4.1 Systematics
The original stem morphogenera include Lepidodendron, Lepidophloios and Sigillaria, in which Lepidodendron as defined by Sternberg (1820) was recently subdivided into several genera such as Diaphorodendron, Synchysidendron, Paralycopodites (= Bergeria) and Ulodendron. Diaphorodendron and Synchysidendron were described based on the anatomically preserved specimens from coal balls (DiMichele 1985; Bateman et al. 1992; DiMichele and Bateman 1992; Phillips and DiMichele 1992; Cleal and Wang 2002; Álvarez-Vázquez and Wagner 2014; Moore et al. 2014), while Paralycopodites and Ulodendron were separated due to the absence of anatomical characters. In Diaphorodendron, the leaf cushions are flattened against the stem and there is no inter area between each other (Moore et al. 2014), these characters are absent in the present fossil and thus different. For Synchysidendron, the upper and lower angles of a leaf cushion are rounded, the leaf cushions become equidimensional on aerial axes as the stem diameter decreases (Cleal and Wang 2002; Álvarez-Vázquez and Wagner 2014; Moore et al. 2014). These features are different from those of the current fossil. The genus Ulodendron only includes the specimens with typical anatomical characters absent. The genus Bergeria, as recently described based on type of a compression form (Álvarez-Vázquez and Wagner 2014), encompasses specimens that lack a well-differentiated leaf scar, and it has usually been applied to partially decorticated lycopsid stems with lepidodendroid leaf cushions. Thus, the specimens with lepidodendroid leaf cushions would be assigned to the genus Bergeria if its leaf cushions have no clearly visible leaf scars. The current fossil, the decorticated Lepidodendron stem with no clear leaf scars, is most likely to be assigned to the genus Bergeria.
Two species of Bergeria, B. dilatata and B. worthenii have similar morphological characters with the current fossil; but B. dilatata has leaf cushions with small, irregular scars at or near the top of the cushion, while B. worthenii has the area above and below the leaf scar ornamented with coarse, transverse, discontinuous wrinkles, which are easily distinguishable from B. wenquanensis.
The leaf cushions of the new fossil material are narrowly fusiform in shape, with its length much longer than the width. Among Cathaysian Lepidodendron, four species have similar leaf cushion like the present fossil: L. dabieshanense Wu (Fig. 4a), L. kirghizicum Zalessky, L. ninghsiaense Sze et Lee (Fig. 4b) and L. szeianum Lee (Fig. 4c). However, the first three species were excluded easily due to the unmatched leaf cushion size or length/width ratio. For example, in L. dabieshanense, the width of leaf cushion is no more than 1 mm (usually 0.5–1 mm); in L. kirghizicum, the length of leaf cushion is no more than 10 mm and width no more than 2 mm; in L. ninghsiaense, the length/width ratio of a leaf cushion is no more than 3 (Sun et al. 2010).
The shape and size of the leaf cushions in the current fossil is most similar to those of Lepidodendron szeianum which was found from the Upper Mississippian of Liaoning, Shanxi, Shandong and Gansu and Lower Permian of Inner Mongolia and Henan (Li 1963; Sun et al. 2010). Their leaf cushions are narrowly fusiform in shape, with a length more than twice the width; the upper and lower apexes of their cushions are acuminate. But the current fossil differs from L. szeianum clearly in the following characters: in the new fossil, the leaf cushions are narrower and long lanceolate in shape with length/width ratio of 7–8; and the upper and lower apexes of a cushion are elongated to lines and appear like needles. In L. szeianum, the leaf cushions are fairly wider, and mostly have a length/width ratio of about 2; and the two ends of a cushion are not elongated or appear like needles. Moreover, in the new fossil, the leaf cushions have clear boundaries between each other; the lower acuminate apex and the upper apex of two adjacent leaf cushions are mostly staggered, occasionally connected; and each side overlaps 1/3 to 1/2 length with its adjacent cushions. In L. szeianum, the leaf cushions are densely arranged on the stem and have no clear boundary between each other, the lower acuminate apex and the upper one of the two adjacent leaf cushions are mostly connected, or only slightly staggered (Sun et al. 2010).
Therefore, the new fossil from the Wenquan County cannot be assigned to any known species. It was thus designated as a new species and named as Bergeria wenquanensis.
