Gondwana Research, V 7, No. I, pp. 153-164. 0 2004 International Association for Gondwana Research, Japan. ISSN: 1342-937X

Diversity of the Woody Vegetation of Gondwanan Southern Africa Marion K. Bamford

Bernard Price Institute for Palaeontology, School of Geosciences, University of the Witwatersrand, Private Bag 3, WlTS 2050, Johannesburg, South Afyica, E-mail: bamfordm@geosciences. wits.ac.za

(Manuscript received August 30,2002; accepted May 12,2003)

Abstract

The flora of Gondwanan southern Africa is represented in the rock record by micro-fossils, macro-fossils and petrified woods. All these types of fossils are seldom preserved together in any one particular facies because of taphonomic and preservational biases. In order to obtain as accurate a picture as possible of the woody vegetation, both the fossil woods and other macroplant fossils, such as leaf impressions, fructifications and cuticle, of woody plants, have been correlated. This was done for each Formation in the Karoo Supergroup in order to illustrate the changes in diversity of woody vegetation over time. Sediments of the Karoo Supergroup represent the terrestrial fossil record of the period Upper Carboniferous to the Lower Cretaceous when Africa finally separated from South America. In the Upper Carboniferous to Lower Permian (Dwyka Formation) there are at least five described genera of woods from South Africa and Namibia. Early to Middle Permian woods (Ecca Group) are a little more diverse with six genera, representing the glossopterids, cordaitaleans and possibly other seed fern groups. Late Permian to Early Triassic (Beaufort Group) woods show very little change in diversity in spite of the major floral and biotic turnover evident from the rest of the fossil record. Although the Late Piassic (Molteno Formation) macro-flora has been shown to be an example of explosive diversification, the generdly poorly preserved woods do not reflect this. Lower Jurassic fossils (Clarens Formation) are also poorly preserved but have araucarian characteristics. Early Cretaceous woods represent the Araucariaceae, Cheirolepidiaceae and Podocarpaceae with a number of species. The diversity of the woods has not changed as much as the rest of the floral components in southern Africa from the Late Carboniferous to the Early Cretaceous. Possible reasons for this apparent stasis are the conservative nature of wood, functional restrictions, limitations of suitable conditions for petrifications and the fact that very little research has been done on southern African woods.

Key words: Macroflora, fossil wood, palaeo environments, taphonomy, southern Africa.

Introduction Southern Africa has some of the oldest records of life

in the form of cyanobacteria of the Onverwacht Formation, and some of the earliest land plants, but it is best known for the long and almost complete biotic record of the Upper Carboniferous to Lower Jurassic in the Karoo. The main Karoo Basin, covering more than two thirds of the surface area of South Africa, as well as outlying basins in South Africa, Namibia and Zimbabwe, were huge inland seas receiving the melt water and sediments from the polar glaciers to the south and highlands to the north, during the Upper Carboniferous and Lower Permian. Plants and animals were preserved along the ever-shrinking shoreline.

Palaeobotanical research in southern Africa has concentrated on the plant impressions of the Permian and Triassic because these are most abundant and accessible. Preservation is best in the northern part of the basin where coal deposits abound. The fossil plants from various localities in southern Africa have been described by several local palaeobotanists as well as some researchers from

other countries. Edna Plumstead brought the Karoo plants to the attention of the world with her glossopterid evidence for continental drift and her interpretation of the glossopterid fructifications (Plumstead, 1956). Eddie van Dijk, Bill Lacey, Kathleen Gordon Gray, Anna Beneke, Shirley Smithies, Heidi Anderson, John Anderson and Eva Kovacs-Endrody, all based locally, have studied aspects of the flora from the Ecca and Beaufort Groups. John and Heidi Anderson havecollected extensively from the Upper Triassic Molteno Formation (Anderson and Anderson, 1983, 1985). Palaeobotanists have concentrated on one plant form or another, pollen, macroplant impressions or petrified wood, and usually these researchers have limited their research to a particular region or a time period. The exceptions are Heidi and John Anderson who have assembled several prodroma on the macroplant flora as a whole. Since their major works were completed in 1983, 1985 and 1989 (other volumes in progress), much more research has been done on the petrified woods of southern Africa (Bamford, 1999, 2000). In this paper an attempt

154 M.K. BAMFORD

is made to correlate the different plant parts, for example leaves that belong to woody taxa, in order to obtain as realistic a picture as possible of the true diversity of the woody vegetation. Taphonomic and preservational biases are considered.

