Shougang Hao

A Late Devonian arborescent lycopsid Sublepidodendron songziense Chen emend. (Sublepidodendraceae Krausel et Weyland 1949) from China, with a revision of the genus Sublepidodendron (Nathorst) Hirmer 1927

Abstract The study of a Late Devonian arborescent lycopsid, Sublepidodendron songziense Chen 1977, from the Hsiehchingssu (Xiejingsi) Formation of Hubei Province and the Wutung (Wutong) Formation of Anhui Province, PR China, provides new insight into its holistic architecture and taxonomic affinity. The study allows several interrelated organ species to be integrated into a conceptual whole-plant species, Sublepidodendron songziense Chen emend. Wang Q., Hao, Wang D.-M., Wang Y. et Thomas Denk. The plant consists of a monocaulous trunk growing from a stigmarian rhizomorph, the trunk bearing biseriate, sub-opposite to possibly alternate lateral branches that expand by means of isotomous to slightly anisotomous dichotomies, forming an excurrent canopy. Lateral branches bear terminal Lepidostrobus -like strobili. These strobili are dimorphic, implying separate mega- and microsporangiate strobili considered with the associated organ assemblage, including Triletes -type megaspores, Lepidocarpopsis -like sporophylls, and Lepidophloios -like cauline anatomy. Moreover, lateral branches are caducous, and ulodendroid scars mark the attachment sites of former branches onto the trunk. The plant diagnostically possesses spirally arranged, small, vertically elongated leaf cushions, which have false leaf scars. The presence of ligule cannot be excluded definitely. The characteristics of leaf cushions show considerable variation corresponding to different taphonomic processes, preservational modes and ontogenetic stages so that this plant megafossil has been classified in various ways previously. Based on organic connection and cauline anatomy, a reconstruction of the whole-plant species is tentatively given. We compare this whole-plant species to the type species S. mirabile (Nathorst) Hirmer, and revise Sublepidodendron (Nathorst) Hirmer 1927, suggesting that the genus and its own family Sublepidodendraceae sensu Krausel et Weyland 1949 should be separated from the order Protolepidodendrales sensu Pichi-Sermolli 1958 and referred to the order Isoetales sensu lato and the suborder Dichostrobiles sensu DiMichele et Bateman 1996. Present Ulodendron specimens are the earliest record in the Late Palaeozoic strata, demonstrating that the pseudomonopodial branching architectural form in the arboresent lycopsids had evolved by the Late Devonian (Famennian). Such a woody trunk together with lateral branches (or cauline peduncles), separate mega- and microsporangiate strobili, and rootlet-bearing rhizomorph epitomize the major structural units (or architectural modules) of a true arborescent lycopsid, and S. songziense appears to be the oldest arborescent lycopsid showing an apparently centralized rhizomorph. We suggest that phylogenetically more advanced arborescent lycopsids did not precipitately occur in the Carboniferous, rather they had already evolved by the Late Devonian (Famennian). The widespread genus Sublepidodendron (Nathorst) Hirmer 1927 emend., especially abundant in China, among the Late Devonian–Early Carboniferous floras may represent one of the progenitors of the phylogenetically advanced arborescent lycopsids.

Earliest rooting system and root : shoot ratio from a new Zosterophyllum plant

The enhanced chemical weathering by rooted vascular plants during the Silurian-Devonian period played a crucial role in altering global biogeochemical cycles and atmospheric environments; however, the documentation of early root morphology and physiology is scarce because the existing fossils are mostly incomplete. Here, we report an entire, uprooted specimen of a new Zosterophyllum Penhallow, named as Z. shengfengense, from the Early Devonian Xitun Formation (Lochkovian, c. 413 Myr old) of Yunnan, south China. This plant has the most ancient known record of a rooting system. The plant consists of aerial axes of 98 mm in height, showing a tufted habit, and a rhizome bearing a fibrous-like rooting system, c. 20 mm in length. The rhizome shows masses of branchings, which produce upwardly directed aerial axes and downwardly directed root-like axes. The completeness of Z. shengfengense made it possible to estimate the biomass allocation and root : shoot ratio. The root : shoot ratio of this early plant is estimated at a mean value of 0.028, and the root-like axes constitute only c. 3% of the total biomass. Zosterophyllum shengfengense was probably a semi-aquatic plant with efficient water use or a strong uptake capacity of the root-like axes.

