q 2004 Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or editing@geosociety.org.Geology; August 2004; v. 32; no. 8; p. 693–696; doi: 10.1130/G20515.1; 3 figures; 1 table. 693

Community heterogeneity of Early Pennsylvanian peat miresRobert A. Gastaldo* Department of Geology, Colby College, Waterville, Maine 04901, USAIvana M. Stevanovic-Walls Department of Earth and Environmental Science, University of Pennsylvania, Philadelphia,

Pennsylvania 19104-6316, USAWilliam N. Ware 1109 Wynterhall Land, Dunwoody, Georgia 30338, USAStephen F. Greb Kentucky Geological Survey, University of Kentucky, Lexington, Kentucky 40506, USA

ABSTRACTReconstructions of Pennsylvanian coal swamps are some of the most common images

of late Paleozoic terrestrial ecosystems. All reconstructions to date are based on data fromeither time-averaged permineralized peats or single-site collections. An erect, in situ EarlyPennsylvanian forest preserved above the Blue Creek Coal, Black Warrior Basin, Ala-bama, was sampled in 17 localities over an area of .0.5 km2, resulting in the first tem-porally and spatially constrained Pennsylvanian mire data set. This three-tiered forest washeterogeneous. Lycopsid and calamitean trees composed the canopy, and lepidodendrids,Lepidophloios, and sigillarians grew together at most sites. More juvenile than maturelycopsid biomass occurs in the forest-floor litter, indicating a mixed-age, multicohort can-opy. Pteridophytes (tree fern) and pteridosperms (seed fern) dominated as understoryshrubs, whereas sphenophyllaleans, pteridophytes, and pteridosperms composed theground-cover and liana tier. The proportion of canopy, understory, and ground-coverbiomass varied across the forest. Low proportions of ground-cover and liana taxa existedwhere canopy fossils accounted for .60% of the litter. There is a distinct spatial clusteringof sites with more or less understory (or ground cover) where canopy contribution was,60%. Where canopy biomass was low (,50%), understory shrubs contributed morebiomass, indicative of light interception and/or competition strategies. Sphenopteris potts-villea, a ubiquitous ground-cover plant, is abundant in all sites except one, where pteri-dosperm creepers and lianas dominate the litter, interpreted to indicate total suppressionof other ground-cover growth. Ecological wet-dry gradients identified in other Pennsyl-vanian swamps do not exist in the Blue Creek mire, with the interpreted wettest (Lepi-dophloios), driest (Sigillaria), and intermediate (Lepidodendron sensu latu) taxa coexistingin most assemblages.

Keywords: Carboniferous, coal, paleobotany, peat mire, wetland.

INTRODUCTIONPennsylvanian peat-accumulating forests

are one of the most intensively studied andoften-reconstructed Phanerozoic ecosystems;models appeared soon after coal exploitationincreased following demands that accompa-nied the Industrial Revolution. Unfortunately,most nineteenth and twentieth century recon-structions present a ‘‘family portrait’’ of thesemires and mainly depict principal trees andunderstory plants. Although plant-growth ar-chitectures have been refined over the pastcentury as insight was gained from both ad-pression and permineralized specimens, mire-ecosystem reconstructions remain coarse. Themost plausible coal swamp reconstructions todate are based on permineralized plants fromcoal-ball assemblages taken from discretestratigraphic horizons (e.g., DiMichele et al.,2002) supplemented, at times, with palynolog-ical data (Greb et al., 1999). However, evencoal-ball assemblages do not provide an in-stantaneous snapshot of the mire community.Coal-ball floras represent time-averaged as-

*Corresponding author. E-mail: ragastal@colby.edu.

semblages, consisting mainly of aerial plantparts (stems, branches, reproductive struc-tures) with rarely preserved leaves (Gastaldoand Staub, 1999); these floras represent theresistant biomass contribution from severalplant generations to the peat. Decay rates ofaerial parts that fall to the surface of Holocenetropical mires are accelerated by high temper-atures and rainfall as well as by fungal anddetritovore activity; leaf half-life is often lessthan a few months (Gastaldo, 1994; Gastaldoand Staub, 1999). As competition for newlyopened space following the death of a plantmay change the systematic composition in thesample location, resistant plant parts that ac-cumulate and are buried may represent a cen-tury or more of biomass contribution in anyone coal ball. Hence, the most accurate recon-structions must be based on assemblages thatwere buried in a geologic instant, freezing theplant relationships in space and time. Such as-semblages are found in some types of roofshale floras, where erect trees are preservedabove the coal (Gastaldo et al., 1995; Di-Michele et al., 1996; Calder et al., 1996). Suchcollections commonly are restricted to what isrecoverable from one or two localities along