Biological diversity can be measured at many different levels, from totaI biotic diversity down to a few or a single phylum, order, family or genus. Obviously this ranges from an impossibly large task if all species are considered, down to a small task but with questionable relevance. The fossil record, however, enables a time factor to be introduced and, therefore, changes in diversity over a length of time, can be studied. The floral diversity changes in southern Africa during the post glacial Permo-Carboniferous period to the breakup of Gondwana are considered in this paper but there are some major obstacles even in this defined group, region and time, which are outlined in the next paragraph,

First of all there is the problem of natural separation of dead plants into their constituent components: pollen, leaves, fruits, cuticle, wood. Secondly, the different taxonomic approaches of different researchers makes correlations between different types of plant material difficult unless all collections of a particular flora are restudied by one person or a like-minded group of researchers. This has been done for some floras (Bamford and Philippe, 2001; Adendorff et al., 2002) but is nowhere near completion. Thirdly, some floras have been very well sampled, for example the Molteno flora, whereas others have received only cursory attention. Finally, the pollen record, which has been studied in great detail for only a few localities (Aitken, 1994; Anderson, 1977; Falcon, 1975, 1978, 1980; MacRae, 1988; Millsteed, 1994), is extremely difficult to relate to the macroplant flora because of the lack of biostratigraphic correlation. Multidisciplinary projects have been proposed to address this problem. This paper is, therefore, a preliminary compilation of the data available in order to identify the woody vegetation (from petrified wood and leaf impressions), assess diversity trends and outline the gaps where future research should be directed.

Tectonic Setting and Palaeoenvironments During the Carboniferous, Africa was sandwiched

between South America to the west and Antarctica and India to the east, the whole Gondwana landmass being positioned over the southern pole and under extensive ice sheets. Gradually the land mass moved northwards, the ice melted forming a shallow inland sea and as the region dried up, the huge river systems dwindled to meandering rivers, braided streams and eventually dry sand dunes and playa lakes. The Drakensberg volcanic

eruptions, during the Jurassic, occurred at the same time as the final separation of Africa from South America and put an end to the sequence of Karoo deposition, thus we have virtually no fossil record for the Middle and Upper Jurassic. During the Early Cretaceous a few lacustrine deposits were formed on the southern margin of Africa, Algoa and Gamtoos Basins, where marine transgressions also took place. These southern deposits are now mostly on land but those on the west coast are mostly below sea level (Fig. 1).

The southwestern Karoo Basin is much deeper than the northeastern and represents the proximal basin deposits. The stratigraphy of the Karoo is continually being refined and the biozones, based on vertebrate fauna, have been correlated basin wide. This detailed research has shown that there is some discrepancy in correlating the strata in the north eastern basin with respect to the south western part of the basin. Catuneanu et al., (1998) proposed that this was due to reciprocal flexural behaviour in the retroarc foreland system of the Karoo Basin. This has resulted in much more deposition and thicker deposits in the south western part, or proximal basin, compared with the thinner deposits in the north eastern, or distal, part of the basin. By the Upper Triassic there was no distinction between the amounts of deposition throughout the shrinking basin so the Molteno, Elliot and Clarens Formations (Stormberg Group) are considered as distal facies confined to the more northerly part (Catuneanu et al., 1998). In spite of the recent and better understanding of the Karoo Basin, palaeobotanical data are not yet well correlated with the vertebrate biozones or with the flora from the rest of Gondwana. Futhermore, the fossil plant exposures in the north east are more accessible and better sampled. In this paper, the plant data are collated and compared at the broader scale of Groups or Formations and not the more narrowly defined vertebrate biozones.

Dwyka Group deposits outcrop around the periphery of the basin and the successive Ecca Group deposits outcrop just internal to the Dwyka deposits and so on up to the Drakensberg lavas (Fig. 1; Table 1). The northwestern and southeastern formations are listed in table 2 together with the general palaeoenvironmental conditions. Deep and shallow water environments with a cool climate predominated during the Ecca times with coal deposits forming in alluvial fan, fan-delta and fluvial systems of the smaller basins in the central and northeastern parts (Cadle et al., 1993). Beaufort Group deposits show river systems downgrading to braided rivers and meandering streams as the regional climate dried up and basins filled with sediments from the southern Cape Fold Belt. Drying out continued through the Molteno and Elliot times until the near desert conditions of the Clarens Formation. The Drakensberg lavas have disrupted the

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WOODY VEGETATION OF GONDWANAN SOUTHERN AFRICA 155

Middle and Upper Jurassic

Early to middle Jurassic

Late Triassic to Early Jurassic

Late Triassic

fauna and flora, and the Lower Cretaceous floras are quite distinct from their early precursors.

