The Jurassic fossil plant record of the UK area

The Jurassic fossil plant record of the UK area Johanna H. A. van Konijnenburg-van Cittert VAN KONIJNENBURG-VAN CITTERT, J. H. A. 2008. The Jurassic fossil plant record of the UK area. Proceedings of the Geologists' Association, 119, 59-72. Plant life in the UK area during the three major epochs of the Jurassic (Early, Mid and Late) is discussed with emphasis on some of the groups that played an important role and/or were first described from British sediments. Palaeobotanists usually include the Rhaetian (late Triassic) in their discussions of the Early Jurassic because the Triassic/Jurassic extinction event is not very distinct in fossil floras. Rhaetian floras are often similar to Early Jurassic ones, and this overview starts with Rhaetian examples. Early Jurassic floras are rare as most of England and Wales was covered by a shallow sea at this time. However, Mid Jurassic floras are well known and diverse, especially those from Yorkshire and Stonesfield in Oxfordshire; smaller floras have been described from Scotland as well. Our knowledge about Late Jurassic floras comes mainly from Scotland and southern England. Key words: palaeobotany, Jurassic, Rhaetian Laboratory of Palaeobotany and Pa/ynology, Budapestlaan 4, 3584 CD Utrecht, The Netherlands (e-mail: [email protected]) and National Natural History Museum 'Naturalis', PO Box 9517, 2300 RA Leiden, The Netherlands (e-mail: [email protected])

1. INTRODUCTION

After the major Permian/Triassic extinction event, it took c. 4-5 million years before plant life in Europe was fully recovered (Looy et al., 2001). Early and early Mid Triassic floras in Europe are rather rare and show a low biodiversity (Dobruskina, 1994). In the UK, the poverty of the floras was exacerbated by desert conditions during most of the Triassic (Cleal et al., 2001). Only a small number of taxa has been recorded, mainly horsetail (Equisetites, Schizoneura) and conifer (Voltzia) (Wills, 1910). Another extinction event took place in the animal kingdom at the Triassic/Jurassic boundary (see Wignall & Bond, 2008). This extinction event is less evident in plant life, although at least one major group of seed ferns (the Order Peltaspermales) died out at this time (Dobruskina, 1994). During the last stage of the Triassic - the Rhaetian - a number of plant groups appeared that would play an important role during the remainder of the Mesozoic, e.g. a number of living fern and conifer families (Dobruskina, 1994). In this paper, the plant life in the UK area during the three major epochs (Early, Mid and Late Jurassic) will be discussed in general terms, with emphasis on some of the groups that played an important role and/or were first described from British sediments. However, before this, it will be helpful to introduce briefly the main plant divisions. The Kingdom Plantae is divided into as many as 25 divisions (all names ending in -phyta); divisions are subdivided into orders (ending in -ales) and these, Proceedings of the Geologists' Association, 119, 59-72.

in their turn, into families (ending in -aceae). The first six divisions form the Algae, and another one is for the other phytoplankton. Three divisions form the bryophytes (including the liverworts or Hepatophyta, and the mosses or Bryophyta). Some orders of the Division Lycophyta (the Lycopodiales or clubmosses, Selaginellales, and Isoetales or quillworts) have a role during the Jurassic, although these herbaceous plants, closely resembling the living genera Lycopodium, Selaginella and Isoetes, have never been encountered in large quantities. In the Jurassic, only one order of the Division Sphenophyta was present (the Equisetales or horsetails). Horsetails are characterized by division of their axes into nodes and internodes. The Division Pteridophyta (or ferns) was represented by several orders and a large number of (mainly living) families. Ferns reproduce by spores (usually isospores, but water ferns have micro- and megaspores) formed in sporangia; these sporangia are placed on the frond, often in small groups called sori. The fossil Division Pteridospermophyta or seed ferns (fronds often resemble those of the ferns) is distinguished by its reproductive method, which is by seeds and pollen grains. In this, it is the most basal group of the so-called Gymnospermae, a group of seed plants with 'naked' seeds. This is in contrast to the Angiospermae, which have their seeds enclosed in an ovary. Three more divisions played an important role in the Jurassic: the Cycadophyta, Ginkgophyta and the Coniferophyta. The Cycadophyta were divided into at 0016-7878/08 $15.00

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Geologists' Association

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least two orders, the Cycadales (with the still-living families Cycadaceae and Zamiaceae, and the fossil family Nilssoniaceae) and the fossil Bennettitales (with the families Williamsoniaceae and Cycadeoidaceae). Some palaeobotanists consider the Nilssoniaceae worthy of an order of its own, the Nilssoniales. The leaves of all these cycadophyte orders are very similar, but their reproductive organs are rather different. The Division Ginkgophyta is nowadays represented by one species only, the living fossil Ginkgo biloba Linnaeus, but during the Jurassic, these plants were represented by a number of genera and species (all placed in the Order Ginkgoales) and occurred all over the world in areas with a warm-temperate or subtropical climate. The last Division of the Gymnospermae, the Coniferophyta (Order Coniferales) was important during the Jurassic. Most living families were represented in the UK area, but some fossil families occurred as well; of these the Cheirolepidiaceae were really common. A whole silicified forest of stumps and branches formed by remains of this family is known from the Purbeck Group of Dorset (see later). Finally, the Division Magnoliophyta is the equivalent of what is generally known as the Angiospermae. This group of plants was virtually unknown during the Jurassic although some fossil plants may represent primitive angiosperms or their ancestors; they will not be discussed in detail here. A last remark must be made about the nature of plant fossils. Fossil plant remains are rarely found as complete specimens. To cope with this, isolated plant parts, such as leaves, stems, roots, reproductive organs, are assigned to different so-called 'morphogenera'. Thus, the various fossilized parts of an individual plant may have very different names. They can be brought together if the parts: (I) consistently occur together; (2) have the same internal or cuticular anatomy; or (3) have all the same special features such as a certain type of hairs or scales. However, this combining of separate fossil plant parts remains conjecture, until we are lucky enough to find some parts actually connected, when we can then make a realistic reconstruction of a whole plant specimen. Figured specimens come from either the Laboratory of Palaeobotany and Palynology, Utrecht (prefixed S) or the Natural History Museum, London (prefixed V). 2. PLANT LIFE IN THE LATEST TRIASSIC AND EARLY JURASSIC

