Chemical characterization of Lower Devonian vascular plants

Org. Geochem. Vol. 25, No. 8, pp. 461-473, 1996

Pergamon PII: S0146-6380(96)00140-4

© 1997 Elsevier Science Ltd All rights reserved. Printed in Great Britain 0146-6380/'96 $15.00 + 0.00

Chemical characterization of Lower Devonian vascular plants G R E G EWBANK ~*, DIANNE EDWARDS 2 and G E O F F R E Y D. ABBOTT~t 'Fossil Fuels and Environmental Geochemistry (Postgraduate Institute): NRG, Drummond Building, University of Newcastle upon Tyne, Newcastle upon Tyne, NEI 7RU, U.K. and 2Department of Geology, University of Wales, College of Cardiff, P.O. Box 914, Cardiff, CF1 3YE, U.K.

(Received 13 September 1995; returned to author for revision 25 January 1996; accepted 18 October 1996) Abstract--Vegetative remains of three coalified Lower Devonian vascular plants (Zosterophyllum, Psilophyton, Renalia) were analyzed using flash pyrolysis-gas chromatography-mass spectrometry. The distributions of pyrolysis products are compared with those from younger vascular plant fossil xylem (Cordaixylon, Callixylon) and cuticle (Pachypteris). The likelihood of the chemical preservation of characteristic higher plant macromolecules (e.g. lignin and cutan) in the Lower Devonian plant fossils is considered in light of this comparison and associated thermal maturity assessments. Reflectance values from vitrinite-like macerals, which may not be vitrinite sensu stricto in the Lower Devonian host rocks for the fossils selected for this study, are shown to provide a reasonable assessment of the thermal maturity of these early vascular plant fossils. Although lignin altered through burial maturation is the most likely source of the prominent alkylphenols and aromatic hydrocarbons in the Lower Devonian tracheophyte flash pyrolysates, a contribution from thermally modified tannins cannot be ruled out. Comparison of the highly aliphatic pyrolysates from the Zosterophyllum and Psilophyton axes with that of a thermally mature fossil gymnosperm leaf revealed that cutan was an important component in the Devonian plant remains. This is the earliest chemical evidence for the presence of cutan in vascular plants. © 1997 Elsevier Science Ltd

Key words--Lower Devonian plants, vascular plant evolution, fossil wood, fossil leaves, lignin, cutan, flash pyrolysis

INTRODUCTION

The appearance of vascular plants in the fossil record has been considered virtually synonymous with the advent of phytoterrestrialization (Gray and Boucot, 1977; Edwards et aL, 1978), tracheophytes being regarded as unequivocal land plants. However, terrestrialization is now perceived as a more gradual process, consisting of overlapping phases of progressively more complex photosynthesizing organisms (Edwards and Selden, 1993; Gray, 1993). Pioneering phases of phytoterrestrialization culminated with the appearance of vascular plants in the Silurian and their major diversification in the Devonian. The elucidation of lineages involved in the various phases of land plant evolution remains contentious, with most advances being made for the tracheophytes. By the late 1960s, four major lineages (rhyniopsids, zosterophyllopsids, trimerophytopsids and lycopsids sensu law) had been distinguished within the Devonian fossils on the basis *Present address: EC6S Environmental Ltd., Unit 4,

Hebble Brook Industrial Estate, Hays Lane, Halifax HX2 8UL, U.K. tAuthor to whom all correspondence should be addressed. OG 25/8--8

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of morphology. Of these four groups, only the lycopsids have extant counterparts (Banks, 1968). The earliest tracheophytes were the rhyniopsids [e.g. Cooksonia, Rhynia, Renalia, see Fig. l(a)], with slender dichotomously branched, leafless axes and terminal sporangia. The zosterophyllopsids possessed creeping and erect axes, with sporangia attached laterally to the axes, the axes themselves being either smooth or with spines or teeth [e.g. Sawdonia, Crenaticaulis, Zosterophyllum, see Fig. l(b)]. The zosterophyllopsids may have been ancestral to the lycopsids (e.g. Baragwanathia, Asteroxylon, Leclercqia), which were characterized by the presence of abundant microphyllous leaves and sporangia borne singly on leaves or in their axils. The trimerophytopsids are thought to have descended from the rhyniopsids and were the probable ancestors of the ferns and seed plants (Banks, 1980). The trimerophytopsids had more complex three-dimensional branching systems, with needlelike, spine-like or forked emergences, and clusters of terminal sporangia on lateral branches [e.g. Pertica, Psilophyton, see Fig. l(c)]. However, Banks (1968, 1992) recognized combinations of morphological and anatomical features which were incompatible with this taxonomic scheme. Such disparity has been further emphasized with new information