Floras

west (see Table l), and the lithological Groups form the framework for time correlations. The fossil plants are divided into macroplants and woods in order to give an indication of the whole flora, but more specifically to show the authors interpretation of which leaves, fruits or seeds belong to woody vegetation. This is indicated in the table by placement on the same horizontal line. For example in

The floras in table 3 are also grouped according to the Formations because they differ in the north east and south

Drakensberg no sedimentary deposition

Stormberg

Fig. 1. Map of southern Africa showing the main formations of the Karoo Basin (modified after Johnson et al., 1996; Catuneanu et al., 1999).

Table 1. Simplified stratigraphy of the Karoo Basin in South Africa.

middle Permian

Early Permian

International Time Scale

middle Ecca Whitehill

lower Ecca Prince Albert

Karoo Groups

Late Carboniferous to Early Permian

Formations: SW Karoo Basin

Dwyka Dwyka

Formations: NE Karoo Basin

Lower Cretaceous Uitenhage ~~

South: Kirkwood Formation Northeast: Makatini, Mngazana Formations

Middle Triassic 1 upper Beaufort I Burgersdorp, Early Triassic I middle Beaufort I Katberg Late Permian lower Beaufort Balfour, Koonap

Teekloof. Middleton

Late Permian upper Ecca Waterford, Fort Brown Collingham

Clarens

Elliot

Molten0

Driekoppen,

Verkykerskop

Normandien

Volksrust, Estcourt

Vr yheid,

Pietermaritzburg

Dwyka

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156 M.K. BAMFORD

table 3, in the Clarens Formation, the horizontal line for conifers has two genera for macroplants and one genus for wood, meaning that the fossils are coniferous but not necessarily from the same plant. As more research is done these interpretations, of what is woody and what is not, could change. Pollen is not included. The overall impression is that the macroplant diversity and especially the wood diversity are very low (Table 4). Unless otherwise stated, the data for the macroplants is from Anderson and Anderson (1985), and the data for the woods from Bamford (1999,2000).

Dwylca Group: Late Carboniferous to Early Permian

The Dwyka Group has not been divided into formations as it comprises predominantly glacial facies and some deep water facies.

Macroplants

Fossil plants and woods are better represented in Namibia in the southern Karasburg and Kalahari Basins and northern Huab Basin. These deposits are of glacial origin and comprise diamictites interbedded with shales (Pickford, 1995). The lycopod Cyclodendron leslii, has been recorded from Namibia but no details were provided (Pickford, 1995). Leaf impressions of Glossopteris, (Gangamopteris) and Noeggerathiopsis were collected by various people from the northern central part of the Karoo Basin but there is some debate about their identity (Anderson and McLachlan, 1976). They probably represent woody taxa.

Woods

Woods have been described from several localities in Namibia by Krausel and Range (19281, Krausel (1956) and summarised by Pickford (1995), Bangert and Bamford (2001). Most of these woods have a central pith preserved and secondary xylem of the Agathoxylon type but cannot be related to any particular plant group. According to Krausel et al. (1961) woods with mesarch protoxylem and centripetal xylem are non-coniferous and those with endarch protoxylem and no centripetal xylem are coniferous but it not possible to verify this from the fossil flora.

Palaeoenvironment and diversity

A low diversity of plants is to be expected from a cold glacial climate and poor conditions for preservation so the comparatively high diversity of woods is unexpected (see Table 4 for summary of diversities). This can be explained as a taphonomic bias towards the woods being preserved in deep waters (drift wood or moraine) and in nodules (McLachlan, pers comm.). As the wood anatomy of this time period is complex with various types of internal pith it is not possible to relate the woods to any modern plant group.

Ecca Group: Midd le Permian

Macroplants

The Ecca Group macroplant flora consists of mainly Glossopteris leaves, some fruits and seeds, cordaitalean leaves some species of which represent woody plants, and non-woody lycopods, sphenophytes and ferns. There are common elements of these groups in most of the deposits but there are also regional differences and endemics. The leaves have been described at the species level on the basis of midrib, venation characteristics and leaf shape but there are many intermediate forms and no consensus on species. Correlating leaves with the fructifications has been attempted (Anderson and Anderson, 1985) on the basis of assemblages but this is not wholly satisfactory as it is not consistent. Renewed research on the morphology and taxonomy of the fructifications (Adendorff et al., 2002) as well as morphometric analysis of the leaves will soon add to our understanding of the Glossopteris flora (Adendorff, pers. comm). The palynostratigraphy of several separate coal deposits has been published (Falcon, 1975, 1978, 1980; Anderson, 1977; MacRae, 1988; Aitken, 1994) but as yet there is no Karoo-wide correlation.

woods

There are only two fossil wood types occurring in the Ecca Group deposits. Prototaxoxylon africanum (Walton) Krausel and Dolianiti is quite distinctive in having fine spiral thickening on the walls of the tracheids. This genus extends into the Beaufort Group (Abrahamskraal and Koonap Formations). Potential macroplant associations are the glossopterids throughout the Ecca sequence. The cordaitales and some plants of unknown affinity (Taeniopteris, Pagiophyllum, Benlightfootia) have only been recorded from the upper Ecca while the glossopterid macrofossils continue beyond the apparent extinction of Prototaxoxylon. One conclusion to be drawn is that Prototaxoxylon is the wood of one or a few glossopterids which became extinct, while non-Prototaxoxylon-type- glossopterids survived for longer. Alternatively, the fossil record is incomplete. Nonetheless the Glossopteridales is a large and probably unnatural taxonomic group and more detailed information is needed on the phylogeny and distribution of all the glossopterid plants.

