DSpace at VNU: The oldest flora of the South China Block, and the stratigraphic bearings of the plant remains from the Ngoc Vung Series, northern Vietnam

Journal of Asian Earth Sciences 43 (2012) 51–63 Contents lists available at SciVerse ScienceDirect Journal of Asian Earth Sciences journal homepage: www.elsevier.com/locate/jseaes The oldest flora of the South China Block, and the stratigraphic bearings of the plant remains from the Ngoc Vung Series, northern Vietnam Paul Gonez a,⇑, Hung Nguyên Huu b, Phuong Ta Hoa c, Gaël Clément d, Philippe Janvier d a Palaeobiogeology, Palaeobotany, Palaeopalynology, Department of Geology (B18), University of Liège, allée du aout, Sart Tilman, 4000 Liège 1, Belgium Vietnam National Museum of Nature, 18 Hoang Quoc Viet Str., Hanoi, Viet Nam c Vietnam National University, Department of Geology, 334 Nguyên Trai Street, Thanh Xuan, Hanoi, Viet Nam d Muséum National d’Histoire Naturelle, UMR-CNRS 7207 CR2P, Bâtiment de Paléontologie, CP 38, 47 rue Cuvier 75231 Paris cedex 05, France b a r t i c l e i n f o Article history: Received 14 October 2010 Received in revised form 12 July 2011 Accepted 25 August 2011 Available online 10 September 2011 Keywords: Early land plants Zosterophylls Late Silurian Basal euphyllophyte Early Devonian Phytogeography Vietnam a b s t r a c t Several outcrops of the Late Silurian and Devonian of the Ngoc Vung Series, northern Vietnam, yielded plant remains The Late Silurian localities delivered the earliest known flora of the South China block Although the fossils are fragmentary, they complement our knowledge about the global composition of the flora The major components of the flora are plants with dichotomous habit and terminal bivalvate sporangia, which are close relatives to zosterophylls, and zosterophylls Plants with possible euphyllophyte affinities and bryophytes are occasionally present This floral composition is similar to that of the rich, younger South China block assemblages from the Posongchong and Xujiachong Formations of China, considered Pragian in age The South China block flora is therefore likely to have been dominated by zosterophylls and pre-zosterophylls at least from the Late Silurian to the Pragian (i.e a 20 million years long period) It also strengthens the hypothesis that more derived plants were present on eastern Gondwana earlier that elsewhere, in the first steps of tracheophyte evolution The Devonian localities of the Ngoc Vung Series delivered a thick fibrous stem fragment and a basal euphyllophyte These latter plant remains provide some stratigraphic data The large stem fragment is consistent with an Eifelian age for the Duong Dong Formation (part of the Ngoc Vung Series), as suggested by the brachiopod fauna The accompanying basal euphyllophyte displays a combination of characters (axes 3–4 mm wide and lateral branchings) that is also consistent with an Eifelian age, but possibly more characteristic of the Emsian flora It is therefore suggested that the stratigraphic range of the Duong Dong Formation might be extended down to the Emsian Ó 2011 Elsevier Ltd All rights reserved Introduction Early land plants of Vietnam have hitherto barely been studied: only three accounts have been published In central Vietnam, Dovjikov (1965) reported the occurrence of two zosterophylls (Zosterophyllum sp and Gosslingia sp.) in the Tay Trang Formation, Lower Devonian Concerning northern Vietnam, Tran et al (1964) mentioned the presence of Eogaspesia sp in the Lower Devonian Si Ka Formation, but specimens are not figured Janvier et al (1987) mentioned the presence of two specimens of Cooksonia sp in Dô Son peninsula, in an outcrop of the Ngoc Vung Series (according to the latest stratigraphic zonation by Nguyên et al (2007)) This Dô Son plant locality is currently regarded as Late Silurian (probably Pridolian) in age (Janvier et al., 1987, 2003; Braddy et al., 2003) and belongs to the South China block The fossil ⇑ Corresponding author Tel.: +32 43665260 E-mail address: paul.gonez@hotmail.fr (P Gonez) 1367-9120/$ - see front matter Ó 2011 Elsevier Ltd All rights reserved doi:10.1016/j.jseaes.2011.08.007 plants from Dô Son thus represent the oldest known flora of the South China block The Ngoc Vung Series not however only include Upper Silurian sediments Its stratigraphic extension is likely to encompass Upper Silurian to Eifelian strata (Hung et al., 2007) The stratigraphic scheme of the series is still debated and commented in several recent publications (Tong et al., 2006, in press; Nguyên et al., 2007) More precise stratigraphic data are still badly needed for these plant localities of northeastern Vietnam Here we present a detail study of new material sampled in several fossil plant localities (including Dô Son) of the Ngoc Vung Series, northeastern Vietnam, along with a reinvestigation of the material figured by Janvier et al (1987) The work on the Dô Son locality aims to assess the floral composition of the oldest flora of the South China block We discuss its affinities with the younger South China block floral assemblages, such as those of the Early Devonian Posongchong and Xujiachong Formations, and with the coeval Khazakhstanian assemblage from the Wutubulake Formation of China The second goal of this work is to consider the bearings of the other palaeobotanical samples of the 52 P Gonez et al / Journal of Asian Earth Sciences 43 (2012) 51–63 Ngoc Vung Series on the stratigraphy of the Middle Palaeozoic of the Quang Ninh area Geology 2.1 Main features of the Ngoc Vung Series The Ngoc Vung Series mainly yield exposures in two areas: the Haiphong area and Bai Tu Long Bay (Fig 1) In Bai Tu Long Bay, exposures are restricted to the easternmost islands, including Tra Ban, Quan Lan and Ngoc Vung (Fig 1A) In the Haiphong area, the main exposure is the hill of Ngoc Xuyen townlet, on the Dô Son peninsula (Fig 1B) The Ngoc Vung Series are comprised of two formations (from base to top) The Van Canh Formation is about 400 m thick Its lower boundary has been observed during recent fieldwork (December 2010) in Kien an The Van Canh Formation is comformably underlain by the Kien an Formation, which is Ludlow-Pridoli in age (Tong et al., 2006) The