Theinsitu Glyptostroboxylon forestofHoegaarden(Belgium ...€¦ · strati¢ed clay with tiny black...

The in situ Glyptostroboxylon forest of Hoegaarden (Belgium)at the Initial Eocene Thermal Maximum (55 Ma)

M. Fairon-Demaret a;�, E. Steurbaut b, F. Damblon b, C. Dupuis c, T. Smith b,P. Gerrienne a

a Universite¤ de Lie'ge, De¤partement de Ge¤ologie, Pale¤obotanique et Pale¤opalynologie, Alle¤e du 6 Aou“t, Ba“timent B18,B-4000 Lie'ge, Belgium

b Institut royal des Sciences naturelles de Belgique (IRSNB), De¤partement de Pale¤ontologie, 29, rue Vautier, B-1000 Brussels, Belgiumc Faculte¤ Polytechnique de Mons, Ge¤ologie Fondamentale et Applique¤e, 9, rue de Houdain, B-7000 Mons, Belgium

Received 19 June 2002; accepted 6 March 2003

Abstract

Hundreds of silicified standing stumps have been discovered within a lignitic horizon in the middle of the TienenFormation near Hoegaarden in northeast Belgium. The anatomical features of the fossil stumps, as those of thenumerous silicified secondary xylem remains collected since the last century from this area, demonstrate that they allbelong to a single taxodiaceous taxon. The stumps bear characteristics shared by Taxodioxylon gypsaceum andGlyptostroboxylon tenerum, but affinities with the latter appear closer. They are attributed to Glyptostroboxylon sp.Calibration of the sedimentological, stratigraphical and organic carbon isotope data reveals that these taxodiaceousfossil trees developed in a swampy lowland environment most probably during the Initial Eocene Thermal Maximumat ca. 55 Ma.3 2003 Elsevier B.V. All rights reserved.

Keywords: Early Eocene; fossil wood; gymnosperm; Glyptostroboxylon ; taphonomy; Belgium

1. Introduction

The Tienen Formation, occurring throughoutnorthern Belgium and in the east of the MonsBasin (Steurbaut, 1998), has been intensivelystudied in recent years because of its positionclose to the Palaeocene/Eocene boundary (Steur-baut et al., 1999, 2000, in press). Its base, in the

more proximal continental settings known as theDormaal Sands, is extremely rich in remains ofland mammals (Smith and Smith, 1996; Smithet al., 1999; Steurbaut et al., 1999; Smith,2000). Records of land plant macro-remainsfrom the Tienen Formation are relatively rare,and usually consist of secondary xylem fragments(Stockmans and Willie're, 1934; Doutrelepont etal., 1997). Current palaeobotanical information ismostly based on palynological analyses (e.g.Roche in Steurbaut et al., 1999, and in Steurbautet al., in press), providing a generalised, homoge-neous picture of the vegetation. Noticeable excep-

0034-6667 / 03 / $ ^ see front matter 3 2003 Elsevier B.V. All rights reserved.doi:10.1016/S0034-6667(03)00062-9

* Corresponding author. Fax: +32-4-366-5338.E-mail address: [email protected]

(M. Fairon-Demaret).

PALBO 2534 15-8-03 Cyaan Magenta Geel Zwart

Review of Palaeobotany and Palynology 126 (2003) 103^129

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tions are the angiosperm leaf assemblage of Levaldescribed by Marty (1907), which is in need ofrevision in the light of modern concepts of foliarmorphologies, and the seeds and fruits recentlydescribed from Dormaal (Fairon-Demaret andSmith, 2002). Here we report the discovery ofan exceptional apparently monospeci¢c fossil for-est at Goudberg^Hoegaarden, 3 km southwest ofTienen, in the Belgian province of Vlaams-Bra-bant (Fig. 1). Hundreds of silici¢ed stumps arepreserved in situ within a lignitic horizon (unit5) in the middle of the Tienen Formation, in anarea extending over more than one hectare.

2. Geographical setting and lithological description

The stumps were exposed during excavationsfor the new High Speed Train railway track atGoudberg^Hoegaarden, extending parallel to theLie'ge/Brussels E40 motorway (50‡47P27.4QN,04‡54P06.1QE; altitude: 55 m; Institut Ge¤ogra-phique National Map 32/7^8: coordinates x=187,y=164; Figs. 1 and 4). An 8-m-thick sectionhas been exposed (Figs. 2 and 3). Its upper part,down to the fossil-tree-bearing lignite, was seen inoutcrop and the lower part in a series of 3-m-deeptemporary trenches dug out at the railway track.This composite pro¢le, known as the GoudbergSection, consists of nine distinct units, of whichthe lithological description is given in Fig. 2. Themiddle part of the section, including units 4^7, is

very complex and needs some more detail. Unit 4,made up of 0.5 m of leached whitish sands, locallyincludes quartzitic silcretes (‘quartzite d’Overlaar-lez-Tirlemont’ of Ledoux, 1910). Occasional rootcasts with unidenti¢able structure are observedperforating this thick but discontinuous white,massive sandstone layer with a bumpy upper sur-face. The embedding whitish sands are devoid oforganic remains. Unit 5 consists of a 45-cm-thicklayer of heterogeneous more or less lignitic depos-its, enclosing the stumps and showing demonstra-tive compaction patterns. It includes in ascendingorder (Fig. 3) : (1) 5.1a, a basal layer with darklignite wrapping the bases of the trunks (Plate I);(2) 5.1b, small sandy lenses interstrati¢ed withlignitic layers towards the base, and with brownto grey clay towards the top; (3) 5.1c, grey^brownstrati¢ed clay with tiny black pieces of fusain;(4) 5.2, an intermediary clay layer with a moreor less breccia-like texture, underlain by a whiteband progressively becoming grey^blue and dark-er, and towards the top interstrati¢ed with small,millimetre-thick lenses of brown clay; (5) 5.3, anupper light grey to brown clayey layer capped by2^3 cm of dark brown clay with numerous rootdepartures; and (6) 5.4, 17 cm of blackish silt.Unit 6 consists of an about 45-cm-thick layer ofbrown sandy to silty mud with in the middle 10cm of greyish clayey silt with a black organic-richbase. It is overlain by unit 7, consisting of a 5-cm-thick layer of white clayey silt, locally ¢nely lami-nated.The succession is not always entirely preserved

along the excavation front because of subsequentlocal erosion. Units 6 and 7, next to the upper 17cm of unit 5 were missing in the eastern part ofthe excavation, reducing the thickness of the Tie-nen Formation by about 65 cm.

3. Stratigraphy and age of the fossil forest

The stratigraphical interpretation of the di¡er-ent units of the Goudberg Section is exclusivelybased on lithofacies analysis, as the entire sectionis devoid of stratigraphically diagnostic microfos-sils (no dino£agellate cysts and calcareous micro-fossils ; a few pollen, spores and freshwater algae

Fig. 1. Geographical location of the localities mentioned inthe text, especially of the Goudberg Section (black star).


M. Fairon-Demaret et al. / Review of Palaeobotany and Palynology 126 (2003) 103^129104

Fig. 2. Lithology of the Goudberg Section with position of the silici¢ed stumps; compare with Fig. 3.


M. Fairon-Demaret et al. / Review of Palaeobotany and Palynology 126 (2003) 103^129 105

Fig. 3. Field picture of units 4^8 (photograph by Professor N. Vandenberghe).