The second Ecca wood type is Australoxylon teixeirae Marguerier and it occurs from the Collingham Formation (Table 1) to the end of the Permian. It is quite a distinctive wood with clusters of pits on the radial walls of the tracheids and some araucarian tracheid pits. The glossopterids occur throughout its range too and so Australoxylon could well be the wood belonging to some of the glossopterid macroplants. As there are 21 species of Australoxylon distributed throughout the Permian of

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Table 2. Lithology and palaeoenvironments of the Formations in the Karoo Basin (based on Cadle et al., 1993; Catuneanu et al., 1998; Cole, 1992; McLachlan and McMillan, 1976; McLachlan et al., 1976; Visser, 1992).

Age Southwestern Basin (Proximal) Northeastern Basin (Distal)

Palaeoenvironmcnt Litholow Palaeoenvironment Litholow

Early Cretaceous

Early to Middle Jurassic

Late Triassic to Early Jurassic

Late Triassic

Upper Beaufort/ Middle Triassic

Upper Beaufort/ Middle Triassic

Middle Beaufort/ Late Permian

Lower Beaufort / Late Permian

Kirkwood Formation reddish-brown varigated silty mudstone with subordinate grey shale and sandstone

Burgersdorp Formation thick upward-fining units of olive-grey medium to coarse grained sadstones overlain by dark red siltstones and mudstones

Katberg Formation thick laterally extensive, light olive-grey, coarse- grained sandtone with transverse and longitudinal macroforms containing horizontal and trough

cross-bedding

Balfour Formation one fining upward sequence of green-grey sandstones with bands of darker mudstones; above and below are sub-aerial unconformities

Koonap Formation greenish silty mudstones and

fluvial system but interfingers with Enon formation (marine transgressions)

mixed-load meandering river and floodplain

shallow, braided environment with pulsatory discharges; also have abandoned channel fills and braidplain environments

braided rivers graded upwards into meandering stream systems

Mngazana, Formation very tough grey-green conglomerates with

highly lignitic sandy lenses, minor shales and calcified muds

Makatini Formation basal conglomerates, glauconitic siltstones and sandstones; with marine invertebrates

Clarens Formation cream, yellow, fine-grained sandstones, sandy siltstones and mudstones; some coarse-grained sandstones

Elliot Formation reddish floodplain mudstone; channel and crevasse slay sandstones wind-blown loessic dust deposits

Molten0 Formation tabular sheets of medium to coarse-grained sandstone

with horizontal and cross- stratified macroforms

high energy braided river system

Driekoppen Formation thin, fine grained channel sandstones with horizontal cross-bedding, overlain by thick, massive to diffusely laminated siltstones to mudstones

Verkykerskop Formation thin, laterally extensive, medium to fine grained sandstones, dominated by transverse bar macroforms which are internally structured by planar cross bedding

Normandien Formation interbedded sandstones and mudstones (like upper Balfour Fm.)

-.

sandstones in a fining upward grading into a lower energy

turbid shallow water depositional environment, open to the sea

lagoonal and playa lake?

desert environment, wind-blown dunes; shallow playa lakes in wetter areas

meandering systems

transition to desert

braided streams on a vast braid plain, rare coal deposits; few filled in abandoned channel tracts and some ponded bodies of water

suspended-load-dominated meandering river deposits

braidplain

Meandering streams with channels in wide semiarid floodplains

sequence meandering stream system

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Plant Group

Table 2. Contd.

