Journal of the Geological Society Latest Mid-Pennsylvanian tree-fern forests in retrograding coastal plain deposits, Sydney Mines Formation, Nova Scotia, Canada H.J. Falcon-Lang Journal of the Geological Society 2006; v. 163; p. 81-93 doi:10.1144/0016-764905-003

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Journal of the Geological Society, London, Vol. 163, 2006, pp. 81–93. Printed in Great Britain.

Latest Mid-Pennsylvanian tree-fern forests in retrograding coastal plain deposits, Sydney Mines Formation, Nova Scotia, Canada H . J. FA L C O N - L A N G Department of Earth Sciences, University of Bristol, Bristol BS8 1RJ, UK (e-mail: [email protected]) Abstract: A latest Mid-Pennsylvanian (early Cantabrian) tree-fern forest is reported from Cape Breton Island, Nova Scotia. The fossil forest is rooted below the Point Aconi coal seam of the Sydney Mines Formation at Cranberry Head. The tree-ferns are preserved in the deposits of a coastal plain, subject to brackish incursions. They were constructed of psaronid-type trunks and massive pecopterid fronds, and locally intergrew with sigillarian lycopsids, neuropterid pteridosperms, and calamiteans. Basal stump diameter measurements, including the root mantle, range from 12 to 89 cm, indicating that the ferns were mostly large forest trees, rather than shrubs. Stump distribution measurements on palaeosols indicate that localized patches of tree-ferndominated forest attained densities of c. 3850 trees per hectare when scaled up to standard forestry units, much denser than typical Mid-Pennsylvanian lycopsid forests. Tree-ferns dominantly grew in aggrading floodbasin settings, and a few of the largest trees show evidence for post-burial regeneration. Being approximately coeval with the late Mid-Pennsylvanian extinction event, which resulted in tree-fern forests rising to dominate tropical lowlands for the first time, the Cranberry Head fossil forests provide insight into community composition and structure during a critical phase of ecosystem reorganization.

Willard 1997). These mire-dwelling tree-ferns, whose remains typically form 60–80% of the peat biomass, inter-grew with sigillarian lycopsids and pteridosperms (Willard & Phillips 1993). Less well known are the tree-fern communities that dominated late Mid- to Late Pennsylvanian mineral soils. Autochthonous tree-ferns have never been described from such settings (possible occurrences have been given by Dawson (1868), Rothwell & Blickle (1982) and Gastaldo (1990)), and knowledge of trunk size and architecture is largely based on prone adpression (i.e. impression and/or compression) fossils. Trunk adpressions are variously assigned to Megaphyton, Caulopteris, or Artisophyton depending on the arrangement and type of leaf scars (Pfefferkorn 1976). Although tree-fern-dominated adpression fossils have been quantitatively analysed (Pfefferkorn & Thomson 1982), results are too coarsely resolved to distinguish community composition. Furthermore, in the absence of autochthonous trees, the density and distribution of tree-fern vegetation remains unknown. This paper describes fossil forests from Cape Breton, Nova Scotia, Canada (Fig. 1a), which improve knowledge of Pennsylvanian tree-fern communities. More than 100 upright fossil trees, first figured by Brown (1849), occur rooted in mineral soils below the Point Aconi coal seam (Gibling et al. 2004), and are described within their sedimentary context. Analysis of these autochthonous fossil trees, which are dominantly marattialean ferns with subordinate calamiteans and lycopsids, permits detailed new insights into vegetation composition, density, structure, and ecology.

Pennsylvanian coal measures in North America, Europe, and Asia contain remains of the earliest tropical rainforests to develop on Earth (DiMichele et al. 2001). Two main lowland vegetation types are generally recognized, those communities centred in nutrient-depleted peatlands (DiMichele & Phillips 1994; Gastaldo et al. 2004b) and those that preferred poorly drained mineral soils (Gastaldo 1987; Calder et al. 1996). A third dryland–upland community is relatively poorly known (FalconLang 2003). Long-term stability in community composition, punctuated by extinction and rapid ecosystem reorganization, was a notable feature of Pennsylvanian rainforests (DiMichele et al. 1996b, 2004; Pfefferkorn et al. 2000). Following a major wave of extinctions that began in late Mid-Pennsylvanian mineral -substrate communities (Pfefferkorn 1979; Pfefferkorn & Thomson 1982; Peppers 1997), and subsequently wiped out 87% of peatland tree species by the Mid–Late Pennsylvanian boundary (Phillips et al. 1974, 1985; DiMichele & Phillips 1996), marattialean tree-ferns came to the fore. This group rapidly increased in diversity and abundance, rising to dominate lowland rainforests for the next 10 Ma (DiMichele & Phillips 2002). Their extraordinary ecological success has been attributed to a relatively ‘cheap’ construction (the trunk largely consisted of an arenchymatous root mantle), and prolific reproductive ability (Baker & DiMichele 1997). Late Pennsylvanian tree-fern communities are best understood in peat-forming settings where they comprised medium-sized trees with a basal diameter of <1 m (Willard & Phillips 1993) and projected heights of up to 12 m high (Millay 1997). Permineralized trunks preserved in coal balls are referred to Psaronius, and are known to have borne large leaf fronds (up to four times pinnate) of which Pecopteris is the most common genus (DiMichele & Phillips 2002). Fertile fronds possessed a variety of synangia, the most diverse and widespread of which was Scolecopteris (Millay 1997). Spores are assigned to Laevigatosporites, Punctatisporites, Thymnospora and Torispora, amongst others (DiMichele & Phillips 1994; Lesnikowska &

