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• W2968625128 abstract "•The earliest forests in the Devonian can be very large and potentially abundant•Devonian Xinhang forest consists of dense and new tree lycopsid Guangdedendron•Guangdedendron may be monocarpic and dioecious, showing the earliest stigmarian root•Devonian forests could contribute greatly to CO2 decline and coastal consolidation Since the Late Paleozoic, forests have become distributed worldwide and significantly changed the Earth’s climate and landscapes, but the record of forests is rare in the Devonian (419–359 Ma in age) when they first appeared. From the Upper Devonian (Famennian with the age of 372–359 Ma) of Xinhang, Anhui, China, we report a very large in situ forest, which includes locally dense stands of lycopsid plants. The Xinhang forest is monospecific with a small tree lycopsid Guangdedendron gen. nov., probably dioecious with monocarpic reproduction. The plant shows the earliest stigmarian rooting system typical of giant tree lycopsids dominating Carboniferous forests. It colonizes coastal clastic wetlands that were influenced by floods. This significantly increases the paleogeographical coverage of in situ Devonian forests, and contributes to our understanding of atmospheric CO2 decline and coastal consolidation. Since the Late Paleozoic, forests have become distributed worldwide and significantly changed the Earth’s climate and landscapes, but the record of forests is rare in the Devonian (419–359 Ma in age) when they first appeared. From the Upper Devonian (Famennian with the age of 372–359 Ma) of Xinhang, Anhui, China, we report a very large in situ forest, which includes locally dense stands of lycopsid plants. The Xinhang forest is monospecific with a small tree lycopsid Guangdedendron gen. nov., probably dioecious with monocarpic reproduction. The plant shows the earliest stigmarian rooting system typical of giant tree lycopsids dominating Carboniferous forests. It colonizes coastal clastic wetlands that were influenced by floods. This significantly increases the paleogeographical coverage of in situ Devonian forests, and contributes to our understanding of atmospheric CO2 decline and coastal consolidation. During the Devonian, the upland forests with roots had greatly accelerated the atmospheric CO2 drawdown, leading the Earth into a major icehouse [1Retallack G.J. Early forest soils and their role in Devonian global change.Science. 1997; 276: 583-585Crossref PubMed Scopus (179) Google Scholar, 2Berner R.A. The carbon cycle and CO2 over Phanerozoic time: the role of land plants.Phil. Trans. R. Soc. Lond. B. 1998; 353: 75-82Crossref Scopus (205) Google Scholar], and the river banks were stabilized by the root development of riparian vegetation [3Gibling M.R. Davies N.S. Palaeozoic landscapes shaped by plant evolution.Nat. Geosci. 2012; 5: 99-105Crossref Scopus (181) Google Scholar, 4Xue J.Z. Deng Z.Z. Huang P. Huang K.J. Benton M.J. Cui Y. Wang D.M. Liu J.B. Shen B. Basinger J.F. et al.Belowground rhizomes in paleosols: The hidden half of an Early Devonian vascular plant.Proc. Natl. Acad. Sci. U. S. A. 2016; 113: 9451-9456Crossref PubMed Scopus (46) Google Scholar]. Trees originated in the Mid-Late Devonian when they were abundant and include three main types, i.e., archaeopteridalean progymnosperms, fernlike plants, and lycopsids (club mosses) [5Meyer-Berthaud B. Decombeix A.L. L’évolution des premiers arbres: les stratégies dévoniennes.C. R. Palevol. 2009; 8: 155-165Crossref Scopus (23) Google Scholar]. However, the earliest forests are rare in the Devonian, limited to Euramerica [6Mintz J.S. Driese S.G. White J.D. Environmental and ecological variability of Middle Devonian (Givetian) forests in Appalachian basin paleosols, New York, United States.Palaios. 