Macroplants Wood

Upper Ecca / Late Permian

Middle Ecca / upper Early Permian

Lower Ecca / Early Permian

Late Carboniferous - Early Permian

Middleton Formation dark rcd and green-grey mudstoncs interbedded with sandstones in an overall fining upward sequence

Fort Brown, Ripon, Collingham Formations grey-green shales, silts and subordinate sandstones, graywackes

Wliitehill Formation carbonaceous shale, weathering white with chert bands and lcnses

Prince Albert Formation dark green-grey shale with some graded silty layers

Dwyka Group tillites cyclically grading upwards into finer grained clastic rocks; 9 cyclcs of terrestrial - subaqucous transition; uniform and laterally cxtensive

low energy meandering and lacustrine system

shallow marine, deep water proximal submarine and distal submarine facies

deep water, pelagic and reducing environment

deep water environment

Glacial environment with deposition from both grounded and floating ice, the latter dominant

Volksrust (Estcourt) Formation dark shales with intercalations of fine- grained sandstone

Vryheid Formation interbedded sandstone, shales and underlying coal beds

Pietermaritzburg Formation shales

Dwyka Group tillites but only 2 cycles recognised; complex facies; irregular thickness; local extension of grounded ice lobes with ponds and outwash fans

deep to shallow marine environment

fluviodeltaic system

moderate to deep water environment

continental glaciation and grounded ice dominated deposits

Table 3. List of macroplant and wood taxa from the Formations of the Karoo Basin. Numbers in brackets are the minimum number of species. (Based on all palaeobotanical references cited in the text).

liverworts ferns

cycads bennettitaleans conifers

incertae sedis

Marchanitites , Ricciopsis ( 2 ) Cladop hle bis, Sp henopteris, Onychiopsis (11+ species) Pseudoctenis (3) Zamites, Dictyozamites (6 ) Araucarites, Brachyphyllum (3 ) more species based on new cuticular work Taeniopteris (3)

Agathoxylon Podocarpoxylon Brachyoxylon West coast: Podocarpoxylon (5), Protocupressinoxylon (1) 1

Clarens Formation (Early - middle Jurassic) sphenophytes conifers

Equiseturn ( 1 ) Podocarpoxylon (1 +)

Elliot Formation (Late Triassic to Early Jurassic)

sphenophytes bennettitaleans conifers incertae sedis

Equisetites (I) Otozamites (1) Sphenolepidiurn (2), Pinus (1) Phoenicopsis (1)

~~

Agathoxylon (1 +)

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I conifers ? I I Agathoxylon ( Z ) , AustraIoxylon (1)

Table 3. Contd.

Marchantites, Muscites Phyllotheca, Neocalamites, Schizoneura, Equisetites. Todites, Asterotheca, Cladophlebis, Dictyophyllum. Dicroidium (lo+), Lepidopteris, Yabiella, Taeniopteris, Dejerseya, Pseudoctenis. Nilssoniopteris. Ginkgoites, Sphenobaiera Rissikia, Voltziopsis, Heidiphyllum, Pagiophyllum ( 9 ) Yabiella, Jungites, Gontriglossa, Linguifolium, Saportaea (14)

mosses sphenophytes

ferns

seed ferns

cycads bennettitaleans gingkos conifers

gymnosperms

Agathoxylon, Podocarpoxyton, Rhexoxylon (5 +)

Gregicaulis (1) Calamites (1) Asterotheca, Cladophlebis ( 2 ) Lepidopteris Dicroidium ( 2 ) Pseudoctenis, NiIssonia (2 ) Ginkgoites, Sphenobaiera (2 ) Sewardistrobis (1)

lycopods sphenophytes ferns seed ferns cycads ginkgos conifers Agathoxylon, Podocarpoxylon ( 2 )

lycopod sphenop hytes

ferns glossopterids

Lepidodendron (1) Phyllotheca, Raniganjia, Schizoneura, Equisetum (4) Asterotheca (1) Glossopteris - 1 leaf type Australoxylon (l), Prototaxoxylon (1)

mosses sphenophytes,

lycopods sphenophytes ferns glossopterids

ferns glossopterids

Australoxylon (1)

cordaitales incertae sedis

lycopods sphenophytes ferns glossopterids

Volksrust (Estcourt) - Teekloof - Middleton Formations (Upper Ecca. Late Permian)

Cyclodendron Solenoxylon (3), Lobatoxylon (l), Megaporoxylon (3), Kaokoxylon (2), Phyllocladopitys (1) undescribed stems

Buthelezia (1) Sphenophyllum, Raniganjia, Phyllotheca, Schizoneura ( 6 ) Sphenopteris (1) minimum: 11 leaf types, 6 fructifications Noeggerathiopsis (1) Taeniopteris, Pagiophyllum, Benlightfootia (3)

Waterford - Fort Brown - Collinj

Australoxylon (1) Prototawoxylon (1)

im Formations (Upper Ecca, Late Permian)

lycopods sphenophytes ferns glossopterids

Cyclodendron (1) Phyllotheca (1) unidentified Glossopteris leaves Australoxylon (1)

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Gondwana (Giraud, 1991, Weaver et al., 1997) there is a strong association with the glossopterids, all of which are typically Gondwanan in distribution. Some of the woods previously described as Dadoxylon are actually members ofAustraloxylon. This means that there are no woods from the Ecca and lower Beaufort Groups with typical araucarian secondary xylem. Such a result is unexpected because the Dwyka woods with central piths have araucarian secondary xylem. An incomplete record is more likely to bc the reason for this gap.