Geological setting The fossil forests are located at Point Aconi (468209N, 608179W) and Cranberry Head (468169N; 608139W), near Sydney Mines, Boularderie Island, Cape Breton (Fig. 1b). They occur within the Sydney Mines Formation (Morien Group), a unit of the Sydney Basin of Nova Scotia (Fig. 2a; Gibling 1995). This basin lay 81

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Fig. 1. Location details of fossil sites. (a) Cape Breton, Nova Scotia, Canada; (b) NE Boulardarie Island, Cape Breton. Point Aconi and Cranberry Head fossil sites are indicated by the tree-fern symbol. Dotted lines indicate road network.

close to the palaeo-equator and drained eastwards into the North Variscan foreland of Europe (Pascucci et al. 2000; Oplusˇtil 2004). The Sydney Mines Formation extends offshore for tens of kilometres (Hacquebard 1976), and has a total thickness of 520 m, although only the lower 410 m is exposed onshore (Gibling & Bird 1994; Gibling et al. 2004). These strata are assigned to the Asturian (formerly Westphalian D; Wagner et al. 2002) and Cantabrian substages of the Westphalian–Stephanian, and contain 14 sequences or cyclothems (Fig. 2b). Sequences are 12–83 m thick, and typically bounded by palaeovalley incisions, or mature interfluve palaeosols, their correlative equivalent (Tandon & Gibling 1997). This study focuses on the highest onshore strata, which are assigned to Sequence 14 (Gibling et al. 2004). This sequence consists of a dominantly terrestrial succession. Regional analysis based on borehole and outcrop data (Gibling & Wightman 1994) show that it comprises an Exxon-type architecture of lowstand, transgressive, and highstand system tracts (LST, TST, HST; Fig. 2c). The lower part of the sequence consists of multistorey, valley-fill sandstone bodies, 20 m deep and >7 km wide, which laterally correlate with a 1 m thick calcrete palaeosol. These units form the LST and record major baselevel fall and the onset of climatic aridity (Gibling & Wightman 1994). The middle part of Sequence 14 consists of red, vertic mudstones containing channel sandstone bodies overlain by grey mudstone and sandstone beds with thin coals. The succession is capped by the metre-thick Point Aconi coal seam (split into a lower and upper bed at Cranberry Head; DeWolfe 1906) overlain by platy, dark grey shales containing rich plant assemblages (Cleal & Thomas 1997; Wimbledon et al. 1999). These units form the TST and record, first, the aggradation of a seasonally dry alluvial plain (early stage TST), and, second, the retrogradation of a poorly drained coastal plain (late-stage TST) in response to rising base level. The platy shale above the Point Aconi coal probably represents the maximum flooding surface (Gibling & Wightman 1994). The upper part of Sequence 14 consists of grey mudstone and coarsening-upward channel sandstone bodies. These units form the HST, and record coastal plain progradation during

times of elevated stillstand base level (Gibling & Wightman 1994). The top of the sequence (defined by evidence for renewed base-level fall) is not exposed onshore, so the total thickness of this unit is uncertain (preserved thickness is 45 m). The inferred coupled fluctuations in climate and base level suggest a glacial–interglacial origin for this sequence (Tandon & Gibling 1994), although tentative estimates of sequence duration lie at the upper end of the Milankovitch Band (Gibling et al. 2004).

Biostratigraphy Floral assemblages indicate an early Stephanian (early Cantabrian) age for Sequence 14. Palynological analyses place the Westphalian–Stephanian (Asturian–Cantabrian) boundary in the upper part of Sequence 12, 25 m above the Hub Seam (Gibling et al. 2004), and some 115 m below the base of the fossil forest interval described here, based on the first appearance of Columinisporites ovalis (Dolby 1988). Based on megafloral analyses, the boundary was initially placed at the level of the Lloyd Cove Seam in the lower part of Sequence 13 (Zodrow & Cleal 1985), c. 70 m below the fossil forests, coinciding with first appearance of Odontopteris cantabrica. However, subsequent revision has brought megafloral zonation into general agreement with palynological data (Fig. 2; Zodrow 1986, 1989a, b; Cleal et al. 2003). North American literature commonly erroneously assumes that the Westphalian–Stephanian boundary coincides exactly with the Mid–Late Pennsylvanian boundary (e.g. DiMichele et al. 2001). In fact, the Mid–Late Pennsylvanian boundary is probably timeequivalent to the mid- to late part of the Cantabrian (Wagner & Lyons 1997). Sequence 14 of the Sydney Mines Formation is of early Cantabrian age (Cleal et al. 2003), and is therefore probably time-equivalent with the Upper Freeport coal measures (uppermost Allegheny Formation) of the latest Mid-Pennsylvanian of the Appalachian Basin (Wagner & Lyons 1997). Thus the fossil forests described in this paper date from the critical evolutionary phase immediately preceding the Mid–Late Pennsylvanian boundary, which witnessed tree-fern forest ecosystems replace previously dominant lycopsid vegetation (DiMichele & Phillips 2002).

