The relationship between Euramerican and Cathaysian tropical floras in the Late Palaeozoic: Palaeobiogeographical and palaeogeographical implications

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Earth-Science Reviews 85 (2007) 85 – 116 www.elsevier.com/locate/earscirev

The relationship between Euramerican and Cathaysian tropical floras in the Late Palaeozoic: Palaeobiogeographical and palaeogeographical implications J. Hilton a,⁎, C.J. Cleal b b

a School of Geography, Earth and Environmental Sciences, University of Birmingham, Edgbaston, Birmingham, B15 2TT, UK Department of Biodiversity and Systematic Biology, Amgueddfa Cymru - National Museum Wales, Cathays Park, Cardiff, CF10 3NP, UK

Received 6 October 2006; accepted 31 July 2007 Available online 9 August 2007

Abstract Wetland plant communities persisted though much of the Pennsylvanian in Euramerica and are the dominant coal forming vegetation in this region. Distribution of these floras show a dramatic decline at the end of the Carboniferous with many of the plant genera and species becoming extinct by the onset of the Permian. This has been correlated with climate change and in particular aridification associated with northwards plate motion and Euramerica moving into the doldrums, but is also associated with the Variscan orogeny and forefront destroying large areas of formerly lowland basinal settings. Other factors may include Pennsylvanian glaciations as evidenced by cyclothem and rhythmical deposition in lowlying wetland settings, and change in eustatic base level. Evidence from Euramerica demonstrates extinction of this kind of wetland biota by the earliest Permian and the development of drier floras including conifer dominated assemblages. However, new data from other parts of the world, most notably North China, confirm this model and highlight the presence of similar coal swamps ranging from the Late Pennsylvanian through the Permian. In this paper we summarise and synthesize recent taxonomic and systematic investigations undertaken on the plant fossils from the Pennsylvanian Benxi Formation – the oldest recognised wetland plant community in China – and the Early Permian Taiyuan Formation – the best preserved wetland plant community from China. Results indicate a remarkable similarity of the Pennsylvanian–Early Permian floras of North China with the older assemblages in the Pennsylvanian of Euramerica, and the presence of typical ‘Euramerican’ coal swamp plant families, genera and in some cases species in China. Conclusions include the presence of the Ameriosinian phytogeographical realm uniting Euramerica and northern Cathaysia at this time, coal swamps in the Permian of North China evolving from a ‘Euramerican’ origin, and the dramatic floral turnover at the end of the Carboniferous representing a regional event rather than a global extinction episode. Patterns of plant distribution through this interval have profound implications on established palaeogeographical models and support continental connection between Euramerica and Cathaysia before the end of the Carboniferous, contradicting ideas of Cathaysian island biogeography and biotic distinction. Continental connection appears to be related to glacial eustatic low-stand and previously shallow marine environments becoming vegetated. Also important is the fact that in both Euramerica and North China the pattern of floristic demise within wetland plant

⁎ Corresponding author. E-mail address: [email protected] (J. Hilton). 0012-8252/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.earscirev.2007.07.003

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communities are similar to each other, implying the same causal mechanisms, with plants occupying waterlogged positions being the most severely devastated. However, ecosystem demise occurred at the end of the Carboniferous in Euramerica and in the middle Permian in North China, but in both cases the primary cause was climate change. © 2007 Elsevier B.V. All rights reserved. Keywords: Carboniferous; Permian; palaeobotany; biogeography; palaeogeography; coal-swamp

1. Introduction From its first appearance in Early Palaeozoic times, terrestrial vegetation showed a progressive increase in global provincialism (e.g. Vakhrameev et al., 1978; Wnuk, 1996) largely driven by latitudinal variation in climate. Until this ‘diversification clock’ was re-set by the end-Permian extinction, this provincialism became largely expressed during the Palaeozoic as four major Palaeozoic ‘phytochoria’ (palaeokingdoms sensu Cleal, 1991): the Gondwanan floras in the southern middle and high palaeolatitudes, the Angaran floras in the northern middle and high palaeolatitudes, and the Euramerican and Cathaysian floras in low, mainly tropical palaeolatitudes (Fig. 1a). Although there are some so-called ‘mixed’ floras which combine elements of high and low latitudinal vegetation (e.g., Wagner, 1962; Durante, 1983; Berthelin et al., 2003), the Gondwanan and Angaran floras are, on the whole, quite distinct and easily recognised. The two floras from tropical palaeolatitudes are more problematic, though. Wetland vegetation occupied large tracts of palaeotropical lowlands, the oldest being recorded in the Serpukovian (latest Mississippian) fossil record in Europe, and the latest in upper Permian strata in southern China (Fig. 2). There have been conflicting opinions as to whether the Euramerican and Cathaysian realms should be recognised as distinct phytochoria (e.g. Halle, 1937; Li, 1963; Havlena, 1970; Chaloner and Meyen, 1973; Li and Yao, 1982; Chaloner and Creber, 1988; Wu, 1995; Tian et al., 1996; Sun, 1996; Tian et al., 2000; Sun, 2001; Fluteau et al., 2001). In the most recent review of the Cathaysian floras, Sun (2006) considered them to be distinct from the Euramerican floras, being characterised by the endemic genera Gigantopteris, Gigantonoclea, Cathaysiopteris, Tingia, Paratingia, Fascipteris, Emplecopteris and Emplecopteridium. In contrast, a number of recent detailed investigations have suggested a remarkable similarity in floristic composition within Euramerican and Cathaysian wetland plant communities (e.g. Hilton et al., 2001a,b, 2002; Cleal and Wang, 2002; Hilton et al., 2004). Evidently, the relationships between these palaeofloristic units remains uncertain.

In this paper, we take a critical look at the evidence for floristic differences between the Euramerican and Cathaysian floras. We will in particular focus on the wetland peat-forming plant communities, and will discuss their consequences for understanding the vegetational and geological history of Late Palaeozoic times. They contributed significantly to climate change during Late Palaeozoic times as the resulting build-up of peat represented a major carbon-sink, limiting atmospheric CO2 levels and thus helping suppress global temperatures (Cleal and Thomas, 2005). The distribution of the forests appears to have been reflecting wider geological changes in the area caused by continental plate movements and the resulting tectonic activity (Cleal and Thomas, 1999; Cleal, 2004). Furthermore, present day China also contains representatives of the previously recognised Late Palaeozoic terrestrial phytochoria now in juxtaposition to each other through subsequent tectonics (Fig. 1b), and thus represents a key area for understanding Late Palaeozoic palaeofloristic connections. However, understanding the dynamic floristics of these Palaeozoic wetland forests is also of economic interest, as they were the main peat-forming ecosystems of Late Palaeozoic age, and have resulted in extensive and economically-important fossil fuel deposits. Our knowledge of Palaeozoic vegetation from the palaeotropics comes from three main datasets: palynology, the adpression record (i.e. compressions/impressions – see Shute and Cleal, 1986), and various anatomicallypreserved macrofloras. Palynology has proved an important biostratigraphical and palaeoecological tool (e.g. Clayton et al., 1977; Dimitrova et al., 2005), but in the majority of cases we still do not know which plants (even at the rank of genus) produced many Late Palaeozoic pollen and spores (Balme, 1995), and so it is difficult to use them for detailed palaeofloristic analysis. The adpression record of Euramerica is in contrast extensively studied and provides a robust and on the whole taxonomically-consistent dataset for palaeofloristic comparisons (e.g. see discussion in Cleal et al. (2007)). This adpression record indicates generally increasing wetland plant diversity from late Mississippian to Middle Pennsylvanian times, but with episodes

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Fig. 1. (a) One of the various latest Pennsylvanian–Early Mississippian palaeogeographical models annotated to show position of Northern Hemisphere mid-high latitude Angaral phytochorion, southern hemisphere mid-high latitude Gondwanan phytochorion, and Low Latitude Euramerican phytochorion. Cathaysian phytochorion not indicated but comprising Northern and Southern Cathaysian blocks. Modified from Gastaldo et al. (1996). (b) Carboniferous–Permian phytogeographical realms in China, modified from Shen (1995).

of dramatic floristic demise, notably a regional demise of wetland plant communities at the end of the Moscovian across the Variscan Foreland (Fig. 3). Elsewhere in Euramerica, these peat-forming wetland plant communities persisted in localised refugia until towards the end of the Carboniferous Period in Variscan Euramerica (Cleal and Thomas, 2005) and remained more extensive in cratonic North America where they continued into the Gzhelian (Fig. 2). The Cathaysian

adpression record is, in contrast, far less well studied although notable exceptions exist (e.g. Halle, 1927; Lee, 1963; Cleal and Wang, 2002). Preliminary comparisons between the adpression records of the two areas, undertaken as part of this study, provided highly equivocal results and indicated that there are inconsistencies in fossil plant nomenclature used in the two regions that at the present time preclude a meaningful synthesis.

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Fig. 2. Approximate ages and extent of Late Palaeozoic wetland plant communities in Europe, North America and China.

The anatomically-preserved macrofloras from China have, in contrast, been the subject of a number of recent studies (e.g. Tian et al., 1996), often in collaboration with western palaeobotanists (e.g. Wang et al., 2003; Hilton et al., 2003), and so this type of nomenclatural problem is less of an issue. Hence, although the anatomically-preserved data-set is much smaller than the adpression record, the present paper focuses on the former. The only exception has been the late Moscovian Benxi Formation adpression floras that have been reviewed in recent years (Cleal and Wang, 2002). As the oldest documented peat-forming wetland plant community within Cathaysia, the Benxi Flora, represents a critical time in the evolution of palaeotropical vegetation but for which we have no anatomically preserved plant fossil assemblages. In this paper we therefore concentrate on the Pennsylvanian floras of Euramerica and China, and the Early Permian floras of China. Within China we focus explicitly on the floras from the northern Cathaysian region (Fig. 1b) as the Middle and Upper Permian wetland plant communities of Southern China have yet to be studied in adequate detail to allow synthetic treatments such as this being undertaken at the present time (See Hilton et al. (2004) for summary). The chronostratigraphical positions of the Late Palaeozoic coal-bearing sequences in North China are summarised in Fig. 4.