a high wall (or to what can be seen from be-neath in an underground mine), limiting re-coverable rock volume, or from spoil piles,where samples from various horizons oftenare mixed. Although the data may be tempo-rally constrained in these autochthonous as-semblages, the spatial relationships often can-not be discerned because of logisticalconstraints.

A data set from the Lower Pennsylvanian(Langsettian) Mary Lee Coal zone in theBlack Warrior Basin provides the first tem-porally and spatially constrained perspectiveon Pennsylvanian peat-mire ecosystems at acommunity scale. Standing forests are pre-served above each of the coals (Jagger, BlueCreek, Mary Lee, and Newcastle, in ascendingstratigraphic order), as well as at five separatehorizons between the Blue Creek and MaryLee seams (Demko and Gastaldo, 1992). Themost extensive in situ forest occurs above theBlue Creek Coal (Gastaldo et al., 1991) andconsists of erect lycopsids, sphenopsids, pte-ridophytes (tree ferns), and pteridosperms(seed-bearing gymnosperms) rooted in the un-derlying coal. Cordaites pith casts are in theroof shale flora, yet no standing trees of thisgymnosperm were encountered. Lycopsidtrees occur to heights of 4.5 m, and all stand-ing vegetation is preserved within and cast bytidalite deposits (Gastaldo, 1992; Gastaldo etal., 2004). Tidal rhythmites begin within theuppermost 0.5 cm of the coal, and are respon-sible for burial of the forest floor of the mire,preserving the leaf litter in exquisite detail.Gastaldo et al. (2004) argued that the onlymechanism that can mold and cast erect for-ests in estuarine tidal deposits, and bury anondegraded forest-floor litter, is rapid coseis-mic base-level subsidence. Rapid coseismicsubsidence buried a peat forest by estuarinesedimentation during the Great Alaska earth-quake of 1964. Subsurface peats in TurnagainArm indicate recurrent coseismic subsidenceand burial through time (Combellick, 1991).

LOCALITY AND METHODOLOGYThe roof shale flora was collected above the

Blue Creek coal in the Drummond BrothersCedrum mine, Townley, Walker County, Ala-bama (Fig. 1) during the last phase of exca-vation and exploitation of the Mary Lee in-terval in 1999. The mine was closed andreclaimed beginning in 2000. Mining opera-

694 GEOLOGY, August 2004

Figure 1. Locality map of Drummond Brother’s Cedrum mine (Town-ley 7.5 U.S. Geological Survey Quadrangle Map, T. 14 S., R. 8 W.,sec. 18, and T. 14 S., R. 9 W., sec. 13) in Warrior coal field, Alabama(inset). Ruled polygon indicates active exploitation area in whichBlue Creek mire was investigated. RR—railroad.

Figure 2. Ternary diagram on which proportions of canopy, under-story, and ground-cover and liana biomass are plotted for each sam-ple quadrat. Biomass proportion is based on 32 identified biologicaltaxa. Sites are subdivided into three clusters according to propor-tion of canopy biomass: >60% (triangle), >50% but <60% (star), and<50% (circle). Clusters are identified in inset map of Cedrum mine,where spatial position is indicated by global positioning systemreadings of latitude and longitude. Shaded areas on distribution mapcorrelate with ternary diagram. Sample site 9/3 is where there isequal proportion of understory (US) and ground-cover (GC)biomass.

tions cleared overburden down to the fossil-iferous horizon, which ranged from ,5 to.15 cm in thickness depending upon locality,in 17 sampled sites within the mine (Fig. 1).A 10 m2 bedding surface was cleaned andused as the sampling quadrat for each site, al-lowing for identification of lycopsid trees inthe sample area. (Systematic assignment of ly-copsids cannot be made on either juvenile ormature leaves because all higher taxa pro-duced the same structures. Hence, it is nec-essary to identify lycopsids on leaf-scar pat-terns found on the bark, requiring larger thanthe standard 1 m2 quadrats used in Holocenestudies.) An identification booklet was devel-oped to assist in systematic assignment to as-sure taxonomic consistency among investiga-tors. Split blocks of overburden exposedsequential bedding surfaces from which taxonpresence or absence was recorded. Collectioncurves were constructed to maximize recoveryefforts, resulting in samples ranging from 134to 393 identified specimens (average 281) perlocality.