ZOSTEROPHYLLUM PENHALLOW AROUND THE SILURIAN-DEVONIAN BOUNDARY OF NORTHEASTERN YUNNAN, CHINA

Two new species of Zosterophyllum Penhallow (Zosterophyllum qujingense sp. nov. and Zosterophyllum xishanense sp. nov.), respectively from the Yulongsi Formation (Late Silurian, Přídolí) and Xiaxishancun Formation (Early Devonian, Lochkovian) of Qujing, Yunnan, China, are described. Both are small and naked plants with spikes of radially arranged sporangia. Zosterophyllum qujingense is the earliest recorded plant of Zosterophyllum in South China. It has basal K‐ or H‐shaped branching and erect dichotomous axes. Terminal spikes consist of lateral and stalked sporangia of wedge or somewhat elliptical shape. Arising from the rhizome, fertile axes of Z. xishanense bear laterally stalked and more or less round sporangia. Configuration of the rhizomatous part and K‐ or H‐shaped branching of Z. xishanense are discussed. Comparison with other zosterophylls indicates that the Late Silurian–Early Devonian flora of the paleoequatorial zone (i.e., Bathurst Island and Qujing, Yunnan) may be characterized by a diversity of zosterophylls, probably differing from coeval floras of other regions dominated by rhyniophytoids. With regard to paleophytogeography, we suggest that the zosterophylls bearing radially arranged sporangia (including subgenus Zosterophyllum) were prevalent in South China (northeastern Gondwana phytogeographic unit) and that those with one or two rows of sporangia (subgenus Platyzosterophyllum) were common on the Laurussian paleocontinent.

ANATOMY OF THE LATE DEVONIAN SPHENOPSID ROTAFOLIA SONGZIENSIS, WITH A DISCUSSION OF STELAR ARCHITECTURE OF THE SPHENOPHYLLALES

A previous study of the Late Devonian (Famennian) sphenopsid Rotafolia songziensis Wang, Hao, and Wang provided detailed descriptions of the morphology and a sketchy illustration of a three‐ribbed primary xylem. The present anatomical data show that the protostele of this plant also has four‐ribbed primary xylem of exarch maturation. Located at the tip of each xylem rib, the tracheids of the protoxylem strand bear helical wall thickenings. Tracheids of metaxylem and secondary xylem possess scalariform pits and/or bordered pits. Ray cells seldom occur in secondary xylem. In contrast to fertile organs demonstrating great diversity in bract shape, structure, and sporangiophore number, the stelar architecture of the Sphenophyllales is consistent in ribbed primary xylem of fundamentally exarch maturation, although secondary xylem indicates moderate structural variations. Stelar architecture of the Sphenophyllales is compared with that of basal euphyllophytes and lycophytes. The position of peripheral protoxylem strands in the Sphenophyllales and Iridopteridales corresponds to the presence of leaves or leaf precursors. The origination of these strands differs from that of radiate protoxylem strands in the Aneurophytales.

Yuguangia ordinata gen.et sp.nov., a new lycopsid from the Middle Devonian (Late Givetian) of Yunnan, China, and its phylogenetic implications