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McCandlish (1980), taking into account taxonomic revisions (e.g. Zodrow & McCandlish 1978; Cleal & Zodrow 1989; Zodrow 1989a, b, 1990; Cleal et al. 1990; Psenicka 2005). In most cases only impressions or organic-poor compressions were preserved, such that taxonomic assignments were made with reservation in several cases. The relative abundance of plant taxa was recorded as follows: present on <3 bedding surfaces, scored as rare; 3–10 surfaces, common; >10 surfaces, abundant (Table 1). The fossil forest interval comprises two main sedimentary facies.

Mudstone-dominated facies The first facies occurs at two intervals at Point Aconi (1.1– 1.9 m; 4.2–6.6 m; Fig. 4) and at one interval at Cranberry Head (2.2–6.7 m; Fig. 4). It consists of medium- to dark-grey, laminated, or locally root-penetrated, mudstone containing common siderite nodules of 1–2 cm diameter. Interbedded with the mudstone are isolated lenses of very fine-grained, ripple crosslaminated sandstone, <15 cm thick, and coal layers, <1–2 cm thick. Agglutinated Foraminifera (Trochammina, Ammobaculites, Ammotium) occur within several mudstone intervals at Point Aconi, and at one further interval rare thecamoebians (protozoans) are present (Gibling & Wightman 1994). One sandstone lens preserves Diplichnites, the trackway of Arthropleura, on its upper surface, and on another, a Limnopus tetrapod (amphibian) trackway is present. Stigmaria rooting systems and other indeterminate roots occur at a few intervals. A few mudstone-cast lycopsid trunks and abundant tree-fern trunks are present in growth position (Figs 5 and 6a), rooted in dark grey mudstone laminae. Allochthonous adpressed plant assemblages are dominated by tree-ferns with subordinate sigillarian lycopsids, sphenopsids, and neuropterid pteridosperms (Table 1).

Sandstone-dominated facies

Fig. 2. Geological setting of fossil sites. (a) Stratigraphy of the Morien Group in the Sydney Basin of eastern Canada (after Gibling 1995); (b) stratigraphy of the Sydney Mines Formation showing the major coal seams and division into the 14 sequences (after Gibling et al. 2004); (c) sequence stratigraphic diagram depicting Sequence 14 of the Sydney Mines Formation, which contains the fossil trees (after Gibling & Wightman 1994). PA (Point Aconi) and CH (Cranberry Head) indicate regional position of sedimentary logs shown in Figure 4.

Sedimentary facies and fossil assemblages At Point Aconi and Cranberry Head, rock successions crop out in 15–20 m high sea-cliffs with 3D exposure around both headlands (Fig. 3). The autochthonous tree-ferns described in this paper occur in a 5.5–9.6 m thick unit (hereafter termed the ‘fossil forest interval’), positioned immediately below the Point Aconi Seam (Fig. 4; Gibling & Wightman 1994; Gibling et al. 2004). These strata represent part of the late-stage TST of Sequence 14. Upright tree-ferns are also present in HST strata overlying the Point Aconi Seam, but were not studied because they occur in inaccessible upper cliff faces. The sedimentary facies and plant assemblages of the fossil forest interval were described in detail. Fossil plants were identified in these beds using the guidebook of Zodrow &

The second facies occurs at Point Aconi (1.9–4.2 m; Fig. 4) and Cranberry Head (6.7–11.8 m; Fig. 4). It comprises fine-grained channelized sandstone bodies, lenticular sandstone bodies and laminated siltstones. Primary channelized sandstone bodies, <1.6 m thick and <85 m wide (Fig. 5), show upward coarsening, upward fining, or no systematic changes in grain size; erosional bases, typically horizontal over tens of metres; and low-angle channel margins. Channel-fills contain inclined stratification, oriented subperpendicular to the channel margins and extending from the top to the base of each body (Fig. 5); trough crossbedding and ripple cross-lamination with palaeocurrents parallel to channel margins; and symmetrical ripple marks on the channel-tops. Locally mudstone-filled channels, <1 m thick and <6.5 m wide, are also present. Secondary channels, <0.4 m thick and <1–2 m wide, divide from primary channels, and where exposed in plan view these bodies are sinuous with a distributary form. Secondary channels laterally grade into very fine-grained sandstone and siltstone lenses, <0.3 m thick and several metres in diameter. Back-filled Taenidium burrows are present in one primary channel sandstone body. On the top of another, a Diplichnites trackway is preserved. Rooted within several lenticular sandstone bodies are upright, sandstone-cast calamiteans (Fig. 6b), rare Pinnularia and Stigmaria, and other indeterminate roots. Lenticular strata also enclose autochthonous lycopsid and tree-fern trunks with preserved heights of up to 1.9 m high, the trees being mostly rooted in mudstone or siltstone beds. Siderite-replaced