lithology of the Van Canh Formation consists of coarse-grained quartzitic sandstones, interbedded with grey clay and siltstone lenses Fossils are found in clay and siltstone lenses, Fig Location of the investigated localities from the Ngoc Vung Series: (A) in Bai Tu Long Bay; (B) on the Dô Son peninsula, Haiphong area (redrawn from Chu et al., 2001; Dang et al., 2001) Grey areas: outcrops of the Ngoc Vung Series Localities 1–3 are part of the Van Canh Formation; localities and are part of the Duong Dong Formation P Gonez et al / Journal of Asian Earth Sciences 43 (2012) 51–63 mostly at Dô Son They are mainly eurypterids (Janvier et al., 1989; Braddy et al., 2003), but placoderms, lingulids and early land plants fragments are also common (Janvier et al., 1987) Sedimentology suggests that the Van Canh Formation represents alluvial channels in a deltaic system, with a slight marine influence in Dô Son, as suggested by the presence of eurypterids, acritarchs and scolecodonts (Braddy et al., 2003) The Van Canh Formation is comformably overlain by the Duong Dong Formation The latter is 130 m thick in the type locality but lateral variations of thickness exist (Fig 2) Although its lithology is roughly similar to that of the Van Canh formation, its palaeontological facies is different The siltstone lenses mostly contain various brachiopods, which suggest a plainly marine environment (Nguyên et al., 2007) The Duong Dong Formation is uncomformably overlain by the Middle-Upper Devonian Dô Son Formation On the Dô Son peninsula, the Duong Dong Formation does not appear There is most probably a lack of sedimentation between the Van Canh Formation and the Dô Son Formation, which are only separated by an erosion surface at Xom Chê quarry (Janvier et al., 2003) 2.2 Age of the Ngoc Vung Series Palynological samples were collected in each locality of the Ngoc Vung Series we prospected, but did not yield any spore (contra Braddy et al., 2003) Fossils are relatively rare in the Van Canh Formation They are mostly recovered from the Dô Son peninsula A Late Silurian age is suggested for the Van Canh Formation (at any rate on the Dô Son peninsula) on the basis of several macrofossils, namely the eurypterid Rhinocarcinosoma (Braddy et al., 2003), the inarticulate brachiopod Laima (Janvier et al., 1987) and several placoderm fish remains, one of which can be referred to the nomen nudum ‘Wangolepis’ hitherto recorded exclusively from the Silurian of China (Nguyên et al., 2007) However scarce are these macrofossils, fishes are clearly different from the now classical Lochkovian-Pragian taxa of northern Vietnam and South China, but closer to those found in 53 association with undoubtedly Ludlow-Pridoli marine invertebrates at My Duc, Quang Binh Province (Janvier et al., 2003) Moreover, Braddy et al (2003) reports that palynological samples from locality (Fig 1B) studied by Marshall yielded spores that indicate a Late Silurian age for this locality Yet we failed to find any evidence for palynomorphs from the same locality The overlying Duong Dong formation is regarded as Eifelian in age, at least in its uppermost beds, according to the brachiopods fauna However, it is strongly suspected to be diachronic, as some lower beds yield fossils that are characteristic of the Pragian Euryspirifer tonkinensis assemblage (Janvier et al., 2003; Tong et al., 2006; Nguyên et al., 2007) The Duong Dong Formation is most likely to encompass Pragian to Eifelian sediments Therefore, the Ngoc Vung series are comprised of deltaic deposits that may range in age from the Upper Silurian to the Lower Middle Devonian 2.3 Palaeogeographic location of the Ngoc Vung Series The Ngoc Vung Series are part of the South China block It is considered as a perigondwanan terrane It was located on the equator throughout the Devonian times, probably very close to the Gondwana, on the eastern side of the continent (Scotese, 2010) Materials and methods We have investigated five outcrops of the Ngoc Vung Series Three of them are located in the Bai Tu Long Bay area (Fig 1A) (1) The road cut of the path that runs along the western coast of Tra Ban Island is part of the Van Canh Fm It exhibits thin bedded silts and clays, rather crumbly and light brown coloured, in which occur plant fragments (4) The natural exposure on the eastern beach of Quan Lan Island north of Minh Chau belongs to the Duong Dong Fm It consists vertical strata of grey bluish silts scarcely popping out from the sand of the beach They only released one fossil of vegetal origin (5) The quarry along the road heading from the Ngoc Vung Island harbour is also part of the Duong Dong Fm It displays a vertical outcrop in which grey bluish silt lenses are Fig Stratigraphic sketch of the investigated localities and position of the studied samples Tk: thickness Dotted layers: sandstones; dashed layers: siltstones and clays; layers with circles: conglomerates; rippled lines: unconformity 54 P Gonez et al / Journal of Asian Earth Sciences 43 (2012) 51–63 interbedded in thick horizontal sandstones layers The silt lenses yield large plant fragments, but they are difficult to access, so only small rock fragments have been sampled The other two exposures are on the Dô Son peninsula (Fig 1B): (2) an old quarry along the northern slope of the hill of Ngoc Xuyen and (3) the Xom Che quarry Both of those outcrops are part of the Van Canh formation and consist of grey bluish clay silt lenses interbedded in coase grained sandstone The silt lenses deliver some small plant fragments and abundant eurypterid remains Localities 1, and are part of the Van Canh Formation; localities and are part of the Duong Dong Formation (Fig 2) The material is housed in the Geology Museum (Bao Tang Dia Chat), Hanoi, Vietnam We also re-observed the material figured by Janvier et al (1987), which was sampled in the locality The material belongs to Geology Museum, Hanoi, Vietnam, some specimens being still provisionally deposited in the Museum National d’Histoire Naturelle, Paris All the fossil plants we collected are adpressions sensu Shute and Cleal (1987), e.g ‘‘plant fossil specimen showing a mixture of compression and impression states’’ They were prepared and studied