A new heterosporous and ligulate lycopsid, Yuguangia ordinata gen. et sp. nov., is described from the Haikou Formation (late Givetian) of Yunnan Province, southwestern China. Its slender stems branch isotomously. Vegetative leaves are arranged in pseudowhorls. Compact bisporangiate strobili terminate distal branches and have megasporangia located proximally and microsporangia distally. Sporophylls are differentiated into horizontal pedicel, upturned lamina, and downward “heel,” and they are pseudowhorlly arranged, with about eight per gyre. Each sporophyll bears adaxially one mega‐ or microsporangium. Megasporangia and microsporangia are obovoid, round to elliptical in shape. Each megasporangium contains four or more Trileites‐type megaspores. Microspores are comparable, to some extent, to those of the dispersed spore genus Acinosporites. A ligule is located at the distal end of the horizontal pedicel of a sporophyll. Distal axes have an exarch primary xylem strand, phloem, and a two‐zoned cortex. This study provides a new temporal framework for ligule acquisition and early diversification of heterosporous lycopsid lineages and implies that homosporous, ligulate lycopsid lineages (i.e., Leclercqia of Protolepidodendrales) and heterosporous, ligulate lycopsid lineages (i.e., Selaginellales and Isoetales) had diverged by the late Middle Devonian.

Observations on anatomy of Adoketophyton subverticillatum from the Posongchong Formation (Pragian, Lower Devonian) of Yunnan, China

Abstract C.-S. Li and Edwards [Palaeontology 35 (1992) 257–272)] revised Zosterophyllum subverticillatum X.-X. Li and C.-Y. Cai (1977) as Adoketophyton subverticillatum . This distinctive plant, of Pragian age (Lower Devonian) from the Posongchong Formation at Zhichang village of Gumu in the Wenshan district of Yunnan Province, China, has terminal strobili composed of decussately arranged, fan-shaped sporophylls, each of which bears a single sporangium attached to the adaxial surface at its base. Three outcrops yielding permineralized parts of this plant, and revealing anatomical details, have been discovered in the Wenshan and Guangnan districts of Yunnan. Observations with optical and scanning electron microscopes reveal that the stem contains a columnar centrarch primary xylem. Metaxylem tracheids are characterized by annular and helical secondary wall thickenings between which are simple perforations. The sporophylls contain parenchyma cells, polygonal or hexagonal in transverse section, and possible tracheids in veins. This combination of anatomy and morphology (a centrarch xylem composed of tracheids of perforated wall structure, and a strobilus of fan-shaped sporophylls bearing adaxial sporangia) illustrates the complexity of land plants that lived during the Early Devonian.

Anatomy of the Late Devonian Denglongia hubeiensis, with a Discussion of the Phylogeny of the Cladoxylopsida

A previous study of the Late Devonian cladoxylopsid Denglongia hubeiensis Xue et Hao provided a detailed morphological description, while new material from the type locality of the Huangjiadeng Formation, Hubei Province, China, reported here reveals details of anatomy. Main axes are actinostelic in transverse sections, showing a primary xylem column with five or six xylem ribs separated by deep embayments. First‐order axes have a deeply ribbed primary xylem column with four xylem ribs. Each stelar rib has two permanent, mesarch protoxylem strands near its periphery. Tracheids interpreted as protoxylem show annular or helical thickenings. Metaxylem tracheids have scalariform and elliptical bordered pits. Anatomically, Denglongia is more similar to the iridopteridaleans than to other plants in the actinostele, the peripheral protoxylem strands, the disintegration of protoxylem tracheids, and the pitting pattern of metaxylem tracheids. The phylogeny of the Cladoxylopsida was evaluated by parsimony and Bayesian cladistic analyses. The results showed that the Pseudosporochnales constitute a very robust monophyletic group supported by a suite of characters: the presence of arborescence; digitate branching; straight tips of sterile ultimate appendages; dissected primary xylem; elliptical to strap‐shaped primary xylem strands; and sclereids in the cortex. The monophyly of the Iridopteridales is questioned in that the previously suggested defining characters, whorled branching and peripheral protoxylem strands, are considered more plesiomorphic in the cladistic context. The relationship between the iridopteridaleans and the sphenophyllaleans is weakly supported. The Bayesian analysis recognized a tentative “iridopteridalean‐based group,” in which the iridopteridaleans, Denglongia, Metacladophyton Wang et Geng, Polypetalophyton Geng, and their probable relatives are nested within an unresolved polytomy.