2. Methods The analyses presented in this paper are based on comparisons of the plant fossils at the generic rank (the raw data are given in Tables within this paper). There are two main reasons why we have used genera rather than species. Firstly, we are comparing floras over a significant range of ages (c. 35 Ma). Given that the longevity of many of the species is the Late Palaeozoic tropical wetlands was something in the order of 5– 10 Ma (based on ranges in Wagner (1984) and Cleal and Thomas (1994)) differences in species would be as much due to temporal changes in the vegetation as biogeographical factors. Secondly, there are subjective differences in the identification of the taxa between palaeobotanists working in different parts of the world and subject to different scientific traditions (e.g. Chaloner and Lacey, 1973). Although increasing collaboration between Chinese and western palaeobotanists is helping resolve many of these issues at the species-rank, some problems still exist. Generic identifications are much less of a problem, though, and we feel that it is now possible to make real and detailed comparisons of the Late Palaeozoic floras at that rank. Various analytical methods have been developed to determine biogeographical patterns both in living and fossil organisms each of which has its merits and weaknesses (see Brenchley and Harper (1998), for

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Fig. 3. Species turnover in fossil plant assemblages though the British coal measures. Diagram based on data presented in Cleal and Thomas (1994).

review and Chaloner and Creber (1988) for earlier example of Late Palaeozoic floristic analysis). We have used three different methods in this study, to show different aspects of the data. Our choice of methods has been to an extent subjective, but has been strongly influenced by preference for conceptually simple techniques, in which the results are less likely to be distorted by the methods themselves. 2.1. Cluster analysis This remains one of the standard classificatory methods for biogeographical analysis (Kovach, 1988; Hammer et al., 2001). Traditionally, Jaccard Coefficients have generally been regarded as the most reliable

similarity coefficient for presence–absence data such as we are examining (e.g. Shi, 1993). However, Jaccard Coefficients can produce misleading results, especially if there is a strong asymmetry in the species-numbers in the two samples (‘floras’) being compared. An alternative approach is to use Raup–Crick Similarity Coefficients. These provide an estimate of the probability of the two samples having been drawn from the same parent population using ‘Monte Carlo’ random-sampling. As they are not prone to the same problems with asymmetrical samples as Jaccard Coefficients, Raup– Crick Coefficients clearly have the potential for providing more robust results. In the past the problem has been the heavy computing power required (Raup and Crick, 1979; Shi, 1993); for instance, the anatomically-

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different the groups (in this case, of floras) are that are revealed by the analysis. However, by grouping the floras according to the pattern revealed by the cluster analysis, it is then possible to perform a Non-Parametric Multivariate Analysis of Variance (NPMANOV – see Anderson, 2001) that calculates an F-statistic (analogous to univariate ANOVA) from which the level of significance of the differences between the groups can be determined. 2.2. Detrended correspondence analysis

Fig. 4. Suggested correlation of the Carboniferous–Permian Formations in northern China with the IUGS Global Chronostratigraphy for these systems.

preserved floras analysed here required over 30,000 randomly-generated samples to be drawn from the original population (based on the fairly conservative 200 samples per pairwise comparison on which the statistical package that we used was based). However, modern software packages enable such calculations to be performed in seconds and Raup–Crick Coefficients now provide a powerful tool for palaeofloristic analysis such as that attempted in this study. Clustering was by the Unweighted Pair Group Average strategy, as recommended by Kovach (1988). The results are illustrated using the traditional type of dendrograms. For the analysis of the anatomicallypreserved floras, the clustering was stratigraphicallyconstrained, i.e. only stratigraphically-adjacent floras (or groups of floras) were clustered during the agglomerative procedure. Although this might be deemed to have been imposing a preordained pattern on the output, bootstrapping in fact revealed that stratigraphicallyconstrained cluster analysis provided a more robust reflection of the data-structure than unconstrained clustering. The analysis was performed using PAST (Hammer et al., 2001). One of the perceived weaknesses of cluster analysis is that there is no statistical test of how significantly

For ordination analysis (i.e. demonstrating trends within the data) we opted for Detrended Correspondence Analysis (DCA), as widely used in other palaeobiogeographical analyses (e.g. Kovach, 1988; Hammer et al., 2001; Summer, 2006). We initially considered using the even simpler Reciprocal Averaging technique, but initial results appeared to be exaggerating the differences between the groupings identified by cluster analysis. This distortion in the output is a widely-recognised drawback of Reciprocal Averaging. Detrended Correspondence Analysis uses essentially the same approach as Reciprocal Averaging, but transforms the output to remove this distortion (Hill and Gauch, 1980). Some authors have queried the legitimacy of this type of transformation of Reciprocal Averaging output (e.g. MacLeod, 2006) but others have argued that DCA is effective at extracting trends from multivariate data (Holland, 2006) and it certainly makes the output easier to interpret (Hammer and Harper, 2005). The analysis was again performed using PAST (Hammer et al., 2001). The data was also analysed using Principle Coordinates Analysis and Nonmetric Multidimensional Scaling, but these did not produce substantially different results from the Detrended Correspondence Analysis. 2.3. Vicariance (cladistic) biogeography This method utilises parsimony (cladistic) analysis as typically employed for determining phylogenetic relationships but for biogeographical analyses uses locality based assemblages rather than taxa, and replaces characters from phylogenetic analysis with taxa scored as either absent (0) or present (1) (Humphries, 1992). Missing or uncertain data are entered as unknown (?) such that results are not skewed by ambiguous data. Analysis was undertaken using PAUP v. 4.0b (Swofford, 2002), using heuristic search, TBR branch swapping, mulpars in effect, and using 1000 random additions as a search strategy to identify islands of most parsimonious trees. The analysis was first run unrooted,

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indicating that the ‘x’ assemblage was basal in the topology produced, from which the ‘x’ assemblage was subsequently selected as outgroup for the subsequent analysis in order to provide unambiguous polarity. This methodology effectively follows those outlined by Humphries (1992) but utilises more advanced heuristic search strategy (see Hilton and Bateman, 2006).

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Taiyuan Formation (see below), this is taken as the oldest wetland plant community in North China, and is therefore of critical importance in documenting the composition of pre-Taiyuan Formation floras of North China. Fossil plant composition of the Benxi Formation is shown in Table 1, and below we consider these accounts as well as comparing them with plants from other locations.

3. Materials 3.1. Adpression floras 3.1.1. Benxi Flora The only Pennsylvanian adpression flora in China to be reviewed recently is that of the Benxi Formation (Cleal and Wang, 2002), reported from a number of localities across the Sino-Korean Platform. The Cleal and Wang (2002) review combined a detailed analysis of specimens with an extensive literature search, and thus provides a robust basis for a comparison with contemporaneous Euramerican floras. Furthermore, the late Moscovian age of the Benxi Flora places it at a critical time in the evolution of palaeotropical vegetation, from where the anatomically-preserved evidence is limited in Europe and absent in China. The most recent revision of this flora (Cleal and Wang, 2002) was based on fossils from the Benxi (formerly Penxi) Formation collected form the Western Hill section and Liuziguo section near Taiyuan in Shanxi Province (Figs. 5c, 6). However, the flora has also been reported from a range of other localities across North China, mostly as allochthonous assemblages in marine deposits, and so is assumed to represent lowland vegetation. In the Western Hill section, the fossils occur in a 10 m thick black–grey shale horizon in the Bangou Member, between the Bangou Limestone and Jinci Sandstone. The shale is interpreted as representing an estuarine lagoonal facies (Chen and Niu, 1993). In the Liuzigou section (Fig. 6), the plant fossils occur in a roof-shale of a thin coal but, as they are fragmentary, they are almost certainly allochthonous. Fossils plants within the Benxi Formation occur as adpressions with some fusinised preservation that yields epidermal features (Cleal and Wang, 2002). Within the Chinese lithostratigraphical sequence the Benxi Formation is considered to be of C22 (late Moscovian) age, and this is supported by fusilinid evidence (see Cleal and Wang, 2002). This makes the Benxi Formation equivalent to floras of the late Asturian (‘Westphalian D’) to early Cantabrian regional substages of the upper Moscovian in Europe. Using this stratigraphical framework, and in agreement with the early Permian age of the overlying

3.1.1.1. Lycopsids. Following the work of Thomas (1978) and Phillips and DiMichele (1992) it is now possible the correlate the principle morphogenera of lycopsid stems and cones. These pairs are shown in Table 1. Unless coal ball evidence is available, the presence of Diaphorodendron is only reliably recognised by the presence of its distinctive microspores Granasporites; these have been identified in the lowland Euramerican floras (Peppers, 1996) but not in the Benxi Formation. Bothrodendron has been listed although its cone is unknown. The root and leaf morphogenera Stigmaria and Cyperites have not been listed as these appear to have little taxonomic significance beyond identifying the presence of the lycopsids. Following Thomas (1997) the herbaceous lycopsids in these floras are identified as Selaginella rather than Selaginellites. 3.1.1.2. Sphenophytes. Table 1 only includes the cone morphogenera from sphenopsids. The stem morphogenus Calamites has little taxonomic significance beyond identifying the presence of sphenophytes. The separation of the foliage morphogenera Asterophyllites and Annularia is also probably artificial and may relate to the same whole plant taxa, and as a consequence these have also not been listed. 3.1.1.3. Bowmanitaleans. This order is identified in Table 1 through the generalised foliage characteristic of morphogenera belonging to bowmanitaleans. Although this is an essentially ‘natural’ taxon in that it refers exclusively to foliage of this order, evidence from reproductive structures (e.g. Zodrow and Gao, 1991) and cuticles (Barthel, 1997) indicate that it hides undoubted taxonomic heterogeneity. However, cones are extremely scarce, and the cuticles are mostly very delicate and have not been widely studied, and so this taxonomic heterogeneity has yet to be properly resolved. 3.1.1.4. Ferns. The taxonomy of Pennsylvanian fern adpressions depends on a combination of reproductive and vegetative characters (e.g. Brousmiche, 1983). Studies on anatomically-preserved material, especially of the Marattiales (e.g. Millay, 1997) has resulted in even

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Fig. 5. Maps showing locations of key assemblages as discussed in the text. (a). Outline map of China showing position of Shanxi Province (b) and Beijing/Hebei boundary enlarged in (d). B. Shanxi Province enlarged from (a) showing position of Taiyuan. (c). Enlargement of boxed area from B showing plant bearing localities within the Xishan coal field. (d). Enlargement from (a) showing Beijing/Hebei border and the position of yangshuling mine in Pingquan district.