(Botryococcus sp.) were found in the top of unit8; see Fig. 2 for location of samples). It includesidenti¢cation of the lithofacies characteristics,trends and junctions, and relies on the lithostrati-graphical classi¢cation of Steurbaut (1998), takinginto account previous studies carried out in theTienen area by Rutot (1887), Van den Broeckand Rutot (1894) and Gulinck (1948).The glauconitic obliquely laminated sands of

unit 1 are attributed to the Hoegaarden Sands, anot yet o⁄cially de¢ned term for the estuarinesands occurring in the Hoegaarden area (Steur-baut, study in progress). These sands are sand-wiched between the Upper Thanetian GrandgliseSands and the Tienen Formation. They are a lat-eral equivalent of the fully marine (infralittoral)Bois-Gilles Sand Formation, cropping out in thesouthwest of Belgium and calibrated with themiddle part of Biochron NP 9 (Steurbaut, 1998),dated at 55.8 Ma (Berggren et al., 1995).The overlying interval, ranging from unit 2 to

unit 7, is marked by an essentially £uviatile depo-sitional regime. Units 3 and 4 represent palaeo-sols. The white clean sands of unit 4 are due tointense leaching of a soil pro¢le in a well-drained,warm, wet environment. Units 5 and 6 representthe peat swamp environment that developed assubsidence caused regional rising of the water ta-bles, whereas unit 7 is thought to be lacustrine.The palaeoenvironmental characteristics of thisessentially £uviatile interval, its heterogeneityand the presence of intraformational lignite referto the Tienen Formation. Lignite beds are knownto occur in many sections of the Tienen Forma-tion (e.g. in northern Belgium in an essentiallylagoonal context; see Steurbaut et al., in press)and its correlatives in northern France (theMont Bernon Group; Dupuis et al., 1998; Magi-oncalda et al., 2001a) and in southern England(the Woolwich Formation; Bone, 1986; Ellisonet al., 1994). In proximal settings (e.g. at Dor-maal) these lignites overlie sandy deposits belong-ing to the Dormaal Sand Member (Steurbaut etal., 1999). This member, at the base of the TienenFormation, is often rich in mammal remains, rep-resenting the well-known Dormaal mammal fauna(Steurbaut et al., 1999; Smith et al., 1999; Smith,2000).

In the most complete settings of northwest Bel-gium (Knokke area), the Tienen Formation is un-conformably overlain by the Zoute Silt Member,the lowermost member of the Kortrijk Clay For-mation (Steurbaut, 1998; Moorkens et al., 2000).The latter is part of the Ypresian stratotype. TheZoute Silt Member, through calibration known tobe deposited within Biochron NP 10, posterior tothe well-known Apectodinium acme, is missing inthe Goudberg Section. Instead, a clayey unit oc-curs (unit 8), which on the ground of its geometryand lithofacies may belong to the Kortrijk ClayFormation. The overlying unit 9, of which severalmetres are locally preserved, is incorporated in theBrussel Formation (Sintubin et al., 2000).The lignite of the Goudberg Section in which

the fossil stumps were found does not contain anybiostratigraphical marker. However, its age canbe reliably inferred from organic carbon isotopeanalysis. The isotope values measured in theGoudberg lignite present a trend towards morepositive values from 325.7x to about 325.0xPeeDee Belemnite (Magioncalda, pers. commun.).The same isotopic trend and values are recordedat ¢ve stratigraphically distinct places in the MontBernon Group of the Cap d’Ailly Section innorthern France, just below the lignitic complexL1, in its lowermost part (L1a), at its top (L1c),within unit SP3 and within unit SP4, respectively(Magioncalda et al., 2001a,b). However, compar-ison with the isotope curves of the Kallo and Doelboreholes, 75 km northwest of the Hoegaardenarea (Steurbaut et al., in press), suggests that thethird option is the most plausible one, and thatthe Goudberg lignite should be correlated withlignite L1c of the Cap d’Ailly Section and withthe lignitic interval L in the middle of the TienenFormation at Kallo and Doel. The pollen andspore records support this correlation. The ligniticinterval encountered at Kallo and Doel and thosefrom other localities in the vicinity of Hoegaarden(Loksbergen, Budingen) contain similar palyno-morph associations, marked by the highest occur-rence of Subtriporopollinites magnoporatus tecto-psilatus, by the presence of Intratriporopollenitesmicroreticulatus and I. pseudoinstrictus, and bythe absence of Subtriporopollenites spissoexinus(Hauregard, 1999; Ostojski, 2001). As a conse-



quence, these lignite levels are believed to be con-temporaneous or only slightly di¡erent in age.Carbon isotope shifts from 326.0x to

325.0x are known to occur within the organicCarbon Isotope Excursion (CIE) in terrestrial, la-goonal, as well as marine settings (Steurbaut etal., in press; Magioncalda et al., 2001a, and un-published research). Estimates for CIE durationvary greatly (Norris and Ro«hl, 2000), althoughrecent high-resolution studies point to a durationof ca. 84 000 years, ranging from 55.50 to 55.41Ma using the age model of Bains et al. (1999) andfrom 55.00 to 54.92 Ma using the model of Bowenet al. (2001, ¢g. 7). The base of the CIE has re-cently been chosen as the criterion for the Palaeo-cene/Eocene boundary (Luterbacher et al., 2000).Integration of these data indicates that the fossiltrees of Hoegaarden probably developed duringthe Initial Eocene Thermal Maximum (IETM,formerly LPTM) about 55 Ma, when climatewas the warmest of the entire Cenozoic (Kochet al., 1992; Norris and Ro«hl, 1999). Accordingto Godinot (2000), based on data from Besse andCourtillot (1991), the Hoegaarden forest was sit-uated at a palaeolatitude of approximately 40‡N.

4. Historical background

Silici¢ed pieces of secondary xylem have beenknown in the Tienen area since the middle of thenineteenth century (Lyell, 1852). Big stumps wereencountered in the quarry of Overlaar (Fig. 1),located at about mid-distance between Hoegaar-den and Tienen (Rutot, 1887), while extractingthe ‘Landenian’ massive sandstone, and in aroad section at Wommersom 10 km to the east(Rutot, 1887). At Overlaar, the stumps were ob-served in upright position in a bed of lignite over-

lying white sands located above a sandstone layer.As early as 1887, Rutot discussed the origin of the‘fossil forest of Overlaar’. In his opinion it repre-sented the remnants of an in situ forest that hadbeen growing in a littoral, swampy environment.Quickly a controversy arose about the ‘in situ’position of the ‘fossil forest’. Stainier (1909) putforward several observations justifying the al-lochthonous character of the stumps that wouldhave been transported by £uvial currents beforedeposition. He particularly pointed out the ab-sence of appreciable lengths of anchoring organs,and the presence of big root departures only atthe base of the stumps. He also noticed a relation-ship between the shape of the tree fragments andtheir orientation in the embedding matrix: cylin-drical trunks, which are the longest, were found inhorizontal position while the conically shapedones with a wide, spreading base were foundstanding. As the centre of gravity of such conicalstumps is lower than their centre of ¢guration,they could well have been carried upright in mud-£ows and have consequently been deposited up-right. On the other hand, Ledoux (1910) arguedthat these stumps were never seen prostrate orgathered into irregular piles, one above another,as a consequence of £ood rafting and accumula-tion. As the distances between adjacent stumpswere similar to those observed in a present-dayforest, he claimed that the fossil trees of Overlaarwere autochthonous. Since that time, members ofboth ‘schools’ maintained their positions due tolack of new observations (Hauregard, 1999).

5. Materials and methods

The studied specimens, which have all been col-lected from the same horizon in the Tienen For-

Plate I.

1^3. Goudberg Section, Hoegaarden (Belgium), earliest Eocene. In situ trunks.1. Lateral view of a specimen still partly embedded in the sediment, showing marked de£ection of the lignite layer around

the trunk base. Scale bar= 20 cm.2. Apical view of a trunk base with well-developed buttress-like roots. Scale bar= 20 cm.3. Lateral view of a large trunk base showing the usual 20^30‡ dip towards the east. Note the thin underlying lignite layer.

Scale bar= 20 cm.



1

3

5 6

4

2



mation, belong to three distinct groups. The ¢rstgroup is made up of 23 specimens from the Stock-mans’ collection of stumps and branches retrievedduring the last century from the now disusedOverlaar quarry, and preserved in the palaeobo-tanical collections of the Royal Institute of Natu-ral Sciences at Brussels. The second one includesmore than thirty isolated dispersed pieces of sec-ondary xylem recently collected at and around theGoudberg Section, and more particularly alongthe motorway which runs parallel in places tothe new railway track (Fig. 1). The tiny silici¢edpieces obtained while macerating the lignitic ma-trix are added to this second group. The thirdgroup consists of silici¢ed secondary xylem frag-ments collected from six in situ tree bases atGoudberg.Two areas about 200 m apart in southeast di-

rection along the motorway were systematicallydug out up to the standing stumps at the baseof unit 5 (Fig. 4). The ¢rst one, rectangular inshape (Fig. 4A,C, 1), is 70 m long and 20 mwide. Some 74 irregularly distributed stumpsand dispersed pieces of trunk were recorded(about 500 trees/ha). Additional stumps were no-ticed outside this area, indicating that many moreare still embedded in the undisturbed surround-ings. The second area, with a semicircular bound-ary, of which about 0.15 ha was excavated (Fig.4A, 2), demonstrated an irregular distribution of67 stump and/or trunk remains. Nearby this sec-ond area (Fig. 4A, 3) the embedded stumps werecarefully uncovered in an area of 7.5 m2 in order

to measure their height, diameter at the base andtop, direction and angle of inclination, as well asthe spacing between individuals (Fig. 4C).The stumps from Goudberg share the same

characteristics. They rest on a thin layer of lignite

Plate II. Transmitted light micrographs of thin sections of the secondary xylem Glyptostroboxylon sp. from the Tienen Forma-tion, Belgium.