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cm

J

Fig. 1. (a) Reconstruction of Renalia hueberi (redrawn from Gensel, 1976), showing part of branching axis. (b) Reconstruction of Zosterophyllum rhenanum (redrawn from Kr~iusel and Weyland, 1935). (c) Reconstruction of Psilophyton dawsonii (redrawn from Banks, 1975), showing portion of aerial axis with sterile and fertile branches. (d) Reconstruction of Archaeopteris (Callixylon) (redrawn from Beck, 1962). (e) Reconstruction of a Cordaitean plant (redrawn from Cridland, 1964). from China (e.g. Hao and Beck, 1991; Li and Edwards, 1992). The classification of early vascular plants is thus still somewhat tentative (cf. Banks, 1975, 1992; Edwards and Edwards, 1986; Kenrick and Crane, 1991), but all such tracheophytes are assumed to possess the same basic chemical adaptations necessary for the successful functioning of

the Structural innovations associated with the colonization of land. Vascular plant evolution is linked to the evolution of lignin (associated with water conduction and support) and the constituents of the cuticular matrix (desiccation inhibitors). The characteristic flash pyrolysis products of lignin are 2-methoxyphe-

Chemical characterization of vascular plants nol and 2,6-dimethoxyphenol, as well as various substituted counterparts of these two compounds (e.g. Obst, 1983). However, these methoxyphenols are absent from the flash pyrolyzates of coalified lignin having a rank of subbituminous coal. This is a result of the gradual demethylation of lignin during coalification (e.g. Hatcher et al., 1988). Other workers have suggested that the cuticular matrix of vascular plant leaves is composed of either the polyester, cutin (e.g. Holloway, 1982) or a substantial amount of the biomacromolecule termed cutan, or a mixture of both cutan and cutin (Nip et al., 1986a, 1989; Tegelaar et al., 1989). However, the preservation potential of cutin sensu stricto has to be regarded as low (Tegelaar et aL, 1991). The frequent occurrence of cutan in fossil plant cuticles indicates that it is the most resistant constituent of plant cuticles (Nip et al., 1986b, 1989). The major flash pyrolysis products from cutan include homologous series of n-alkanes, naik-l-enes and ~,to-alkadienes (e.g. Tegelaar et al., 1991; Collinson et al., 1994); however, treatment by tetramethylammonium hydroxide (TMAH) chemolysis has recently suggested a chemical structure for cutan which also contains functionalized benzene rings which are not detected using flash pyrolysisgas chromatography-mass spectrometry (McKinney et al., 1996). This paper reports a study on the chemical characterization of a selection of Lower Devonian early land plants (Renalia, Zosterophyllum, Psilophyton) using flash pyrolysis-gas chromatography-mass spectrometry. The pyrolyzates for the vegetative remains of these plants are compared with those from younger fossilized tracheophytes which contain unequivocal cuticular and lignified tissue. The objective of this comparison is to test the assumption that the early vascular plants possessed iignin and cutan. Because the effects of thermal maturation on the chemistry of the early tracheophyte fossils will be significant, values of optical and molecular maturity parameters were

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compared to assess the reliability of light reflectance values from vitrinite-like particles in the Lower Devonian fossil host rocks.

EXPERIMENTAL

Samples The fossil plant materials analyzed in this study by flash pyrolysis-gas chromatography-mass spectrometry are listed in Table 1. The three major Lower Devonian tracheophyte groups (rhyniopsids, zosterophyllopsids and trimerophytopsids) are represented by coalified axes. The younger samples include silicified progymnosperm wood and coalifled gymnosperm wood from the Carboniferous as well as coalified leaves from a Jurassic gymnosperm. Fossils preserved as coalified compressions were either picked off manually from rock surfaces or recovered after treatment of the rock matrix with hydrochloric acid and hydrofluoric acid. They were then washed with doubly distilled water and stored dry. Extraction and fractionation The powdered host rocks and a portion of the powdered fossil for which host rock was unavailable, the silicified Callixylon wood, were Soxhlet extracted with dichloromethane/methanol (93:7, v/ v) for 72 h. An aliquot of each total extract was separated by thin-layer chromatography (TLC; Kieselgel 60G) using light petroleum ether (boiling point about 40-60°C) as developer, into aliphatic hydrocarbon, aromatic hydrocarbon and NSO/resin fractions. Gas chromatography (GC)/gas mass spectrometry ( G C - M S )

chromatography-

The aliphatic and aromatic hydrocarbon fractions were analyzed using a Carlo Erba Mega Series 5160 gas chromatograph (GC) equipped with an on-col-

Table 1. Summaryof fossilplant materialsanalyzedin this study Genus

Age

Systematicposition

Locality

Pachypteris

Mid Jurassic

Gymnosperm

Cordaixylon*

Lower Carboniferous

Gymnosperm

Kinderhookian,Lower

Progymnosperm

Hill House Nab, Leaf, coalified,in mudstone Scarborough,North Yorkshire, England Orrock Quarry, Central Fife, Wood, coalified,in ash-filled Scotland volcanicpipe Amherst, OH, U.S.A. Wood, silicified