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Fig. 3. South face of Cranberry Head (from rope access point) showing fossil forest interval within the late-stage TST strata and illustrating the superb exposure.

tree-fern petioles with a subvertical orientation commonly occur in the lenticular sandstone beds associated with upright tree-fern trunks. Sigillarian lycopsid trunks, sphenopsids (Fig. 6c), and rare cordaitalean leaves occur in allochthonous plant assemblages, together with abundant tree-fern foliage and trunks (Fig. 6d–g; Table 1).

Interpreted environment and ecosystems Positioned between alluvial plain red beds (below) and the Point Aconi Seam with its platy shale cap (above), the fossil forest interval originated in a generally retrograding setting (Gibling et al. 2004). The presence of agglutinated foraminifers at several intervals indicates that beds formed on the brackish coastal margins of the North Variscan epicontinental seaway (Gibling & Wightman 1994; Falcon-Lang 2005). Sandstone-dominated facies represent fluvial deposits characterized by channels of <6.5 m width and <1.6 m depth (based on the dimensions of mud-filled abandonments and plan-view exposures). Although some studies of Pennsylvanian fluvial systems have indicated that channels of such dimensions represent minor crevasse splay feeders lateral to much larger distributary channels (Fielding 1986; Hartley 1993), there is no outcrop or borehole evidence for the existence of any larger channels in the fossil forest interval. The channel systems are therefore considered to represent the small primary drainages of a narrow coastal plain. Inclined stratification indicates that channels were sinuous and migrated laterally by point bar accretion. Upward coarsening and the occurrence of symmetrical ripple marks indicate that a few channel bodies may represent the mouth bars of small deltas on the brackish coastal margin, whereas other channels showing upward fining represent fluvial drainages positioned further back from the coast. Very small secondary channels with a distributary form, together with sandstone lenses, represent crevasse splay deposits. Associated mudstone-dominated facies formed in poorly drained floodplain depressions, positioned between drainage channels, where mud accumulated from suspension. These wetlands dominantly comprised immature mineral soils with localized peat mires. Despite the occurrence of brackish incursions, which periodically introduced agglutinated foraminifers onto the coastal

plain, conditions were mostly freshwater, as indicated by sedimentary indicators and thecamoebians. This retrograding coastal plain environment was vegetated by tree-fern forests as indicated by common trees in growth position (see below) and an allochthonous plant assemblage dominated by Pecopteris fronds. Other plants included sigillarian lycopsids, and a diversity of neuropterid pteridosperms and calamiteans, the latter showing a preference for aggrading settings (Gastaldo 1992). Vegetated wetlands were occupied by arthropleurids and tetrapods, based on the trace fossil record (Archer et al. 1995).

Fossil tree-fern forests The remainder of this paper is devoted to describing the fossil trees, which occur in growth position within coastal plain deposits underlying the Point Aconi Seam. A variety of vegetation-induced sedimentary structures (sensu Rygel et al. 2004) are preserved around these trees, the most common being upturned beds. In total, 107 autochthonous trees occur in this interval at Cranberry Head (Fig. 5), of which 89.7% (n ¼ 96) are tree-ferns, 5.6% (n ¼ 6) are calamiteans, and 4.7% (n ¼ 5) are sigillarian lycopsids. At Point Aconi only two trees occur in the fossil forest interval, but both are tree-ferns. These discoveries are important because autochthonous tree-fern trunks have been described only once before from Pennsylvanian strata (Gastaldo et al. 2004a), and never before from clastic facies (although see Gastaldo 1990). The tree-fern trunks are easily distinguishable from the other autochthonous trees in the fossil forest interval. Lycopsid trunks are distinguished by their simple, sediment-filled, cylindrical trunks (28–65 cm diameter, preserved heights of <1.1 m), which show flared bases and locally attached Stigmaria, whereas calamitean trunks are distinguished by their distinctive nodes, horn-shaped bases, and locally attached Pinnularia (Fig. 6a and b). In contrast, tree-fern trunk architecture is more complex.

Tree-fern trunk architecture Autochthonous tree-fern trunks show a range of morphologies primarily dependent on the size of the tree and the quality of preservation. Trunks have basal diameters ranging from 12 to

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Fig. 4. Graphic sedimentological logs at Point Aconi and Cranberry Head. TST, transgressive systems tract; HST, highstand systems tract; mfs, maximum flooding surface. Foraminifera assemblages after Gibling & Wightman (1994).