according to standard palaeobotany techniques All specimens were freed from the sediment that partially embeds them by means of the technique called ‘‘dégagement’’: sediment covering the fossil surface is removed by sharpened needles under a dissection microscope (Fairon-Demaret et al., 1999) The specimens were measured from digital photographs taken with a Zeiss Stemi 2000C stereomicroscope equipped with a CCD camera and the software AxioVision digital image processing We attempted scanning electron microscopy observations on two three-dimensionally preserved specimens, but no cellular detail was available Results 4.1 Localities from the Van Canh Formation 4.1.1 Locality 1: vegetative features Locality only yielded tiny, unbranched axes portions, the width of which ranges from 0.25 to 0.5 mm 4.1.2 Localities and 4.1.2.1 Vegetative features The localities, close to each other, are here described and discussed together Locality delivered the best preserved fossils Slender axis fragments were recovered They are about 1–1.5 mm wide and exhibit a distinct central strand, 0.1–0.12 mm wide, the cells of which are not preserved (Fig 3A) A bulbous structure sticking out from an unbranched axis was also released by ‘‘dégagement’’ technique (Fig 3B) The vascular trace displayed by the axis does not penetrate the bulged structure This can either be a preservation artefact or the indication of an unvascularized structure The bulbous structure looks like a slightly vertically stretched bladder It is 0.65 mm wide and 0.8 mm high Sixty-five samples were collected at Xom Che quarry, locality Vegetative remains are very abundant Most of the vegetative fragments are unbranched axes, sometimes of a great length: the longest is 14 cm They occasionally exhibit a coalified central strand, 0.1–0.15 mm wide Sometimes, the central strand preserves the outline of elongated cells (Fig 3C and D) The cells are about 400 lm long and 20 lm wide All axes are smooth and of constant diameter Many instances of branching axes are recovered, and commonly display K-type branchings (Fig 3E) Those axes are 0.25–0.7 mm wide Some trichotomies (Fig 3F) are also encountered, but they most probably result from the folding of a K-type branching They display the same width range The presence of lateral branchings that dichotomize (Fig 3G) is noteworthy The main axis of this system is 0.55 mm wide The lateral branch is 0.15 mm wide and its two daughter-axes are 0.1 mm wide Isotomous branchings are rare Two minute, delicate fossils display peculiar features Specimen ST390-7 exposes a slender axis, 0.2 mm wide and mm long This axis bears a lateral pointed structure on each of its sides The pointed structures are 1.5–2.2 mm long and are roughly bananashaped (Fig 3H) There are two possible interpretations for those unbranched lateral structures (1) Those banana-shaped emergences could be sporangia Rebskia musaeformis Schweiter 2000, from the Emsian of Germany, also displays banana-shaped sporangia However, those are much bigger (5–7 mm), terminal, and exhibit a rounded structure at their apex (Schweitzer, 2000) Thus our specimens are clearly different from Rebskia Moreover, we are reluctant to favour the sporangia hypothesis, because the only known plants with lateral sporangia in the Late Silurian are lycophytes or lycophytes-like plants, i.e with reniform and dehiscent sporangia (2) The second hypothesis is that those structures are microphylls The size and shape are consistent with this interpretation This hypothesis is more parsimonious, because microphylls are the characteristic vegetative lateral organs of lycophytes; and early lycophytes had already evolved by the Late Silurian (Kotyk et al., 2002) The oldest occurrence of microphylls is gedinnian (i.e the equivalent of the Lochkovian) (Schweitzer, 1982) Specimen ST390-8 also consists of a slender axis, 0.2 mm wide and 4.9 mm long It shows a 0.8 mm long, bifurcating lateral structure The basal part is 0.25 mm long; the two furcas are longer They have different lengths: 0.25 and 0.55 mm They both demonstrate a pointed end (Fig 3I) Once again two interpretations are possible for this structure: either it can be interpreted as a sporangium or as a lycophyte lateral vegetative organ The sporangium hypothesis is not favoured, according to the same arguments Though bifurcating sporangia are reported in the Late Silurian (Fanning et al., 1991), their shape is rather different and they are borne terminally We consider that the vegetative organ hypothesis is more parsimonious This structure will therefore be tentatively interpreted as a bifurcating microphyll, yet multifurcate microphylls only appear by the Emsian in the fossil record (Gensel and Kasper, 2005) 4.1.2.2 Sporangia Eleven complete sporangia were recovered in locality They demonstrate a significant variation in size and shape (Fig 4), thus testifying of the diversity of the original flora The biggest sporangium (Fig 5A) is 3.9 mm high and 2.35 mm wide It is borne by a 7.9 mm long portion of unbranched slender axis, 0.5 mm wide The lower portion of the sporangium consists in an abrupt widening of the subtending axis, at an angle of 60° The upper part of the sporangium has a more irregular and curved outline This structure resembles the body illustrated by Edwards et al (2001, Fig 36) The authors compare this structure to Tarrantia Fanning et al 1992 We disagree with this comparison Tarrantia exhibits a more regular oval or ovoid outline, and the dimensions of its sporangium are smaller (around mm high) We would rather attribute our specimen to the genus Sporogonites Halle, 1916 Most of the features match Halle’s diagnosis (1916): ‘‘spore producing body consisting of a simple stalk and a terminal capsule Stalk 0.5 mm in diameter and up to at least 50 mm long, faintly striated longitudinally Capsule elongated or clavate, 6–9 mm long and 2–4 mm in diameter in the thicker upper part, with a rounded apex and a tapering base [ .]’’