositely or sub-oppositely on the upper part of the primary branch. In this example, these fertile secondary branches appear to form a triseriate branching pattern. The fertile secondary branches (e.g., Figure 4E,I) are quite similar to the vegetative ones (e.g., Figure 3A,B); their only difference lies in the separate occurrence of fertile and vegetative ultimate appendages. A complete secondary branch shows alternate fertile ultimate appendages (fertile organs) and terminates in one fertile organ (Figures 5D and 6a). No aphlebiae are found inserted at the base of the secondary branches.
Tertiary branches were discovered only in one specimen ( Figure 1H). They are alternately arranged and bear fertile organs (Figure 2b, tb), resembling fertile secondary branches (e.g., Figure 4G-I) in shape and size.

Fertile Organs
Fertile organs are inserted mostly on the secondary branches ( Figure 4) and rarely on the tertiary ones (Figure 2b). A fertile organ consists of two parts, i.e., the terminal sporangia and a basal axis. The sporangia with pointed tips are elongate in shape and borne in pairs (Figures 5 and 6). They range 0.3-0.5 mm wide and 0.9-1.5 mm long. The smooth basal axes within the fertile organs measure 0.2-0.4 mm in diameter and usually dichotomize but sometimes do not. In the distal area of the secondary branches, such axes lack dichotomy and terminate in one pair of sporangia ( Figure 5E,N). In most of the examples, the basal axes dichotomize once or twice to produce two or four pairs of terminal sporangia ( Figure 5H,O,P,T and Figure 6g). Occasionally, the basal axes dichotomize thrice to form possibly eight pairs of sporangia ( Figure 5I,J and Figure 6e). As to one fertile organ with twice dichotomizing, the serial dégagement shows two pairs of terminal sporangia in three dimensions ( Figure 5T-W and Figure 6h).

Anatomy
Two limonitized axes containing only the xylem, ca. 2 mm in diameter, were embedded and transversely sectioned into 18 and 15 slices, respectively ( Figure 7A). Of these, 11 slices are selected to show the relatively complete structure ( Figure 7B-L). The stele has a primary xylem surrounded by a radial secondary xylem. Though these two axes were preserved as isolates without an organic connection with the morphological parts, we believe that they belong to Xinhangia, with a high probability, because of the close relationship of the preservation and correspondence between the two-poled pattern in the anatomy and the alternate branching pattern in the morphology. For the limonitized axis observed by SEM ( Figure 8A, left arrow), a spiny axis was preserved beside it ( Figure 8A, right arrow), indicating the close relationship among the axes in Figure 8A. Considering the tissues outside the xylem and the diameter of the main axes and branches, it is assumed that these limonitized axes represent the main axes.
The primary xylem, 720-870 µm by 220-360 µm in the transverse section, is mesarch in maturation and has a clepsydroid shape in the cross-section ( Figure 7M-Q). At each end of the primary xylem, there is a prominent protoxylem pole ( Figure 7M-Q, arrows), which may have been originally filled with parenchymal cells. The smallest tracheids surrounding the protoxylem poles indicate the protoxylem and measure 10-25 µm in diameter. The bigger tracheids between and surrounding the protoxylem represent metaxylem and 30-51 µm in diameter.
The tracheids of the secondary xylem are arranged in radial files ( Figure 7R-T) and are 32-73 µm in diameter. In the transverse sections, the rays can be recognized between the rows of the secondary xylem tracheids ( Figure 7R-T, arrows). With SEM, the uniseriate rays appear to occur between every row of the secondary xylem tracheids ( Figure 8D, J,N,P). A single ray is one to six cells in height ( Figure 8C,F,H,K). The secondary wall of the secondary xylem tracheids may possess scalariform thickenings ( Figure 8L,M). In the tangential section ( Figure 8D), circular to elliptical bordered pits are visible in the walls of the secondary xylem tracheids ( Figure 8J,O,P). Table 2 indicates comparisons among Xinhangia, other fern-like plants and related groups. The comparisons involve the vegetative branching pattern, basal aphlebiae, fertile organs (fertile ultimate appendages) and stelar architecture.
Xinhangia and most cladoxylopsids s.l. share similar fertile organs (three-dimensional dichotomous branches, terminated by elongate and paired sporangia) and mesarch primary xylem. However, Xinhangia is characterized by a triseriate branching pattern and clepsydroid stele, which are absent in the cladoxylopsids s.l.
Whorled organs (the branches and ultimate appendages) and actinostele (a protostele with the primary xylem ribs arranged radially and more or less connected in the stelar center) occur in all the iridopteridaleans but not in Xinhangia. The digitate branching and dissected stele characterizing the pseudosporochnaleans are absent in Xinhangia. Furthermore, the iridopteridaleans and pseudosporochnaleans usually have no secondary xylem. Dissected stele typifies (most) nonpseudosporochnaleans but is not recorded in Xinhangia.
Among the nonpseudosporochnaleans, Panxia [36,37] is known for its morphology. As with Xinhangia, Panxia has alternate branches and simple vegetative ultimate appendages with distal recurving. Nevertheless, its discoidal sporangia with stalks are borne laterally and closely. Metacladophyton [12,31] is treated as a nonpseudosporochnalean [23]. It morphologically differs from Xinhangia mainly in the whorled and decussate branches.
Denglongia [13,14] is placed in the cladoxylopsids s.s., but is not allied with the nonpseudosporochnaleans or nonpseudosporochnaleans [23]. It shows clear differences with Xinhangia in the whorled branches, complex and unique fertile organs with alternate segments and actinostele.
As in Xinghangia, Eocladoxylon [19], Melvillipteris [6] and Protopteridophyton [20] show elongate and sometimes paired sporangia terminating in three-dimensionally dichotomous axes; Ellesmeris [18], Eocladoxylon, Melvillipteris and Rhacophyton possess basal aphlebiae; Ellesmeris, Eocladoxylon and Rhacophyton demonstrate clepsydroid xylem. However, all of these plants lack triseriate branching and some present quadriseriate branching; Rhacophyton has fertile organs paired at the branch base, and such arrangement occurs sometimes in Eocladoxylon; the fertile organs of Rhacophyton are very complex and unique, with pinnate segments, and those of Protopteridophyton are quadriseriate in arrangement and also complex.