Field identifications of 47 form taxa weremade from plant debris consisting of random-ly oriented trunks, stems, and branches of ly-copsids (club mosses) and calamiteans (horse-tails); juvenile (Kosanke, 1979) and maturefoliage of lycopsids and calamiteans withor without attached reproductive cones (ajuvenile-to-mature lycopsid-leaf ratio was cal-culated for each site); mature foliage anddehisced stems or branches of ferns andpteridosperms (seed ferns); disseminated re-

productive structures of lycopsids, calami-teans, and pteridosperms; small-diameter axeswith attached leaves assignable to spheno-phyllaleans (horsetails), ferns, and pterido-sperms (lyginopterids, medullosans, and?callistophytaleans); and unidentifiable, de-corticated stems and branches (Stevanovic-Walls, 2001; Ware, 2001). Form taxa werecondensed within recognized biological taxa,where applicable, allocating the dispersed can-opy detritus proportionally to known biologi-cal affinities. For example, where two or morelycopsid stem taxa were identified in a site,the numbers of leaves, leaf-bearing branches,strobili, etc., were apportioned proportionatelybased on the number of each identified stemtaxon. Hence, if Lepidodendron aculeatumwas identified three times and Lepidophloioslaricinus was found seven times at the collec-tion site, then 30% and 70% of the total num-ber of vegetative and reproductive parts weretransferred to these taxa, respectively. Thisplacement of form taxa within a parent taxonresulted in an interpretation that 32 biologicaltaxa contributed to the mire (Gastaldo et al.,2004). These were categorized relative to theirgrowth form and include a canopy (lycopsids,calamiteans, cordaites), an understory or sub-canopy (tree ferns and pteridosperms), andground-cover and liana tier (ferns and pteri-dosperms); the latter two categories are basedsolely on sterile foliage leaf architecture, frondsize, and diameter of parent stem, where avail-able (e.g., Gastaldo and Boersma, 1983). Asin modern ecosystems, many ground-cover

plants that develop along a rhizome or pros-trate stem can grow as a vine into the canopy.Therefore, taxa considered to be creepers canoccupy either or both positions within the for-est, depending upon space, light, and plantdensity. The systematic composition betweensample locations was compared by using thebiological data set with x2 statistics to deter-mine forest homogeneity or heterogeneity, andthe growth-habit proportions for each localitywere plotted on a ternary diagram.

RESULTSWhen the biomass of a tier in each locality

is compared to the average tier proportion forthe forest, there is a statistical differenceamong collection sites (canopy x2 5 40.82, df[degrees of freedom] 5 16, p 5 0.0005; un-derstory x2 5 114.14, df 5 16, p 5 7.3 31027; ground cover x2 5 102.76, df 5 16, p5 1.1 3 10214). Hence, the Blue Creek mireis a heterogeneous forest. A ternary diagramdepicts the forest-structure variation, and thespatial distribution is highlighted on a sample-site map (Fig. 2). Six sites delimit a centralsoutheast trend where canopy elements com-pose ;60% of the biomass; the understoryvaries from 10% to 40%, and the remainderconsists of ground cover and liana (Table 1).Here, the canopy is most diverse sys-tematically and dominated by a mix oflepidodendrids, Lepidophloios, sigillarians,and abundant calamiteans. The ratio ofmature-to-immature lycopsid-leaf biomassindicates a high contribution from juvenile

GEOLOGY, August 2004 695

TABLE 1. FOREST CHARACTERISTICS OF SAMPLE SITE CLUSTERS

C . 60% C . 50%U . 30%

C . 50%G . 25%

C , 50%U . 40%

C , 50%U 5 G

Canopy (average %) 66 58 55 36 43Understory (average %) 22 35 13 52 27Ground cover (average %) 12 7 32 11 30Canopy taxa (N) 7 6 6 5 6Understory taxa (N) 6 5 8 7 8Ground cover taxa (N) 6 6 9 6 8Immature:mature lycopsid-leaf ratio 19:1 9:1 14:1 18:1 16:1

Note: Sample site clusters are delimited from the plot of canopy (C) : understory (U) : ground cover (G) ratioson the ternary diagram (Fig. 2). The average proportion of biomass from each tier is provided in addition to thenumber of taxa responsible for the contribution on the basis of identified ‘‘biological’’ taxa. A juvenile:maturelycopsid leaf ratio is calculated from raw field data.