Callixylont Psilophyton~;

Carboniferous

Emsian,LowerDevonian

Zosterophyllum~ Emsian,Lower Devonian Renalia~i Emsian, Lower Devonian

Mode of preservation

Trimerophyte

North Shore, Gasp~ peninsula, Canada

Axes, coalified, in mudstone

Zosterophyllophyte

Westhall Terrace, Scotland

Rhyniophyte

North Shore, Gasp~ peninsula, Canada

Axes. coalified, in mudstone Axes, coalified, in mudstone

*Provided by Dr A. C. Raymond (see Raymond et aL, 1989). tProvided by Professor C. B. Beck. :[:Provided by Professor P. G. GenseL

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umn injector and a flame ionization detector. The carrier gas was hydrogen, and the oven temperature was held at 50°C for 2 min and then programmed at a rate of 4°Cmin -l to 300°C at which it was held for 30 min. Aliphatic hydrocarbon fractions were analyzed on a fused silica capillary column (25 m x 0.32 mm i.d.) coated with a methylsilicone bonded stationary phase (OV-1, film thickness 0.1 #m), whereas the aromatic hydrocarbon fractions were analyzed on a fused silica column ( 2 5 m x 0 . 3 2 m m i.d) coated with a 5% phenylmethylsilicone bonded stationary phase (HP-5, film thickness 0.25#m). A Multichrom (VG Data Systems, Micro VAX 2000) data handling system was used to acquire and process the GC analyses. G C - M S was performed using a Fisons 8060 GC connected to a Fisons Trio 1000 Mass Spectrometer (electron energy 70eV, mass range m/z 50-550, cycle time 1.2 s). Helium was used as the carrier gas. The GC was fitted with a fused silica capillary column (25 m x 0.2 mm i.d.) coated with a methylsilicone bonded stationary phase (HP-1, film thickness 0.11 #m) for the separation of aliphatic hydrocarbons. Aromatic hydrocarbons were separated using a fused silica capillary column ( 2 5 m × 0 . 2 m m i.d.) coated with a 5% phenylmethylsilicone bonded stationary phase (HP-5, film thickness 0.25/~m). The following oven temperature programmes were employed for the analysis of specific biological marker compounds: (aliphatic hydrocarbons) 40°C (2 min) to 175°C at 10°Cmin-1, 175-225°C at 6°Cmin -1, and finally 225-300°C (30 min) at 4°C min-l; (aromatic hydrocarbons) 40-300°C (30 min) at 4°C min-l.

ference to angular phytoclasts or those exhibiting cellular structure.

Flash pyrolysis-gas chromatography-mass trometry ( P y - G C - M S )

spec-

The homogenized fossil plant samples for PyG C - M S analysis were Soxhlet extracted with dichloromethane/methanol (93:7, v/v) for 72h. Subsequently flash pyrolysis experiments were performed on the dried residues using a coil pyrolyzer, specifically a Chemical Data Systems (CDS) 200 Pyroprobe unit fitted with a platinum coil probe, directly connected to the injector of a Hewlett Packard 5890A gas chromatograph (GC). Samples (0.5 10 mg) were placed in a quartz tube plugged with Soxhlet extracted silica wool. Pyrolysis was performed at 610°C for 20 s. The pyrolysis products were separated using a fused silica capillary column ( 2 5 m × 0 . 2 m m i.d.) coated with a 5% phenylmethylsilicone bonded stationary phase (HP-5, 0.11/~m film thickness). The oven temperature was programmed from 0°C (2min; CO2 cooled) to 300°C (30 min) at 3°C min-l. Helium was used as the carrier gas. The GC was directly coupled to a Hewlett Packard 5970 Series mass selective detector. The spectrometer was operated in the electron impact (El) mode at 70 eV with a mass range m/z 50-550 and a cycle time of 1.0 s. Pyrolysis products were identified by comparison of their mass spectra and relative retention times with those of compounds reported in the literature (e.g. Hatcher et al., 1988; van Bergen et al., 1994a). RESULTS AND DISCUSSION

Thermal maturity Petrographic analyses Polished particulate blocks were made from samples of the host rock matrix (with the exception of that containing fossil plant material from Amherst, OH, which was unavailable). Mean maximum vitrinite reflectances (abbreviated to "Ro") were then measured from the blocks using a Zeiss standard reflected light microscope with an oil immersion objective (x40), a glass plate reflector as a vertical illuminator and an E.M.I. 98448 1l-stage Venetian blind-type photomultiplier with an aperture of 5/~m. The reflectances were measured in Zeiss immersion oil (518C, refractive index 1.518 at 23°C) at 546 nm and compared with two standards: sapphire (0.590% Ro) and yttrium aluminium garnet (0.917% Ro). Reflectance measurements were made on elongate wisps or stringers of the lowest grey material (often in close association with framboidal pyrite and dark bitumen staining). As the host rocks of the early tracheophyte fossils were Lower Devonian in age, this lowest grey material was taken to be the maceral closest in composition to primary vitrinite sensu lato and measured in pre-