2010; 25: 85-96Crossref Scopus (44) Google Scholar, 7Stein W.E. Berry C.M. Hernick L.V. Mannolini F. Surprisingly complex community discovered in the mid-Devonian fossil forest at Gilboa.Nature. 2012; 483: 78-81Crossref PubMed Scopus (127) Google Scholar, 8Berry C.M. Marshall J.E.A. Lycopsid forests in the early Late Devonian paleoequatorial zone of Svalbard.Geology. 2015; 43: 1043-1046Crossref Scopus (57) Google Scholar], relatively small in exposed surface areas, and their relationship with environments needs to be known in more detail. By contrast, the Carboniferous (Pennsylvanian Subperiod) forests yielding extensive coal seams are common (including hundreds of sites), even exposed in hectares and among the best-understood Phanerozoic terrestrial ecosystems [9DiMichele W.A. Pfefferkorn H.W. Gastaldo R.A. Response of Late Carboniferous and Early Permian plant communities to climate change.Annu. Rev. Earth Planet. Sci. 2001; 29: 461-487Crossref Scopus (182) Google Scholar, 10Falcon-Lang H.J. Jud N.A. Nelson W.J. DiMichele W.A. Chaney D.S. Lucas S.G. Pennsylvanian coniferopsid forests in sabkha facies reveal the nature of seasonal tropical biome.Geology. 2011; 39: 371-374Crossref Scopus (49) Google Scholar]. The arborescent lycopsids (extinct trees) originated in the Late Devonian and dominated Carboniferous and Permian lowland environments, especially wetland habitats such as tropical swamps [11DiMichele W.A. Phillips T.L. Arborescent lycopod reproduction and paleoecology in a coal-swamp environment of late Middle Pennsylvanian age (Herrin coal, Illinois, U.S.A.).Rev. Palaeobot. Palynol. 1985; 44: 1-26Crossref Scopus (136) Google Scholar, 12Phillips T.L. DiMichele W.A. Comparative ecology and life-history biology of arborescent lycopsids in Late Carboniferous swamps of Euramerica.Ann. Missouri Bot. Gard. 1992; 79: 560-588Crossref Google Scholar, 13Pigg K.B. Isoetalean lycopsid evolution: from the Devonian to the present.Am. Fern J. 2001; 91: 99-114Crossref Scopus (98) Google Scholar, 14Dimichele W.A. Elrick S.D. Bateman R.M. Growth habit of the late Paleozoic rhizomorphic tree-lycopsid family Diaphorodendraceae: Phylogenetic, evolutionary, and paleoecological significance.Am. J. Bot. 2013; 100: 1604-1625Crossref PubMed Scopus (30) Google Scholar]. These plants, termed rhizomorphic lycopsids, are placed in the Isoëtales sensu lato (the most derived lycopsid clade) and represented by herbaceous Isoetes as their only extant relative [14Dimichele W.A. Elrick S.D. Bateman R.M. Growth habit of the late Paleozoic rhizomorphic tree-lycopsid family Diaphorodendraceae: Phylogenetic, evolutionary, and paleoecological significance.Am. J. Bot. 2013; 100: 1604-1625Crossref PubMed Scopus (30) Google Scholar, 15DiMichele W.A. Bateman R.M. The rhizomorphic lycopsids: a case study in paleobotanical classification.Syst. Bot. 1996; 21: 535-552Crossref Scopus (78) Google Scholar]. The rhizomorph is a root-producing subterranean structure of the isoetaleans, and the Stigmaria-type or stigmarian rhizomorph extends out from the stem base, dichotomizes, and bears laterally arranged rootlets in helices [16Taylor T.N. Taylor E.L. Krings M. Paleobotany: The Biology and Evolution of Fossil Plants. Academic press, New York2009Google Scholar]. The stigmarian rhizomorph characterizes the arborescent lycopsids, but its early evolution in the Devonian has long been obscure due to the paucity of fossil record [17Gensel P.G. Berry C.M. Early lycophyte evolution.Am. Fern J. 2001; 91: 74-98Crossref Scopus (83) Google Scholar]. Although a lycopsid forest had been documented in the Late Devonian, the plant growth is localized, and some traits including reproduction and habit are unclear [8Berry C.M. Marshall J.E.A. Lycopsid forests in the early Late Devonian paleoequatorial zone of Svalbard.Geology. 