1^3. Transverse sections.4^6. Longitudinal tangential sections.

1. General view; growth rings and false rings. Specimen IRSNB 67855. U50.2. Detail showing the shape of the tracheids. Specimen IRSNB 67680. U100.3. General view of secondary xylem devoid of false ring. Notice the distribution of the xylem parenchyma cells. Specimen

IRSNB 67590. U30.4. General view of the vascular rays. Specimen IRSNB 67680. U85.5. Detail of a vascular ray biseriate on one row. Specimen IRSNB 67590. U350.6. Uniseriate rays of various heights. Notice the small bordered pits on the tangential walls of the tracheids. Specimen

IRSNB 67590. U190.

Fig. 4. (A) Position of the three areas, 1, 2 and 3, systemati-cally excavated up to the standing stumps. (B) Detail of theposition of the 13 stumps of area 3. Abbreviations: D, direc-tion of inclination of each stump; T, angle of inclination.(C) Detail of the irregular distribution of 74 stumps (blackdots) in area 1.



1 2

3 4

5 6 7



interstrati¢ed with black clay, and are surroundedby a thin layer of dark and glossy lignitic clay.They are silici¢ed, more or less conical in shape,truncated at the top which appears scalloped andcrushed (Plate I, 1^3), and generally range be-tween 60 and 100 cm in height. They stand up-right, and most of them, when fully detachedfrom the clayey embedding matrix, show a 20^30‡ dip towards the east (Fig. 4B; Plate I, 1,3).Their distal diameter usually ranges from 30 to 60cm, the biggest specimens reaching up to 80 cm.The stumps are decorticated and an unknownthickness of the outermost wood layers is not pre-served. The diameter of the trees at breast height(dbh; necessary for reporting stand basal area)could not be valuably estimated. The base ofeach upright stump is enlarged with buttress-like£anges (Plate I, 1,2). No attached roots have beenobserved penetrating the underlying layers for anappreciable length, and only the departure ofsturdy, horizontal basal roots has occasionallybeen seen, spreading laterally from the enlargedbase of the stumps. Several specimens are re-corded lying in-between the standing stumps.These are trunk-like, cylindrical in shape butusually are dorso-ventrally £attened, being pre-served over a longer length than the standingtree bases.The wood was studied using conventional thin

sections combined with observation of fracturedfragments under the scanning electron microscope(SEM). A transverse thin section of every speci-men was prepared, and when preservation ap-peared promising, tangential and radial sectionswere made. Light microscopy was performed us-

ing the photomicroscope Reichert Polyvar. In ad-dition, pieces of fractured silici¢ed secondary xy-lem were prepared on stubs and gold-coated.Observation and photographs were made usinga Jeol JSM-5800 scanning electron microscope.Pieces of lignite were also soaked in hot waterwith Na2CO3 added and the dark brown solutionobtained was sieved. No remains of diasporeswere preserved and only naturally macerated sili-ci¢ed fragments of secondary xylem were re-trieved. These were also studied under the SEM.Maceration of the lignite embedding the bases

of the tree as well as of the clayey matrix was alsoperformed using standard palynological proce-dures (Streel, 1965). No palynomorphs havebeen found in the samples of the Tienen Forma-tion, including the lignite of unit 5 (see Fig. 2 forlocation). This is probably due to severe oxida-tion.

6. Mineralogy of the fossil wood

Any attempt at demineralisation of the in situand dispersed silici¢ed secondary xylem fragmentswas disastrous. Microdi¡raction studies show thatpolycrystalline ¢ne-grained quartz is the maincomponent of the specimens, mixed with amor-phous opal (Kuczumov et al., 1999). Quartz oc-curs both in cell walls and lumina, and very little,if any, of the original organic matter is stillpresent. Apparently, the secondary wood patternhas not been lost to an appreciable extent duringthe transition from the opal-like to the micro-quartz structure.

Plate III. Transmitted light micrographs of radial thin sections of Glyptostroboxylon sp. from the Tienen Formation, Belgium.

1. A vascular ray, two cells high with 1^4 rounded, eroded pit apertures/cross-¢eld. Notice the shape of the cross-¢elds.Specimen IRSNB 34865. U350.

2. An early wood vascular ray, four cells high; the marginal cells show four cross-¢eld pits arranged in two tiers; themedian cells show two, side by side, taxodioid to glyptostroboid pits. Specimen ULg I2. U300.

3. Glyptostroboid to taxodioid early wood cross-¢eld pits. Specimen IRSNB 67590. U300.4. Detail of the bordered pits on the horizontal walls of the tracheids. Specimen IRSNB 67590. U870.5. Paired bordered pits. Crassulae are obvious. Specimen IRSNB 34865. U350.6. Radial wall of a wood parenchyma cell. Specimen IRSNB 67642. U870.7. Slightly knotted radial wall of a wood parenchyma cell. Specimen ULg I3. U870.



1

3

5 6

4

2



7. Taxonomy

Family Cupressaceae Gray, nom. cons.Subfamily Taxodioideae Endl. ex K. KochGenus Glyptostroboxylon ConwentzGlyptostroboxylon sp.

8. Description

The characters of all secondary xylem speci-mens studied are distributed along a continuumof variation, and despite some apparent dissimi-larity it was impossible to separate them into dif-ferent structural entities. They are consequentlyinterpreted as belonging to a single taxon, anddescribed accordingly.The secondary xylem is homogeneous and

shows no normal resin canals (Plate II, 1,3). De-spite the high number of specimens studied, notraumatic resin ducts were observed. Growthrings are clearly distinct (Plate II, 1^3), and ofvery variable width. Within the same specimen,some growth rings show a gradual early^to latewood transition, whereas others show an abrupttransition (Plate II, 1^3). False rings may occur,generally in the outermost part of a growth ring(Plate II, 1) ; their number and frequency arehighly variable. Early wood tracheids of widegrowth rings are generally hexagonal to more orless rounded (Plate II, 2,3). They are more quad-rangular in the early wood of narrow growthrings (Plate II, 1), and are tangentially compressedin late wood. Early wood tracheids have a radialdiameter of 24^56 Wm (usually 40 Wm) and a tan-gential breadth between 32^44 Wm (usually 40Wm). As a rule the tangentially broadest tracheids

occur near the middle of a growth ring. The radialwalls of early wood tracheids bear uniseriate torarely biseriate opposite bordered pits, 14^17 Wmin diameter (Plate III, 1,4,5; Plate IV, 1). Crassu-lae are common (Plate III, 4,5). Bordered pit pairshave a well-developed torus which is scalloped(Plate III, 4,5). Their apertures are slightly ellip-tical. On the tangential walls of late wood trache-ids slightly smaller uniseriate bordered pits, 12^15Wm in diameter, are common (Plate II, 6).Axial parenchyma is di¡use (Plate II, 2,3) or

loosely grouped in tangential rows, 1 cell wide(Plate II, 1). In longitudinal sections the second-ary xylem parenchyma cells are typically barrelshaped (Plate IV, 1). Brown to black depositsare frequent and usually obliterate the wall details(Plate III, 7; Plate IV, 1). The horizontal walls arethin (Plate IV, 5) and smooth, slightly knottedwhen pits are present (Plate III, 6,7). They neverappear strongly beaded. Small rounded half-bor-dered pits occur on their vertical walls often ar-ranged in two irregular, more or less parallel rows(Plate IV, 5).Vascular rays are homocellular (Plate II, 4^6;

Plate IV, 1,2). They are mostly uniseriate, rangingfrom 1 to 34 cells high, although typically only up

Fig. 5. Maximum likelihood phenogram stemming from acladistic analysis using the characters listed in Table 1.