.The only absent characters are the striation of the stalk and the spores The smaller dimensions of our specimen are characteristic of S yunnanense Li 1966 Yet our specimen does not show any pointed apex, and the elongated epidermal cells of the wall of the capsule mentioned in Li’s diagnosis are not perceptible Therefore, we provisionally refer to the specimen as cf Sporogonites yunnanense P Gonez et al / Journal of Asian Earth Sciences 43 (2012) 51–63 55 Fig Remarkable vegetative features of the plant remains from the Dô Son peninsula, Van Canh Formation: (A) specimen ST390-1, axis exhibiting a central strand; (B) specimen ST390-2, bulbous structure emerging from an unbranched axis; (C) specimen ST390-3, axis preserved as the outlines of the cells are perceptible, general view; (D) specimen ST390-3, detailed view; (E) specimen ST390-4, K-type branching; (F) specimen ST390-5, the central daughter internodes is dichotomous; (G) specimen ST390-6, the main axis bears a lateral branch that is dichotomous, only the coalified central strand of the whole branching system is preserved; (H) specimen ST390-7, axis portion that bears two structures tentatively interpreted as microphylls; (I) specimen ST390-8, axis portion that bears a lateral original structure tentatively interpreted as a bifurcating microphyll 56 P Gonez et al / Journal of Asian Earth Sciences 43 (2012) 51–63 and Gerrienne, 2010b) The rounded bodies are more likely to be oval to reniform sporangia similar to those described above, i.e probably bivalved and dehiscent The lack of morphological detail prevents from a more precise determination 4.1.2.3 cf Filiformorama Wang et al., 2006 The specimen (Fig 5M and N), found in locality 3, is 26 mm long Axes are constant in diameter and their curves hint that they were flexible in life The main axis is 0.3 mm wide Our specimen displays one single anisotomous branching at the base of the main axis The lateral axis is 5.6 mm long for 0.2 mm wide and unbranched (Fig 5M) The terminal body is 0.45 mm high for 0.5 mm wide, poorly and incompletely preserved (Fig 5N) It is interpreted as the base of a terminal structure, most probably a sporangium Those features are reminiscent of Filiformorama Wang et al., 2006 However, Filiformorama sporangia are bigger and tongue-shaped or reniform (Wang et al., 2006) Here poor preservation prevents from a secure three-dimensional reconstruction of the terminal body Therefore, we refer this specimen to cf Filiformorama sp with great reservation Fig Outlines of the sporangia population from Xom Che quarry, Van Canh Formation (A) ST390-11; (B) ST390-12; (C) ST390-13; (D) ST390-14; (E) ST390-15; F: ST390-17; G: ST390-16; (H) ST390-6; (I) ST390-10; (J) ST390-9 Another terminal body is clearly vertically stretched It is 0.42 mm wide and 0.67 mm high, and is tentatively interpreted as an elongated sporangium of unknown affinity (Fig 5B) The remaining nine sporangia are all rounded to reniform in shape, but exhibit different characteristics Their width ranges from 1.17 to mm, and their height from 1.78 to 2.57 mm The limit between the sporangia and the subtending axis could not be defined clearly in these specimens, due to preservation Two specimens terminate isotomous axes (Fig 5C–E) The terminal branches of the largest specimen (Fig 5C and D) are 5.18 mm and 4.27 mm long The sporangia are rounded, respectively 2.75 mm high–3.0 mm wide, and 2.57 mm high–3.23 mm wide They are the largest ones found in this locality Specimen ST390-12 (Fig 5E) is more slender: the only complete terminal branch is 3.15 mm and the single preserved sporangium is 1.7 mm high Sporangium width cannot be measured with certainty due to deformation Nevertheless, this sporangium shows a conspicuous rim parallel to the sporangial outline This feature is interpreted as a sub-distal dehiscence line It is one of the smallest sporangia of the flora The other specimens are borne by unbranched axes Sporangial dimensions show significant variation: their width ranges from 1.17 mm to 2.22 mm, and their height from 1.78 mm to 2.21 mm (see also Fig 4) Among the latter, three exhibit a longer subtending axis (Fig 5F–H): respectively 5.18 mm, 4.0 mm and 2.9 mm A sub-distal dehiscence line is perceptible on specimens ST390-14 and ST390-13 (Fig 5F and G) The latter (specimen ST390-15, Fig 5H) shows that its two valves are separated and are clearly unequal in size The last three specimens are borne by short portions of subtending axis, whose length ranges from 1.75 to 2.17 mm These three sporangia consist of oval to reniform bodies (Fig 5I–K) One of them exhibits a clear sub-distal dehiscence line (Fig 5I) Sporangial wall of another is ornate with a reticulum (Fig 5K–L) We also re-observed the material figured by Janvier et al (1987) who mentioned the presence of Cooksonia Lang 1937 sporangia in the flora These sporangia are weak impressions of rounded bodies with a slightly blurred outline, due to poor preservation They not match with the updated definition of Cooksonia, which states that the sporangium is characteristically trumpet-shaped (Gonez 4.2 Localities from the Duong Dong Formation 4.2.1 Locality Locality yielded larger axes, 1–6 mm wide, one of which could possibly be trifurcate, but bad preservation prevents from assessing it with certainty A poorly preserved specimen is of much larger size The fragment is 16 mm wide for 14 mm high The texture of the preserved organic matter is porous and the outline of the fibres is clearly perceptible (Fig 6A) This fossil could be a wood fragment However, the preservation prevents from assessing that with certainty: neither aligned tracheids, nor vascular rays can be demonstrated We will therefore consider that this fragment is part of a large plant, stems at least 1.5 cm large, whose stem anatomy is fibrous 4.2.2 Locality Locality yielded plant fragments of different sizes The larger remains are 3.3–4 mm wide and display anisotomous branchings, the lateral emissions are 1.7–2 mm wide They are inserted at 120° (Fig 6B) The width of the main axis is large and constant The fragment is thus interpreted as a proximal or median part of the stem In one fragmentary specimen, the main axis (3.2 mm wide), tapers to mm after two successive lateral emissions that are 1.7 mm wide Here the width of the main axis is less important and tapers distally The fossil is thus interpreted as a distal part of a main axis, close to the apex The 