Jinzhuang Xue

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.

Diversity dynamics of silurian-early carboniferous land plants in South china.

New megafossil and microfossil data indicate four episodes in the diversification of Silurian–Early Carboniferous land plants of South China, a relatively continuous regional record. Plant diversity increased throughout, but the rising curve was punctuated by three major falls. There were peaks of origination in the Ludlow–Pragian, Givetian, late Famennian and Visean and peaks of extinction in the Pragian–Emsian, Givetian and early Tournaisian. Speciation and extinction rates were highest in the Lochkovian–Pragian and became progressively lower in subsequent stages. High correlation coefficients indicate that these events are associated with the availability of land habitat contingent on eustatic variations and increasing numbers of cosmopolitan genera. Meanwhile, proportions of endemic genera declined gradually. Due to less endemism and more migrations, both speciation and species extinction rates reduced. The changes of diversity and the timing of the three extinctions of land plants in South China are similar to those known already from Laurussia. However, the largest events in the Lochkovian–Pragian and subsequent smaller ones have not been seen in the global pattern of plant evolution. These land plant events do not correspond well temporally with those affecting land vertebrates or marine invertebrates. In South China, the diversity curve of land plants is generally opposite to that of marine faunas, showing a strong effect of eustatic variations. The increasing diversity of both land vertebrates and plants was punctuated above the Devonian–Carboniferous boundary, known as Romers Gap, implying common underlying constraints on macroevolution of land animals and plants.

Cosmosperma polyloba gen. et sp. nov., a seed plant from the Upper Devonian of South China.

Seed plants with ovules were abundant in the Late Devonian of Euramerica and they contribute significantly to our understanding of their early history. However, coeval ovules have been scarce in other regions of the world. Specimens of the seed plant Cosmosperma polyloba gen. et sp. nov. Wang et al. were recently obtained from the Upper Devonian (Famennian) Wutong Formation, at Fanwan Village, Changxing County, Zhejiang Province, China. This new seed plant has cupulate ovules, the uniovulate cupules with up to 16 distal segments and with minute spines on the outer surface, synangiate pollen organs bearing six to eight microsporangia fused only at the base, and planate and highly dissected pinnules in alternate arrangement. It differs from other Devonian seed plants mainly in the organization and position of the uniovulate and ornamented cupule, and in the highly dissected pinnules. Cosmosperma Wang et al. represents the first Devonian ovules recovered from China or eastern Asia and further illustrates the diversity of early spermatophytes. As for the Late Devonian seed plants, it is suggested that the pollen organs are synangiate and simple in organization, and the branches and leaves are generally planate.

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.