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Fig. 6. Composite log of the Carboniferous–Permian boundary sequences in Shanxi Province showing the position of plant bearing horizons, modified from Cleal and Wang (2002).

finer generic divisions but at present these cannot be recognised in most of the fossil record (see comments by Stubblefield, 1984) and so cannot be incorporated in this analysis. Filicalean ferns of Bashkirian–Moscovian age are normally referred to the morphogenera Alloiopteris if sporangia have not been reported in attachment, or Corynepteris where sporangia are known, although Corynepteris is also used for anatomically preserved stems. However, there can be little doubt that they refer

to the same group of plants, and so for the purposes of this comparison they are regarded as equivalent, as in Table 1. Many authors still use the generalised foliar morphogenera Sphenopteris and Pecopteris for sterile specimens of Pennsylvanian ferns. These morphogenera have little systematic significance other than identifying the presence of fern (and sometimes pteridosperm) families, and are not included in the comparative information in Table 1.

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Table 1 Table showing adpression comparison data

3.1.1.5. Pteridosperms (seed ferns). The most abundant pteridosperm macrofossils are foliar adpressions and a number of morphogenera have now been established for them based on a combination of frond architecture, epidermal and pinnule morphological characters (e.g. Laveine et al., 1977; Cleal and Shute 1995; Zodrow and Cleal, 1998). These can be assigned to one of three orders: Medullosales, Lagenostomales and Callistophytales. Reproductive organs, especially ovules, are also found but, without evidence of anatomy, they are difficult to identify. The comparison given here has therefore been based exclusively on the foliage morphogenera.

Cordaites and dicranophylls. Cordaite-like leaves are known from a number of plant groups. Those from the Euramerican floras appear to belong to the Cordaitanthaceae, and their cones are well-documented in these floras (Trivett and Rothwell, 1991). However, cones have not been reported from the Benxi Flora and so we cannot be certain which family these leaves belong to. The record of Dicranophyllum from China quoted by Cleal and Wang (2002) (Schenk, 1883) is based on isolated leaves and must therefore be regarded as suspect. In Europe, this morphogenus is known primarily from extrabasinal vegetation (e.g. Barthel, 1977). The relationship of

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dicranophylls and Angaran Vojnovskyalean Cordaites is at present unknown. 3.1.2. Euramerican floras The late Moscovian age for the Benxi Formation makes it contemporaneous with the Asturian and lower Cantabrian Substages in the western European regional chronostratigraphy (often referred to as the Heerlen Classification – Wagner, 1974). Much of the lowland parts of Euramerica had ceased to support wetland communities by this time, and so adpression floras contemporaneous with the Benxi Flora are relatively uncommon. Many of the floras of this age that do exist, especially in Europe, represent upland intramontane basins that do not provide an obvious comparison with the lowland Benxi floras (e.g. Saar-Lorraine and Central Bohemia). Other floras occur in North America but in most cases have poor taxonomic resolution for comparisons, including the Allegheny floras of the Appalachians. While these floras in North America form extensive coals, their fossil plant assemblages are in general poorly known, precluding detailed floristic comparisons to be undertaken. There are nevertheless three Euramerican adpression floras of this age, which have been reasonably well documented and which provide the basis for a comparison: the Mazon Creek Flora in Illinois, the upper Morien Group in eastern Cape Breton, and the Radstock Flora in southern Britain. The classic Middle Pennsylvanian macroflora from Mazon Creek is found in Illinois, USA, and includes two distinct facies containing fossil that are interpreted as representing lowland marine influenced facies; the Braidwood biota contains abundant plants and is accompanied by a freshwater faunal component, and the marginal marine Essex biota contains transported terrestrial organisms that includes occasional plant fossils (Baird et al., 1986). The fossils from Mazon Creek occur as authigenic mineralisations in siderite, which tends to limit the size of the specimens but can often yield better preservation than adpressions, including occasional anatomical detail (e.g. Drinnan et al., 1990). The flora is early Cantabrian in age (Cleal et al., 2003). The best documentation of the Mazon Creek flora is by Darrah (1969), Pfefferkorn et al. (1971), Pffefferkorn (1979) and Wittry (2006). The Upper Morien Group Flora refers to the plant adpressions found in the upper part of the coal-bearing sequence in the Sydney Coalfield of Cape Breton, Canada. They can be found underground and surface workings for coal, as well as in extensive natural coastal outcrops. Although there are no marine bands as such in this sequence, foraminifera have been reported which

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indicate some marine influence (Wightman et al., 1994). The flora is late Asturian to early Cantabrian in age (Zodrow and Cleal, 1985; Cleal et al., 2003). Although similar in general composition to the Radstock Flora in the Bristol coalfield of England, the preservation is much better, including the presence of cuticles (e.g. Cleal and Zodrow, 1989; Zodrow and Cleal, 1993, 1998). The composition of the Upper Morien Group Flora has been reviewed by Bell (1938), Zodrow and McCandlish (1980) and Zodrow (1986). The Radstock Flora is one of the classic late Moscovian adpression floras in Europe, having been investigated since the very early part of the 19th century. It occurs in the Radstock and Farringdon Formations in the Somerset Coalfield, southern England, and is latest Asturian to early Cantabrian in age in the Heerlen Classification (Cleal et al., 2003). There is no direct evidence of marine influence in these sequences, but the close comparison of the macrofloras of the Morien Group (Zodrow and Cleal, 1985) suggests that the Radstock Flora also represents a lowland setting. There is no modern monograph on the macroflora, but specimens from here have been extensively documented by Kidston (1888) and Thomas and Cleal (1994), and most notably by Kidston (1923-1925). These and other studies quoted therein have allowed us to document with a high degree of confidence the Radstock adpression flora to generic rank. 3.2. Anatomically-preserved floras 3.2.1. Cathaysian data Due to the abundance of palaeobotanical information from the Pennsylvanian–Permian sequences of North China, we focus on the key palaeobotanical data and those floral assemblages for which we consider the taxonomic and systematic investigations to be robust, notably the anatomically-preserved macrofloras of the Taiyuan Formation. The Taiyuan Formation is best known from outcrops in Shanxi Province of North China (Fig. 5a–b) and is a succession of deltaic deposits comprising predominantly siliclastic sediments, with inter-bedded coals, occasional volcaniclastic tuffs, and thin limestones towards the top of the Formation (e.g. Hilton et al., 2001b). Plant compressions occur in several lithologies, including darkgrey coloured mudstones, grey coloured fine-grained sandstones, and pale grey-yellow palaeosols, the latter dominated by poorly preserved and unidentifiable rooting organs. However, within the Taiyuan Formation, permineralized plant fossils occur in coal balls in Shanxi (Fig. 5b–c) and Shangdong Provinces (Tian et al., 1996;

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Wang et al., 2003) as well as fine-grained green-grey lithic tuffs from Pingquan district of Hebei Province (Hilton et al., 2001a,b; Fig. 5d). To date the coal ball assemblages from Shangdong Province are largely undescribed, but where known are similar to the more comprehensively characterised assemblage from Shanxi Province (Wang SJ, pers. comm. 2003), and include a number of co-occurring species. In Shanxi Province, coal balls are found in coal mines near Xiedao village where they occur in the No. 7 Xishan coal seam of the Xishan coal field (Li et al., 1995; Tian et al., 1996; Fig. 5c). This seam is part of the Dongdayao member of the Tiayuan Formation (Pan et al., 1985), that is immediately overlain by the marginal marine Xiedao Limestone. This limestone represents a regional-scale transgressive event that abruptly terminates coal accumulation (Pan et al., 1985). Within the No. 7 Xishan seam, Pan et al. (1985) noted organisms including the coral Pseudozaphrentoides sp. that indicate a degree of marine influence on the coal seam. Coal balls from the Taiyuan Formation in Shangdong Province have been collected from Zhaozhuang, Yanzhou and Xinwen (Tian et al., 1996). They have not been independently dated but are considered as the same age as those from Shanxi Province based on plant fossil biostratigraphy. Taiyuan coal balls from both Shanxi and Shangdong Provinces have carbonate as the permineralising agent, and localised pyritisation. While detailed taphonomic investigations have not been conducted on the composition and structure of the coal balls, they include a large percentage of roots and decayed amorphous plant matter, and appear to represent a preserved peat (Wang et al., 1995b; Wang SJ and J Hilton, work in progress). The sedimentological succession in the main plant bearing outcrops in Shanxi Province, the Liuzigou and Kaihuagou sections, is shown in Fig. 6 illustrating the position of adpression and permineralised assemblages from the Taiyuan and Benxi formations. The Pingquan assemblage (Fig. 5d) is preserved in a green lithic tuff that weathers to a grey colour, and includes well sorted angular feldspar crystals, as well as bright-green authigenic glauconates, and a carbonate matrix. Because tuff samples were found loose their precise position within the Taiyuan Formation is unknown (Hilton et al., 2001b) as are details of their sedimentary setting, but the bedding and petrology are consistent with reworking of an ash fall tuff (J. Hilton and P. Turner, work in progress). Permineralisation is by an early-stage carbonate replicating celluar features of plant tissues. Permineralisation occurred rapidly after deposition of the plants considering the low levels of