Plate IV. Scanning electron micrographs of fractured longitudinal radial plane of Glyptostroboxylon sp. from the Tienen Forma-tion (Belgium). Specimen ULg I3.

1. General view with two vascular rays and two vertical rows of parenchyma cells. U95.2. Detail of wide, rounded to oval, cross-¢eld pits. Notice the outline of the cross-¢elds. U490.3. Glyptostroboid cross-¢eld pits (casts) irregularly arranged, up to ¢ve per cross-¢eld. U490.4. Variation in shape, size and arrangement of cross-¢eld pits with a very narrow border. U490.5. Detail of a wood parenchyma cell with small half-bordered pits. U290.6. Detail of a vascular ray showing sparsely pitted (arrow heads) horizontal walls and occasionally, an enlargement at the

junction between horizontal and vertical walls (small arrows). U875.



1

2 3

4 5



to 2^4 cells (Plate II, 4,6); some are biseriate onone row (Plate II, 5). They occasionally containblack deposits. On tangential sections the outlineof the ray cells looks more or less like a £attenedellipse, the pointed, marginal cells being higherthan the median ones (Plate II, 4^6). The heightof the median ray cells measures between 20 and28 Wm while the marginal ones reach 32^40 Wm.Their relatively thick (1^3.4 Wm wide) horizontalwalls are smooth, and appear unpitted under thelight microscope. The tangential walls are mostgenerally perpendicular (Plate IV, 6). Indenturesare not present. At the point of junction of tan-gential with horizontal ray cell walls, a triangularwidening is sometimes noticed (Plate III, 2; PlateIV, 6, arrows).Cross-¢eld pits vary from taxodioid to glypto-

stroboid. Cupressoid pits have not been observed.The pit aperture is often rounded (Plate III, 1^3;Plate IV, 2^4), although wide oval pit aperturesare not rare either (Plate II, 3; Plate IV, 3,4,6).Their long axis is inclined at an angle of 0 toabout 25‡ to the horizontal wall of the ray cell.The border, if present, is always narrow. The di-ameter of the cross-¢eld pits is 8^14 Wm. Theirnumber ranges from 1 to 6 (exceptionally 7) percross-¢eld. When more then two are present, thecross-¢eld pits are usually arranged in two tiers,occasionally three tiers, and are irregularly dis-posed (Plate IV, 3,4,6). Numerous irregularly dis-posed cross-¢eld pits are particularly obvious onthe higher marginal cells but they also occur onthe median ones.Incidentally, small roots (Plate V, 1) were ob-

served inside branch pieces, 15/10 cm wide byabout 30 cm long, lying in-between the stumps.These have no secondary xylem; they vary fromdiarch and triarch (Plate V, 4) for the smallest

rootlets less than 1 mm wide to tetrach (Plate V,2,3) and exceptionally pentarch for the ‘widest’ones 3.3 mm in diameter. Tiny frequentlybranched rootlets are seen attached to largerones (Plate V, 2). In all sections epidermal cellslack root hairs. A mantle of hyphae surroundseach root (Plate V, 4,5) ; tiny Hartig net hyphaeoccur around the cortical cells, reaching the endo-dermis (Plate V, 3). These small roots with ecto-mycorrhizal association might belong to the sametaxon as the secondary wood.

9. Identi¢cation

The combination of distinct growth rings,smooth to sparsely pitted, horizontal and tangen-tial walls of ray parenchyma, 1^30 cells high rayswith occasionally 1^2 paired cells in the body, and1^6 taxodioid to glyptostroboid pits in the cross-¢elds, in association with the absence of resinducts and spiral thickening, indicates taxodia-ceous secondary xylem. In the absence of otherevidence, these woods are notoriously di⁄cult toassess because of the overlapping characters (Bas-inger, 1981). Furthermore, there are di¡erentopinions about the diagnostic value of these char-acters (Figueiral et al., 1999).Usually, determination of taxodioid fossil sec-

ondary xylem puts special emphasis on the cross-¢eld pits (Kra«usel, 1949). However, it has beenshown that the type of cross-¢eld pitting is rathervariable within a single species according to thepart of the tree considered (Bailey and Faull,1934), and only the most common type of earlywood cross-¢eld pitting should be considered(Kra«usel, 1949). In addition, even in modern sec-ondary xylem, the di¡erence between taxodioid,

Plate V. Transversal sections through Glyptostroboxylon sp. from the Tienen Formation, Belgium, with roots in the decayed pith.Specimen ULg Re¤servoir 1.

1. Transverse section through a branch, general view showing sections of small roots in the decaying pith. Re£ectedpolarised light micrograph of the polished surface. U3.

2. Detail of a branching rootlet. U20.3. Detail of a tetrach root. Cortical cells up to the endodermis are enclosed in the Hartig net. U90.4. A triarch root with a thick mantle of hyphae. The arrows point to the secondary xylem of the branch. U35.5. Detail of the mantle of ectomycorrhizal hyphae. U200.



Table 1Comparison of the characteristics of extant Taxodioideae genera, Taxodioxylon gypsaceum, Glyptostroboxylon tenerum, and the Hoegaarden wood

Cunning-

hamia

C Sequoia C Sequoia-

dendron

C Metase-

quoia

C Crypto-

meria

C Taxodium C Glypto-

strobus

C gypsa-

ceum

C tenerum C wood C

Tracheids Number

of pits

1 (rarely 2),

(2), (3), (6)

1 scattered,

rarely 2 side

by side (3)

1 1 (3) 0 1^2 (3) 1 1^2^(3), (3) 2 up to quadri-

seriate (3)

3 1^2, in long

rows (3) up to

4 in thick root,

(10)

1 1^2^(3^4), (6),

(8), 1^2^3 (9)

3 uni- to

biseriate (9)

1 uni- to

biseriate

1

Diameter of

pits

9^11 (3) 0 16^18 (3) 1 14^17 (3) 1 20^22 1 13^14 (3) 0 12^14 (3), 8^12

(9)

0 13^16 (3) 1 14^17 (8) 1 11 (10); 14^20

(9)

1 14^17 1

Torus present 1 present 1 absent (1) 0 present 1 present 1 present 1 present 1 present 1 present 1 present 1

Tang. diam.

(early wood)

30^55 (3), 30^

60 (2)

0 40^50 (3) ^80

(7), 24^60 (2)

1 30^90 (3) 1 65^85 (3), 40^

85 (4)

1 30^50 (2) 0 35^90 (2), (7) 1 40^50 (3), 20^

70 (2)

1 40^54^70 (8),

45^48^60 (9)

1 40^44^50 (9),

32^50 (10)

0 32^40^44 0

Rays Height 1^24, 30 (2) 0 1^30 (2)^ 40+

(7)

1 1^20+ (3) 0 1^30 (3) 0 1^30 (2) ; 1^6

(7), (3)

0 up to 60 cells 1 1^18 (330) (3) 0 1^30 (8), 7^12

(9)

0 1^8^14 (9) up

to 24 (10)

0 1 up to 34 0

Width partly biseriate

(2)

1 biseriate

(partly or

compl.)

1 uniseriate, rare

biseriate

(partly or

compl.) (3)

1 uniseriate, rare

partly biseriate

(3)

1 partly biseriate

(rare)

1 ? rare 1^2 paired

cells (3)

1 rare paired

cells (9)

1 occas. paired

cells

1 rare 1^3 paired

cells

1

Ray tracheids present (1) 1 present (1), (4) 1 present (4); ?