2015; 43: 1043-1046Crossref Scopus (57) Google Scholar]. Here, we record the earliest forest of Asia, which is exceptionally large in outcrop, with small and isoetalean lycopsid trees of locally high density. This forest contributes to our understanding of the reproductive mode, growth architecture, and evolution of the rooting system in early lycopsids. We also discuss the relationship between Devonian forests and environments, including habitat, climate, and landscape. The fossil forest occurs mainly in the Jianchuan clay mine near Jianchuan village, Xinhang Town, Guangde County, southeastern Anhui Province, China (Figures 1A and 1B ). The mine includes a west quarry (WQ) and east quarry (EQ), where most in situ lycopsid plants were buried in growth position (Figures 1B and 1C) and thus indicate T0 deposits [18DiMichele W.A. Falcon-Lang H.J. Pennsylvanian ‘fossil forests’ in growth position (T0 assemblages): origin, taphonomic bias and palaeoecological insights.J. Geol. Soc. Lond. 2011; 168: 585-605Crossref Scopus (86) Google Scholar]. The forest was also found in Yongchuan clay mine, which is ca. 1 km northwest of Jianchuan mine and preserves some in situ lycopsids (Figure 1B). The outcrops in these two mines represent the Wutong Formation. This formation is widespread in the Lower Yangtze Valley, including Anhui Province, and comprises two parts [19Li X.X. Cai C.Y. Ouyang S. Recent advances in the study of the Wutong Formation of the Lower Yangtze Valley.Bull. Chin. Acad. Sci. 1984; 9: 119-136Google Scholar], i.e., Leigutai Member characterized by interbedded quartz sandstone and mudstone (Figure 1D) and underlying Guanshan Member mainly with quartz sandstone. Paleontological evidence, including spore zonation, indicates that the Wutong Formation (perhaps excluding the uppermost part of Leigutai Member) is Famennian (Late Devonian) [19Li X.X. Cai C.Y. Ouyang S. Recent advances in the study of the Wutong Formation of the Lower Yangtze Valley.Bull. Chin. Acad. Sci. 1984; 9: 119-136Google Scholar, 20Gao L.D. The Late Devonian–Early Carboniferous miospore zonation in the Lower Yangtze Valley, China and the Devonian–Carboniferous boundary.Acta Geoscient. Sin. 2015; 36: 7-21Google Scholar]. At Jianchuan mine, Archaeopteris, a ubiquitous Late Devonian progymnosperm plant, was discovered in the lower part of Leigutai Member with upper part missing and 60-meter-thick strata. Local companies have been excavating clay or mudstone in Leigutai Member for pottery manufacture. According to satellite images, the successive excavations at Jianchuan mine began in 2009 and the boundaries of WQ and EQ are recorded in 2009 and 2013–2017 (Figure 1C). In 2016–2019, the excavation at WQ was toward north and east and that at EQ was toward directions excluding east (other sides of quarries have been backfilled); we have surveyed the localities 13 times and found numerous erect lycopsids exposed in quarry highwalls or in fallen blocks from highwalls or on the ground (Figures 1C, 2, 3, 4, and S1–S5; Data S1). The stems or rooting systems of these in situ lycopsids are located below and above a 4-meter-thick sandstone bed (Figure 1D) and occur in several beds of blue-gray or gray-yellow silty mudstones and pelitic siltstones of the exposed strata (20 m thick or more) of the Leigutai Member (Figure 1D). Some stems or branches are horizontally disposed on the bedding plane (Figures 5A–5C), and the strobili sometimes