Figure 3. Reconstruction of three-locality transect in central position within mire. Vegetation at 9/2 (far left) consists of 51% canopywith high proportion of understory contribution; canopy tier represents only 43% of assemblage at 9/3 (middle), whereas canopycontributes 53% of biomass to site 9/23 (far left). Mire forest consists of more juvenile than mature lycopsid trees with abundantcalamiteans. Well-developed understory of seed-fern shrubs and rare tree ferns allowed for nearly ubiquitous ground cover by S.pottsvillea. Reduced light penetration in these sites limited other ground-cover and liana taxa abundance. Vertical scale in metersapplies to foreground plants.

trees. The understory is dominated by pteri-dosperms, including Eusphenopteris andNeuralethopteris, and has a small proportionof small ferns (Sphenopteris brongniartii).The ground-cover pteridosperm Sphenopterispottsvillea (Gastaldo, 1988) is ubiquitous, andonly the ground-cover and liana lyginopteridvine, Lyginopteris hoeninghausii, occurs inabundance. Other ground-cover taxa are rare.

In areas where there is .50% but ,60%canopy biomass, two distinct floral assemblag-es exist (Table 1). One group (Fig. 2) is char-acterized by .30% biomass of ground-coverand liana taxa and a low proportion of under-story. This canopy is dominated by Lepidoph-loios and Sigillaria as well as calamiteans, andthe ratio of lycopsid-leaf biomass indicates ahigh contribution from juveniles. Understorytaxa are rare ferns and pteridosperms—Car-diopteridium, Eremopteris, S. brongniartii,Alethopteris—with a S. pottsvillea groundcover. The ground-cover and liana taxa Lygi-nopteris, Palmatopteris, Sphenopteris pseu-docristata, and Sphenopteris schatzlarensiseach may compose ;5% of the biomass. The

second group averages 35% understory bio-mass and shows very low proportions (;10%)of ground cover and liana. Here, Lepidoph-loios, Sigillaria, and Calamites contribute bio-mass, and the juvenile:mature leaf ratio islowest in the forest (Table 1). Pteridospermsconspicuously dominate the understory, whichcontains abundant Neuralethopteris but a nearabsence of Alethopteris, and there is a minorEusphenopteris contribution. As elsewhere, S.pottsvillea is very abundant, along with smallground creepers and/or vines (Palmatopteris,Sphenopteris, and Sphenophyllum). No sam-pling sites exist intermediate between thesetwo extremes.

In sites averaging ;40% canopy biomass,one cluster of three sites is characterized by.50% understory biomass, and one localityhas a near-equal contribution from bothground cover and understory that is spatiallyintermediate (Fig. 2; Table 1). Canopy con-stituents are mainly lepidodendrons and cal-amiteans, although Lepidophloios occurs in allbut one site. Pteridosperms (Neuralethopteris)dominate the understory in the former group,

whereas a mix of pteridosperms and ferns(Neuralethopteris, Alethopteris, and sphenop-terids) accounts for the low biomass contri-bution in the latter. Where the proportion ofground-cover and liana taxa is highest (sample9/3; Fig. 2), Lyginopteris dominates alongwith high proportions of other lianas (Alloiop-teris and Sphenophyllum), and there is a pro-nounced absence of S. pottsvillea. This is theonly collection site where S. pottsvillea wasabsent.

DISCUSSIONThere is a considerable range of vegetation-

al heterogeneity in the forest-floor litter of theBlue Creek mire, reflecting contribution fromthree forest tiers. Most localities are dominat-ed by canopy biomass with varying contri-butions from understory and ground cover(Fig. 2; Table 1). Overall, juvenile lycopsidswere intermixed with a few mature trees, andcalamiteans were ubiquitous. Where the great-est proportion of canopy-biomass exists, ju-venile lycopsids grew as pole trees, envelopedby linear leaves up to 1 m in length (Kosanke,1979), generally with an understory of scat-tered tree ferns and abundant pteridospermsalong with an omnipresent ground cover ofSphenopteris pottsvillea. Here, there are scat-tered to abundant occurrences of taxa capableof a liana growth habit (i.e., Lyginopteris,Sphenophyllum), and an average of 12 othertaxa beneath the canopy, many of which areencountered rarely (,2.5%). Hence, tree den-sity in these sites probably limited systematicdiversity owing to space competition.