4.2 Spatiotemporal distribution of the Cathaysian Lepidodendron
Based on the distributions of Cathaysian lepidodendroid plants (including the species of genus Lepidodendron and of the current Bergeria wenquanensis; Fig. 5), during the Early Mississippian, most of the species were grouped in an old land (South China Plate) near the Equator; only two Xinjiang species (Lepidodendron kirghizicum and B. wenquanensis) are discrete in the north and far from the Equator (Fig. 5a). Thus, the palaeoequatorial hot and humid tropical zone was probably the main distribution area of Cathaysian lepidodendroid plants during the Mississippian (Li and Wu 1996; Naugolnykh and Jin 2014).
In the Middle and Late Mississippian, with the northward drift of the South China Plate, the Cathaysian Lepidodendron also expanded northwards into the Northern Hemisphere, taking the north block of the South China Plate as the center, but most of the species were still distributed near the Equator, except for L. szeianum which was distributed in the north and far from the palaeo-Equator (Fig. 5b). During this time interval, the Cathaysian Lepidodendron flourished and had the highest species diversity (Li and Wu 1996; Sun 2002; Wang and Pfefferkorn 2013).
During the Late Pennsylvanian–Cisuralian transition interval, the Euramerican Lepidodendron became extinct because of dry climate, but the Cathaysian Lepidodendron survived due to the persistent hot and humid climate in the Cathaysian Floral Province (Li and Wu 1996; Li 1997; Sun 1999, 2002; Cleal and Wang 2002; Hilton and Cleal 2007; Taylor et al. 2009; Cleal et al. 2009; Wang et al. 2012; Wang and Pfefferkorn 2013). Since the Cisuralian to the Guadalupian, the distribution of the Cathaysian Lepidodendron expanded northwards continuously with the northward drift of the South China Plate, and as a result, most of the species were distributed in the northern block of the South China Plate rather than near the palaeo-Equator (Fig. 5c–d). The fossil sites of Cathaysian Lepidodendron became sparser gradually in all distribution areas, probably due to strong climate change, related to dry conditions (Cleal and Thomas 2005; Taylor et al. 2009) or due to a glaciation (Wang 2010; DiMichele et al. 2011; DiMichele 2014).
During the Late Permian, the Cathaysian Lepidodendron ceased to expand northwards, but migrated southwards, to the southern block of the South China Plate, obtaining a broader distribution southwards (Fig. 5e). This migration was induced by the formation of the land bridge during this time interval (Li 1997; Rigby 1998; Fluteau et al. 2001; Stevens et al. 2011; Yang and Wang 2012; Bercovici et al. 2012). To the end of the Lopingian, the Cathaysian Lepidodendron eventually became extinct (Sun 1999; Wang and Chen 2001; Rees 2002; Stevens et al. 2011).
4.3 Evolution of leaf cushions in Cathaysian Lepidodendron
Leaf cushions of sixty reported Cathaysian lepidodendroid species were studied. We report that the Mississippian cushions (Fig. 6a) were mainly characterized by the vertically elongated fusiform shape, representing the primitive morphological state (DiMichele and Bateman 1992). During the Pennsylvanian, the cushion shape was dominated by the rhomboidal form. Since the Late Pennsylvanian to the Cisuralian, the species of Lepidodendron experienced a rapid decrease, and only a few of them survived (Booi et al. 2008; Stevens et al. 2011). The cushions of these species show three types of shape: rhombus, fusiform and hexagon. The hexagonal cushion shape appeared after the fusiform and the rhomboidal shapes. During the Guadalupian, the shapes of leaf cushions are more complicated, as five types were reported (Fig. 6a). Most of the species show hexagonal leaf cushions, some other species also have fusiform, rhomboidal, inverted water-drop or horizontal rhomboidal leaf cushions. During the Lopingian, only four types of leaf cushions were left: fusiform, rhomboidal, hexagonal and horizontal rhomboidal (Sun 1999; Wang and Chen 2001; Rees 2002; Stevens et al. 2011).
Based on this evidence, we report that the fusiform leaf cushions (represented by Lepidodendron wenquanensis or L. ninghsiaense) were the earliest during the Mississippian, then the rhomboidal (L. quadratum) and the inverted water-drop shape (L. volkmannianum) during the Late Mississippian. The trapeziform (Lepidodendron liulinense), horizontal rhombus (L. acutangulum) and hexagonal (L. hexagonum) leaf cushions appeared during the Pennsylvanian. The Permian species had the more diverse leaf cushions than the Carboniferous species, as a new, square shape (Lepidodendron hejinense) occurred beside older types (Fig. 6b). It is significant that the cushion shape showed a consistent lineage, represented by the gradual decrease of the length/width ratio, indicating that the height of the arborescent lycopsids decreased.