1.7 mm wide lateral emissions of this specimen are branched isotomously (Fig 6C) The smaller remains are dichotomous systems that sometimes terminate in pointed recurvations (Fig 6D) The latter can also be found as dispersed organs in the sediment (Fig 6C) Discussion 5.1 Floral composition of the localities 5.1.1 Locality The fragmentary preservation of the vegetal remains prevents from providing a precise floral composition However, the fact that all, very abundant, axis fragments are all very short and narrow suggests that the flora was likely to be comprised of early embryophytes, probably with a rhyniophytoid architecture, i.e with dichotomous axes and terminal sporangia ‘‘Rhyniophytes’’ have been demonstrated to be polyphyletic (Kenrick and Crane, 1997), P Gonez et al / Journal of Asian Earth Sciences 43 (2012) 51–63 57 Fig Fertile remains from Xom Che quarry, Van Canh Formation: (A) specimen ST390-9, cf Sporogonites yunnanense; (B) specimen ST390-10, terminal structure interpreted as a sporangium of previously unknown morphology; (C) specimen ST390-11, rounded sporangia borne on isotomous axes; (D) specimen ST390-11, counter-part; (E) specimen ST390-12, reniform dehiscent sporangium borne by isotomous axes; (F) specimen ST390-13, rounded dehiscent sporangium; (G) specimen ST390-14, reniform dehiscent sporangium; (H) specimen ST390-15, sporangium with unequal valves, the larger upper valve is broken, and thus reveals part of the lower smaller one; I, specimen ST390-16, rounded dehiscent sporangium; (J) specimen ST390-6, oval sporangium; (K) specimen ST390-17, sporangium with a reticulate wall; (L) specimen ST390-17, detail of the reticulum; (M) specimen ST390-4, cf Filiformorama sp., general view, the arrow points toward the anisotomous branching; N, specimen ST390-17, cf Filiformorama sp, detail of the terminal body interpreted as the base of a sporangium the preferred term ‘‘rhyniophytoid’’ (Pratt et al., 1978) only refers to a morphology, and does not convey any taxonomic bearing 5.1.2 Localities and Our fossils demonstrate a rather remarkable diversity in the flora of the Van Canh Formation on the Dô Son peninsula Bulbous structures similar to those we found in Dô Son have hitherto only been demonstrated in two taxa (1) In the tracheophyte Bitelaria dubjanskii Istchenko & Istchenko 1979, similar bulbous structure are present, but they occur as clusters (Johnson and Gensel, 1992) (2) Stockmansella remyi Schultka and Hass, 1997 is a Rhyniopsida sensu Kenrick and Crane, 1997 (Paratracheophyta Gerrienne et al., 2006) It consists in dichotomous creeping axes with lateral hazelnut-shaped sporangia, and rhizoid-bearing bulges (Schultka and Hass, 1997) The rhizoid-bearing bulges look similar in shape to that we illustrate on figure 3B However, they are much smaller: 0.2–0.3 mm wide for 0.3–0.5 mm long (Schultka and Hass, 1997) As those bulbous structures would be the only and weakest clue for the presence of this taxon, we are reluctant to postulate that the flora might have contained Rhyniopsida/ Paratracheophyta In brief, this bulged structure cannot be referred to any existing taxon Sporogonites is believed to be a plant at a bryophytic grade of organization (Halle, 1916; Andrews, 1960; Kenrick and Crane, 1997) The rounded to reniform sporangia terminating dichotomous axes are relevant to plants with rhyniophytoid architecture At least two taxa were present in this locality Firstly, a taxon represented by plants with large, rounded sporangia; secondly a taxon with sporangia showing a sub-distal dehiscence line Compression 58 P Gonez et al / Journal of Asian Earth Sciences 43 (2012) 51–63 Fig Remarkable vegetative features of the plant remains of the Duong Dong Formation: (A) specimen ST390-18, large fibrous stem fragment from locality 4; (B) specimen ST390-19 from locality 5, main axis showing lateral branches inserted at 120°; (C) specimen ST390-20 from locality 5, main axis that tapers after two successive lateral emissions, inserted at 120°, the lateral emissions are dichotomous systems, the tapered axis branch near the edge of the fossiliferous block, arrow points to at an isolated recurved tip; (D) specimen ST390-21 from locality 5, vegetative ramified system, the terminal branches are recurved tips, some of them are indicated by arrows fossils of plant with rhyniophytoid architecture and rounded to reniform sporangia are represented by three genera: Uskiella Shute & Edwards 1989, Renalia Gensel 1976, Hsüa Li 1982 (Gonez and Gerrienne, 2010a) The specimen with large rounded sporangia cannot be attributed to any of those taxa, because of the lack of morphological details on its sporangia Our second specimen shares a feature in common with Aberlemnia Gonez & Gerrienne 2010: it possesses a sub-distal dehiscence line (Gonez and Gerrienne, 2010a) Yet, other sporangial characters defining the genus are not displayed: the features of the sporangium/subtending axis limit are not preserved We will therefore refer to the specimen as cf Aberlemnia sp Aberlemnia, and possibly other plants of rhyniophytoid architecture with reniform dehiscent sporangia, are most probably a stem-group of early lycophytes, i.e zosterophylls (Gonez and Gerrienne, 2010b) Other incertae sedis plants with different or unknown architecture (such as cf Filiformorama or the putative elongated sporangia) are present in this locality Other rounded to reniform sporangia are borne by unbranched portions of axis Three specimens each demonstrate a distinctive feature (reticulate sporangial wall, subdistally dehiscent sporangium, unequal valves), which suggests that there were at least three different taxa of parent plants Two interpretations are possible for those sporangia borne by unbranched short portions of axis (1) The fossils are broken parts of terminal dichotomous axis In this case, the fossils represent other taxa of plant with rhyniophytoid architecture and reniform sporangia, i.e stem-group of zosterophylls (2) The fossils that possess the shortest subtending axes may represent detached laterally borne dehiscent and reniform sporangia, a diagnostic feature of zosterophylls It is not possible to produce a more precise determination of these