89 cm (mean 27.35 cm; n ¼ 98) when measured to the margin of the outer root mantle (see below). Preserved tree trunk height ranges from 0.13 to 1.91 m (mean 0.49 m; n ¼ 98) and a pronounced taper in diameter (by 54–76%; n ¼ 98) occurs above the basal 10–40 cm of the trunk. In general, smaller diameter specimens show higher quality preservation than larger specimens. The trees are described in terms of three morphotypes, which in fact represent a single continuum. Morphotype 1 tree-fern trunks are well preserved and have basal diameters in the range of 12–29 cm (n ¼ 3). They consist of a vertical, sediment-filled, axial cylinder (here termed the ‘stem’), of 15–25 mm diameter, containing coalified strands of

indeterminate morphology (e.g. Fig. 7). Rare, poorly preserved, C-shaped petiole traces are recognizable on the outer surface of the stem, but preservation is too sporadic to determine leaf arrangement. The stem is surrounded by a compact, coalified layer constructed of small, cylindrical roots, <1–3 mm in diameter, oriented subparallel to the stem (here termed the ‘inner root mantle’). Roots are locally sediment-filled, and may be taphonomically widened to form features up to 10 mm in diameter. In this compact zone, it is rare to find sediment between adjacent roots. Large, coalified petioles, 20–30 mm in diameter and C-shaped in cross-section, are also rarely interspersed within the inner root mantle. At a greater radius from

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Table 1. Plant assemblages within the fossil forest interval below the Point Aconi Seam

Lycopsids Sigillariophyllum sp. Sigillaria elegans Stigmaria ficoides Sphenopsids Annularia stellata Asterophyllites equisetiformis Calamites sp. Sphenophyllum oblongifolium Tree-ferns Asterotheca sp. Pecopteris sp. P. aplina (¼ Lobatopteris lamuriana) P. arborescens P hemitelioides cf. Psaronius sp. Indeterminate petioles Indeterminate trunks Pteridosperms Linopteris obliqua Macroneuropteris scheuchzeri Neuropteris ovata var. aconiensis Neuropteris sp. Sphenopteris cf. suspecta Indeterminate petioles Cordaitaleans Cordaites principalis

Mudstone facies

Sandstone facies

x xx xx

x xx

xx x x xx xxx x xxx xx x

xx xx x x xxx x xxx xxx x xx x

xx xx x x x x x

x, rare; xx, common; xxx, abundant; see text for explanation.

the stem a more diffuse zone of sediment-filled roots occur (here termed the ‘outer root mantle’). The original size of these roots cannot be easily determined because of taphonomic effects, but preserved structures resembling individual roots have diameters of 5–20 mm. These depart from the trunk at an angle of 5–508, giving rise to a distinctly cone-shaped basal trunk morphology (Fig. 8a and d). Clay and silt are commonly associated with the outer root mantle, a sediment grade significantly finer than that seen outside the trunks (typically fine sand).

Morphotype 2 tree-fern trunks are moderately well preserved and have basal diameters in the range of 12–32 cm (n ¼ 27). They consist of an axial, sediment-cast cylinder, 4–11 cm in diameter, which shows an external structure of anastomosed fibres, and a slight upward taper in diameter (Fig. 8b and e). A thin coalified layer locally adheres to the outer surface of the cylinder. The absence of external leaf attachment scars indicates that this axial cylinder does not correspond precisely to the ‘stem’ of Morphotype 1 specimens, but instead represents a decay-expanded stem surrounded by the remaining part of the inner root mantle. The surface morphology of the inner root mantle is cast on the external surface of the axial cylinder. The axial cylinder is surrounded by the outer root mantle zone. In one tree, an impression of a 2 cm wide petiole was observed to laterally depart from the trunk. A poorly preserved pecopterid frond impression was observed between two sandstone lenses lateral to this trunk, but its connection to the attached petiole could not be proven. Morphotype 3 tree-ferns are poorly preserved and have basal diameters of 13–89 cm (n ¼ 68). These specimens comprise the outer root mantle only, the tree being preserved as a sandstone cast with an impressed or coalified structure of anastomosed fibres marking the exterior of the original trunk. In three of the largest trees (basal diameter range 60–89 cm), the diameter of the outer root mantle abruptly increases at a certain height above the trunk base, corresponding to the level of a palaeosol overlying a sandstone lens deposited around the tree base (Fig. 8c and f ). Trunks of Morphotypes 1–3 are attributed to marattialean ferns based on the fact that trunks are largely constructed from a root mantle and locally contain large C-shaped petioles. Larger diameter trunks, in general, probably represent older trees rather than different tree-fern species, especially bearing in mind that the smallest trees show the best preservation. Older trees might be expected to be subject to more advanced tissue decay before burial. That said, it cannot be ruled that two or more taxa may be represented by the trunks given the diversity of adpressed treefern foliage preserved in association. Morphotype 1 tree-fern trunks, and therefore by association Morphotype 2 and 3 trunks as well, exhibit a general similarity to the genus Psaronius Cotta (see Fig. 7). Particularly notable is the tripartite division of trees into stem, inner root mantle, and

Fig. 5. Bed architecture and distribution of fossil trees. (a) Thumbnail map of Cranberry Head; (b) field sketch of south face of Cranberry Head based on photomontages (see Fig. 3), showing the fossil forest interval dominated by small sandstone bodies containing inclined stratification.