The degree of reflectance of vitrinite macerals is a property used to assess the thermal maturity of kerogens as well as the rank of coals (Teichmiiller and Teichm~iller, 1968; Bostick, 1971). It may be argued that reflectance values from vitrinite-like macerals in Devonian host rocks (i.e. from North Shore, Gasp6 peninsula, Canada or Westhall Terrace, Scotland) cannot be related to Ro values in younger samples (i.e. Cordaixylon, Orrock Quarry, Scotland, Carboniferous; host rock of Pachypteris, Hill House Nab, England, Jurassic) because the observed particulate organic matter from the Devonian may not be vitrinite sensu stricto. Molecular maturity parameters, believed to be independent of geological age, are also used as a measure of thermal maturity (e.g. Mackenzie, 1984). Calibration of optical with molecular maturity parameters derived from the host rocks is an important exercise prior to interpretation of the PyG C - M S analyses of the fossils, given that characteristic higher plant macromolecules, such as lignin, undergo changes in chemical composition arising from the temperature rise associated with thermal

Chemical characterization of vascular plants

465

Table 2. Summary of samples used for correlation of thermal maturity measurements shown in Fig. 2 Sample

Age

Locality

Gymnosperm (conifer) wood, in mudstone Mudstone

Lower Cretaceous Lower Cretaceous

Hanover Point, Isle of Wight, England Hanover Point, Isle of Wight, England

Gymnosperm (cycad) frond, in mudstone Gymnosperm (bennettitalean) frond, in mudstone lncertae sedis wood, in mudstone Mudstone Incertae sedis wood, in mudstone

Mid Jurassic Mid Jurassic Mid Jurassic Mid Jurassic Mid Jurassic

Mudstone* Mudstone

Mid Jurassic

Cayton Bay, Scarborough, North Yorkshire, England Cayton Bay, Scarborough, North Yorkshire, England Cayton Bay, Scarborough, North Yorkshire, England Cayton Bay, Scarborough, North Yorkshire, England Hill House Nab, Scarborough, North Yorkshire, England Hill House Nab, Scarborough, North Yorkshire, England Hasty Bank, Scarborough, North Yorkshire, England

Gymnosperm (cordaitean) wood*

Lower Carboniferous

Orrock Quarry, Central Fife, Scotland

Mudstone Sandstone Calcareous mudstone* Mudstone* Mudstone Limestone

Lochkovian, Lower Devonian Lochkovian, Lower Devonian Lochkovian, Lower Devonian Emsian, Lower Devonian Emsian, Lower Devonian Lower Devonian

Hudwick Dingle, Welsh Borderland, England Devil's Hole, Welsh Borderland, England Westhall Terrace, Scotland North Shore, Gasp6 peninsula, Canada Ballunacator Farm, Scotland Prague Basin, Czech Republic

Mid Jurassic

*Maturity data derived from these samples is also presented in Table 3. m a t u r a t i o n (e.g. H a t c h e r et al., 1988). Both vitrinite reflectance and molecular parameters were m e a s u r e d o n a selection (see Table 2) o f coalified woods, f r o n d remains a n d rocks o f varying ages (Lower D e v o n i a n to L o w e r Cretaceous) including the fossil host rocks. T h e values o f these p a r a m e t e r s were c o m p a r e d to test the reliability o f reflectances determined from vitrinite-like macerals present in the Lower D e v o n i a n host rocks as a measure of the t h e r m a l m a t u r i t y of these samples (Fig. 2). M a t u r i t y - d e p e n d e n t molecular p a r a m e t e r s were evaluated from peak areas within the G C - M S distributions o f the following c o m p o u n d s : steranes a n d t r i a r o m a t i c steroids (e.g. Mackenzie, 1984); tricyclic terpanes a n d h o p a n e s (e.g. Seifert a n d M o l d o w a n , 1980); m e t h y l p h e n a n t h r e n e s (e.g. K v a l h e i m et al., 1987); a n d m e t h y l d i b e n z o t h i o p h e n e s (e.g. R a d k e et al., 1986). T h e host rocks for the Lower D e v o n i a n fossils c o n t a i n e d vitrinite-like macerals with reflectances greater t h a n 0.7% (Table 3). E n d points