Either of these woods could be members of the Cordaitales or from the other possibly woody taxa but at this stage there is no evidence to support any particular association.

Diversity and palaeoenvironrnent

There are only two wood genera represented in the southern African Ecca Group yet thcre are more than 11 Glossopteris leaf taxa, a t least one cordaitalean and several of unknown affinity. This most probably means that the diversity of the woody vegetation is greater than the two wood genera but again the unresolved Glossopteris taxonomy is a problem when trying to consider diversity. Most of the Ecca plant deposits are in the form of thick coal seams in the north eastern part of the Karoo Basin and the plant components are rarely distinguishable. This implies a very prolific vegetation cover with swampy conditions suitable for the formation of coal. The seams are not uniform in thickness or quality; Cadle et al. (1993) interpret the Ecca coals as having been deposited in a variety of environments, from deep waters, deltas, and proximal and distal flood plains.

Beaufort Group (Late Permian to Middle Triassic)

Macroplants

The lower Beaufort Group macroplant flora is dominated by glossopterids but they diminish in diversity and number as other groups appear in the fossil plant record, the cycads, benettitaleans, ginkgos and other seed ferns. The transition is poorly represented, especially the Permian-Triassic boundary with its global extinction, as far as the flora is concerned. The macroplant flora is undescribed from the Balfour and Normandian Formations but is abundant in the Late Permian Estcourt Formation of the north eastern part of the Karoo basin. I t would appear that Anderson and Anderson (1985) use different formations from the vertebrate biostratigraphers (Rubidge et al., 1995). Abundant plants and insects have been recovered from the Estcourt Formation localities (van Dijk, 1997; Geetsema et al., 2002). Detailed sampling through the Permian-Triassic boundary is planned for future research.

The macroplant flora from the Upper Beaufort is devoid of glossopterids and cordaitaleans but is rich in ferns and new seed ferns, Lepidopteris and Dicroidium. No typical podocarpaceous foliage has been described from these strata, yet wood is known from the Burgersdorp Formation (see below). Again it is difficult to correlate the woods with the macroplants. There is no apparent Ginkgo wood here but there are three species of leaves, and it is impossible to tell what wood belongs to the isolated sample of the conifer (?) cone Sewardistrobis.

woods

Two species of Araucarioxylon (Bamford, 1999; = Agathoxylon see Bamford and Philippe, 2001, for nomenclatural revision) occur in the Balfour and Normandien Formations, with A. karooensis occurring only in these formations but A. africanum extending into the Cretaceous (Bamford, 2000). These formations are also the upper limit of the range of Australoxylon in southern Africa.

Higher up in the Beaufort, the Burgersdorp Formation, the first podocarpaceous type of wood occurs, described as Mesembrioxylon (= Podocarpoxylon Bamford and Philippe, 2001). Although this wood is poorly preserved the large, single tracheid and cross field pits are evident (Bamford, 1999).

Diversity and palaeoenvironment

The Beaufort floras are less diverse than the Ecca floras with half as many taxa (Table 4). As there is poor correlation with the s t ra t igraphy used by the palaeobotanists and the vertebrate biostratigraphers, the low diversity of the Beaufort times may not be real. During this time the huge inland sea was drying up and braided rivers and flood plains decreased to meandering streams (Catuneanu et al., 1998). Such environments, however, are usually considered to have a good potential for fossilization of vegetation. Under representative sampling could also be a factor.

Stormberg Group, Mol teno Format ion (Upper Triassic)

Macroflora

This flora is the most extensively sampled one and many specimens from many sites have been collected by Heidi and John Anderson. They have used the palaeodeme approach to the taxonomy (Anderson and Anderson, 1985) so it is difficult to equate their numbers of species with the more conventional taxonomic approach used by the other palaeobotanists who have studied the South African flora. Nonetheless there are many more species described from the Molteno Formation than from any other southern African strata. Anderson and Anderson (1989)

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described some new leaves which they attribute to the Pinopsida, Voltziopsis, Heidiphyllum, Rissikia and Pagiophyllum, so these have been treated as conifers here.