containing megaspores were discovered (Figures 5D–5K). The excavation of Yongchuan mine began in 2016; we have observed in situ lycopsids in highwalls, fallen blocks, and observed stems or branches lying parallel on the bedding plane and strobili with megaspores (Figure S6; Data S1). Sedimentary structures, such as ripple mark, rock stratification, and oriented alignment of plants, are recorded at Jianchuan mine (Figure S7; Data S1).Figure 3In Situ Lycopsid from East Quarry at Jianchuan MineShow full caption(A) Highwall in September of 2016, the same as Figure S2D; for explanations of arrows, see Figure S2D.(B) Rock in (A) (arrow 16) with stem bases and rhizomorphs (arrows and rectangle).(C) Side view of rhizomorph in (B) (arrow a). PKUB16153.(D) Rhizomorph in (C) after removal of rock cast in stem base.(E) Four rhizomorphs in (B) (rectangle).(F) Side view of arrowed rhizomorph in (E).(G) Side view of rhizomorph in (B) (arrow b).(H) Rhizomorph in (B) (arrow c). Scale bars: 2 cm (H and coin diameter).See also Figures S2, S4, S5, and S7 and Data S1.View Large Image Figure ViewerDownload Hi-res image Download (PPT)Figure 4Rooting Systems and Erect Stems of GuangdedendronShow full caption(A–J and L–U) The same as in Data S1.13F, S1.25C, S1.19C, S1.19F, S1.14H, S1.14G, S1.8I, S1.12J, S1.12L, and S1.12M and Figure S3H, Data S1.4B, S1.4F, S1.4G (arrow), S1.18A–S1.18D, S1.18H, and S1.16I, respectively.(A–G) Rooting system with (dichotomous) rhizomorph axes bearing rootlets.(A) Stem base with three rhizomorph axes (bases) visible. PKUB16167d.(B) Dégagement of stem in Data S1.25C (left arrow) and its connected rhizomorph.(C) Stem base with two rhizomorph axes visible.(D) Dégagement of stem base and rhizomorph in Data S1.19F. PKUB16169.(E) Side view of arrowed part in (F). PKUB16166a and PKUB16166c.(F) Top view of the stem base with four dichotomous rhizomorph axes.(G) Stem base with four rhizomorph axes. PKUB16199.(H) Single rhizomorph axis bearing rootlets. PKUB16164 and PKUB16165.(I and J) Two sides (I) and one side (J) of a rhizomorph axis in (H), respectively, showing rootlet scars. PKUB16165.(K–O) In situ erect stems showing leaf bases.(K) Enlargement of arrowed stem in (B).(L) Helically arranged leaf bases in parasiticides.(M) Thick stem. PKUB16062.(N) Stem with leaves.(O) Thick stem.(P–U) Erect stems (in fallen blocks) that show dichotomies.(P and Q) Lateral view of a dichotomous stem.(R) Face view of arrowed part in (Q), showing two branches of stem.(S) Mid-upper part of the stem in (Q) after dégagement.(T) Arrow indicating dichotomous point of a stem.(U) Arrow indicating dichotomous point of a stem and extension of branches along bedding plane.Scale bars: 1 cm (I and J), 2 cm (D, F–H, K–N, and coin diameter), and 5 cm (C and T).See also Figures S1–S3 and Data S1.View Large Image Figure ViewerDownload Hi-res image Download (PPT)Figure 5Stems, Branches, and Strobili of GuangdedendronShow full caption(A) Stem with leaf bases and leaves. PKUB16156.(B) Dichotomous stem.(C) Terminal branch with leaves. PKUB16052c.(D) Strobili lying vertical to bedding plane and probably in a pair. The same as in Data S1.14K is shown.(E) Two terminal strobili. PKUB16051.(F) A pair of terminal strobili.(G) A single terminal strobilus. The same as in Data S1.24N is shown. PKUB16001a.(H) Distal part of strobilus in (G), showing sporophylls and megaspores.(I) Counterpart of right strobilus in (D). PKUB16177.(J) Enlargement of (I) (arrow) showing megaspores.