0.75

were observed at Ro ~ 0.6% for p a r a m e t e r s based u p o n steranes, tricyclic terpanes a n d hopanes, limiting their use in this exercise. However, p a r a m e t e r s based u p o n the relative a b u n d a n c e s o f methylphen a n t h r e n e s a n d m e t h y l d i b e n z o t h i o p h e n e s show a greater d y n a m i c range. A good correlation between these molecular p a r a m e t e r s a n d Ro is achieved, with b o t h showing a systematic increase with increasing Ro u p to a reflectance value o f a b o u t 1.2% (Fig. 2). Therefore, within the limitations o f this sample set a n d for the purposes o f this paper, the reflectance values o b t a i n e d from vitrinite-like particles in these L o w e r D e v o n i a n host rocks provide a reasonable assessment of their t h e r m a l maturity. M e a s u r e m e n t s o f Ro a n d the molecular parameters for the fossil host rocks are collated in Table 3. M o l e c u l a r m a t u r i t y p a r a m e t e r s for the fossil sample where vitrinite reflectance could n o t be m e a s u r e d (Amherst, OH), due to unavailability o f the host rock, are also presented in Table 3. The

3.75 3.25

0.65

2.75

0.55

2.25 0.45 0.35 0.25 0.2

1.75 * I 0.4

1.25 i 0.6

I 0.8

I 1

I 1.2

0.75 0.2

A

t I

I

I

I

I

0.4

0.6

0.8

1

1.2

!io Fig. 2. Correlation of MPDF and MDR with vitrinite reflectance for samples listed in Table 2. Key: (e) Lower Devonian; (1) Carboniferous; (0) Middle Jurassic; (&) Lower Cretaceous. For abbreviations of molecular parameters see Table 3.

G r e g E w b a n k et al.

466

Table 3. Measurements of thermal maturity parameters for the sampling sites of the fossil plant material listed in Table 1. Ro is the mean reflectance of vitrinite under oil

Locality

eC29~tfl/ Ro (%) ~C29ctctc¢S/ bC20/ cC23Tri/ C23Tri dTs/Tm ctfl + fl~ fc31~ S +~RS/ gc320tfl S + RS/ S + R C20 + C28 +C30ctflH

hMPDF

iMD R

Hill House Nab, North Yorkshire, England Orrock Quarry, Scotland Amherst, OH, U.S.A.

0.57

0.51

0.16

0.02

0.05

0.94

0.56

0.60

0.42

1.86

0.39 n.d.

0.16 0.26

n.d. 0.11

n.d. 0.47

0.21 0.43

0.50 0.90

0.37 0.48

0.30 0.57

0.32 0.46

n.d. 1.51

North Shore, Gasp~, Canada Westhall Terrace, Scotland

1.03

0.45

0.96

0.95

1.07

0.91

0.52

0.59

0.57

2.71

0.78

0.52

0.84

0.61

0.67

0.81

0.52

0.58

0.48

1.73

aC29~tct~tS/S + R, 5~t(H),I4~(H),I7ct(H) 24-ethylcholestane 20S/20S + 20R (from m/z 217 mass chromatogram); bC2o/C2o + C2s,C20 triaromatic steroid/C2o triaromatic steroid + C28 triaromatic steroids (20S + 20R) (from m/z 231 mass chromatogram); cC23Tri/C23Tri + C300tflH,C23 tricyclic terpane/C23 tricyclic terpane + 17ct(H),21fl(H) hopane (from m/z 191 mass chromatogram); aTs/Tm, 18et(H)-22,29,30-trisnorneohopane/17~(H)-22,29,30-trisnorhopane(from m/z 191 mass chromatogram); ec 29otfl/o~fl + fl~,17ct(H),21fl(H) 30-norhopane/17ot(H),21fl(H)- 30-norhopane + 17fl(H),21~(H)-30-normoretane (from m/ z 191 mass chromatogram); fCalctflS/S + R,17~t(H),21fl(H)-homohopane 22S/22S + 22R (from m/z 191 mass chromatogram); gC32~tflS/S + R,17~(H),21fl(H)-bishomohopane 22S/22S + 22R (from m/z 191 mass chromatogram); hMPDF, Methylphenanthrene (MP) distribution fraction: 2-MP + 3-MP/2-MP + 3-MP + I-MP + 9-MP (from m/z 192 mass chromatogram); iMDR, Methyldibenzothiophene (MDBT) ratio: 4-MDBT/I-MDBT (from m/z 198 mass chromatogram); n.d.--* not determined.

host sediments of the Lower Devonian fossils are more mature than those of the Hill House Nab Pachypteris leaves (Jurassic), although the C29ctctct S/S + R sterane as well as the C31 and C32 ctfl S/ S + R hopane maturity parameters have reached their end points in the Jurassic samples. The Orrock Quarry and Amherst samples (Carboniferous) are relatively immature.