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Fig. 6. Adpression plant fossils in the fossil forest interval. (a) Sandstone-cast sigillarian lycopsid trunk, scale: 50 cm; (b) sandstone-cast calamitean stem, scale: 5 cm; (c) Annularia stellata, scale: 2 cm; (d) Pecopteris sp., scale: 2 cm; (e) Pecopteris, cf. P. hemitelioides, scale: 1 cm; (f) Pecopteris aplina (¼ Lobatopteris lamuriana), scale: 4 cm; (g ) tree-fern stem with helically arranged leaf scars, scale: 4 cm.

outer root mantle zones as in Psaronius (Andrews 1961). However, given that the trunks do not exhibit cellular preservation, they are referred to this genus with reservation. Although previous studies have identified diverse epiphytic vegetation associated with psaronid root mantles at other sites (Rothwell 1991; Ro¨ßler 2000), no epiphytes were observed in the course of this study, despite targeted searches. Two organizational types of psaronid trunks are known, polycyclic trunks bearing petioles with a helical arrangement (Morgans 1959), and less common monocyclic trunks showing a distichous arrangement (DiMichele & Phillips 1977). Unfortunately, preservation is insufficient to confidently determine which organizational type is present at Cranberry Head. Poorly preserved trunk impressions bearing putative pecopterid petiole scars may suggest a helical arrangement (Fig. 6g ). This is further supported by the size of the trees (<89 cm basal diameter), which may suggest that they belonged to the polycylic organizational type because known monocyclic forms were typically much smaller (DiMichele & Phillips 1977). Dawson (1868) described a monocyclic tree-fern trunk specimen from Sydney Mines Formation (Megaphyton humile), but polycyclic forms (Caulopteris) are more common.

Siderite-replaced fossils Siderite nodules are very common in the fossil forest interval and preserve additional features that complement the above description. These siderite-replaced plant fossils do not show cellular anatomical features but rather preserve gross morphological characters. One distinctive type of fossil comprises clusters of <1–2 cm diameter, cylindrical, siderite-replaced roots on mudstone palaeosol surfaces. The roots are vertically to subvertically oriented and occur in 20–35 cm diameter clusters, subcircular in plan view (Fig. 9a). These features clearly mark the position of tree-fern trunk bases, and in the absence of axial, siderite-replaced stems, probably represent the roots in the subsoil immediately beneath the trees. Much more abundant are isolated, siderite-replaced axes (Fig. 9b and c). The most common type are subhorizontally to vertically oriented axes, oval in cross-section, 12–43 mm in diameter, and containing a distinct C-shaped stele. These axes are particularly abundant in the lenticular sandstone bodies that enclose the upright tree-fern trunks. Although direct attachment with the trunks cannot be proven, they are closely similar to treefern petioles found in coal balls (Stidd 1971; DiMichele & Phillips 1994), and show evidence for lateral rachis branching

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exception is an area near the SE corner of Cranberry Head, between the cliff and the low tide mark. At this locality, a siltstone surface dissected by small secondary channel sandstone ribbons bears autochthonous tree-ferns (Fig. 10a and b). This sedimentary association is interpreted as part of a small crevasse splay complex. Tree-fern stumps are dominantly rooted in the mudstone beds between the minor crevasse channels, although one example occurs within a crevasse channel (Fig. 10c). They comprise planview exposures of the Morphotype 2 and 3 tree-fern fossils (Fig. 11), and range in basal diameter from 12 to 32 cm inclusive of the outer root mantle (mean 18.25 cm; n ¼ 27). Over the 70 m2 of exposure, which includes areas covered by sandstone ribbons, 27 tree-fern stumps occur. Parautochthonous plant assemblages in the mudstone beds adjacent to the stumps are dominated by abundant impressions of Pecopteris arborescens and P. aplina together with Annularia stellata and Sphenophyllum oblongifolium (Fig. 6c and g). Scaling the limited stump distribution data to standard forestry units indicates that this portion of crevasse splay was very densely covered with tree-ferns (c. 3850 trees per hectare). At this site, and more generally throughout Cranberry Head, the close co-occurrence of tree-fern stumps with sandstone lenses indicates that the tree-ferns thrived in disturbed and aggrading sedimentary environments. Burial in decimetre-thick sandstone lenses appears to have been lethal for small, presumably juvenile, tree-ferns (Morphotypes 1 and 2), but the largest specimens (Morphotype 3) show clear evidence for post-burial regeneration (Fig. 8f). Nevertheless, regardless of size, most if not all trees were tolerant of some clastic input, as indicated by the accumulation of muddy sediment within the outer root mantle. This observation implies that tree-fern trunk bases were occasionally submerged in muddy floodwaters. Sphenopsids, another plant group well adapted to aggrading environments (Gastaldo 1992), probably formed the understorey layer of most tree-fern forests, as indicated by common calamitean fossils in palaeosols associated with tree-fern stumps.