(1)

1 present (1), (4) 1 absent (1) 0 absent (1) 0 absent (1) 0 absent (8) 0 absent (9) 0 absent 0

Hor. wall

pitting

thin and pitted 1 sparsly pitted

(4)

1 sparsly pitted

(4)

1 sparsly pitted

(4)

1 thick and

smooth (3)

0 thick and

pitted (3)

1 thin+warts (3) 1 smooth (9) to

pitted (6)

1 smooth (9) 0 thin and

sparsly pitted

1

Indentures prominent in

early wood (2)

1 absent (2),

occasional (3)

1 shallow (3) 1 not noted (3),

present (7)

1 prominent (2) 1 absent (3) 0 present (2);

none (3)

1 not mentioned ? not mentioned ? absent 0

Peculiarity relatively small

cross-¢eld pits

(3)

resin

tracheids+

medull. stone

cells (11)

rad.+tg. walls

thickn. to the

same extent (3)

tg. wall thin,

smooth (3)

medull. stone

cells (11)

highest rays

within

taxodiaceae (2,

3)

triang.

widening at

junction (3)

occasional

triang.

widening at

junction

Cross-¢eld pits Type glypto-cupresso taxodioid (3)

(5), taxo-

glypto-

(cupresso-

podocarpo) (4)

taxodioid (3)

(5), taxo-

(glypto-

cupresso-

podocarpo) (4)

taxodioid (3),

taxo-glypto-

(cupresso-

podocarpo) (4)

taxo to glypto

(2)

taxodioid (2) glypto (3),

taxo to glypto

(2), (4)

taxo-(glypto)-

(cup-

ressoid) (6)

taxo to glypto

(9)

taxo to glypto

Number 2^4 (2) 0 2^5 (2); 1^6

(3)

1 1^2 and 4^6

(3)

1 1^2 (3) 0 2^4 (2) 0 2^6 (2) up 8

(3)

1 1^8 (3); 2^6

(2), (4)

1 2^6^(5), (8);

2^3 (9)

1 1^4^8 (9), (10) 1 1 to 7 1

Disposition one horizontal one horizontal one horizontal

series (6)

side by side or

superposed

one horizontal mostly in twos 2 tiers, occas.

3,

side by side in

one

several tiers,

and

2 tiers, occas.

3

series (2);

superposed in

terminal cell

(3)

0 series (2) 0 or two (3),

aperture often

horizontal.

‘elongated (6)

1 (3), (7) 1 series (2), 1 side by side (3)0 irregularly

distributed (2)

1 row (8), (9),

two tiers

1 irregularly

distributed

1 irregularly

distributed

1

PALBO

253415-8-03

Cyaan

Magenta

Geel

Zwart

M.Fairon-D

emaret

etal./R

eviewof

Palaeobotany

andPalynology

126(2003)

103^129118

half-bordered pits with wide oval aperture andnarrow border, and glyptostroboid pits similarto the taxodioid ones but with a very narrow,almost invisible border (Schwarz and Weide,1962, p. 177), is not always easy to assess, as avariety of transitional forms occur within eachspecies. Measurements of the length of the pitand width of the margo, followed by calculationof the Schwarz and Weide (1962) index, plus con-sideration of the inclination of the aperture givegood clues. On the Belgian fossil secondary xylemthe Schwarz and Weide index is rarely applicable,as the cross-¢eld pits often appear corroded andwith ill-de¢ned margins when observed under thetransmitted light microscope (Plate III, 1^3).Nevertheless, on the same piece of secondary xy-lem both types of cross-¢eld pitting ^ 4^7 more orless superposed glyptostroboid pits (Plate III, 1,4;Plate IV, 3,4) or 1^2 side by side taxodioid ones(Plate III, 1,2) ^ are observed on early wood raycells. Amongst the many specimens studied, bothtaxodioid and glyptostroboid cross-¢eld pits arethus present, the glyptostroboid type of pittingoccurring at least as frequently as the taxodioid.We have not observed typical cupressoid pits. Ac-cording to Kra«usel’s key, the Hoegaarden second-ary xylem falls between Taxodioxylon Hartigemend. Gothan and Glyptostroboxylon Conwentz.According to the last ‘diagnoses’ (sic) providedfor Taxodioxylon and Glyptostroboxylon (Su«ssand Velitzelos, 1997), the di¡erence betweenboth genera is narrow indeed. The segregation,once again, rests on the early wood cross-¢eldpits: they have broad apertures of more or lesselongated axis (taxodioid pitting) in Taxodioxylonwhile they are ovoid in Glyptostroboxylon, all theother characters listed in both ‘diagnoses’ beingidentical. Su«ss and Velitzelos (1997) do not dis-cuss the taxonomic value of this single feature,which is known to be rather variable even withina single extant species (Bailey and Faull, 1934).As both types of pitting occur in the Hoegaardenwood, the morphology of the cross-¢eld pits givesno clue for its identi¢cation.In taxodioid fossil (and extant) secondary xy-

lem, emphasis is also put on the morphology ofthe horizontal wall of the secondary xylem paren-chyma cells. In Glyptostroboxylon these walls areTa

ble1(C

ontinued).

Cun

ning

-

hamia

CSequo

iaC

Sequo

ia-

dend

ron

CMetase-

quoia

CCrypto-

meria

CTax

odium

CGlypto-

strobu

s

Cgy

psa-

ceum

Ctenerum

Cwoo

dC

superposed

(3)

1ho

rizontal

series

(2)

oninternal+

marginalray

cells

(2)

whenmore

than

4(8)

also

twoside

byside

(9)

also

twoside

byside

smoo

th(1),

thin,pitted

largeno

dules

(1)

smallno

dules

(1)

largeno

dules

(1)

largeno

dules

(1)

largeno

dules

(1)

smallno

dules

(1),coarse

thin,sm

ooth

(9)

smoo

th(10)

thin

and

sparslypitted

Parenchym

aHoriz.wall

tono

dular(3),

slightly

0thin,un

iform

(5)

0thin

with

simplepits

(5)

0sm

ooth

to

nodu

l.(4)

?rare

coarse

pits

(2)

?coarse

pits

(2),1

pits(2),1^2

occasion

alpits

?to

pitted

with

0pitted

(beaded)

(6)

?sm

ooth

to

pitted

with

0

thickn

.(2)

1sm

ooth

(3)

1sm

ooth

(3)

1sm

ooth

(3)

0slightly

nodu

lar(3)

1thick,

nodu

lose

(5)

1(3),sm

ooth

to

nodu

lar(4)

1sm

allno

dules

(6),(8)

1irregularly

thickn

.(9)

1sm

allno

dules

1

(1)Gadek

etal.,2000;(2)Peirce,

1936;(3)Greguss,1955;(4)Schw

arzandWeide,1962;(5)Henry

andMcIntyre,

1926;6vanderBurgh

andMeijer,

1996;

7Basinger,1981;8Huard,1966;9Gottwald,

1992;10

Scho«nfeld,1955;11

BaileyandFaull,

1934.

C,coding

ofthecharacters.



described as smooth, sparsely pitted (see Scho«n-feld, 1955 for a listing of Glyptostroboxylon speci-men characters) to irregularly thickened (Gott-wald, 1992). A similar range of variation is alsonoticed in Kra«usel’s descriptions of Taxodioxylongypsaceum Go«ppert, the most common EuropeanTertiary taxodiaceous secondary xylem, where thehorizontal walls of the axial parenchyma are re-garded as smooth to granularly thickened (Su«ssand Velitzelos, 1997) or as frequently nodular,the nodules being small and uneasy to observe(van der Burgh and Meijer, 1996). The validityof Glyptostroboxylon has even been questionedbecause G. tenerum (Kraus) Conwentz appearsdi⁄cult to distinguish from T. gypsaceum (vander Burgh and Meijer, 1996, and references there-in). Actually, when considering the variability ofboth key features, i.e. cross-¢eld pits and woodparenchyma horizontal walls (Table 1), it istempting indeed to synonymise G. tenerum and

T. gypsaceum. The formalisation of such amove, however, requires ¢rst-hand knowledge ofthe type-specimens plus an in-depth analysis of avariety of formerly described fragments from dif-ferent localities and ages. This is beyond the scopeof this paper.Meijer (2000), while describing late Cretaceous

silici¢ed driftwood from the Aachen Formation(northeast Belgium), faced a similar problem ofoverlapping characteristics. Despite occurrenceof so-called ‘Eiporen’ (sensu Kra«usel, 1949) ar-ranged in more than one horizontal row in me-dian ray cells, he chose assignation to Taxodi-oxylon as Taxodioxylon cf. gypsaceum instead ofGlyptostroboxylon mainly because his specimendemonstrated rootwood features, and Sequoiarootwood according to Bailey and Faull (1934)possesses similar cross-¢eld pits. We have second-ary xylem from branches and trunk bases, notfrom roots. These wood fragments show a variety