Recognition of markers for cuticle and xylem in unequivocal vascular plant fossils A series of n-alk-l-enes and n-alkanes dominates the partial TIC for the flash pyrolyzate from the fossil leaves of the mid-Jurassic gymnosperm Pachypteris [Fig. 3(c)]. These range from C8 to C28 with a maximum at C14. Co Cl alkylphenols and C1-C2 aikylbenzenes are also present in this trace. Other workers have suggested that homologous series of n-alk-l-ene and n-alkane pyrolysis products from fossil leaf cuticle remains are evidence for the presence of the biomacromolecule cutan (e.g. Tegelaar et al., 1991). Co-C1 alkylphenols may originate from lignin structural units that have been modified by loss of methoxyl groups (Hatcher et al., 1988) and/or from thermally altered tannins (see later). The main flash pyrolysis products from the Carboniferous gymnosperm wood Cordaixylon [the generic name now used for anatomically preserved stems of the extinct Cordaitales gymnosperm group, reconstruction of plant presented in Fig. l(e)] are the C0-Cl alkylphenols and the Co-CI alkylcatechols [Fig. 3(a)]. Although the wood is of immature rank (Ro = 0.39%, see Table 3), its pyrolyzate does not contain any peaks related to guaiacyl units which are characteristic of lignin biosynthesized by

gymnosperms (Saiz-Jimenez and de Leeuw, 1986). Hatcher et al. (1988) have shown that during the early coalification of gymnosperm wood to a rank of lignite there is a progressive loss of methoxyl carbon resulting in increasing proportions of catecholbased products in the lignitic gymnosperm flash pyrolyzate. Assuming that the sclerotic tissue present in this fossil material contained lignin similar in composition to that of modern gymnosperms, the presence of catechoMike structures in the pyrolyzate is consistent with the demethylation of methoxy groups, rather than demethoxylation, during the early coalification process (Hatcher et al., 1988). Decreases in methoxyl content, with the subsequent formation of catechol units, also occur in sweetgum lignin (Kirk and Adler, 1970) and spruce wood (Kirk, 1975) when decayed by brown-rot fungi. In addition, several soft- and white-rot fungi have been shown to degrade phenols and dehydropolymers of t4C-labelled coniferyl alcohol (as well as plant material) at the methoxyl group, the aromatic ring and the propyl side chain (Haider and Trojanowski, 1975). Raymond et al. (1989) reported evidence for microbial degradation, attributing some of it to soft-rot fungi, in this particular Cordaixylon wood sample. Hence, the absence of methoxyphenols and the dominance of alkylcatechols in its flash pyrolyzate may be partly a result of fungal degradation prior to burial. A simple distribution of low-intensity alkylbenzene and alkylphenol peaks characterizes the TIC of the flash pyrolyzate of the silicified Callixylon sample [Fig. 3(b)]. It is well established that Callixylon is the trunk of the frond genus Archaeopteris and that the whole plant, based on morphological studies, had a gymnosperm type of

Chemical characterization of vascular plants secondary wood and pteridophytic reproduction [Beck, 1962; reconstruction of plant shown in Fig. l(d)]. If the progymnosperm lignin contained guaiacols characteristic of modern gymnosperms, then the absence of both guaiacols and catechols from this flash pyrolyzate may be a result of their modification through diagenesis (including fungal degradation), thermal maturation and silicification.

467

Hence, the alkylbenzene and alkylphenol pyrolysis products may originate from lignin structural units. The molecular and optical parameters presented in Table 3 indicate that its maturity is equivalent to that of a sub-bituminous coal. Other workers have shown that coalification of xylem through to lignite and finally to the rank of sub-bituminous coal results in the ultimate disappearance of methoxy-

(a)

od

!

20

RetenUon tlme (mlnl

80

100

(b)

6

q

-I-

I

2O

Retention time (rain)

Fig. 3(a,b).

8~)

468

Greg Ewbank et al. 14

(c)

? 28

4,._ 20

Retention time (min)

8'0

Fig. 3. (a) Partial trace for the TIC of the Cordaixylon (Orrock Quarry, Scotland) flash pyrolyzate. (b) Partial trace for the TIC of the Callixylon (Amherst, OH) flash pyrolyzate. (c) Partial trace for the TIC of the Pachypteris (Hill House Nab, northern England) flash pyrolyzate. Numbers indicate total number of carbon atoms for n-alk-l-enes and n-alkanes. phenol and catechol-like flash pyrolysis products with a concomitant increase in the amounts of alkylphenols (Hatcher et al., 1988). A simple distribution of C2 and C3 alkylphenols from the pyrolysis ?.+

of fossil lignified plant material has been recorded before (Hatcher, 1990). The low intensity of the TIC shown in Fig. 3(b) is probably a result of the replacement of the majority of the fossil plant

?,

17 19

? I

2021 24 2526

Retention time (mln)

I

80

Fig. 4. Partial trace for the TIC of the Psilophyton (Gasp6, Canada) flash pyrolyzate. Numbers indicate total number of carbon atoms for n-alk-l-enes and n-alkanes.

Chemical characterization of vascular plants

469

; 16 17 18

19 20

21 2223

0 I

I 20

Retention •ne (rain)

I

8O

Fig. 5. Partial trace for the TIC of the Zosterophyllum (Westhall Terrace, Scotland) flash pyrolyzate. Numbers indicate total number of carbon atoms for n-alk-l-enes and n-alkanes. organic carbon with silicon. The nature and mechanism of such a transformation are, however, not clear.