degradation and decay noted in the fossils. Within the tuff, plant fossils are overall fragmentary, suggesting that the fossils were transported prior to deposition, or that the tuff preserved an in situ detrital layer such as a soil that included fragmentary plant organs. The Taiyuan Formation is considered to be of P11 age in the Chinese litho-stratigraphical scheme (Tian et al., 1996), and thus correlates with the Asselian-lower Sakmarian Stages of the IUGS chronostratigraphical scheme. This is in contrast to some previous palaeobotanical reports that considered the Taiyuan Formation to be Pennsylvanian in age (e.g. Zhao, 1989; Li, 1993), and elsewhere in North China the base of the Taiyuan Formation may be Pennsylvanian through diachronous sedimentation (Deng SH, pers. com. 2004). Although precise biostratigraphical correlations are currently unavailable in this part of the north China coal basin, microfaunal occurrences within the marine limestones of the Taiyuan Formation and overlying it support an Early Permian age (Pan et al., 1985; Li et al., 1995; Wu, 1995; Tian et al., 1996). Radiometric and chemostratigraphic ages are presently unavailable for the Taiyuan Formations, and correlation between the CarboniferousPermian terrestrial sequences in North China are the subject of ongoing research programs to provide a more precise method of stratigraphical correlation. As anatomically-preserved plant remains occur in two distinct preservational-setting in the Taiyuan Formation, in coal balls and in volcanic tuffs, they will be briefly reviewed separately. Anatomical data is shown in Table 2. 3.2.1.1. Coal ball data. Following a series of systematic and taxonomic investigations undertaken by Tian, Wang and colleagues over the past two decades, we are confident about the majority of identifications previously made from Shanxi coal balls, and have personally studied many peels from this assemblage to confirm the identity of the taxa we discuss. Ongoing research conducted on the Taiyuan Formation means that new discoveries and taxonomic identifications will continue at pace in the future, but we are certain of the following occurrences. 3.2.1.1.1. Lycopsids. Shanxi coal balls contain a diversity of lycopsid morphogenera and include several whole plant taxa, and are the most numerous and constitute the largest part of the volume of all species encountered in many coal balls. Palaeoecological studies indicate these occupy wet parts of the coal swamp (Wang et al., 1995b). Stems include Lepidodendron spp. (Wang et al., 2002b; Zhou et al., 2004) and Sigillariopsis Scott (Wang et al., 2002a), megasporangiate disseminules of

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Achlamydocarpon (Zhou et al., manuscript in progress), microsporangiate cones of Lepidostrobus (Wang et al., 1995a), and leaves of Lepidophylloides Snigirevskaya (Wang et al., 2002c). In addition, the stem Diaphorodendon DiMichele has been reported from Shandong coal balls by Wang (2004) and is one of the few species detailed from this assemblage to date. While species are generally different from those reported in Euramerica, some of the Taiyuan species have characters that intergrade with those from Euramerica (Zhou et al., manuscript in progress), but all of the genera cited above co-occur between Euramerica and Cathaysia. 3.2.1.1.2. Sphenophytes. These are only known in the Shanxi coal ball assemblages from vegetative remains, and include at least two whole plant species (pers. obs.). Recognised organs include stems of Arthropitys Göpp., foliage of Sphenophyllum Brongn., and two kinds of the root Astromyelon Reed. 3.2.1.1.3. Ferns. These are an important component of the Shanxi coal ball assemblage, of which Marattialean ferns account for 10–20% of the total volume of fossils encountered (Tian et al., 1996), and include stems and root mantles of Psaronius Cotta and vegetative pinnules of Pecopteris (Brongn.) Sternb. and rachises of Stipitopteris Grand’Eury (Tian et al., 1996). Other ferns include Etapteris Bertr., Anachoropteris Corda and Botryopteris Renault (Tian et al., 1996). 3.2.1.1.4. Pteridosperms (seed ferns). As currently recognised, pteridosperms are a minor component in Shanxi coal balls, but include the callistophytalean ovule Callospermarion Eggert and Delevoryas (Hilton et al., 2002), stems of the medullosalean Medullosa Cotta (Tian et al., 1996), and lyginopteridalean ovules closely resembling Lagenostoma Williamson that warrant further investigation (see Hilton et al., 2001a, 2003). The ecological position of these plants is at present unknown. 3.2.1.1.5. Cordaites. These are a sub-dominant component of the Shanxi coal balls after lycopsids, and are volumetrically important in the plant assemblage. Where they occur, the cordaites appear to have occupied drier parts of the coal swamp, and in one case are interpreted as representing a rare example of a transported upland element (Wang et al., 1995b; Wang et al., 2003; Hilton and Wang, work in progress). Three whole-plant taxa have been recognised (Wang, 1998; Wang et al., 2003): the Shanxioxylon sinense, Shanxioxylon taiyuanense and Pennsylvanioxylon/Cordaixylon tianii plants named after their distinctive stems (see Wang et al., 2003 for a taxonomic summary). The Shanxioxylon sinense plant possesses stems of Shanxioxylon sinense Wang and Tian, roots of Amyelon taiyuanensis Wang and Tian, male shoots of Cathayanthus ramentrarus Wang et al.,

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Florinites-type pollen, ovulate cones Cathayanthus sinensis Wang et al., and ovules of Cardiocarpus samaratus Wang and Tian. The other two whole plants are currently being reconstructed and require reinvestigation of the different components, but are briefly summarised by Wang (1998). Of these plants, both species of Shanxioxylon are perhaps most similar to vojnovskyalean plants from Angara (Meyen, 1988, 1997) although comparative information from the Angaran species is lacking. Present evidence shows the genus Shanxioxylon to be endemic to China. In contrast the genus Pennsylvanioxylon/Cordaixylon (see Wang et al., 2003 for taxonomic discussion) also occurs in Euramerica, although the species Pennsylvanioxylon/Cordaixylon tianii is only known from China. Other cordaite organs include wood of Dadoxylon taiyuanensis Li, but this could be a different ontogenetic stage of the known whole plant cordaites. The ovule Parataxospermum taiyuanensis Li (Li, 1993) could be either a cordaite or pteridosperm, and requires reinvestigation so cannot be included in this analysis. 3.2.1.2. Volcanic tuff data. Although known from a single assemblage with limited diversity, the Taiyuan Formation tuff assemblage is important as it preserves plant fossils in another geological setting from the coal ball assemblage, but contains a comparable wetland plant floral association. 3.2.1.2.1. Lycopsids. The only identifiable lycopsid in the Taiyuan Formation tuff assemblage are megasporophylls and foliar fragments illustrated by Hilton et al. (2001b) that have subsequently been reconstructed as a single species of Achlamydocarpon Schumacker–Lambry, A. pingquanensis (Zhou et al., 2006). However, it is unknown if this species of Achlamydocarpon was produced by the Diaphorodendraceae or Lepidodendraceae, or a group intermediate to the two (Zhou et al., 2006). 3.2.1.2.2. Sphenophytes. Although fragmentary, calamite stems of Arthroxylon Reed and Sphenophyllum, roots of Astromyelon, and cones belonging to sphenopsids were documented by Hilton et al. (2001b). 3.2.1.2.3. Ferns. The Pingquan assemblage contains the only definite proof of the fern Botryopteris tridentata (Felix) Gothan, with its characteristic tridentate foliar xylem-strand and adjacent cauline/ pinna trace (Hilton et al., 2001b). Other organs are less complete but include Corynepteris-type sporangia, marattialean ferns including Psaronius root-mantles and sporangia, and the enigmatic fern Rastropteris pingquanensis Galtier et al. (2001) possibly related to the Osmundaceae and closely related to the similarly

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Table 2 Table showing permineralised comparison data

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Anatomical data.

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enigmatic genus Grammatopteris Renault (Rößler and Galtier, 2002). 3.2.1.2.4. Pteridosperms (seed ferns). The medullosalean ovule Pachytesta Brongn. has been recognised from the Pingquan tuff assemblage, but other organs of the parent plant are unknown. Some of the cardiocarpalean ovules present in this assemblage may be seed ferns but are incompletely characterised such that confirmation of their affinity remains speculative (Hilton et al., 2001b, 2003). 3.2.1.2.5. Cordaites. Cordaitalean organs include stems, leaves, fertile shoots and roots, with stems of Cordaixylon Grand'Eury, roots of Amyelon Williamson, leaves of Cordaites cf. crassus Renault, and fertile shoots with characteristic distichious bract/shoot complexes of Cordaitanthus Feistmantel (Hilton et al., 2001b). Although ovules of the morphogenus Cardiocarpus produced by both cordaites and pteridosperms, C. dabiziae Hilton et al. (2001a) is considered to have been produced by a cordaitalean plant based on the extended sporophyll-like pedicel. Other ovules present in this assemblage are incompletely known and cannot be assigned to any group of seed plant. 3.2.2. Euramerican data The Pennsylvanian (Late Carboniferous) fossil record from the tropical palaeolatitudes mainly preserves plants from wetland habitats. The wettest terrestrial habitats were dominated by arborescent lycopsids and sphenopsids during the early and middle Pennsylvanian, and by Mararrialean ferns during the late Pennsylvanian, all unlike the extant members of their respective lineages in their large size, arborescence, and in some cases having sophisticated seed-like monomegasporic sporangia (see Bateman and DiMichele, 1994). In contrast, the relatively drier areas had various ferns (both arborescent and herbaceous) and gymnospermous seed-plants (including trees, and scrambling and lianescent plants). From the fossil record, we have been able to build-up a picture in which a remarkably stable series of plant associations (here termed wetland plant communities) thriving over large areas of lowlands under generally warm and humid conditions (Cleal and Thomas, 2005). In Euramerica, floras generally become less common towards the top of the Pennsylvanian and into the Permian, and where known mainly present evidence from riparian environments in close proximity to lakes and rivers rather than wetland swamp settings. The history of change in geographical spread of these wetland plant communities is reviewed by Cleal and Thomas (2005). The earliest known are of latest Mississippian (Serpukhovian) age, in Europe (e.g.,