Woods

In contrast to the macroplants, the woods are poorly represented. Although the author has sectioned numerous samples of Molteno wood, most of the internal anatomy has been greatly distorted by heat (dykes?) and no cellular details have been preserved. Rhexoxylon Bancroft is a wood type exclusively from the Molteno in southern Africa, and probably other Gondwanan localitites too, for example Argentina from where five species have been described (Artabe et al., 1999). Rhexoxylon has been attributed to being the wood of Dicroidium, Corystospermales, (Archangelsky and Brett, 1961; Archangelsly, 1968; Herbst and Lutz, 1988) but this has not been convincingly substantiated. Meyer-Berthaud et al. (1993) described a new and different wood type, Kykloxylon, attached to Dicroidium leaves from Antarctica. From South Africa there are two species of Rhexoxylon (Archangelsky and Brett, 1961; Artabe et al., 1999) and a third one yet to be described. I t is possible that Rhexoxylon is the wood of several members of the Corystospermales and that there could be more species of the wood.

Wood of the Agathoxylon type also occurs in the Molteno Formation and may or may not belong to a conifer since the typical conifer leaf types are missing. Nonetheless several new gymnosperm taxa are being described by Anderson and Anderson (pers. comni.) and some may have been woody. Podocarpoxylon wood and the possibly podocarpaceous leaves Rissikia Townrow (Anderson and Anderson, 1985), occur in separate Molteno sites.

Diversity and palaeoenvironment

Table 4 shows a conservative estimate of the diversity of the vegetation of the Molteno Formation. The seed ferns are very diverse or variable in leaf frond morphology. There are only three genera of petrified woods but if the conifers that the Andersons have found (Anderson and Anderson, 1989) are frorn woody plants, this number increases to six. The Molteno palaeoenvironment is postulated to have had seven ecozones, ranging from

Dicroidium riparian forest to Sphenobaiera woodland to Ginkgophytopsis meadow (Anderson et al., 1998). They have also described associated vertebrate and invertebrate faunas for each of their ecozones. This Formation has the greatest diversity of fossil plants in the Karoo. The diverse palaeoenvironments and good conditions for preservation (rapid burial in fluvial and overbank deposits), may be the reasons for this high diversity. An even greater diversity of all biota has been postulated by Anderson et al. (1998) by extrapolating from the probabilities of organisms, being buried and being recovered by palaeontologists.

Stormberg Group, Elliot Formation (Late Triassic to Early Jurassic)

Macroplants

The macroplant flora is very scarce and comprises only one sphenophyte, one bennettitalean and three conifers.

Woods

The woods are mostly poorly preserved with only Agathoxylon being represented but new collections being studied should add to this.

Diversity and palaeoenvironment

The climate was drying out and so the vegetation would have been localised and chances of preservation greatly reduced. The diversity is low for woody and non-woody taxa (Table 4).

Stormberg Group, Clarens Formation (Early to Middle Jurassic)

Macroflora

stratum.

Woods

Only one sphenophyte has been recorded from this

Only one wood type, Podocarpoxylon (=Mesembrioxylon Bamford, 1999) has been described from the main Karoo Basin. Although there are sites, such as Rhebokhoek, near the town of Clarens, with whole trunks preserved, the preservation is too poor to see the diagnostic features in the wood anatomy (pers. observation.) and it can only be

Table 4. Diversity Numbers of genera of Gondwanan fossil plants, southern Africa.

Dwyka Ecca Beau fort Molteno Elliot Clarens Kirkwood Group Group Group Fm Fm Fm Fm

Herbs Leaves Wood Conifers Total

1 8 6 13 1 1 5 10 2 7 2 4

5 2 2 3 1 1 4 4 1 6 2 5

6 24 11 29 6 2 18

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162 M.K. BAMFORD

said that the woods are coniferous. From the lower part of the Clarens Formation in the n l i Basin, northern South Africa - southern Zimbabwe, logs ofAgathoxylon have been recovered (Bordy and Catuneanu, 2002).

Diversity and palaeoenvironment

This impoverished flora is likely to be partly a result of poor exposures and poor conditions for silicification of the woods. The environment is said to be that of a desert with playa lakes in the main Karoo Basin (Smith, 1990) terminating with aeolian dune deposits. The Tuli Basin also shows progressive drying out (Bordy and Catuneanu, 2002).

Drakensberg Group (Middle and Upper Jurassic)

No plants have been discovered from this time period.

Kirkwood Formation (Early Cretaceous)

Macroplants

A greater floral diversity is evident in the Kirkwood Formation than from the earlier deposits, with liverworts, ferns, cycads, bennettitaleans and conifers (Table 3). Renewed studies of old collections and recent collections of this flora show that there is a greater variety of conifers represented by the cuticle fragments (Gomez et al., 2002a), as well as amber (Gomez et al., 2002b). With moderate confidence some macroplants can be associated with fossil wood: Araucarites rogersii cone scales may belong to the wood Agathoxylon but there is no physical attachment. The leaf type Brachyphyllum is coniferous but the exact family affinities depend on the species and revision is needed of this genus worldwide. There are more conifer leaf types than conifer wood types. There is no obvious podocarpaceous leaf type yet there are several podocarpaceous wood types. Thus the associations are incomplete and the number of genera shown in tables 3 and 4 are only tentative.