Stand age and structure

Fig. 7. Well-preserved Morphotype 1 tree-fern trunk (cf. Psaronius), south face of Cranberry Head. (a) Photograph of trunk cross-section, 55 cm above palaeosol surface; (b) interpretative sketch showing the axial stem, inner root mantle containing departing C-shaped petiole, and outer root mantle.

(Fig. 9c). The diameter of the petioles (,43 mm diameter) suggests that the tree-ferns bore very large fronds. Axes with a more simple cylindrical structure are not easily identifiable. In gross morphology they resemble roots of treeferns and lycopsids, and possibly lycopsid axes (DiMichele & Phillips 1994). Other uncommon axes, 5–13 mm in diameter, showing a distinctly triradiate structure, resemble Sphenophyllum stems (Phillips et al. 1976).

Some of the fossil forests described, such as those on the wavecut platform SE of Cranberry Head, represent a single-age vegetation stand, based on the uniform trunk diameter and the fact that all trees are rooted in the same lamination. However, in other parts of the Cranberry Head exposure, mixed age forests are preserved. At such sites it is clearly demonstrable that some trees have regenerated following partial burial, and grew alongside those plants that later colonized the newly deposited sediment. It is extremely difficult to estimate the probable age of the tree-ferns preserved at Cranberry Head. Modern tree-ferns, which are distantly related to Pennsylvanian psaronids, are extremely slow-growing, with mean annual height increases of the order of only 1–7 cm (Mueck et al. 1996). Applying these data suggest that the smallest and most completely preserved tree-ferns were, at most, a few decades old, and the largest specimens, whose complete height is uncertain, were probably several hundred years old when they died.

Late Mid-Pennsylvanian tropical ecosystems Tree distribution and density Determining the density of tree-fern trunks on palaeosol surfaces is difficult because of limited bedding plane exposure. One

The results of this paper show that some Mid-Pennsylvanian coastal plain environments in Atlantic Canada were covered by dense (3850 trees per hectare) tree-fern forests. This vegetation

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Fig. 8. Autochthonous tree-fern trunks, all south face of Cranberry Head. (a) Morphotype 1 specimen, scale: 10 cm; (b) Morphotype 2 specimen, scale: 10 cm; (c) large Morphotype 3 specimen showing regrowth after partial burial, scale: 50 cm; (d)– (f) interpretative sketches of tree-fern stems and roots shown in (a)– (c). Only minimal sedimentary features are shown, to improve clarity of the trees.

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Fig. 9. Siderite-replaced fossils. (a) Root cluster (25 cm diameter), presumably marking a tree-fern base in the subsoil; (b) vertically to subhorizontally oriented axes (2–4 cm diameter); (c) sketches of a variety of common axis-types. Axes showing C-shaped steles are most common and probably represent tree-fern petioles. Axes showing a triradiate structure may represent Sphenophyllum stems. The identity of other axes is uncertain. Scale bar is 2 cm.

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Fig. 10. Tree-fern stump distribution. (a) Thumbnail map of Cranberry Head; (b) plan view of sandstone ribbon geometry; (c) distribution of stumps on mudstone palaeosol between minor crevasse splay channels, plotted on a metre-scale grid.

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ing, floodbasin settings, another contrast to lycopsid forests, which were generally restricted to slowly aggrading floodbasin settings (Gastaldo 1987; Fig. 12). Following their origination in latest Mississippian times (Gerrienne et al. 1999), the abundance and diversity of treeferns markedly increased in poorly drained floodplain environments in late Mid-Pennsylvanian times both west and east of the Appalachians (Gastaldo & Zodrow 1982; Pfefferkorn & Thomson 1982). In the Sydney Basin of Nova Scotia, tree-fern abundance and diversity increased in poorly drained floodplain environments at the level of the Hub Seam in the Sydney Mines Formation, just below the Asturian–Cantabrian boundary (Fig. 13; Cleal et al. 2003). At the boundary of the Mid- and Late Pennsylvanian (mid-Cantabrian), tree-fern forests expanded from poorly drained floodbasins into adjacent peat mires at sites west

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Fig. 11. Tree-fern stumps. (a) Stumps distributed across part of palaeosol shown in Figure 10; (b) close-up of Morphotype 2 tree-fern stump in oblique plan view.

was much denser than typical Mid-Pennsylvanian lycopsid forests, where tree density was in the range of 500–1800 trees per hectare (DiMichele et al. 1996a, 2001). In addition, the fern forests, which comprised large trees rather than shrubs, had a distinct preference for periodically flooded, moderately aggrad-

Fig. 12. Palaeoenvironmental reconstruction. Location of splay channels and position of tree-ferns are based directly on field data shown in Figure 10. Tree-fern canopy would be probably <12 m high given stump diameter data (see Millay 1997). The presence of a calamitean understorey is inferred from parautochthonous assemblages.