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Purkyňová, 1970) but soon after they rapidly spread into North America, during Bashkirian times (e.g., Blake et al., 2002). Collectively these regions constituted large parts of lowland Euramerica, and the characteristic and distinctive wetland vegetation has become known as the Euramerican flora. The flora reached its peak during late Moscovian times, when it occupied some upland (intramontane) areas (e.g., Laveine, 1989) as well as the classic coastal lowland areas. However, at the end of the Moscovian age the wetlands started to undergo a dramatic contraction coincidental with the onset of significant global climatic warming. This change occurred earlier in Europe, where tectonically-driven changes to swamp hydrodynamics resulting from the Variscan orogeny caused changes in drainage patterns (Cleal and Thomas, 2005; Opluštil and Cleal, 2007). In North America extensive coal formation continued until the end of Moscovian times but then declined significantly but persisted through the Kasimovian and into the Gzhelian (Fig. 2). Refugial pockets of the Euramerican flora persisted during the rest of Carboniferous times in the European intramontane basins (Doubinger et al., 1995) and on parts of the cratonic platform of North America (e.g. Willard et al., 2006), albeit with changed dominance-patterns of the plant-groups (arborescent marattialean ferns replaced lycopsids as the dominant plants), but eventually disappeared from these areas in early Asselian (Early Permian) times. The best reviews of the Euramerican coal ball evidence are by Phillips (1980, 1981) and Phillips et al. (1985) for North America, and Galtier (1997) for Europe, and we have used their data extensively in the following analysis. There is also an extensive coal ball record from the Donetz Basin in Ukraine (e.g. Snigirevskaya, 1972) but these data remain under-developed and are at the present time difficult to reliably assess. Other than these Donetz floras, the Euramerican coal ball floras essentially fall into two classes: (1) those from upper Serpukhovian to lower Moscovian (Namurian and lower Westphalian) coals of Western Europe, most notably the Union/Lower Foot/ Halifax Hard Coal from northern England; and (2) those from the upper Moscovian and Kasimovian coals of the USA. In the present investigation we have not included all of the distinct coal ball assemblages into the analysis, and have selected representative assemblages from each region to provide a geographical and stratigraphical range for the Euramerican floras. In doing so we have aimed to avoid duplication of near-identical associations occurring closely to each other in time and space, and have also aimed at eliminating assemblages from which insufficient information is known for us to characterise

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them as comprehensively characterised. The assemblages we have included are: the Koksflöz flora (early Bashkirian; Namurian C) from the Czech Republic, the Haupflöz flora (middle Bashkirian; Kinderscoutian) from Germany, the Bouxharmont (late Bashkirian; Langesettian) from Belgium, the First, Halifax, Union and Upper Foot floras (late Bashkirian; Langesettian) from England, the Katharina flora (late Bashkirian or early Moscovian; Langesettian-Duckmantian boundary) from the Netherlands, the Aegir flora (middle Moscovian; Duckmantian-Bolsovian) from the Netherlands, the Rock Island flora (middle Moscovian; middle Atokan) from North America, the Mineral Fleming flora (middle Moscovian; Atokan-Desmoinesian), the Springfield flora (middle Moscovian; Desmoinesian), the Herrin Flora (middle–upper Moscovian; Desmoinesian) from North America, and the Calhoun and Duquesne floras (Kasimovian–Gzhelian; Missourian–Virgillian) from North America, and the Shanxi and Shangdong floras (Asselian–Sakmarian) from North China. The palaeobotany of these coal balls has been extensively studied for over a century and needs no detailed review here (details of the plants can be found in most modern palaeobotanical textbooks, e.g., Taylor and Taylor, 1993; Stewart and Rothwell, 1993). In essence, the Middle Pennsylvanian floras are heavily dominated by arborescent lycopsids, with only occasional exceptions (e.g. the flora from Shore Mine in Lancashire) that probably represent ecotonal settings on the margins of the peat-forming habitats. In contrast, the Late Pennsylvanian coal ball floras are dominated by arborescent ferns and sometimes gymnospermous seedplants (notably medullosaleans and cordaites). We have not included the Grand’Croix Gzhelian flora preserved in cherts from France (Doubinger et al., 1995) as it appears to represent an essentially clastic-substrate vegetation (e.g. Doubinger et al., 1995), rather than a peat-substrate vegetation such as preserved in the contemporaneous coal ball of North America. The anatomical data used in the analysis are presented in Table 2 and in this we have retained the essential basis of Phillips' earlier taxonomic system rather than adopting more recent alternatives such as the re-classification of lycopsid taxa based on whole plant associations (e.g. DiMichele and Bateman, 1992; Bateman, 1994; DiMichele and Bateman, 1996). While this may open our work to a certain amount of criticism, it is necessary as comparable whole plant data do not exist from China at the present time whereas we are confident about the identification of the individual morphospecies (see Zhou et al., 2006 for summary). As this paper focuses on the correlation with Euramerican and

Chinese taxa a system that is internally consistent and from which individual taxa from the various assemblages can be reliably identified has been necessary, and other whole-plant based approaches are currently impossible from China. 4. Comparison between the Pennsylvanian–Early Permian floras of North China and Euramerica 4.1. Comparison between Benxi Flora and contemporaneous Euramerican floras The genera have been scored for each of the eight Benxi assemblages and the three Euramerican assemblages (Table 1). Not all morphogenera were included in the analysis adopting a similar approach to that employed by Cleal (2005, 2007) in diversity analyses of British Westphalian aged macrofloras; morphogenera of only one organ per plant group were used — mostly foliar, except for the lycopsids where stems were used. This eliminates the effects of plant organ duplication within whole-plant species. A cluster analysis of the Raup–Crick Similarity Coefficients generated from these data clearly separatesout the Benxi and Euramerican floras into two quite distinct groups (Fig. 7). This would appear to give support to the notion that the Cathaysian floras are significantly different from the Euramerican floras even in this early

Fig. 7. Cluster analysis of the adpression floras of the Benxi Formation in North China, and contemporaneous adpression floras in Euramerica. Details of the analytical method used are given in the text, and the raw data is given in Table 1.

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Fig. 8. Axes 1 and 2 of a Detrended Correspondence Analysis of Benxi Flora assemblages and contemporaneous Euramerican adpression assemblages. Details of the analytical method used are given in the text, and the raw data is given in Table 1.

phase of their evolution; perhaps the concept of the Benxi Flora as a Protocathaysian Flora as suggested by Li et al. (1993) is correct. However, if we instead ordinate the data using Detrended Correspondence Analysis (DCA), these two groups break-down (Fig. 8). The Euramerican and Benxi floras form a continuum along Axis 1, with the scores along that axis showing a significant negative correlation (probability N99.9%) with the number of genera in each assemblage (Fig. 9); they do not separate at all along axes 2 or 3. The only exception is the Kaiping assemblage, which has a significantly higher score on Axis 2 than the other Chinese assemblages. However, the taxa which are ‘pulling’ Kaiping away from the others on Axis 2 (Kaipingia, Poacordaites, Dicranophyllum) are all extremely rare and therefore of limited floristic significance. This biasing by ‘rare’ taxa is an inevitable problem with studies of presence–absence data from unevenly-sampled assemblages. If we were studying modern-day vegetation, the problem might be solvable using quantitative data. However, the virtual absence of suitable autochthonous assemblages in the regions covered by this study, and the bias introduced in most allochtonous assemblages by biostratinomy, have made the use of relative abundance data in the present study impossible in any meaningful sense. Even the introduction of quadrat studies, if that were practically possible, would only introduce a superficially systematic set of observations on a data-set that has been irrevocably randomised by the processes of taphonomy; quadrat studies of allochthonous assemblages may assist in purely

sedimentological investigations, but cannot be used meaningfully to reconstruct original vegetation structure. This issue emphasises the importance of having a direct understanding of the composition of the assemblages before attempting multivariate analyses of this type; it is not a ‘black-box’ that will produce meaningful results from an exclusively desk-based study. These results thus indicate that there is an undoubted difference between the Benxi Flora assemblages and the

Fig. 9. Correlation of Axis 1 scores of the DCA of late Moscovian adpression floras from China and Euramerica (see Fig. 8) with the number of species in each assemblage.

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contemporaneous assemblages from Euramerica; a NonParametric Multivariate Analysis of Variance (NPMANOVA) indicates that the possibility that they originated from the same parent population is b0.5%. There are some Benxi taxa which are absent from Euramerica, notably Paripteris, Conchophyllum and Tingia (although it should be noted that Paripteris does occur in Euramerica, but only in stratigraphically-older assemblages). However, the bulk of the apparent difference can be explained by the species diversities in the different assemblages, with low diversity assemblages in China correlating poorly with those from other regions, hence, being of very limited palaeofloristic significance. It would seem, therefore, that the Benxi Flora can in essence be regarded as a subset of the Euramerican Flora, supporting the conclusions of Cleal and Wang (2002). Is the reason for only having a subset of the Euramerican flora in China in late Moscovian times just due to poor sampling of the latter? We believe that it is probably not. The absentee genera from Benxi include many of the most abundantly-found taxa in the contemporaneous Euramerican floras. Perhaps most noticeable is the total absence of the Medullosales (e.g. Neuropteris, Macroneuropteris, Alethopteris, Laveineopteris ) except for the Potonieaceae (a family that unlike the rest of the order probably had its origins in China and may in fact not be truly medullosalean – see discussion by Laveine et al., 1993). Also notable is the absence of the arborescent ferns of the Tedelaceae (Senftenbergia). For the other plant groups, the Benxi Flora does contain representative genera, but lacks some of the most widely-found in the Radstock, Upper Morien Group and Mazon Creek floras, such as Sigillaria, Lepidophloios, Annularia, Acitheca, Mariopteris and Dicksonites. So, why is there only a limited subset of the Euramerican taxa in the Benxi Flora? One possible explanation could be that the plants were unable to disperse widely because of limitations imposed by their disseminules. The large seeds of the Medullosales, for instance, might be considered to have made it difficult for them to extend their range from Euramerica into China. As pointed out by Laveine et al. (1993), however, the Potonieaceae were able to make the reverse migration from China to Euramerica during Early and Middle Pennsylvanian times, so this seems like an unlikely explanation. It certainly cannot be used to explain the virtual absence of a number of the major Euramerican arborescent marattialean and lycopsid genera, as their spores were undoubtedly dispersed by wind. A more likely explanation is that there was a fundamental difference in the physical habitats available in