When canopy detritus is ,60% of the bio-mass, there is more systematic diversity in the

696 GEOLOGY, August 2004

other two tiers (Table 1). Where understorycontribution is greater than ground-cover bio-mass, 11 other taxa occur and are similar tothe diversity noted here. But, where ground-cover plants contributed more biomass, thereare at least 17 taxa that resided beneath thecanopy, equally distributed between understo-ry and ground-cover and liana growth strate-gies. Diversity in these sites probably wascontrolled by light penetration because of ei-ther lower plant density or a more juvenilelycopsid canopy.

The forest had a better-developed understo-ry where there is the least canopy contribu-tion, but this biomass originated from thesame number of taxa as found in the remain-der of the forest. The number of ground-coverand liana taxa equals that of the understory,and S. pottsvillea was abundant across the for-est floor. In only one site is S. pottsvillea ab-sent; here the biomass is dominated by theliana or ground creeper Lyginopteris. This re-lationship indicates a near-total cover of theforest floor by lianas, preventing the growthof ground cover. Such high proportions ofground-cover and liana biomass are associatedwith very open areas within modern forests,as the result of either canopy-tree death orblow-down that allows light penetration (Oli-ver and Larson, 1990).

A reconstructed transect through the BlueCreek mire at the time of its burial depicts amixed-age, multicohort forest (Oliver and Lar-son, 1990) consisting of more juvenile thanmature trees in an open-canopy configuration(Fig. 3). At one extreme where the densestcanopy formed, more understory than ground-cover biomass was produced, indicating lim-ited light penetration to the forest floor. Theunderstory seed-fern and tree-fern taxa grewto sizes that maximized light interception, lim-iting the establishment of a well-developedand diverse ground-cover tier. At the other ex-treme where there was minimal canopy bio-mass, understory taxa dominated except inone site, where there is an equal contributionfrom both understory and ground-cover tiers.Intermediate between these extremes are areasin which either understory or ground covercontributed more biomass to the litter.

Previous mire reconstructions have empha-sized a soil-moisture gradient in Pennsylva-nian swamps on the basis of lycopsid distri-bution (DiMichele et al., 1985; Gastaldo,1987; DiMichele and Phillips, 1994). The wet-test extreme was interpreted as a monocultureof Lepidophloios; the driest extreme was dom-inated by Sigillaria; lepidodendrids grewwithin intermediate sites. The spatial resolu-tion of the Blue Creek mire data allows a testof this hypothesis wherein the previously in-terpreted gradient is not present. Rather, thecurrent data indicate that these lycopsids had

a wider range of environmental tolerances andcommonly grew in close proximity within thesame Histosol. Hence, the ecological parti-tioning interpreted in Late Pennsylvanianmires either may be a sampling artifact, anecophysiological adaptation of these plants tochanging climatic conditions (Gastaldo et al.,1996), or increasing habitat specialization aspeat mires become more extensive in the LatePennsylvanian.

ACKNOWLEDGMENTSWe thank Drummond Brothers Coal Company,

Jasper, Alabama, for access to the Cedrum mine,and particularly M. Hendon, P. Hubbard, J. Garri-son, and J. Fowler, who facilitated the logistics ofthis study over a six-month period. The followingagencies, societies, and private foundations are ac-knowledged for project support: National ScienceFoundation (grant EAR-8618815 to Gastaldo), Geo-logical Society of America (Stevanovic-Walls,Ware), Gulf Coast Association of Geologic Socie-ties (GCAGS) (Stevanovic-Walls, Ware), Paleobio-logical Fund (Stevanovic-Walls, Ware), and the Pa-leontological Society (Stevanovic-Walls). We alsothank John Calder and Howard Falcon-Lang fortheir critiques of the manuscript.

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Manuscript received 26 January 2004Revised manuscript received 23 April 2004Manuscript accepted 4 May 2004

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