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R9""2L$/(.Q2"5(.$.&) Fig. 13. Changing patterns of tree-fern dominance through the late Mid- to early Late Pennsylvanian. Patterns are semiquantitative only and redrawn from data in Gastaldo & Zodrow (1982), Pfefferkorn & Thomson (1982), and Phillips et al. (1985). Tree-ferns initially rose to dominance in floodbasin (clastic) environments before expanding into peat mire (coal) environments. Star indicates position of the Cranberry Head fossil forests described here.

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of the Appalachians (Phillips et al. 1974). East of the Appalachians in the Sydney Basin, the picture is less clear because of poor sample resolution and the absence of outcrop (Dolby 1988). This rapid sequence of step-changes in tropical lowland ecology, and the possible factors that triggered it, have been discussed at length elsewhere (e.g. DiMichele & Phillips 1996, 2002; DiMichele et al. 2004). One widely accepted model invokes a trend towards drier tropical environments as the key trigger for lycopsid decline and tree-fern expansion (DiMichele & Phillips 1994). However, data from the Sydney Mines Formation present a problem for this model. Poorly drained floodplain forests at the level of the Harbour Seam (late Asturian) are dominated by lycopsids, calamiteans and pteridosperms with only minor tree-ferns (Calder et al. 1996; Falcon-Lang, unpubl. data), whereas the deposits of closely similar environments below the Point Aconi Seam (early Cantabrian) are tree-fern dominated (this paper). Clearly, vegetation change cannot be explained in terms of a simple trend to increasingly drier environments when facies analysis suggests constant edaphic conditions. A key to unravelling the cause of the observed vegetation step-change may be the occurrence of high-frequency glacial– interglacial cycles recorded within the Sydney Mines Formation (Tandon & Gibling 1994). These coupled changes in climate and base level are known to have driven major cyclical changes in vegetation composition and ecology (Falcon-Lang 2004). Cool, dry glacial phases supported vegetation dominated by tree-ferns, cordaitaleans, and pteridosperms, whereas warm, wet interglacials were generally lycopsid dominated. The repeated reassembly of lycopsid rainforests following each glaciation implies that these hydrophilic plants must have survived in small, wetland refugia during the dry glacial phases (DiMichele et al. 1996b). With this in mind, it seems plausible that following a particularly intense glaciation, or series of glaciations, during the late Mid-Pennsylvanian, lycopsid populations may have been sufficiently depleted to make subsequent expansion impossible. This hypothesis is supported by the most recent analyses of Gondwanan glaciation. These data suggest that glacial–intergla-

cial cycles became more intense in the latest Mid-Pennsylvanian, with maximum ice volume increasing during glacial phases (Isbell et al. 2003) at the same time as interglacial phases became warmer (Cleal et al. 1999). Depletion of lycopsid communities, caused by glacially mediated tropical drying, would have left many wetland niches unoccupied in subsequent warm, wet interglacial phases, permitting tree-ferns to expand first into poorly drained floodplain environments, and second, into peat mire settings (DiMichele & Phillips 1996).

Conclusions (1) Autochthonous tree-fern-dominated fossil forests are reported for the first time from latest Mid-Pennsylvanian (early Cantabrian) strata in the Sydney Basin, Nova Scotia, Canada. (2) The tree-ferns, some of which were large (<89 cm basal diameter), inter-grew with sigillarian lycopsids, calamiteans, and pteridosperms in a poorly drained coastal plain environment. (3) Stump mapping indicates that localized patches of treeferns attained forest densities of c. 3850 trees per hectare, much higher than lycopsid-dominated forests of similar age. (4) The tree-ferns preferred an aggrading floodbasin setting, and although burial in decimetre-thick sandstone lenses was lethal for small trees, the largest specimens were able to regenerate. (5) The discovery sheds important light on a critical evolutionary phase during which tree-ferns ousted lycopsids from lowland tropical ecosystems. I gratefully acknowledge receipt of an NERC Post-doctoral Fellowship held at the University of Bristol, UK (NER/I/S/2001/00738), and a Palaeontology Research Grant (2001) from the Museum of Nova Scotia. I thank C. Cleal, W. DiMichele, R. Gastaldo, M. Gibling, J. Psenicka, H. Pfefferkorn, and E. Zodrow for sharing information and stimulating discussion. The reviews of C. Fielding and C. Cleal were also immensely helpful.

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Received 11 January 2005; revised typescript accepted 25 June 2005. Scientific editing by Jane Francis