China at this time compared with Euramerica. Unfortunately, there is little detailed sedimentological/ palaeoecological data available from the Benxi Formation for us to test this idea. However, it is possible to speculate that at this time China did not have the large deltaic areas with clastic-substrate habitats, which would support the Medullosales, the Tedelaceae and lycopsids such as Sigillaria. Their absence in turn limited the ability of the plants with scrambling and climbing (lianescent-like) habits such as the Lyginopteridales to spread into the area, as they probably depended on these mainly arborescent groups for their physical support. Clearly, more sedimentological and palaeoecological data are needed from these Chinese successions. 4.2. Comparison between Taiyuan Flora and contemporaneous Euramerican floras Comparisons of the Taiyuan Formation permineralised assemblages are not possible with similarly aged Permian floras of Euramerica which lacked similar wetland plant communities, and from which permineralised fossil plants are not known. However, the composition of the Shanxi coal ball and Hebei tuff assemblages bear striking similarity with the Pennsylvanian permineralized coal ball floras of Euramerica. Fig. 10 shows the results of a cluster analysis of the Raup–Crick Similarity Coefficients for the anatomicallypreserved floras from Euramerica and China. This reveals five clear clusters of floras, with a level of significance (based on NPMANOVA) of b 0.01%. By using stratigraphical constraining in the agglomerative procedure, the clusters readily arrange in sequence corresponding to decreasing stratigraphical age, suggesting a progressive change with time from early Bashkirian Euramerican floras to Permian Cathaysian floras. The DCA (Fig. 11) reveals a more complex pattern, with a separation along Axis 1 (representing the greatest variation) of the early Bashkirian (Namurian regional substage) Euramerican floras (high scores), all the other Euramerican floras (intermediate scores) and the Cathaysian floras (low scores). The Cathaysian floras are also clearly separated on Axis 3, and in both cases it seems to be based mainly on the presence of Cordaixylon and Cordaianthus. Within the Euramerican late Bashkirian to Gzhelian floras, there is a clear correlation between Axis 2 scores and stratigraphical age (Fig. 12). Significantly, the Axis 2 scores of the Cathaysian floras are nearer to those of the late Moscovian Euramerican floras, than to the Kasimovian–Gzhelian Euramerican floras to which they are nearer in age – from the Axis 2 scores this seems to be

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largely through the presence of lycopsids (Bothrodendron, Lepidodendron, Achlamydocarpon). Results from the vicariance biogeographical analysis are generally consistent with the other methods of analysis used and reveal similar trends and floral groupings. Results are shown in Fig. 13 in which the floral ranges have been plotted in accordance with their stratigraphical age. Starting from the base of the tree the early Bashkirian European assemblages are the first to diverge from the stem, with the Koksflöz flora occupying a basal position followed by the Haupflöz flora. Considering the limited diversity of these floras (see Table 2) and the use of a hypothetical ancestor in which all taxa are absent, this is hardly surprising in itself, but is interesting in that these form a pectinate pattern in which the Koksflöz is basalmost rather than forming a basal sistergroup relationship with the remaining assemblages. Above this node the Aegir flora is next to diverge from the stem, occupying a key position as a transitional flora between the early and middle Bashkirian floras and the remaining late Bashkirian–Moscovian floras. These lowermost branches are

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indicated as 1 in Fig. 13 which represents the earliest diversification within wetland plant communities. The next successive node (2 on Fig. 13) forms a polytomy from which a number of separate European Bashkirian– Moscovian floras arise, as well as two other distinct groups of floras, the North American and Chinese floras. Based on the youngest aged flora that arises from this node, this node can be dated as being no older than Langesettian in age (in the European regional chronostratigraphy, upper Bashkirian Age globally), but may be slightly earlier with the latest age for divergence set by the Haupflöz assemblage (Kinderscoutian in age in the European regional classification). In this arrangement the remaining European Bashkirian–Moscovian floras arise from this node, as shown by 3 in Fig. 13, with these having a sistergroup relationship with distinct North American (4 in Fig. 13) and Chinese (7 in Fig. 13) floral groups. The North American floras are divided into two distinct groups: the Moscovian aged floras (Rock Island, Herrin, Springfield, Mineral-Fleming; 5 in Fig. 13) and the Kasimovian–Gzhelian floras (Duquesne and

Fig. 10. Cluster analysis of anatomically-preserved floras from Euramerica and China. Details of the analytical method used are given in the text, and the raw data is given in Table 2.

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Fig. 11. DCA of anatomically-preserved floras from Euramerica and China. Details of the analytical method used are given in the text, and the raw data is given in Table 2.

Calhoun; 6 in Fig. 13). In this analysis the main period of diversification within North America appears to be during middle Moscovian times, with demise of the North American peat-substrate floras occurring during Gzhelian times (6 in Fig. 13). However, numerous accounts

document this to be a period of floristic stability and low speciation rates within North American floras (e.g. DiMichele and Phillips, 1995, 1996; DiMichele et al., 2001, 2004) from which this result may represent a sampling artefact in which six floras with generally

Fig. 12. Least Squares Regression of Axis 2 scores of the Euramerican early Bashkirian to Gzhelian anatomically preserved floras (see Fig. 11) against approximate age in Ma. Also plotted are the scores for the Cathaysian floras, showing their closer relationship to the late Moscovian floras.

J. Hilton, C.J. Cleal / Earth-Science Reviews 85 (2007) 85–116 Fig. 13. Vicariance biogeographical analysis of anatomically preserved floras with tree plotted against age, showing strict consensus of 15 most parsimonious 162 step trees. Six events are highlighted on the tree, namely: (1). Early Bashkirian wetland plant community origin followed by early diversification within Europe. (2). Latest Bashkirian diversification event. (3). Diversification of European Bashkirian–Moscovian floras with regional demise during the Moscovian (4). Divergence of North American Moscovian from Kasimovian–Gzhelian associations. (5). Diversification and regional demise of North American Moscovian and Kasimovian–Gzhelian associations. (6) Diversification followed by regional demise of Chinese Asselian–Sakmarian wetland association in northern Chinese block. The position of the Taiyuan Formation floras within the range of Euramerican assemblages supports their interpretation as a distinct and recognisable subset of a Euramerican-styled wetland plant community, and strongly refutes the concept of Cathaysian floristic separation during the Mississippian–Early Permian. 105

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similar taxonomic compositions (as scored in this analysis; see Table 2) form a polytomy (i.e. point 4 on Fig. 13) that conceals a better resolved structure with a younger radiation event. The two Chinese assemblages from the Taiyuan Formation form a sister-group (7 in Fig. 13) that arises from the polytomy amongst the North American floras and the bulk of the European Baskirian– Moscovian floras. This position supports the relationship of the Taiyuan Formation floras as a recognisable subset of a Euramerican-styled wetland flora, rather than being a separate floristic unit. Divergence time for the Taiyuan Flora from their Euramerican ancestral stock is not well constrained from the vicariance analysis of the anatomical data and could be anywhere from the middle Bashkirian to the Gzhelian, but is constrained by the age of the Benxi adpression flora (see above), which gives a date no later than late Moscovian. 5. Discussion 5.1. Palaeozoic floristic relationship between Cathaysia and Euramerica Havlena's (1970) concept of an Amerosinian realm unifying the Pennsylvanian and Permian palaeotropical vegetation of Euramerica and China was based essentially on the adpression records from these two areas. It is currently very difficult to corroborate this based on the adpression record as the Chinese records are in need of significant revision; many of the identifications of adpression taxa from there using morphogeneric names originally established for Euramerican species just cannot be verified. Even the Benxi floras that have been the subject of recent taxonomic revision (Cleal and Wang, 2002) give equivocal results, due to differences in sampling and in palaeoecological settings in the two areas. However, the anatomically-preserved floras that have been the subject of more recent work in China, do appear to corroborate Havlena's Amerosinian model. This evidence suggests that here was an essential continuity in the wetland vegetation across large tracts of lowland Euramerica and China, throughout the Pennsylvanian and into the early Permian (Fig. 2). Our findings indicate that the Pennsylvanian and early Permian wetland plant communities of North China represent a clear subset of those wetland communities occurring stratigraphically earlier in Euramerica. Some of our analyses suggest a simple stratigraphical progression from the Pennsylvanian floras of Euramerica to the early Permian floras of China. However, the DCA of the anatomically-preserved floras show that the Chinese Permian floras in fact are closer to the lycopsid-dominated Moscovian floras of

Euramerica, rather than to the Late Pennsylvanian floras with which they are more similar in age. From this we conclude that the Chinese floras probably evolved from the lycopsid-dominated wetlands found in Euramerica during Early and Middle Pennsylvanian times. There are two problems here; firstly that there are no anatomicallypreserved floras of late Moscovian age in China, this being the time in our model when the lycopsid-dominated wetlands would first be starting to invade this region, and secondly that there are no younger coal ball horizons from marine influenced paralic basins in Europe and North America to provide accurate comparisons with those from similarly aged sequences in China. However, the limited adpression evidence from the Benxi Formation suggests that coal-forest type wetlands were starting to occur in North China in late Moscovian times and that these were the forerunners of the more fully-developed wetland forests found in the overlying Taiyuan Formation. In terms of floristic comparisons, the results we present demonstrate that this similarity between floras in Euramerica and China is not just through a small number plant taxa that co-occur throughout the stratigraphical interval we consider, but that a large number of plant families, genera and in some cases species persisted throughout these regions (see Tables 1 and 2 for comprehensive list). What information is known about the palaeoecology of the Benxi flora (Cleal and Wang, 2002) and Taiyuan flora (Wang et al., 1995b, S. J. Wang pers. com. 2002–6) in China shows that they are also remarkably similar to those known from Euramerica, as noted by similar co-occurrences of plant groups within assemblages with lycopsids and sphenopsids occupying wetter parts of mires, cordaites and pteridosperms occupying drier or elevated parts (e.g. Wang et al., 2003). We therefore conclude that it is not just individual plants nor plant groups that co-occur in Euramerica and China throughout this interval, but that a remarkably stable series of closely related plant associations and hence similar ecosystems persisted throughout low latitude settings for a considerable geological time. This is important as few palaeobotanical studies have convincingly demonstrated long term persistence of ecological organisation to provide comparable data to neontological evidence (see DiMichele et al., 2004). We now need to ask is this floristic similarity only apparent in the wetland plant communities or, was it also present in other contemporaneous plant communities at this time? In answer to this question, during the time of the Benxi and Taiyuan floras evidence for other kinds of floras is minimal, and evidence for distinct floral compositions not forthcoming. However, after this period although other floral elements increasingly appear,