A New Lycopsid from the Upper Devonian of Hubei Province, China

A new herbaceous lycopsid, Hubeiia dicrofollia gen. et sp. nov., is described from the Xiejingsi Formation (Late Devonian) of Hubei Province, China. Its stems are isotomously dichotomized. Leaf bases are circular or slightly elliptical in shape and arranged in low helices or alternating pseudowhorls. Persistent leaves are divided into four segments as a result of two successive dichotomies. It has a solid exarch primary xylem strand. Protoxylem tracheids are composed of annular elements that appear as ridges on the stele surface. Metaxylem tracheids are scalariformly thickened and between thickened bars present vertical threads generally termed “Williamson’s striations.” Primary phloem forms a narrow band, four to eight cells thick, surrounding the stele. The cortex is relatively thick, possibly two zoned. Leaf traces are mesarch; they depart directly from protoxylem points and pass through the cortex with a stable oblique angle. This plant possesses a distinctive combination of characters, i.e., forked leaves in morphology and scalariformly thickened tracheids with Williamson’s striations in anatomy, and therefore it is designated as a new stem form genus Hubeiia gen. nov. Taxonomically, it is assigned to Protolepidodendraceae.

Belowground rhizomes in paleosols: The hidden half of an Early Devonian vascular plant

Significance The roots and rhizomes of early vascular plants, and their interactions with soils, are poorly documented. Here we report on the complex, belowground rhizome systems of an Early Devonian plant, and their contribution to the formation of the earliest record of rooted red-bed soils in Asia. Our specimens predate the earliest trees with deep roots from the Middle Devonian by 20 million years. We propose that plant rhizomes have long functioned in terrestrial ecosystems, playing important roles in shaping Earth’s environments by reducing soil erosion rates and thereby increasing the stability of land surface and resilience of plant communities. The colonization of terrestrial environments by rooted vascular plants had far-reaching impacts on the Earth system. However, the belowground structures of early vascular plants are rarely documented, and thus the plant−soil interactions in early terrestrial ecosystems are poorly understood. Here we report the earliest rooted paleosols (fossil soils) in Asia from Early Devonian deposits of Yunnan, China. Plant traces are extensive within the soil and occur as complex network-like structures, which are interpreted as representing long-lived, belowground rhizomes of the basal lycopsid Drepanophycus. The rhizomes produced large clones and helped the plant survive frequent sediment burial in well-drained soils within a seasonal wet−dry climate zone. Rhizome networks contributed to the accumulation and pedogenesis of floodplain sediments and increased the soil stabilizing effects of early plants. Predating the appearance of trees with deep roots in the Middle Devonian, plant rhizomes have long functioned in the belowground soil ecosystem. This study presents strong, direct evidence for plant−soil interactions at an early stage of vascular plant radiation. Soil stabilization by complex rhizome systems was apparently widespread, and contributed to landscape modification at an earlier time than had been appreciated.

Further Study of the Late Devonian Sphenopsid Hamatophyton verticillatum from China

Hamatophyton verticillatum is one of the most widespread plants in the Late Devonian (Famennian) of South China. Using fossils from the coeval Yuelushan Formation, Yiyang District, northern Hunan Province, we reinvestigated this important plant. Its vegetative axes pseudomonopodially branch and possess ribbed internodes and nodes bearing undivided leaves or leaves with one division. Linear leaves are in whorls. Their tips taper or slightly expand and sometimes curve adaxially to form a hooklike shape. Although H. verticillatum has been reported by many workers, knowledge of its fertile characters is still problematic. This article shows that sporangial stalks at nodes of the fertile axis lack bracts. Each stalk is undivided, curves adaxially in the distal part, and terminates in a single spiny and elliptical sporangium. Stalks may occur with a fertile lateral axis at the same node of the fertile main axis. This plant may represent the most primitive type in the Sphenopsida and, further, may prove the greater reproductive diversity of the Sphenophyllales. Detailed anatomical study shows a three‐ribbed primary xylem of exarch maturation in protostele and a secondary xylem lacking parenchyma cells. Anatomical comparison with other members indicates that the Sphenophyllales are consistent in primary‐xylem architecture and are moderately variable in secondary‐xylem structure.