specimens, due to the fragmentary nature of the material The presence of a central strand in some axes, interpreted as vascular on the basis of size and shape of the observable in situ cell outlines can either be part of plants with rhyniophytoid architec- ture and dehiscent sporangia or of zosterophylls Zosterophylls are known to possess G-type tracheids and some of the plants with rhyniophytoid architecture and dehiscent sporangia have been demonstrated to be vascular as well (Li, 1992) On the contrary, the seven occurrences of K-type branchings shed little doubt on the actual presence of zosterophylls Even though those branchings are not an ‘‘official’’ diagnostic character of zosterophylls, they have hitherto only been recorded as part of zosterophylls, when not dispersed Therefore the flora includes two major components: (1) plants with rhyniophytoid architecture and reniform and/or dehiscent sporangia, which are likely to be a stem-group of zosterophylls; and (2) zosterophylls The plants bearing the lateral structures resembling microphylls could also share affinities with lycophytes sensu lato More derived plants, i.e plants with euphyllophytes affinities, have possibly been present in the flora, as suggested by the lateral branching systems Minute plants with lateral fertile systems have recently been discovered in the Late Silurian of North China (Wang et al., 2007) 5.1.3 Locality It is also impossible to deduce a floral composition because of the fragmentary preservation The flora includes large plants with fibrous stem 5.1.4 Locality Plant remains, all considered as belonging to the same taxon, found in locality are comprised of thick axes that branch anisotomously at 120°, and more slender dichotomous systems The slender vegetative dichotomous systems are most likely the continuation of the lateral branches of the thicker axes Indeed, the thick axes lateral emissions width is comparable to that of the basal internodes of the dichotomous systems On the specimen ST390-20, the larger axes and the slender isotomous systems are P Gonez et al / Journal of Asian Earth Sciences 43 (2012) 51–63 59 Fig Global palaeogeography of the Late Silurian Squares: location of floras from the Khazakhstanian unit; circles: floras from the South Laurussian–Northwest Gondwanan unit; triangle: flora from Bathurst Island; star: flora from the Australian unit; rhomb: flora from the Dô Son peninsula (modified from Raymond et al., 2006) connected The plant can thus be understood as main axes that bear lateral, helicoidally-arranged, ramified vegetative systems This fits the definition of euphyllophytes (Kenrick and Crane, 1997) The gross morphology of the plant indeed strongly recalls that of some basal euphyllophytes, especially Psilophyton Dawson 1859 Similar branchings, axes width and terminal vegetative recurvations are present in Psilophyton dawsonii (Banks et al., 1975) Yet we prefer not to name that plant, and refer it to a ‘‘basal euphyllophyte’’, since no sporangia can allow its precise taxonomic attribution, nor warrant the erection of a new taxon This basal euphyllophyte is dominant in the assemblage of Ngoc Vung Island, which may even be monospecific 5.2 Stratigraphic bearings of the plant findings Plants are generally, but unjustly, regarded as poor biostratigraphic tools for the Siluro-Devonian period The fossil record is scarce and characterized by provinciality (Raymond et al., 2006), which may generate biases Nonetheless, the morphological and anatomical characters of the early land plants show remarkably rapid changes throughout the Devonian, so that their biostratigraphic potential has been pointed out (Edwards and Davies, 1990) A biostratigraphic method, based on characters, rather than taxa, was developed by Gerrienne and Streel (1994) Unfortunately our fossils display too few anatomical characters to apply here However, since the stratigraphy of the Siluro-Devonian of the Quang Ninh area needs to be refined, we consider that their stratigraphic bearings are worth to be mentioned The very tiny plants fragments from locality (Van Canh Fm) suggest a Late Silurian age Early Devonian axes are usually larger indeed Size is not a widely used criterion for stratigraphy, but concerning plants, a relationship between axes size and sediment age is clearly established for the Silurian and Devonian (Edwards, 1979; Gerrienne, 1990; Gerrienne and Streel, 1994) The plants from localities and (Van Canh Fm) are mostly zosterophylls and plants with terminal reniform sporangia, which are characteristic of Upper Silurian–Lowermost Devonian proximal deposits This is consistent with the faunal data, which suggest a Late Silurian age for the Dô Son outcrops The large fibrous stem recovered at locality is younger The oldest plant displaying axis around 1.5 cm thick is Pertica Kasper and Andrews, 1972 Pertica is a basal euphyllophyte from the Trout Valley Formation, Maine, USA, which is LateEmsian–Early Eifelian in age (Kasper and Andrews, 1972) This is consistent with Nguyen’s dating (2007) for the Duong Dong Formation, which is supposed to be Eifelian The basal euphyllophyte remain found at locality is also consistent with an Eifelian age However, basal euphyllophytes (i.e with lateral vegetative systems) with such axes width (3–4 mm) are more characteristic of the Emsian (Gerrienne, 1990; Gerrienne and Streel, 1994) Therefore, the exposure of the Duong Dong Fm on Ngoc Vung island is possibly of Emsian age, all the more that the Duong Dong Fm is likely to be diachronic (Janvier et al., 2003) This strengthens the hypothesis stating that the stratigraphic extension of the Duong Dong Formation may therefore be more important than previously thought (Nguyên et al., 2007; Tong et al., Pragian), encompassing accepted for publication to Eifelian sediments 5.3 Comparisons 5.3.1 Comparison with other Late Silurian floras Raymond et al (2006) produced a global phytogeographic analysis of Late Silurian macrofloras The authors led two analyses: one based on taxa and another based on morphological traits Both analyses yielded the same four phytogeographic units, ensuring that the definition of areas is not biased by regional taxonomic usage The