Characterization of Lower Devonian vascular plant fossils The partial TIC trace of the pyrolyzate from the trimerophytopsid Psilophyton [Fig. 4, reconstruction of plant shown in Fig. l(c)] shows a series of n-alkl-enes and n-alkanes, with a maximum at C10 through to CI4 , ranging from C8 to C27. C 0 - C l alkylphenols, CI-C3 alkylbenzenes and C0-Cl alkylnaphthalenes are also prominent pyrolysis products in this trace. These aromatic components may arise from altered iignin. There is also the possibility of a contribution from tannins (see later). The flash pyrolyzate of the zosterophyllopsid Zosterophyllum [Fig. 5, reconstruction of plant shown in Fig. l(b)] is also dominated by a series of n-alk-l-ene/n-alkane doublets extending up to C23. The most intense components are the Cl0 through to C14 homologues. The distribution of the acyclic aliphatic pyrolysis products for both the Psilophyton and the Zosterophyllum is similar to that of the Pachypteris leaves, whose pyrolyzate was attributed to cutan. There are, however, other possible sources for the n-alk-l-ene/n-alkane doublets in these flash pyrolyzates, namely suberan (Tegelaar et al., 1995) and the aliphatic component of sporopollenin (e.g. Guilford et al., 1988; van Bergen et al., !993). The latter is least likely as care was taken to select only sterile axes for analysis. The biopolyester suberin (a

component of cork cells) occurs in very small amounts in the endodermis of vascular plants and more extensively in periderm ("bark"). Suberan is an insoluble non-hydrolyzable polymethylenic type macromolecule which occurs in suberized plant tissue (de Leeuw and Largeau, 1993; Tegelaar et al., 1995). There is, as yet, no anatomical evidence for an endodermis in the two taxa sampled (Raven, 1994). However, localized areas of periderm, in which radially aligned "thin-walled cork cells (phellem) are clearly distinguishable" (Banks, 1981) have been recorded in calcium carbonate permineralizations of Psilophyton dawsonii from the Gasp& These are thought to have developed in response to wounding. Such injuries are rare, extend over very small areas of axes and have not yet been recorded in coalified material. Conventional "natural" periderm, i.e. completely surrounding the stem, is not observed in these particular Lower Devonian fossils. The first record of such a peridermis is in the Givetian (Middle Devonian) (Banks, 1981). The Devonian samples comprised coalified axes, in which the encasing cuticle could be identified optically. It is more than likely that the source of the nalk-l-ene/n-alkane doublets in the Lower Devonian plant pyrolyzates is cutan. Previously, the oldest fossil plant materials that have been demonstrated to contain cutan have been Upper Carboniferous pteridosperm cuticles (van Bergen et al., 1994b). Thus the pyrolyzates of Psilophyton and Zosterophyllum provide the earliest evidence for the biomacromolecule cutan in vascular plants. The alkylbenzenes and alkylphenols are less abundant

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I

2O

Retention time (mln)

I

8O

Fig. 6. Partial trace for the TIC of the Renal& (Gasp& Canada) flash pyrolyzate. relative to the n-alk-l-ene/n-alkane doublets in the Zosterophyllum pyrolyzate when compared with those in the Psilophyton pyrolyzate. This mixture of aliphatic and aromatic products from both samples is consistent with the nature of the fossils, where a cuticular envelope encloses the remains of both parenchymatous and sclerified tissues. Coalified Renalia, of putative rhyniopsid affinity, yielded solely aromatic pyrolysis products including C0-C1 alkylphenols and polycyclic aromatic compounds, namely Co C1 alkylnaphthalenes, fluorene, phenanthrene, anthracene and fluoranthene [Fig. 6, reconstruction of plant shown in Fig. l(a)]. Polycyclic aromatic compounds are often detected in the flash pyrolyzates of coals and some of their constituent macerals (e.g. Nip et al., 1992). Such compounds have been observed during the thermal desorption of inertinites (Crelling et al., 1994). Woody tissues that have been modified by palaeofires are one possible source for these compounds (e.g. Killops and Massaud, 1992). However, there is no direct optical evidence for combustion in this sample. Another suggestion for the origin of some alkylnaphthalenes and alkylphenanthrenes is the thermal breakdown of gymnosperm resinites (van Aarssen and de Leeuw, 1992). Obviously this latter source is impossible given that gymnosperms had not evolved by the Lower Devonian. The absence of significant aliphatic pyrolysis products from the Renalia is somewhat puzzling in that transmitted light microscopy indicated the presence of cuticle in this particular Gasp6 fossil. If lignin was an important structural unit of the xylem in these Lower Devonian tracheophytes then,