Table 2Quanti¢cation of the characters

Characters Coding

Tracheids Pit number 1 02 13 2more than 3 3

Pit diameter less than 16 Wm 0more than 15 Wm 1

Torus no 0yes 1

Tangential diam. less than 61 Wm 0more than 60 Wm 1

Rays Height less than 41 cells 0more than 40 cells 1

Width uniseriate 0biseriate 1

Tracheids no 0yes 1

Hor. wall smooth 0pitted 1

IØndentures no 0yes 1

Cross-¢eld pits less than 5 0more than 4 1

Disposition one row 0two rows 1

Parenchyma Horizontal wall smooth 0nodular 1

Horizontal wall thin 0thick 1



of cross-¢eld features. Their cross-¢eld pits arenevertheless more often organised in irregularlysuperposed tiers rather than side by side in aline, even in median ray cells. Accordingly, weadopted a conservative position and decided forthe inclusion of Hoegaarden secondary xylem inGlyptostroboxylon. In the present state of knowl-edge of both wood arti¢cial taxa, our decision iscomforted by the following observations: (1) theintertracheidal bordered pits are never triseriate,even on the radial walls of the widest elements, (2)the latter are smaller than those of T. gypsaceum(van der Burgh and Meijer, 1996, for a compar-ison of data from di¡erent descriptions of T. gyp-saceum), and (3) the relatively narrow width ofearly wood tracheids and the height of ray cellsgive a peculiar appearance to the radial sectionswith cross-¢elds generally much less radially elon-gated than those usually illustrated for T. gypsa-ceum. Nevertheless, the Hoegaarden wood in-cludes features of both Taxodioxylon and Glyp-tostroboxylon, illustrating the occurrence of a Ter-tiary taxodioid secondary xylem complex in whichsegregation between arti¢cial genera is presentlydebatable.Until recently Glyptostroboxylon tenerum was

the only species referred to Glyptostroboxylon(for synonymy, see Kra«usel, 1949; Su«ss and Ve-litzelos, 1997). Lately two more species have beenadded to the genus: G. microtracheidale Su«ss andVelitzelos 1997 and G. tendagurense Su«ss andSchultka 2001. Glyptostroboxylon tendagurense,known from the Upper Jurassic of East Africa,is remarkable for its extremely narrow tracheidswith variable size and shape. Glyptostroboxylonmicrotracheidale, present in the Upper Oligo-cene^Lower Miocene of Lesbos (Greece), di¡ersfrom the Hoegaarden secondary xylem in the ar-rangement of its radially shortened tracheidswhich are disposed in irregular rows.

Glyptostroboxylon tenerum is considered aswidespread in the European Cenozoic (van derBurgh, 1973, and references therein; Mai, 1995),but no recent detailed description is available.Stockmans and Willie're (1934) recorded G. tene-rum at Leval^Trahegnies in Belgium, a locality ofabout the same age as Hoegaarden, but the de-scription is very short and the drawings uninfor-

mative. The specimens attributed to G. tenerumfrom brown coal of the Lower Rhine Embaymentby van der Burgh (1973) appear to be di¡erentfrom those of Hoegaarden. They are instead re-garded as belonging to Taxodioxylon by Su«ss andVelitzelos (1997), a divergence of opinion illustrat-ing once more the di⁄culty in distinguishing be-tween Taxodioxylon and Glyptostroboxylon. Ourspecimens are more comparable to G. tenerumEocene silici¢ed material from brown coal pitsnear Leipzig (Germany), but on transverse sec-tions the German secondary xylem shows a looserarrangement of the tracheids with open, recognis-able intercellular spaces (Scho«nfeld, 1955, Taf. 7,Abb. 54). The G. tenerum phosphatised driftwooddescribed by Gottwald (1992) from the Late Eo-cene of Helmstedt (Germany) is similar to theBelgian Glyptostroboxylon, but cross-¢elds withsingle pits are apparently not present. We there-fore decided not to refer the Hoegaarden second-ary xylem to G. tenerum. It is designated as Glyp-tostroboxylon sp. to emphasise its unique combi-nation of characteristics pointing to the Glypto-stroboxylon type of fossil wood genus, but withseveral traits, notably the horizontal wall pittingof the wood parenchyma cells, of Taxodioxylongypsaceum.

10. Discussion and comparison with moderntaxodiaceous taxa

Recently, several clades have been identi¢edwithin taxodiaceous taxa leading to a more infor-mative infrafamilial classi¢cation (Gadek et al.,2000; Kusumi et al., 2000), Sciadopitys Sieboldet Zucc. being transferred to the monotypic familyof the Sciadopityaceae Luerss. (Farjon, 1998).The Taiwanioideae (Hayata) Quinn and Athro-

taxidoideae Quinn have secondary xylem di¡erentfrom the Hoegaarden specimens, showing smallerbordered pits on their tracheids, shorter rays(rarely exceeding 10 cells high in Athrotaxis D.Don), and smaller cross-¢eld pits which are dis-tinctly cupressoid in Taiwania Hayata (Peirce,1936; Greguss, 1955).The gross morphology of the secondary xylem

of monogeneric Cunninghamioideae (Siebold et



Zucc.) Quinn is close to the Belgian specimens,especially in the size of the cells and the outlineof the cross-¢elds. Actually, according to Gothan(1905), Glyptostroboxylon is comparable to thewood of both the extant genera CunninghamiaR. Brown and Glyptostrobus Endl. Cunninghamiarays with pointed marginal cells, higher than themedian ones, as seen on tangential sections, havethe same appearance indeed as those of the Hoe-gaarden wood. However, the tracheids usuallyshow only uniseriate bordered pits on the radialwalls, and thus constitute an exception to the usu-al occurrence of biseriate-bordered pits, a featureshared by all other taxodiaceous secondary xylem(van der Burgh and Meijer, 1996; Table 1). More-over, Cunninghamia possesses ray tracheids (Ga-dek et al., 2000), but these are obviously di⁄cultto observe as they are not mentioned in earlierliterature, and non-observation of ray tracheidson fossil taxodiaceous wood may not precludetheir occurrence (Basinger, 1981). In Cunningha-mia, 2^4 glyptostroboid to cupressoid pits, andapparently no taxodioid ones, are arranged inone horizontal series in the cross-¢elds but super-posed pits are also noticed, particularly in theterminal cells of the rays (Table 1). These di¡er-ences between Cunninghamia and Glyptostroboxy-lon sp. wood anatomy are di⁄cult to evaluate,more so as divergence of opinion still exists par-ticularly about the morphology of the radial wallsof the secondary xylem parenchyma cells (see Ta-ble 1). Anyway, both present-day species, C. ko-nishii and C. lanceolata, which are very similarand should perhaps be treated as varieties of asingle species (Farjon, 1998), are often multi-trunked, an observation never made at Hoegaar-den where only stumps standing singly are re-corded.If a consensus exists on the members ^ Sequoia

Endlicher, Metasequoia Hu et Cheng and Se-quoiadendron giganteum Buchholz ^ of the Sequoi-deae clade, on those of the Taxodioideae there isdebate. According to Gadek et al. (2000), theTaxodioideae clade includes Taxodium L.C. Rich-ard, Cryptomeria D. Don and Glyptostrobus,while Kusumi et al. (2000) conclude that Taxodi-um and Glyptostrobus form a clade which is in asister-group relationship to Cryptomeria.

Cryptomeria, with up to 3-seriate bordered pitson the radial walls of its tracheids, strongly nod-ular parenchyma horizontal walls, thickened hor-izontal walls of the ray cells with well discernibleindentures, is clearly di¡erent from the Hoegaar-den wood (Table 1).The Sequoideae have heterocellular rays,

although these are not common. They are consid-ered to have smooth horizontal walls in the xylemparenchyma, as opposed to the dentate ones ofthe Taxodioideae (e.g. Greguss, 1955). However,disagreement is obvious on this key distinguishingfeature (Table 1). We are not in a position todecide between these con£icting observations.Other characters are helpful. Sequoiadendron isthe only taxodiaceous secondary xylem withouta torus on the membrane of intertracheidal pitpairs. A torus is present on the Belgian wood(Plate II, 4; Plate III, 4). The size of the Sequoi-deae tracheids is also di¡erent. Tracheid size de-pends on growth rate and the vertical and lateralposition on the tree of the studied branch ortrunk material, and is thus of questionable taxo-nomic importance. Nevertheless, the maximumtangential diameter of the tracheids in the outer-most part of a stem gives a rough indicationabout stem height (Stockey et al., 2001, and refer-ences therein). The tallest known living trees pos-sess early wood tracheids with the widest tangen-tial diameter (up to 85 and 90 Wm for Sequoia andSequoiadendron ; Table 1). In Glyptostroboxylonsp. from Hoegaarden the widest measured tangen-tial diameter reaches 44 Wm only. Even if it can-not be ascertained that this dimension representsthe widest tracheids, as we know neither the agenor the stage of maturity reached by the stumps,Glyptostroboxylon from Hoegaarden neverthelessappears to belong to smaller trees than the giantSequoideae.Within the Taxodioideae, Taxodium distichum