phytogeographic units are (1) Bathurst Island, (2) a South Laurussian–Northwest Gondwanan assemblage, (3) an East Gondwanan assemblage (Australia), and (4) a Khazakhstanian assemblage We will now briefly compare our South Chinese block assemblage to each Late Silurian phytogeographic unit (Fig 7) More importance will be given to the Khazakhstanian flora, as it is the closest assemblage to South China, in terms of palaeogeography 60 P Gonez et al / Journal of Asian Earth Sciences 43 (2012) 51–63 5.3.1.1 Flora from the Pridoli of Khazakhstania: data from the Wutubulake Formation of China The Wutubulake flora is coeval to that of the Van Canh Formation at Dô Son However Wutubulake was part of the Khazakhstanian block (Edwards, 1990; Edwards et al., 2001) Khazakhstania was located on the equatorial belt of the Gondwana (Scotese, 2010) The South China block, to which belongs the Dô Son plant remains outcrop, is supposed to be a perigondwanan terrane, i.e it was possibly closer to the Gondwana in Silurian-Devonian times Plants showing rhyniophytoid architecture with reniform sporangia were present in both terranes, as Table Composition of Late Silurian–Early Devonian plant assemblages from southeastern Asia Van Canh Fm flora Watabulake Fm flora Xujiachong Fm flora Posongchong Fm flora (Late Silurian) (Late Silurian) (Pragian) (Pragian) Vietnam China China China Plants at a bryophyte grade of organization Sporogonites cf S.yunnanense Plants with a rhyniophytoid architecture At least three taxa, possibly including Aberlemnia sp Lycophytina sensu Kenrick and Crane (1997) or plants with lycophytes affinities Zosterophylls indet Duong Dong Fm flora (? Emsian–? Givetian) Vietnam Sporogonites yunnanense (Hsü, 1966) Junggaria spinosa (Cai et al., 1993) Hsüa robusta (Li, 1992) Salopella xinjiangensis (Cai et al., 1993) Sciadophyton sp (Cai et al., 1993) Hsüa deflexa (Wang et al., 2003) Zosterophyllum sp (Cai et al., 1993) Zosterophyllum australianum (Hao, 1992) Zosterophyllum yunnanicum (Wang, 2007) Zosterophyllum longa (Wang, 2007) Huia gracilis (Wang et al., 2002) Drepanophycus qujingensis (Li and Edwards, 1995) Bracteophyton variatum (Wang and Hao, 2004) Guangnania cuneata (Wang and Hao, 2002) Baragwanathia sp (Hao and Gensel, 1998) Zosterophyllum australianum (Hao and Gensel, 1998) Zosterophyllum sp.1 (Hao and Gensel, 1998) Huia recurvata Gumuia zyzzata (Hao, 1989b) Adoketophyton subverticillatum (Li and Edwards, 1992; Hao et al., 2003) Discalis longistipa (Hao, 1989a) Stachyophyton yunnanense (Geng, 1983; Wang and Cai, 1996) Wenshania zhichangensis (Zhu and Kenrick, 1999) Guangnania cuneata (Wang and Hao, 2002) Hueberia zhichangensis (Yang et al., 2009) Zenghlia radiata (Hao et al., 2006) Halleophyton zhichangense (Li and Edwards, 1997) Demersatheca contigua (Li and Edwards, 1996) Basal euphyllophytes sensu Kenrick and Crane (1997) or plants with euphyllophyte affinities Probably at least one taxon Psilophyton primitivum (Hao and Gensel, 1998) Watabulaka multidichotoma (Wang et al., 2007) Eophyllophyton bellum (Hao and Beck, 1993) Polythecophyton demissum (Hao et al., 2001) Plant with sphenopsid affinities Plant of unknown affinities Estinnophyton yunnanense (Hao et al., 2004) cf Filiformorama sp Filiformorama simplexa (Wang et al., 2006) Hedeia sinica (Wang et al., 2002) Hedeia sinica (Hao and Gensel, 1998) Celatheca beckii (Hao and Gensel, 1995) Catenalis digitata (Hao and Beck, 1991b) Yunia dichotoma (Hao and Beck, 1991a) Basal euphyllophyte P Gonez et al / Journal of Asian Earth Sciences 43 (2012) 51–63 well as early lycophytes with K-type branchings (Table 1) The enigmatic taxon Filiformorama is also common to both floras The originality of the Wutubulake formation flora lies in the early presence of plants with lateral fertile systems (Wang Yi et al., 2007) The Van Canh formation flora also delivers plants with lateral branchings systems, but it is impossible to tell whether they were vegetative or fertile, and their affinities therefore remain unresolved These two floras respectively belong to two different phytogeographic sub-units (Hao and Gensel, 1998), but may have been somewhat linked, according to the dispersed morphological features mentioned above This hypothesis of the relationship between the floras of the two terranes must be considered with caution, mainly for two reasons: (1) the fragmentary nature of the Vietnamese plants does not allow fully reliable comparisons, (2) the Wutubulake Fm flora is also characterized by taxa with rhyniophytoid architecture with non-reniform sporangia (Table 1) This constitutes a major difference from the South Chinese phytogeographic unit, to which the Dô Son, Posongchong and Xujiachong floras belong 5.3.1.2 Floras from South Laurussia–Norwest Gondwana This phytogeographic unit is mostly documented by Welsh and South American floras It is characterized by the dominance of plants of rhyniophytoid architecture (Raymond et al., 2006), some of them exhibiting reniform dehiscent sporangia (Edwards, 1970; Edwards et al., 2001; Gonez and Gerrienne, 2010a); and the occasional presence of zosterophylls Our assemblage also contains occasional zosterophylls and plants of rhyniophytoid architecture, with reniform and dehiscent sporangia, Aberlemnia-type The major difference between the two assemblages lies in the presence of a large panel of rhyniophytoids with non reniform sporangia in the South Laurussian–Northwest Gondwanan assemblage However, this difference may be the result of sample bias, as only one Silurian locality is known on the South China block, whereas numerous sites have been prospected in Wales and South America 5.3.1.3 Flora from the Ludlow of Bathurst Island The assemblage of Bathurst Island is represented by seven taxa, the majority of which are zosterophylls The plants are remarkably complex and derived in regard of their old age (Kotyk et al., 2002) Our assemblage differs neatly from that of Bathurst Island: more basal embryophytes well represented on the South China block flora Thus the Canadian flora remains unique 5.3.1.4 Flora from East Gondwana: data from Australian assemblages The Australian assemblage is also dominated by Lycophytes sensu lato (Baragwanathia and