(K) SEM (scanning electron micrograph), a megaspore with trilete mark.Scale bars are as follows: 0.5 mm (K), 1 mm (J), 5 mm (H), 1 cm (C and I), and 2 cm (A, B, D–G).See also Figure S2 and Data S1.View Large Image Figure ViewerDownload Hi-res image Download (PPT) (A) Highwall in September of 2016, the same as Figure S2D; for explanations of arrows, see Figure S2D. (B) Rock in (A) (arrow 16) with stem bases and rhizomorphs (arrows and rectangle). (C) Side view of rhizomorph in (B) (arrow a). PKUB16153. (D) Rhizomorph in (C) after removal of rock cast in stem base. (E) Four rhizomorphs in (B) (rectangle). (F) Side view of arrowed rhizomorph in (E). (G) Side view of rhizomorph in (B) (arrow b). (H) Rhizomorph in (B) (arrow c). Scale bars: 2 cm (H and coin diameter). See also Figures S2, S4, S5, and S7 and Data S1. (A–J and L–U) The same as in Data S1.13F, S1.25C, S1.19C, S1.19F, S1.14H, S1.14G, S1.8I, S1.12J, S1.12L, and S1.12M and Figure S3H, Data S1.4B, S1.4F, S1.4G (arrow), S1.18A–S1.18D, S1.18H, and S1.16I, respectively. (A–G) Rooting system with (dichotomous) rhizomorph axes bearing rootlets. (A) Stem base with three rhizomorph axes (bases) visible. PKUB16167d. (B) Dégagement of stem in Data S1.25C (left arrow) and its connected rhizomorph. (C) Stem base with two rhizomorph axes visible. (D) Dégagement of stem base and rhizomorph in Data S1.19F. PKUB16169. (E) Side view of arrowed part in (F). PKUB16166a and PKUB16166c. (F) Top view of the stem base with four dichotomous rhizomorph axes. (G) Stem base with four rhizomorph axes. PKUB16199. (H) Single rhizomorph axis bearing rootlets. PKUB16164 and PKUB16165. (I and J) Two sides (I) and one side (J) of a rhizomorph axis in (H), respectively, showing rootlet scars. PKUB16165. (K–O) In situ erect stems showing leaf bases. (K) Enlargement of arrowed stem in (B). (L) Helically arranged leaf bases in parasiticides. (M) Thick stem. PKUB16062. (N) Stem with leaves. (O) Thick stem. (P–U) Erect stems (in fallen blocks) that show dichotomies. (P and Q) Lateral view of a dichotomous stem. (R) Face view of arrowed part in (Q), showing two branches of stem. (S) Mid-upper part of the stem in (Q) after dégagement. (T) Arrow indicating dichotomous point of a stem. (U) Arrow indicating dichotomous point of a stem and extension of branches along bedding plane. Scale bars: 1 cm (I and J), 2 cm (D, F–H, K–N, and coin diameter), and 5 cm (C and T). See also Figures S1–S3 and Data S1. (A) Stem with leaf bases and leaves. PKUB16156. (B) Dichotomous stem. (C) Terminal branch with leaves. PKUB16052c. (D) Strobili lying vertical to bedding plane and probably in a pair. The same as in Data S1.14K is shown. (E) Two terminal strobili. PKUB16051. (F) A pair of terminal strobili. (G) A single terminal strobilus. The same as in Data S1.24N is shown. PKUB16001a. (H) Distal part of strobilus in (G), showing sporophylls and megaspores. (I) Counterpart of right strobilus in (D). PKUB16177. (J) Enlargement of (I) (arrow) showing megaspores. (K) SEM (scanning electron micrograph), a megaspore with trilete mark. Scale bars are as follows: 0.5 mm (K), 1 mm (J), 5 mm (H), 1 cm (C and I), and 2 cm (A, B, D–G). See also Figure S2 and Data S1. At Jianchuan and Yongchuan mines, the plant-bearing strata of the Leigutai Member have an inclination angle of ca. 7°. Based on this small angle, boundary of strata and distribution of in situ lycopsids at WQ and EQ, the minimum area of the fossil forest at Jianchuan mine is estimated as ca. 200,000 m2 (gray-blue part in Figures 1B and 1C). The in situ lycopsids also occur at west corner of Jianchuan mine (Figure 1B, arrow). The exposed area of the fossil forest is ca. 50,000 m2 at Yongchuan mine (gray-blue part in Figure 1B). Therefore, the Xinhang forest is at least 250,000 m2. Many individuals are erect or oblique in quarry highwalls (Figures 2, 4B, arrow, 4K–4O, S1–S3, and S6; Data