(L.) Rich has strongly nodular horizontal wallsof secondary xylem parenchyma cells. It also pos-sesses the tallest rays (up to 60 cells high) and itstracheids are much wider than in Glyptostroboxy-lon sp., with up to 4-seriate bordered pits on theradial walls (Table 1). Similarity with secondaryxylem of Glyptostrobus pensilis (Staunton ex D.Don) K. Koch is greater. Indentures have been



reported as regularly present (Peirce, 1936), atleast in the early wood (Scho«nfeld, 1955). How-ever, according to Greguss (1955), at the point ofjunction with horizontal walls, the tangential raycell walls widen in the shape of a triangle. A sim-ilar triangular enlargement is occasionally noticedon the Belgian Glyptostroboxylon sp. Glyptostro-bus pensilis shows pitting on the horizontal wallsof ray parenchyma cells, a character which ismore dubious on Glyptostroboxylon sp. Owingto the relative tangential diameter and height ofthe ray cells, early wood cross-¢elds are similar inoutline in both fossil and extant woods. Borderedpits on the radial walls of early wood tracheidsare of similar diameter to the lower range of taxo-diaceous secondary xylem pit dimension (Table1). The transverse walls of the vertical wood pa-renchyma cells of G. pensilis are, once more, var-iously described as simply pitted or with incon-spicuous thickenings or as distinctively nodularbut with small nodules (Table 1). Anyway, thedi¡erence between Glyptostroboxylon sp. fromHoegaarden and extant G. pensilis wood is nar-row, the most obvious di¡erence being the usuallyhigher number of pits/cross-¢eld and their nar-rower diameter in the modern species. The impor-tance of these discrepancies in relation to preser-vation bias, nature of environmental signature,physiology and development of the source treesis presently impossible to assess.The a⁄nities of the Hoegaarden wood to some

taxa with roughly comparable secondary xylem(Table 1) were also estimated through a cladisticanalysis. The analysis was based on 10 taxa and13 characters, listed in Table 1. Some charactershave been arbitrarily coded such as the tracheidtangential diameter, coded 0 for diameters under60 Wm and 1 for larger diameters (see coding foreach character, columns marked C in Tables 1and 2). The Branch-and-Bound routine ofPAUP (version 3.1.1; Swo¡ord, 1991) generatedone single tree of 21 steps, with consistency indexof 0.667. This phenogram (Fig. 5) con¢rms ourmore intuitive analysis, as the Hoegaarden woodappears to share the closest similarities with Glyp-tostroboxylon, and to a lesser extent with Taxodi-um and Taxodioxylon. Furthermore, the analysisplaces Glyptostrobus in a sister-group relationship

with a complex comprising Taxodium, Taxodi-oxylon, Glyptostroboxylon, and the Hoegaardenwood.In summary, the secondary wood from Hoe-

gaarden is similar in gross morphology to extantCunninghamia but clearly distinct from the mod-ern genus when considering anatomical details.Several features (more particularly the variabilityin the cross-¢eld pit type) are shared with extantSequoia wood, but Belgian Glyptostroboxylon sp.appears most similar, although not identical, toliving Glyptostrobus secondary wood.The taxodiaceous conifers have a very long fos-

sil history, originating during the Triassic (Yao etal., 1997) and being well established by Jurassictime (Basinger, 1981; Stewart and Rothwell,1993). Evidence for Cunninghamia-type fossils oc-curs as early as the Middle Jurassic with severalgenera of permineralised seed cones, some woodpieces and compressed vegetative and fertile re-mains (Yao et al., 1998, and references therein).The earliest fossil identi¢ed as Cunninghamiadates back to the Hauterivian^Barremian (middleEarly Cretaceous) of Inner Mongolia (Kester,2000) but fossil cones most similar to those ofCunninghamia are known to occur in the LowerCretaceous beds of California (Miller, 1974). Onthe basis of parsimony analysis of nucleotide se-quence data from the plastid genes matK andrbcL, together with non-molecular data consider-ations (Gadek et al., 2000) as well as of nucleotidesequences from three additional chloroplastsgenes (Kusumi et al., 2000), Cunninghamia wasthe ¢rst indeed to separate within the taxodia-ceous lineage. The order of divergence of bothsequoioid and taxodioid clades, although re-solved, receives less support, but apparently theyboth diverged after separation of Cunninghamia,Taiwania and Athrotaxis (Gadek et al., 2000;Kusumi et al., 2000). A glyptostroboid typeof wood is already recognised by the UpperJurassic in East Africa (Su«ss and Schultka,2001). Taxodioxylon gypsaceum is present in theCretaceous (Meijer, 2000). Occurrence of second-ary xylem, the overlapping characteristics ofwhich appear to be shared by both Taxodioxylonand Glyptostroboxylon in the very early Eoceneonce more points to the structural unity of this



taxodioid type of wood, while seeds belonging tothree modern genera, Glyptostrobus, Sequoia andTaxodium, are already recognised in Middle Euro-pean Cenomanian strata (Knobloch and Mai,1986).

11. Taphonomic considerations

Examples of stumps transported in upright po-sition in mud£ows (e.g. in those generated by theMount Saint Helen’s eruption; Karowe and Jef-ferson, 1987) are well known. As is the case forthe Hoegaarden stumps, they are less than 2 mhigh and possess a wide base with no or very fewroots remaining. A matrix di¡erent from the sur-rounding sediment is often noticed in their root-balls, a character by which they di¡er from theHoegaarden stumps (Chapman, 1994, and refer-ences therein). At Hoegaarden the stumps are 1^3m apart, and, as already noticed in the nearbyOverlaar area, distributed in a similar way as insome modern virgin conifer forests. Clearly, thedensity of the stumps is a good argument for theirin situ position. The large geographical extent ofthe lignitic layer on which the stumps are to befound precludes the hypothesis of small rafts offorest £oor with standing trees having been trans-ported several times (Karowe and Je¡erson,1987). In addition, occurrence of roots reachingthe underlying sandstone reinforces the argument.No roots are preserved between the stump basesand the massive sandstone in which undetermin-able root casts only rarely are observed. The al-teration of wood is greatly in£uenced by the em-bedding sediment and its permeability (Weibel,1996), and all the original organic content includ-ing roots have been leached out of the permeablewhite sands, a process most probably resultingfrom variations in the water table level and inits physical and chemical characteristics. As aconsequence also of considerable variations inthe water table levels, accumulated dead organicmaterial in the peat decayed while the stumps,sitting on poorly permeable lignitic clay, were per-mineralised by silica-rich water percolatingthrough the overlying sandy deposits, silica pre-cipitation during petrifaction being templated by

the organic molecules of the dead wood tissues(Martin, 1999).