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typical wetland plant taxa such as arborescent lycopsids, cordaites, sphenophytes still exist (e.g. Halle, 1927; Geng and Hilton, 1999) in addition to incoming endemic taxa such as those considered by Sun (2006) to be characteristic of the Cathaysian flora. 5.2. Floristic demise and its causal mechanisms Perhaps the most obvious conclusion from this work is the demonstration that the progressive demise of wetland plant communities in Euramerica during Late Pennsylvanian times represents a regional loss of diversity rather than a global extinction episode. This episode marks a profound time of change in floristic distribution patterns within the low latitudes, with the wetland plant communities migrating east, and diversifying there after their regional demise in Euramerica. Although the loss of diversity during Late Pennsylvanian times in Euramerica was not an extinction episode sensu-stricto, it was an important regional scaled biotic event. More importantly for this paper, it was not a synchronous event, nor was it alone – it mirrors similar Early Permian losses in plant diversity in North China, as the post-Taiyuan floras were replaced by successively drier floras (Wang, 1985, 1989). We must therefore consider the timing and pattern of this demise in order to evaluate the causal mechanisms for this. In Europe, there was a rapid demise of wetland plant communities towards the end of Moscovian times (Fig. 2), with only a small percentage of species persisting into Late Pennsylvanian times such as the St Etienne and Grand'Croix assemblages (Doubinger et al., 1995; Hilton et al., 2002, 2004). In Britain, this loss of diversity was stepped with an initial episode followed by a more serious and regionally catastrophic event probably at or just before the Moscovian-Kasimovian boundary (Fig. 3; Cleal and Thomas, 1999, 2005; Cleal, 2007). The Moscovian-Kasimovian boundary in North America is also marked by a significant change in the lowland vegetation, with the replacement of lycopsid rheotrophic swamp-forests by fern-gymnosperm dominated forests. In both of these regions, plants inhabiting wetter positions within mires and swamps were the most severely affected, with lycopsids and sphenopsids being hardest hit, and becoming the first groups to disappear from the regional floras. While similar studies are presently unavailable in North China, what evidence is available shows the same trend, with wet inhabiting plant groups becoming less apparent in post-Taiyuan floras, including the Shanxi Formation (e.g. Hilton and Geng, 1998; Geng and Hilton, 1999), Shihhotse Formation (Halle, 1927) and Shishengfeng Formation (Wang, 1985, 1989).

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It has been argued that these profound changes in plant communities were almost certainly related to increased aridification within the wetland areas (DiMichele and Hook, 1992; DiMichele et al., 2001; Hilton et al., 2002). In our view this is highly probable as the plant groups that were hardest hit, the free-sporing lycopsids and sphenopsids, were reliant on available water to enable successful sexual reproduction (e.g. Taylor and Taylor, 1993; Hemsley et al., 1999). While increased water stress may have been tolerated to a certain degree by these plants, it may have severely hampered or even prevented them from reproducing. However, the cause of the aridification is less certain. Global climate change (e.g. Gastaldo et al., 1996), regional mountain-building resulting in a rain-shadow effect (Phillips and Peppers, 1984), the progressive drift of the Euramerica away from the palaeotropics (Bless et al., 1984), and tectonically-induced changes in watertables and drainage-patterns (e.g. Besly, 1987, 1988; Kerp, 1996; Cleal and Thomas, 1999) have been variously advanced as being the underlying cause for this aridification. A recent palaeofloristic analysis of Variscan Euramerica (Cleal, in press-a) has suggested that, although the wetland forests were contracting during late Moscovian times, where they did persist conditions were not significantly different from those in early Moscovian times. Cleal (in press-a) concluded that a regional breakdown of the swamp habitats was probably triggered by tectonic activity, whereby local uplift of certain areas would cause changes in drainage patterns, affecting water-table levels and sediment influx. This would have made conditions locally unsuitable for the dominant plants (especially the arborescent lycopsids and sphenopsids), but in other areas the swamp habitats could persist, at least for a time. There is undoubted evidence for global climatic warming having taken place in early Kasimovian times, especially in higher palaeolatitudes (e.g. Gonzalez, 1990; Stollhofen et al., 2000; Isbell et al., 2003; Cleal and Thomas, 2005). However, it is unclear why this would have had such a catastrophic impact on the arborescent lycopsids and sphenopsids, which were after all adapted to wet-tropical conditions. What would have had an effect was a significant fall in substrate moisture. This could have been caused by a significant reduction in precipitation, but it would have had to have been regional if it was to have caused the observed shift away from lycopsid-dominance across the Euramerican wetlands. Identifying such a regional climatic drying is hampered by problems of stratigraphical correlation between the basins which were becoming increasingly fragmented due to tectonics (for instance, see Wagner and Lyons,

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1997; Eagar and Belt, 2003; Cleal et al., 2003; Opluštil and Cleal, 2007) and the sedimentological interpretation of the successions (e.g. see Besly and Turner, 1983; Besly, 1987, 1988). However, a regional synthesis of Pennsylvanian–Permian palaeoclimatic indicators in central Europe by Roscher and Schneider (2006) suggested that the ‘Kasimovian Dry Interval’ in fact started in middle-late Moscovian times and thus significantly pre-dates the demise of the lycopsid forests. Moreover, where we do see premature (early to middle Moscovian) collapse of the lycopsid forests on the northern margins of the Variscan Foreland, such as in central and northern England (Cleal, 2005, in press-b), it appears to have been due to a lowering of water tables but in still-wet climatic conditions (Besly and Turner, 1983). Whilst evidence for regional reductions in precipitation are at best equivocal, there is unequivocal evidence for major tectonic activity occurring at this time More likely, the damage to these wetlands was the result of a lowering of the water table due to changing drainage and sedimentation patterns following tectonically-induced landscape changes. The increased temperatures during Kasimovian times may have had a negative feedback effect on these tropical wetland communities that had been already damaged by the lowered water tables, hindering any possible recovery if water tables rose again, but in our view it is unlikely to have been the primary cause of the collapse of this habitat. 5.3. Palaeofloristics: the origin and relationships of the ‘Cathaysian flora’ Li (1963) and Li and Yao (1982) considered that the Cathaysian flora had already diversified and become distinct from other global floras before the deposition of the Benxi and Taiyuan floras, which they considered to represent the early stage Cathaysian flora. They considered this to have evolved from Middle Carboniferous Euramerican flora that persisted throughout East Asia at that time. By contrast, Sun (1996, 2001) considered that the Cathaysian flora first became distinct during Bashkirian times, evolving from an Early Carboniferous ‘global’ Lepidodendropis flora. However, Tian et al. (1996, 2000) argued that the anatomical structures of cordaitaleans and lepidodendralean plants from the Cathaysian regions supported a Euramerican origin for the Cathaysian flora, concluding that a number of distinct Cathaysian assemblages evolved in isolation from each other, adapting to different environmental conditions in different parts of China. From the synthesis we present here, we conclude that the Benxi and Taiyuan floras evolved directly from the

Euramerican styled wetland plant communities, of which they represent a subset. The floristic endemism in Pennsylvanian to earliest Permian times of North China is mostly at the species level, occasionally at the generic level, and only very rarely at higher systematic levels. This leads us to conclude that at the time of the Benxi and Taiyuan floras, the Cathaysian flora had not differentiated sufficiently to be considered as a distinct floristic realm. We do not exclude stratigraphically younger floras from being endemic to China and forming a phytogeographically distinct Cathaysian flora, and consider that over time, with further time for evolution to occur, the floras we documented undoubtedly evolved and differentiated into a distinct flora. For instance, the compression floras from the Lower and Upper Shihhotse Formations described by Halle (1927) and others subsequently contain a larger number of endemic species, genera and in some instances families; this is clearly a distinct flora and is perhaps the start of adequately diversified floristic elements to constitute a distinct entity. However, this is probably largely because, by Permian times, suitable lowland wetlands to support this type of vegetation had disappeared from Euramerica. In a recent review covering the origin of the Cathaysian flora Sun (2006) considered an endemic and distinct Cathaysian flora to be present in the Carboniferous and Permian, and considered the Cathaysian flora to be characterised by the genera Gigantopteris, Gigantonoclea, Cathaysiopteris, Paratingia, Fascipteris, Emplecopteris and Emplecopteridium. In the present account we demonstrate that these genera are absent from the Pennsylvanian Benxi and earliest Permian Tauiyuan formations, but acknowledge that they start to play a significant role in subsequent floras where they start to appear and diversify in the Shanxi and Shihhotse floras. Our conclusion remains that that until the end of the Taiyuan flora that the Euramerican aspect of the flora in North China remained evident. Of the available hypotheses on the origin of the Cathaysian flora, our results agree closest with those presented by Tian et al. (1996, 2000). 5.4. Palaeogeographical implications The most straight forward way for the wetland plant communities to migrate from Euramerica into China comes through terrestrial connection of lowland basinal settings, with plants migrating through their geological scaled extension and contraction of their natural ranges as conditions change or as species evolve. If correct, there must therefore have been a terrestrial connection between Euramerican and North China before the deposition of the