by definition, they would be expected to have possessed a significant methoxyphenol content. However, given the thermal maturity of the samples, it is to be expected that the characteristic methoxyphenol markers are absent from their flash pyrolyzates. Therefore the prominence of alkylphenols in the Lower Devonian tracheophyte pyrolyzates may represent a progressive degradation of the methoxyphenol groups through to catechol-like units, and eventually forming carbon-substituted phenols in the same way that lignin in younger gymnosperms and angiosperms is modified upon coalification (Saiz-Jimenez and de Leeuw, 1986; Senftle et al., 1986; Hatcher et al., 1988, 1989; Oygard et al., 1988; Hatcher, 1990). Whilst there is no doubt, from anatomical data, that these Lower Devonian early land plants were vascular in nature, the presence of phenolic moieties in their flash pyrolyzates does not necessarily provide unequivocal evidence for the lignin they are presumed to have contained. Alternative sources for alkylphenol pyrolysis products

Alkylphenols and alkylbenzenes may also be derived from sporopollenin (e.g. Schenck et al., 1981), as it has been shown that sporopollenin, as well as consisting of building units which are aliphatic in nature (Guilford et al., 1988; van Bergen et al., 1993), can also have a component composed of oxygenated aromatics (Schulze Osthoff and Wierman, 1987). These may undergo changes upon coalification, similar to that of lignin, and may ultimately yield alkylbenzenes and alkylphenols upon pyrolysis (Nip et al., 1992). However, sporopollenin

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Chemical characterization of vascular plants was excluded by careful isolation of only vegetative remains for the samples. The phenolic chemistry of bryophytes is well known (e.g. Harborne, 1992). These sources, however, can also be excluded because anatomical observations clearly show that these Lower Devonian fossils were vascular land plants (including Renalia). Another potential source for the alkylphenols, observed in all of the flash pyrolyzates described in this paper, are the tannins which are also derivatives of oxygenated aromatics. There are three main groups: phlorotannins, hydrolyzable tannins, and condensed tannins (proanthocyanidin polymers). Phlorotannins consist of phloroglucinol units linked via diaryl ether bridges--the conventional wisdom is that these are restricted to brown algae (Ragan and Glombitza, 1986). An algal source can be discounted on the basis of the unequivocal vascular nature of the fossils described in this paper. Hydrolyzable tannins comprise a polyhydroxy monomeric unit esterified with a varying number of gallic acids, gallotannins and/or hexahydroxydiphenic acids (de Leeuw and Largeau, 1993 and references therein). Proanthocyanidin polymers are composed of chains of polyhydroxy-flavan-3-ol units linked via C-C bonds. A great variety of proanthocyanidin structures is observed, arising from hydroxylation patterns, stereochemistry, location of interflavan bonds and branching points, and the degree of polymerization (de Leeuw and Largeau, 1993). Proanthocyanidins, while less common than lignins, are widespread amongst gymnosperms and woody angiosperms (Stafford, 1988), although are rare or lacking in non-woody aquatic and herbaceous angiosperms (de Leeuw and Largeau, 1993). Tannins have been detected in fossil barks isolated from brown coals (Wilson and Hatcher, 1988), and have been considered as a major source of the maceral vitrinite, in addition to lignin and cellulose (e.g. Given, 1984). The chemical stability of proanthocyanidins to drastic hydrolysis conditions suggests that a high preservation potential could be anticipated, especially for those based on procyanidin and propelargonidin units (de Leeuw and Largeau, 1993 and references therein). The thermal effects of burial maturation on tannin chemistry in coals of rank greater than brown coal are, however, unknown. Although geothermally altered lignin is the most likely source of the alkylphenols detected in the fossil plant pyrolyzates described above, a contribution from tannins modified by burial maturation cannot be excluded.

CONCLUSIONS Reflectance values from vitrinite-like macerals in the Lower Devonian host rocks can provide a reasonable assessment of the thermal stress experienced by these particular samples. The earliest evi-

dence for the biomacromolecule termed cutan is provided by the aliphatic pyrolysis products for both the Psilophyton and the Zosterophyllum. Although lignin altered through burial maturation is the most likely source of the prominent alkylphenols and aromatic hydrocarbons in the Lower Devonian tracheophyte flash pyrolyzates, a contribution from thermally modified tannins cannot be ruled out. Associate Editor--P. G. Hatcher Acknowledgements--The authors are grateful to the following for the provision of samples: Professor D. L. Dilcher (Incertae sedis angiosperm leaves), Professor P. Gensel (Gasp6 material), Dr A. C. Raymond (Cordaixylon) and Professor C. B. Beck (Callixylon). We thank Ms L. Axe for the isolation of the coalified samples and Mr P. Donohoe for assistance with the Py-GC-MS and GC-MS analyses. This manuscript benefited significantly from constructive comments by Mike Kruge and an unknown reviewer. This work (grant GST/02/500) was conducted as part of the Biomolecular Palaeontology Special Topic financially supported by the Natural Environment Research Council. REFERENCES

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