S1) and on bedding-plane exposures (Figures 3, 4A, 4E–4J, S2D, S4, and S5; Data S1). Some in fallen blocks have the rooting systems (rhizomorphs) or stems across (vertical or oblique to) the bedding plane or have the originally erect stems separated from the rock matrix (Figures 4C and 4D; Data S1). The rhizomorphs were found in five levels at EQ (Figure 1D), shown in Figure S2D (arrows 2, 3, 6, 8, and 10). The in situ plants could be locally seated in a high density (Figures 1C, stars, 2, S2D, S3B, S4, and S5; Data S1), ranging from 8/m2 (Figure 3B) to 38/m2 (Figures S4 and S5). The Stigmaria-type rhizomorph has four axes, which are evenly separated and extended (Figures 4A–4G, S4, and S5; Data S1), and each axis dichotomizes once or rarely twice. The rhizomorphs are 5.6–17.5 cm deep and extend at 30°–60° to ground surface. The rhizomorph axes are 8.3–(14.6)–25.3 cm long, and their first-order and second-order branches are 0.5–(2.0)–5.5 cm and 0.3–(1.4)–3.0 cm in diameter, respectively. The intervals between two orders of branches are 2.7–(4.1)–5.5 cm. The rootlets occur helically all along the rhizomorph axis and extend in different directions (Figures 3G, 4A–4E, 4H, and S3E; Data S1), and they may leave oval scars after abscission (Figures 4E and 4H–4J; Data S1). The rootlets, 9.0–(16.7)–27.2 cm long and 1.3–(3.2)–7.0 mm in diameter, appear unbranched in most cases. No root hairs have been observed. As for 713 in situ stems with 0.4–(3.1)–18.7 cm diameter (measurement made above the expanded bases of stems), ca. 21 stems exceed 10 cm in diameter and ca. 80% stems are less than 5 cm in diameter (Figures 2C–2O, 4K–4O, S3, and S6; Data S1). In situ stems have upper portions removed by sedimentary scouring, with the remained parts reaching 68.9–88.0 cm in height (Figures S3P and S3R), and the stems on bedding plane are up to 230 cm long (Data S1). The stems have their bases expanded downward (Figure 3G; Data S1) at the transition to the rhizomorphs (Figures 4B–4D; Data S1). Some erect stems were found dichotomous (Figures 4P–4T), and the branches may be turned parallel to the bedding plane where the stem dichotomizes (Figure 4U, arrow). On the bedding plane, the upper parts of the stems or terminal branches themselves may dichotomize at least once and usually at 30°–60° into two parts with the diameter decreased by half (Figures 5B and 5C; Data S1). Vegetative leaves, 2.0–(4.5)–9.2 cm long and 1.2–(3.0)–4.5 mm wide, are linear and have entire margins (Figures 2G, 4N, 5A, 5C, 5F, and 5G). Leaf bases are helically arranged in parastichies on many stems or branches (e.g., Figures 4K–4M, 4O, 5A, and S6G). The angle between the parastichy and transverse plane is 40°–65°. Leaf bases are closely disposed and narrowfusiform in shape. Axes terminated by strobili are up to 6.0 cm long and 1.5–4.7 mm in diameter. Some fertile axes are dichotomous (Figure 5E; Data S1), whereas others appear to lack dichotomy (Figures 5F and 5G; Data S1). The strobili are cylindrical in outline and, sometimes with attached axis, lie vertical to the bedding plane and with the strobilar tip pointing downward (Figure 5D; Data S1). They are 5.0–(11.2)–21.9 cm long and 1.0–(1.6)–3.0 cm in diameter (excluding sporophyll length), and the strobilar axes are 1.2–2.0 mm in diameter. The strobili are paired (Figures 5D and 5F; Data S1) or single (Figures 5E and 5G; Data S1) and have closely arranged sporophylls. The known strobili, including distal part, contain only megaspores so they are megasporangiate (Figures 5D and 5G–5K; Data S1). The megaspores of 670–1,200 μm diameter show trilete rays (Figure 5K; Data S1). The plants with small (e.g., less than 1 cm) stem diameters (Figures 2C, 2H–2J, and S6F; Data S1) or with undeveloped rhizomorph bearing short axes (Figures S5E–S5G) are tentatively considered as juvenile and reconstructed in Figure 6 (left). Those with larger diameter or developed rhizomorph or simple crown bearing long terminal strobili are mature and reconstructed in Figure 6 (right). The mature plants may have shed or withered leaves in the lower part of stem. At Jianchuan mine, ripple marks occur in the thick quartz sandstone bed (Figure S7A). Many stems or terminal branches may lie in a same direction in the sandstone (Figures S7B and S7C). The parallel or oblique stratification exists in the thin sandstone interbedded with mudstone (Figures S7D, S7E, and S7H–S7K). Cross-stratification, such as wedge-shaped and probable herringbone crossbedding, characterizes the thick sandstone bed (Figures S7F and S7G; Data S1). Morphological investigation and comparison indicates only one taxon is present. First, the individuals are small and the branches are dichotomous. Second, the stems, if well preserved, bear the same leaves or leaf bases and basal flare attached to a rhizomorph. Third, all rhizomorphs in different sizes are Stigmaria type. Fourth, the strobili found are megasporangiate with same type of spores. The microphyllous leaves and occurrence of only megaspores in a single strobilus, especially its distal part, suggest the plant is a heterosporous lycopsid with monosporangiate strobili. Although the sporophylls are currently unknown in detail, the lycopsid represents a new genus mainly because of the rhizomorph type, simple crown with rare branching points and strobili sometimes in pairs. These distinct traits enable the assignment of current lycopsid to Suborder Dichostrobiles within the Order Isoëtales sensu lato and the establishment of new genus and species as Guangdedendron micrum gen. et sp. nov. (see Systematics). The discovery of the Gilboa forest and in situ roots of archaeopteridalean progymnosperms (perhaps forming monospecific forests) from New York, USA indicates that forests first appeared in the Middle Devonian [6Mintz J.S. Driese S.G. White J.D. Environmental and ecological variability of Middle Devonian (Givetian) forests in Appalachian basin paleosols, New York, United States.Palaios. 2010; 25: 85-96Crossref Scopus (44) Google Scholar, 7Stein W.E. Berry C.M. Hernick L.V. Mannolini F. Surprisingly complex community discovered in the mid-Devonian fossil forest at Gilboa.Nature. 2012; 483: 78-81Crossref PubMed Scopus (127) Google Scholar]. The Gilboa forest comprises fernlike plants and aneurophytalean progymnosperms, i.e., giant cladoxylopsids resembling the habit of modern tree ferns and understory rhizomatous to lianaceous aneurophytaleans. The exposed part of forest is 1,200 m2, and the cladoxylopsid tree density is less than 1/m2. The Late Devonian (Frasnian) Svalbard lycopsid forest in Norway includes Protolepidodendropsis with stems of 5.0–(less than)–20 cm in diameter and of at least 1.3 m in height, and in situ archaeopteridaleans are found in other levels [8Berry C.M. Marshall J.E.A. Lycopsid forests in the early Late Devonian paleoequatorial zone of Svalbard.Geology. 2015; 43: 1043-1046Crossref Scopus (57) Google Scholar]. The exposed surface area of this forest is limited by the outcrop and the tree density is 14/m2. Late Devonian progymnosperm Archaeopteris, as a widespread conifer-like tree, may produce the forest probably with understory fernlike plants [21Greb S.F. DiMichele W.A. Gastaldo R.A. Evolution and importance of wetlands in earth history.Geol. Soc. Am. Special Paper. 