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Benxi Formation during late Moscovian times as evidenced by adpression floras (Cleal and Wang, 2002), and or at the latest by late Bashkirian times as advocated on anatomical data (point 2 in Fig. 12). Such a link between Euramerica and China has in fact been suggested to have existed as early as Viséan times or earlier (Laveine, 1989; Laveine et al., 1992, 1993, 2000) but such ideas have been almost universally rejected because they did not fit in with the existing preconceptions concerning Carboniferous-Permian palaeogeography (e.g. Wang et al., 1998; Rees et al., 1999; Wang et al., 2001; Rees et al., 2002; Scotese, 2005) that postulate North China being an island (e.g. Fig. 14). However, this rejection ignores the very real palaeobotanical difficulties that result from a lack of land-connection between Euramerica and China. If the North China block was an island, how did the wetland plant communities migrate across the sea? While some of the plant species were undoubtedly anemophilous (i.e. were wind pollinated and wind dispersed), we consider it unlikely that the entire community could have migrated in this way. In modern ecosystems, avian vertebrates play a valuable role in plant dispersal, but these animals had not evolved by the late Palaeozoic and during this time interval nothing else fulfilled this particular role. While there are reports of plant species being rafted across marine settings, we find it hard to consider this as a suitable explanation for the transport of an entire wetland plant community from Euramerica to the North China block. From the processes that can be observed in modern day settings, there are no viable means of transporting an entire plant community such as the Late Palaeozoic coal forests other than by some form of land connection. Furthermore, the evidence for the separation of Euramerica from China is largely based on some poorly-constrained palaeomagnetic evidence from tectonically-disturbed areas in the latter area (as discussed briefly by Nie et al., 1990) and it is difficult to see how this can provide a robust refutation of the better documented palaeofloristic data. The case of island or not island may, however, be over simplifying the issue. Deposition of coal-bearing sequences during Pennsylvanian times in Euramerica has been documented as a time of fluctuating high latitude glacial:non-glacial conditions (Gastaldo et al., 1996; Cleal and Thomas, 1999; DiMichele et al., 2001; Cleal and Thomas, 2005). This is evidenced by regular and rhythmical changes in eustatic base level controlling cyclothem deposition as well as other glacial evidence (e.g. Ramsbottom, 1979; Cecil, 1990; Maynard and Leeder, 1992; Gastaldo et al., 1996; Falcon-Lang, 2004). In periods of glacial maximum sea level would be lower, allowing colonisation of shelf areas by plant

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Fig. 14. Palaeogeographic reconstructions showing large Tethys. (a) Serpukhovian–Moscovian reconstruction after Laveine et al. (1992) showing North China in low latitudes north of the equator and trending longitudinally. (b) Late Carboniferous (Pennsylvanian) reconstruction from Scotese (2005) showing North China as mid-latitude island not in connection with other continental masses. (c) Early Permian reconstruction following Rees et al. (1999) showing north China as island and with general situation similar to (b).

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communities, and, possibly facilitating terrestrial connection of previous islands. This is likely to have been a contributing factor, but, is again difficult to test. However, alternative palaeogeographical models do exist, and these in our opinion fit the palaeobotanical data more convincingly. For instance, although they do not show the position of North China in itself, the model of Kalvoda et al. (2003) based on marine faunal assemblages suggests the presence of a low latitude island arc system running to the north of Palaeo–Tethys, and including the Tarim plate (Fig. 15a) that would lead

on to North China to the east. This would, theoretically at least, provide a series of islands that would have been vegetated and moving over time through continental drift in an island arc setting, capable of facilitating migration of wetland plant communities compliant with theories of island biogeographical dispersal. While this model may be accused of being biased towards data from shallow marine faunal communities, it is interesting in showing that we are not alone in our belief that accepted models of Carboniferous palaeogeography need to be independently questioned.

Fig. 15. Palaeogeographic reconstructions promoting either island arcs formation. (a) Mississippian after Kalvoda et al. (2003) and with low latitude island arc spanning Baltica to Turan and Tarim plates, but with reconstruction not extending to show north China. (b) Pennsylvanian–Early Permian after Natal'in and Celâl Şengör (2005) in which the Silk Road arc joins the southern margin of Northern China with the Manchurides, the Tarim block, the Kazakhstan–Tien Shan domain and to the east the North Caspian block and the Russian Craton.

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In our view, however, a more convincing model for floral migration from Euramerica into China comes through the geophysical results of Natal'in and Celâl Şengör (2005) who have shown the presence of the late Palaeozoic to early Mesozoic Silk Road Island Arc that is now buried beneath deep Mesozoic–Cenozoic sedimentary sequences (Fig. 15b). The Silk Road Island Arc joins the southern margin of North China with the Manchurides, the Tarim block, the Kazakhstan-Tien Shan domain and to the east the North Caspian block and the Russian Craton. In terms of continental floral and faunal patterns this tectonic arrangement can be viewed as the ‘Silk Road Corridor’ that presents lowland settings that span from west to east at the southern margin of a continental mass at low latitudes, presenting an ideal set of environments to allow faunal and floral migration. Importantly, the western margin of the Arc is the Russian Craton that is known to have had wetland plant communities similar to those documented from the Donets basin (Snigirevskaya, 1972; Phillips 1980). In our view this presents a viable route for Pennsylvanian migration of wetland plant communities along the lowland basins of this collision zone. If this model is correct, it would be impossible to test palaeobotanically because the islands would not have been preserved in this kind of destructive setting, or would be beneath a great depth of younger sediments. From the work we have conducted it is clear that the island nature of North China in Bashkirian–Moscovian times – which we propose is the latest date at the Euramerican-styled flora could have migrated into China – is essentially unproven and requires detailed investigations to conclusively demonstrate. We prefer to consider North China to have been in continental connection with Euramerica, presumably with floral migration through southern Russia. 5.5. Floristic evolution and mechanisms for regional plant evolution The model of changes in plant distribution over time and space we present in essence advocates that the Benxi and Taiyuan floras of North China represent the continuation of the wetland plant communities that previously occupied lowland areas of Euramerica, and which together represent part of the Amerosinian Realm of Havlena (1970). The persistence and expansion of suitable habitats in North China during Early Permian times allowed this wetland vegetation to continue to flourish and evolve there. In Euramerica, in contrast, continental-scale changes in water availability, driven by a combination tectonic activity and climate change,

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caused the Amerosinian wetland elements to become regionally extinct and be replaced by vegetation better suited to the more arid conditions. These replacing elements (e.g. conifers, peltasperms) probably originated in Carboniferous times in extra-basinal habitats (Lyons and Darrah, 1989; Kerp, 1996) and did not evolve from the wetland vegetation. Consequently, the Permian Amerosinian floras in China and the Permian Euramerican floras adapted to drier habitats were floristically quite distinct and unrelated. If this is correct, as advocated by the analyses conducted here, we hypothesize that the early Permian floras of North China represent what would have grown in Euramerica had aridification not regionally devastated floras in this part of their range. This effectively shows a continuation of the same plant communities and structure, but differences do occur in the Early Permian floras of China. In the plant communities of North China a high number of species originations occur and represent species not present in Euramerica – the majority of the Chinese taxa encountered represent new and putatively endemic species. This is not unreasonable in view of the time gap between the well-documented Moscovian macrofloras in Euramerica and the Asselian macrofloras in China, and the average longevity of similar species in the Euramerican floras of 1–5 Ma (based on ranges given in Wagner (1984) and Cleal and Thomas (1994)). A smaller amount of generic originations occur, with only a handful of endemic Chinese genera occurring such as the Cordaitean plant Shanxioxylon. At the higher systematic levels no new families or orders of plants have been adequately documented from China, suggesting floristic differentiation to only have occurred at the lower systematic levels. This primarily species-level distinction suggests once more that the Chinese floras have not had sufficient time to evolve into a distinct flora comprising entirely distinct taxa. The background process of evolution is slow and on these timescales available by the end of the Taiyuan Formation only species level evolution has occurred. 6. Conclusions (1) Through numerical analyses of Pennsylvanian– Early Permian wetland floras from Euramerica and northern China, we have shown the essential continuity of a single floral type across much of the low latitude world at that time. This supports the presence of the Amerosinian phytogeographical realm and suggests that the floristic distinction between the Euramerican and Cathaysian flora at this particular stratigraphical interval is no longer appropriate.

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(2) Detrended Correspondence Analysis, cluster analysis, and vicariance (cladistic) biogeographical techniques have all provided essentially similar results, and show that the Benxi and Taiyuan floras of North China represent a distinct subset of a ‘Euramerican’styled flora. The results largely agree and show consistency between the methods used. (3) Results presented demonstrate that the Benxi and Taiyuan floras of North China have evolved directly from a Euramerican ancestral flora during Moscovian times, and that a differentiated Cathaysian flora is not a valid concept until after the deposition of the Taiyuan Formation when the numbers of endemic species, genera and families significantly increase. (4) The data presented challenge existing models of global palaeogeography and infer that during the Bashkirian–early Moscovian the North China block was connected directly or indirectly with Euramerica. This most closely fits with the concept of the Silk Road Arc that provides a series of islands suitable for vegetation colonisation running from west to East and joining North China with the southern extent of Russia in which ‘Euramerican’ styled floras are known to exist. Eustatic glacially low-stands are likely to have caused the emergence of previously shallow marine environments, which made them suitable for plant colonisation and provided previously unavailable migratory routes. (5) Demise of wetland plant communities occurred at different times in different places, and approximating to the Gzhelian in Euramerica, and towards the end of the Sakmarian in North China. Although it is presently impossible to pin the demise of these communities on a single causal mechanism, we consider that increased localised aridification was a key contributing factor. This was the result of a combination of global climate change and tectonic activity causing changes to regional drainage patterns and basin configuration. Acknowledgements This work has been enabled by IGCP project 469 Late Variscan terrestrial biotas and palaeoenvironments, allowing the authors to discuss concepts and data presented in this work with other project members over the past 2 years, with the IGCP and Geological Society of London Providing financial support for meeting attendance. We thank Wang Zhi-Qiang (Tainjian Institute of Geology) for discussion on and access to materials from the Benxi Formation, Wang Shi-Jun (Institute of Botany, Chinese Academy of Sciences) and Deng Sheng-Hui (Petroleum Research Institute, Beijing) for discussion on the flora of the Taiyuan

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