Stauropteridales
Late Devonian-Carboniferous Stauropteridales [2] is characterized by quadriseriate branching, usually single sporangium terminating branches and actinostele usually with four primary xylem ribs and without secondary xylem. Such morphological and anatomical features are lacking in Xinhangia. Among the members, Multifurcatus has trichotomous branches, basal aphlebiae and a single sporangium at the node of the branches [39]. Its trichotomous branches appear to form alternate pairs. In contrast, Xinghangia possesses triseriate branches and terminal sporangia in pairs.

Shougangia
Late Devonian (Fammenian) Shougangia is unknown for affinity at the class level. As a derived fern-like plant, it possesses helical branches, laminate vegetative leaves, very complex fertile organs, which terminate in branches with pinnules and dichotomize up to 10 times, and dissected stele [4,5]. These traits show clear differences from Xinhangia, although both genera have terminal and elongate sporangia in pairs.

Aneurophytales
Middle to Late Devonian (Late Eifelian to Frasnian) Aneurophytales is considered the most primitive progymnosperm [2,22], which is characterized by: (1) three-dimensional branching systems with laterals helically or decussately arranged, (2) a primary xylem stele consisting of three or more ribs with protoxylem strands occurring near the tips and along the midplanes of the ribs, (3) elongate sporangia terminally inserted on the ultimate pinnate fertile appendages [2]. Xinhangia exhibits an alternate and sometimes a triseriate branching pattern, which differs from the Aneurophytales. The clepsydroid-shaped primary xylem and protoxylem strands occurring around the two poles in Xinhangia distinguish it from the order. Furthermore, the Aneurophytales shows more complicated style in their fertile appendages, while the fertile organs of Xinhangia are simpler, by contrast.

Discussion
Xinhangia and most fern-like plants share fertile organs with three-dimensionally dichotomous axes terminated by elongate and paired sporangia and mesarch primary xylem with a permanent protoxylem (strand near the periphery of the xylem). Nevertheless, this taxon cannot be placed in the Iridopteridales, with whorled organs and actinostele, or in the Pseudosporochnales, with digitate branching and dissected stele, or in the nonpseudosporochnaleans with dissected stele. It cannot be assigned to the Rhacophytales and Stauropteridales, which typically have a quadriseriate branching pattern. It also cannot be classified into Aneurophytales, for their helical or decussate branching pattern, multiple ribs in the stele and the complex fertile appendages. Thus, Xinhangia is now treated as a fern-like genus of an uncertain affinity at the class or order level.
Rhacophyton ceratangium and Ellesmeris sphenopteroides possess both quadriseriate branching and clepsydroid xylem. Specifically, in R. ceratangium, the common bases of each pair of primary branches are alternately attached to the main axis or stem [17,39]; in E. sphenopteroides, the common bases of the paired primary branches have the same arrangement, and the secondary branches are unpaired and borne alternately or suboppositely [18]. Either the main axes or primary branches in both plants contain a clepsydroid stele. By contrast, in R. zygopteroides, the primary branches are helical on the main axis (probably) containing an actinostele [15]. Alternate branching and clepsydroid stele are also seen in Eocladoxylon [19] and now, in Xinhangia. As to the triseriate primary or secondary branches of Xinhangia, the unpaired branches and the common base of the paired branches are alternately attached. Therefore, in these four plants, the common bases of the paired branches and/or unpaired branches occur in a single plane, correlating to the clepsydroid stele with the primary xylem bearing two peripheral protoxylem strands. This suggests that the clepsydroid stele may not be highly stressed in discussing relationships among plants such as rhacophytaleans.
The co-evolution of the vegetative and fertile organs has been suggested for the fernlike plants [4]. The derivation of the laminate leaves from planate ultimate appendages is accompanied by the increase in complexity of the fertile organs (number of internal divisions and terminal sporangia). Of Xinhangia as a plesiomorphic taxon, the vegetative ultimate appendages, with the distal recurving and rare divisions, are simple and far from planation. Accordingly, the fertile organs divide usually only 1-2 times to produce few terminal sporangia. The secondary xylem provides mechanical support and, fundamentally, water conductance for fern-like plants with planate ultimate appendages or laminate leaves, e.g., Rhacophyton and Shougangia [3,5]. As to Xinhangia lacking leaf-like organs or leaves, the secondary xylem appears necessary to perform a supporting function. Of course, the explanation of such a function in Xinhangia may also depend on the growth habit and understanding of its habitat (under study).

Conclusions
Xinhangia spina gen. et sp. nov. is known from both its morphology and anatomy. It is characterized by primary and secondary branches borne in an alternate and sometimes a triseriate pattern, vegetative and fertile ultimate appendages with rare divisions and a clepsydroid-shaped primary xylem surrounded by secondary xylem. By comparison with other fern-like plants, Xinhangia represents a new and primitive taxon with uncertain affinity. Based on stelar architecture in relation to branching pattern, the anatomical features of some fern-like plants are reconsidered.