Woods

The earlier floras mentioned above are represented only in terrestrial deposits of the Karoo Basin but there are Lower Cretaceous onshore and offshore west coast deposits. These have several conifer woods but no leaves or cones (Bamford and Corbett, 1994,1995). These woods are mostly podocarpaceous but there is one example of Protocupressinoxylon, a member of the extinct family Cheirolepidiaceae. Woods of araucarian affinity have been sectioned and two genera will be described.

Diversity and palaeoenvironment

The silicified woods are numerous, extending from north of the Orange river southwards to the Olifants River

and have probably been transported from the hinterland (Bamford and Corbett, 1994,1995). Dense chatter-marking of the rounded cobbles of silicified woods indicates that they have been reworked (Bamford and Corbett, 1995) into Middle Pleistocene shoreline deposits (Kensley and Pether, 1986). Direct evidence from the woods shows that there are four genera from three families. Cuticles, from the Lower Cretaceous Kirkwood Formation, of coniferous trees indicate that there are two of these families represented (Araucariaceae and Cheirolepidiaceae; Gomez et al., in press). If both the west coast woods and Kirkwood formation woods are considered, there may be at least five genera (Table 4).

Discussion Data on the fossil plants compiled mostly from Anderson

and Anderson (1985) and the woods (Bamford 1999, 2000, Bangert and Bamford 2001) for each of the formations in the Karoo sequence is presented at the generic level, not the species level. The criteria for recognising palaeobotanical species differ from one palaeobotanical field (palynology, palaeoxylotomy, macroplant morpholom, cuticular morphology) to another and it would make any correlations difficult. The criteria for genera, if not less variable, at least reflect a recognisable plant type, and so the generotype is used here as the basis for comparisons. Common genera occurred in the different parts of Gondwana and so will be able to form a basis for future comparisons.

There is hardly any correlation between the Karoo fossil woods and the macroplants at the generic level. Even at the family level there is little correlation. Strong taphonomic biases are probably the cause. Firstly, fossil woods seldom occur with leaf impressions, except in the Kirkwood Formation. The woods may represent a high energy environment such as flooding where trees were ripped up and washed downstream before burial and preservation. The leaf impressions represent a low energy environment with minimal transport and deposition in lakes or ponds. Woods require burial in a mineral rich anoxic solution whereas leaves can be preserved in drier anoxic environments (Buurman, 1972; Cleal, 1991). From table 2 it can be seen that a variety of depositional and preservational environments have occurred throughout the Karoo sequence.

Gymnosperm woods are composed of tracheids, rays and parenchyma in varying proportions. Functional requirements, such as the transport of liquids and solutes by tracheids and rays, storage of substances by parenchyma, and rigidity, are probably limiting factors for the arrangement of cells within a tree trunk. Only once the angiosperm wood tissues have evolved with greatly

Gondwana Research, V. 7, No. 1, 2004

WOODY VEGETATION OF GONDWANAN SOUTHERN AFRICA 163

modified tracheids to form different cell types, vessels and fibres, or different patterns of parenchyma cell distribution and complex rays, is there the potential for a greater variation in trunks. These dicot woods arose in the Late Cretaceous after the evolution of the other plant organs. Karoo woods pre-date these events and so have less potential for variation and this may be the reason why we seem to have more leaf taxa of woody plants than fossil wood taxa.

Macroplant localities at the Permian-Triassic boundary are very rare and not well sampled so the data for or against a major plant extinction, in parallel with the marine and terrestrial vertebrate mass extinction, are absent. This is an area that needs research in the future.

Although the Gondwanan floras have many genera in common, (Anderson et al., 1999) the species are mostly different. This could well be a result of genuine differences but would be exaggerated by differences in taxonomic approach. Cuneo (1996) has dealt with the regional distribution of and variation within the Permian floras. To the best of the author's knowledge, the Gondwanan Triassic floras have not been correlated. Lower Cretaceous floras are probably not as homogeneous as has been postulated and detailed comparisons are underway, starting with the woods (Bamford and Philippe, 2001).

Acknowledgments Wood samples for various projects have been supplied

by many people, but in particular I would like to thank Ian Corbett, Mike de Wit, Ian McMillan and John Ward of De Beers; Johann Neveling of the Council of Geosciences, Pretoria; John Hancox and Bruce Rubidge of the University of the Witwatersrand; Berthold Bangert of the University of Wurzburg, Germany; and Heidi Anderson of the National Botanical Institute, Pretoria. The two referees, Jane C. Shearer and A.P. Kershaw, are gratefully acknowledged for their comments and improving the manuscript.

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