Zosterophyllum), together with plant with rhyniophytoid architecture and non reniform sporangia (Salopella) and the possibly more derived plant Hedeia (Raymond et al., 2006) Therefore this assemblage is radically different from our South Chinese flora 5.3.2 Comparison with younger floras from the South China block: data from the Pragian Posongchong and Xujiachong Formations, China The Xujichong and the Posongchong formations floras are from the South China block They are younger than the Dô Son flora and are dominated by lycophytes (64% of the number of taxa in each case) (Table 1) It is worth to notice that plants with ‘‘rhyniophytoid’’ architecture are also present in the Xujiachong Fm and display reniform, bivalved and dehiscent sporangia (Table 1), like the ‘‘rhyniophytoid’’ plants from Dô Son The Dô Son fossils therefore suggest that the Late Silurian flora of the South China block was roughly similar to the younger Pragian floras of the same terrane, i.e comprised of plants with rhyniophytoid architecture and reniform sporangia, early lycophytes and, occasionally, more derived plants (Table 1) 61 The presence of a basal euphyllophyte in the ?Emsian-?Eifelian of the South China block (the Duong Dong Fm/Ngoc Vung Island flora, locality 5) is also consistent with the Chinese data Indeed basal euphyllophytes are represented by three taxa in the Posongchong Fm flora, including a species of Psilophyton (Table 1) Conclusions (1) The fossil plants from the Dô Son peninsula in northern Vietnam likely represents the oldest flora of the South China Block they indicate that the land flora of this major Asian land mass was already fairly diverse at least since the Late Silurian This is in agreement with data from Ludlow, Pridoli and earliest Lochkovian plant localities all over the world (Edwards, 1979; Tims and Chambers, 1984; Cai et al., 1993; Edwards et al., 2001; Gerrienne et al., 2001; Kotyk et al., 2002) The early flora of the South China block was mainly comprised of early lycophytes and plants with rhyniophytoid architecture and reniform sporangia Some more derived and/or enigmatic plants are also encountered This composition is qualitatively similar to the younger Pragian assemblages from the Yunnan Province, southern China The Pragian flora of Yunnan might thus have been established on the South China block since the Late Silurian It is however frustrating that the fragmentary nature of the fossils only allows to infer a global floral composition of the vegetation More field work is needed in order to investigate new localities with better preserved fossils, which could allow a more accurate taxonomic and palaeobiological considerations (2) The stratigraphic significance of the plant remains in the Ngoc Vung series is in concordance with the previous data for the Van Canh Formation and agrees with the Late Silurian (probably Pridoli) age of the plant-bearing outcrop inferred from faunal data Concerning the Duong Dong Formation, the plant remains strengthen the hypothesis of a broader age range that could also encompass Pragian to Eifelian sediments Acknowledgments We thank Mme Marcela Giraldo for the preparation of the palynological samples, Dr Philippe Gerrienne for his advice and pertinent critics We also thank an anonymous reviewer whose comments substiantially improved the content of this manuscript The field work was made possible by National Geographic Grant #8550-08 Ta Hoa Phuong has been supported by the Vietnamese NAFOSTED (Project 105.01.79.09 and 105.06.60.09) Paul Gonez holds a FRIA Grant References Andrews, H.N., 1960 Notes on the Devonian genus Sporogonites The Palaeobotanist 7, 85–89 Banks, H.P., Leclercq, S., Hueber, F.M., 1975 Anatomy and morphology on Psilophyton dawsonii sp n from the Late Lower Devonian of Quebec (Gaspé), and Ontario, Canada Palaeontographica Americana 48, 77–126 Braddy, S.J., Selden, P.A., Truong, Doan Nath, 2003 A new Carcinosomatid Eurypterid from the Upper Silurian of Northern Vietnam Palaeontology 45 (5), 897–915 Cai, Chong Yang, Dou, Ya Wai, Edwards, D., 1993 New observations on a Pridoli assemblage from north Xinjiang, northwest China, with comments on its evolutionary and palaeogeographical significance Geological Magazine 130 (2), 155–170 Chu, Quang Bong, Dang, Van Doi, Do, Van Tiep, Luu, Doan Nguyen, Ngo, Quang Thang, Nguyen, Huy Dung, Nguyen, Quoc Thanh, Truong, Cong Duong, Vu, Viet Hung, 2001 Ha Long (Hon Gai) sheet, 1/200 000 In: Nguyen, Cong Luong (Ed.), 62 P Gonez et al / Journal of Asian Earth Sciences 43 (2012) 51–63 Geology and Mineral Map of Vietnam Department of Geology and Minerals of Vietnam, Hanoi Dang, Tran Quan, Hoan, Trong De, Le, Duc Kham, Nguyen, Ngoc Minh, Nguyen, Tien Chu, 2001 Haiphong sheet, 1/200 000 In: Hoan, Ngoc Ky (Ed.), Geology and Mineral Resources Map of Vietnam Department of Geology and Minerals of Vietnam, Hanoi Dovjikov, A.E (Ed.), 1965 Geologia Svernoga Vietnama Geological Survey of Vietnam, Hanoi, 665 pp (in Russian) Edwards, D., 1970 Fertile Rhyniophytina from the Lower Devonian of Britain Palaeontology 13 (3), 451–461 Edwards, D., 1979 A Late Silurian flora from the Lower Old Red Sandstone of southwest Dyfed Palaeontology 22 (1), 23–52 Edwards, D., 1990 Constraints on Silurian and Early Devonian phytogeographic analysis based on megafossils In: MacKerrow, W.S., Scotese, C.R (Eds.), Palaeozoic Palaeogeography and Biogeography Geological Society, London, pp 233–242 Edwards, D., Davies, M.S., 1990 Interpretations of early land plant radiations: ‘‘facile adaptationnist guesswork’’ or reasoned speculation? 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Journal of Asian Earth Sciences 43 (2012) 51–63 Ngoc Vung Series on the stratigraphy of the Middle Palaeozoic of the Quang Ninh area Geology 2.1 Main features of the Ngoc Vung Series The Ngoc Vung. .. outcrops of the Ngoc Vung Series Three of them are located in the Bai Tu Long Bay area (Fig 1A) (1) The road cut of the path that runs along the western coast of Tra Ban Island is part of the Van... South China block: data from the Pragian Posongchong and Xujiachong Formations, China The Xujichong and the Posongchong formations floras are from the South China block They are younger than the