2006; 399: 1-40Google Scholar]. The Xinhang fossil forest, at least 25 hectares in exposed area, is the largest in Devonian and even larger than many Carboniferous fossil forests [22Gastaldo R.A. Stevanovic-Walls I.M. Ware W.N. Greb S.F. Community heterogeneity of Early Pennsylvanian peat mires.Geology. 2004; 32: 693-696Crossref Scopus (57) Google Scholar, 23DiMichele W.A. Falcon-Lang H.J. Nelson W.J. Elrick S.D. Ames P.R. Ecological gradients within a Pennsylvanian mire forest.Geology. 2007; 35: 415-418Crossref Scopus (73) Google Scholar]. The density of small trees is locally up to 38/m2, and the plants are thus reconstructed in patches as thickets (Figure 7). A certain degree of uneven tree spacing occurs in the overall forest. However, according to the result of the nearest-neighbor analysis, the local plants in a high-density area are not considered clustered (Data S1). Based on the diameter value (D), the height (H) of Devonian and Carboniferous trees including lycopsids had been calculated by using the formula H = 37.5 × (D/2)2/3 [24Mosbrugger V. Constructional Principles of Tree Trunks. Springer-Verlag, 1990Google Scholar]. Accordingly, in Xinhang lycopsid forest, the small trees with 1.0–18.7 cm diameter are estimated 1.1–7.7 m high and usually less than ca. 3.2 m high. Devonian tree or tree-like lycopsids generally have stems of 0.7–30 cm in maximum diameter and are estimated up to 0.5–10 m high [25Schweitzer H.J. Li C.S. Chamaedendron nov. gen., eine multisporangiate lycophyte aus dem Frasnium Südchinas.Palaeontogr. Abt. B. 1996; 238: 45-69Google Scholar, 26Wang Q. Hao S.G. Wang D.M. Wang Y. Denk T. A Late Devonian arborescent lycopsid Sublepidodendron songziense Chen emend. (Sublepidodendraceae Kräusel et Weyland 1949) from China, with a revision of the genus Sublepidodendron (Nathorst) Hirmer 1927.Rev. Palaeobot. Palynol. 2003; 127: 269-305Crossref Scopus (44) Google Scholar]. The single type of rhizomorph and consistent traits in leaves and megaspores indicate a monospecific forest in Xinhang, where no other groups of in situ plants have been discovered, but dispersed seeds are present. The forest is thus homogeneous at the local scale and does not present ecological gradients at the landscape scale. Many stems, terminal branches, and strobili of Guangdedendron preserved on the bedding plane represent the forest floor litter after these organs were shed or transported. The strobili across bedding planes indicate that they were pendant in life and had fallen near the mother plant. Such strobili characterize tree lycopsids [11DiMichele W.A. Phillips T.L. Arborescent lycopod reproduction and paleoecology in a coal-swamp environment of late Middle Pennsylvanian age (Herrin coal, Illinois, U.S.A.).Rev. Palaeobot. Palynol. 1985; 44: 1-26Crossref Scopus (136) Google Scholar]. In great contrast, the tree lycopsids dominating the Carboniferous forests can be up to 200 cm in stem diameter and 40–50 m high, and they usually form a canopy and are accompanied by understory plant types [22Gastaldo R.A. Stevanovic-Walls I.M. Ware W.N. Greb S.F. Community heterogeneity of Early Pennsylvanian peat mires.Geology. 2004; 32: 693-696Crossref Scopus (57) Google Scholar, 23DiMichele W.A. Falcon-Lang H.J. Nelson W.J. Elrick S.D. Ames P.R. Ecological gradients within a Pennsylvanian mire forest.Geology. 2007; 35: 415-418Crossref Scopus (73) Google Scholar, 27Thomas B.A. Cleal C.J. Arborescent lycophyte growth in the late Carboniferous coal swamps.New Phytol. 2018; 218: 885-890Crossref PubMed Scopus (15) Google Scholar, 28Boyce C.K. DiMichele W.A. Fast or slow for the arborescent lycopsids?.New Phytol. 2018; 218: 891-893Crossref PubMed Scopus (6) Google Scholar]." @default.
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