12. Palaeoenvironmental considerations

At Hoegaarden the bases of the stumps areembedded in a lignite layer (unit 5; Figs. 2 and3). The Hoegaarden trees were lignite buildersconsistently colonising lowland habitats and livingin a swampy but nevertheless irregularly waterstressed environment. Water stresses, consequenceeither of low or high water levels, caused the de-velopment of false growth rings in addition toregular, seasonally induced ones.Present-day Taxodioideae are often found in

swampy environments, particularly both decidu-ous swamp cypresses, Glyptostrobus pensilis andTaxodium distichium. These peat builders sharesimilar ecological requirements. One of the re-maining populations of the globally threatenedG. pensilis, with more than 200 individuals, is tobe found in a swampy area, in the Da Lak prov-ince at about 500 m altitude in the central high-lands of Vietnam (Source book of Existing andProposed Protected Areas in Vietnam, 2001).Glyptostrobus pensilis is scattered also in southernChina and occurs to as far as 13‡N in Vietnam,attesting to its wide amplitude of ecological toler-ance (Li, 1953). It thrives in damp places usuallyat low elevation, on river banks and next to in-undated rice ¢elds. In the eastern US Taxodiumdistichum is widespread in swamps and on water-logged lakeshores where it usually occurs in purestands but with sparse understory angiospermtrees on drier sites. Such a Taxodium consociationcould provide a likely modern analogue for theHoegaarden fossil forest in which the spacing ofthe taxodiaceous stumps is similar as in somemodern intact swamp forests.On the other hand, extant Sequoia sempervirens

(D. Don) Endl is often assumed to be the nearestliving relative to Taxodioxylon gypsaceum (vander Burgh and Meijer, 1996, and references there-in), which also shares characteristics with theHoegaarden Glyptostroboxylon sp. Such an as-sumption is questionable, independent from thedi⁄culties raised by the variability of this taxodia-



ceous wood. The coast redwoods which includethe tallest known trees are now restricted to thefoggy coastal belt of northern California. Theyare not peat builders, as opposed to Taxodioxylongypsaceum which is commonly found in EuropeanTertiary lignite. A hypothetical change in ecolog-ical requirements of S. sempervirens (Jurasky,1936, in Mai, 1995) through time does not pro-vide an answer to the disagreement. Obviouslywithin the fossil taxodioid complex of secondarywood, anatomical information is on itself insu⁄-cient for deduction of precise taxonomic a⁄nitieswith a modern species. This is particularly true forthe Hoegaarden wood.

13. Other macrofossil remains from the TienenFormation

Nearby coeval fossil localities may providehelpful additional macrofossil remains to recon-struct the plant landscape, as more than 30 fossillocalities are known from the Tienen Formationin strata encompassing the Palaeocene/Eocenetransition in Belgium. Yet, most of them, withthe exception of Dormaal, Trieu de Leval andHoegaarden, are known only from small outcropsor boreholes. The Dormaal Sand Member is fa-mous for the richness and diversity of the verte-brate assemblage it yields. It is located at the baseof the Tienen Formation (Steurbaut et al., 1999),and is slightly older than the Hoegaarden ligniticbed. Silici¢ed wood of Populoxylon Ma«del^Ange-liewa (Doutrelepont et al., 1997) plus angiospermfruits and seeds are present at Dormaal, but nogymnosperm remains are recorded. The Dormaalplant assemblage is characterised by the abun-dance of £eshy fruits, drupes and berries pro-duced by climbing plants, especially woody lianas(Fairon-Demaret and Smith, 2002) that areknown to play an important role in forest typesin a warm moist climate (Morley, 2000). Accord-ing to palynological analyses of the DormaalSands, the assemblage that also yields a varietyof fern spores is dominated by Juglandaceae,and includes Myricaceae, Restionaceae, Pinaceae,and two taxa of Normapolles (Steurbaut et al.,1999). The correlative Trieu de Leval locality in

the Mons Basin yields a diversi¢ed leaf assem-blage with a paratropical signature according toMarty (1907). In addition, numerous charred sec-ondary xylem remains have been recently col-lected. Apart from angiosperm secondary xylemremains, several pieces of Glyptostroboxylon sp.identical to the Hoegaarden taxon have beenidenti¢ed. The embedding sediment yields a di-verse palyno£ora (Petricevic, 2000) with, amongstother taxa, many Inarperturopollenites hiatus (Po-tonie¤) Thompson et P£ug, the mother plant ofwhich is Taxodium or/and Glyptostrobus. Hup-paye, located along the Namur^Tienen railwaytrack about 16 km from Tienen, is another fossillocality in the Tienen Formation. Its positionwithin the Formation is not well constrained,but its age appears similar to Hoegaarden. Theplant remains of Huppaye studied by Gilkinet(1925) are preserved in a very hard whitish mi-crite. The assemblage is diverse, yielding a varietyof fern pinnules with a majority of Lygodium re-mains, several types of eudicot and palm leaves,well preserved internal and external moulds ofEricaceae capsules and many gymnosperm re-mains. Taxodiaceous twig fragments and conescalled ‘Athrotaxis (Sequoia) couttsiae’ by Gilkinet(1925) are especially numerous; they occur as 3-Dempty casts without any organic matter pre-served. This association of remains trapped inthe sandy sediment illustrates a rich and diverse£ora with a wet, warm signature.Palynological remains, gymnosperm leaf litter

and roots have been destroyed at Hoegaardenby oxidative taphonomic processes and diagene-sis. It can be argued that the same might explainthe absence of angiosperm remains. Of coursegymnospermous secondary xylem is more resis-tant to decay than angiospermous wood (Lu«ckeet al., 1999; Figueiral and Mosbrugger, 2000).Lignin of the latter is typically degraded intowater-soluble products and hence is rapidly de-stroyed, whereas the gymnosperm lignin decaysinto water-insoluble products (Hatcher and Clif-ford, 1997; Kim and Singh, 2000). However, wethink that the non-occurrence of angiosperm re-mains at Hoegaarden is not to be attributed totaphonomic bias. The monospeci¢c character ofthe Hoegaarden in situ forest is linked to the spec-



i¢city of its swampy coastal alluvial £oodplainenvironment as deduced by sedimentological data.In another Glyptostroboxylon sp. locality de-

scribed in younger, Upper Eocene, sediments onthe Isle of Wight no other plant macrofossils oc-cur with the ligniti¢ed and occasionally heavilypyritised stumps. They also are thought to havegrown as monospeci¢c stands in a waterloggedhabitat, occupying swampy areas within the veg-etation of a coastal alluvial £oodplain (Fowler etal., 1973).

14. Conclusions

The fossil forest deposit of Goudberg^Overlaar,at Hoegaarden, has been known since the middleof the nineteenth century. For the ¢rst time ananatomical analysis of the silici¢ed stumps andtrunk fragments allows their attribution to a sin-gle taxon. The anatomical structure of the second-ary wood specimens shows characteristics sharedby Taxodioxylon gypsaceum and Glyptostroboxy-lon tenerum, but a⁄nities with the latter are clos-er. We chose the conservative attribution Glypto-stroboxylon sp.The long lasting controversy about the autoch-

thonous or allochthonous origin of the stumps isresolved, favouring the in situ character of thefossil forest. Such a conclusion is based on theupright position of the great majority of thestumps, their distribution in the ¢eld, and theembedding of the stump bases within a single lig-nitic layer, implying a continuum in the fossilifer-ous deposit. Thus, the fossil deposit correspondsto remains of a natural forest rather than to arafted secondary accumulation of stumps.This apparently monospeci¢c population of

taxodiaceous gymnosperms is suggestive of aswamp with oscillating water table levels not dis-similar in appearance to a present-day Bald Cy-press dominated swamp. Such an understandingof the palaeoenvironment of the Hoegaarden fos-sil forest is in agreement with sedimentologicaland stratigraphical data demonstrating that thetrees grew in £uctuating £uvio^lacustrine settings.The Hoegaarden taxodiaceous swamp forest mostprobably developed during the IETM (formerly

LPTM) at about 55 Ma, when climate was thewarmest of the entire Cenozoic.

Acknowledgements

All the team members are very grateful to Dr.Piet Laga of the Belgium Geological Survey forhis generous sharing of information and to HugoDe Potter of the IRSNB for his active help in the¢eld. We also thank Mr. M. Dejean, secre¤taire duCercle ge¤ologique du Hainaut, for his generousloan of several specimens to Professor C. Dupuis,and Professor N. Vandenberghe for providing theoriginal picture of the Goudberg Section. Specialthanks are due to Mr. J. Kerkhof and Mr. H.Mestdagh (Vlaamse Landmaatschappij) and Mr.C. Van Du¡el (TUC rail society) for logistic helpin the ¢eld. We also thank Dr. Catherine Prive¤-Gill for her advice in determining the fossil sec-ondary xylem, Dr. Philippe Steemans for his helpin preparing the manuscript, Mr. Stefaan Van Si-maeys for additional information on the palyno-morph content of some samples, and Mrs. Mar-cella Giraldo-Martin and Mr. Fernand Noebertfor their technical support during the preparationof the palynological samples and thin sections.Dr. Chris Berry kindly accepted to correct theEnglish text. We would like to thank also Dr.J.F. Basinger and Dr. I. Figueiral for their helpfulreviews of the manuscript. This work has bene-¢ted from ¢nancial support by Grant FRFC-IC2.4540.99 of the Belgian National Fund of Scien-ti¢c Research. P. Gerrienne is an FNRS ResearchAssociate.

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