Palaeobotanists often include the Rhaetian in their discussions of Early Jurassic floras and, here, two famous plant taxa known from British Rhaetian sediments will be recorded briefly. The first one, the liverwort Naiadita, is known from a thin but persistent bed, the Naiadita Bed, extending about 145 km from east of the Mendip Hills (near Wells) to south Worcestershire and Warwickshire (Cleal et al., 2001). Naiadita has simple, one cell-thick leaves borne spirally

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rhizome

'.• • ,,;.•. Ifl' \'\!\U I' : ',· ; 1 : I I'~ \i '

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. \;i\\ ~\ Fig. 1. Reconstruction of Naiadita (after Cleal et al., 2001, fig. 2.4; courtesy of JNCC).

on a slender stem with basal unicellular rhizoids (rootlike structures) (Fig. I). Naked archegonia (multicellular structures in land plants that produce an egg) are borne laterally on short stalks; later they become enveloped in scales resembling foliage leaves. Asexual reproduction occurs as well (Harris, 1938). This liverwort displays an unusual combination of characters and is not closely related to any living genus. Harris (1938) suggested that Naiadita grew as a submerged plant covering the bottom of shallow parts of lakes. It has never been found after the Rhaetian. A few other plants have been described from the Naiadita Bed, such as the liverwort Hepaticites solenotus Harris and the alga Botryococcus braunii Kiitzing, but only in small quantities (Harris, 1938; Swift & Martill, 1999). A fern from sandstones referable to the Quarella Sandstone (a littoral Rhaetian facies in South Wales) was identified as Clathropteris meniscoides Brongniart by Harris in notes written shortly before his death. The plant has never been described formally (Swift & Martill, 1999). Another Rhaetian flora is that from Cnap Twt in South Wales. Apart from a number of mainly cycadophyte and conifer cuticle fragments, the site has yielded large amounts of charcoaly fragments now assigned to the conifer Hirmeriella airelensis Muir et van Konijnenburg-van Cittert (Harris, 1957; Muir & van

JURASSIC FOSSIL PLANTS OF THE UK

Konijnenburg-van Cittert, 1970). The record is one of the earliest of the Cheirolepidiaceae, an extinct, warmth-loving conifer family that occurred worldwide during the Jurassic and Cretaceous (Vakhrameev, 1991). Another early member of the family is Brachyphyllum scottii Kendall; shoots and male cones of this taxon are known from the Lower Jurassic (or Rhaetian) of Larachbeg (Morvern, Scotland; Kendall, 1949a). Members of this family occur again in the Yorkshire Mid Jurassic flora and in the Late Jurassic Purbeck Group; the latter consisting largely of silicified wood (stumps) of cheirolepidiacean trees. Finally, the record of the bennettitalean leaf species Otozamites bechei Brongniart from the Rhaetian of Waterloo Bay, near Lame, County Antrim, Ireland, should be mentioned (Harris, 1961c; Swift & Martill, 1999). The beds contain a rich fauna including bivalves, gastropods, fish and reptiles, but Oibechei is the only plant fossil recorded from there; its determination was confirmed by the preserved cuticle. Similar leaves have been recorded from Rhaetian sediments near Cowbridge, Glamorgan (Wales) and the marine Lias Group of SW England (from Watchet and Charlton Mackrell in Somerset, and Lyme Regis in Dorset) (Harris, 1961c). A single record of Otozamites obtusus (Lindley et Hutton) Brongniart from the Langport Member at a quarry near Radstock, Somerset was mentioned in Harris (1938) and figured by Swift & Martill (1999). The specimen is so fragmentary that it is difficult to make a determination other than Otozamites sp. During the Early Jurassic, most of England and Wales was covered by a shallow sea and the Lower Jurassic Series contains mainly washed-in plant remains (Cleal et al., 2001). Only in a few areas, around Lyme Regis and in Warwickshire, slightly better plant material (mainly conifer remains, and some cycadophyte and seed fern specimens) has been found, e.g. Pagiophyllum peregrinum Lindley et Hutton and P. sewardii Kendall (see Kendall, 1948), Ctenozamites cycadea (Berger) Schenk, Cycadopteris jurensis (Kurr) Hirmer, Otozamites obtusus (Lindley et Hutton) Brongniart, Cycadites rectangularis Brauns and the stem Cycadeoidea gracilis (Carruthers) Seward (Seward, 1904). In contrast, Scotland was largely a land area and deposition was rare. Non-marine deposition took place during the Early Jurassic along only a small eastern coastal area near Golspie, but no extensive fossil flora has been recorded from there. Very little is thus known of the plant life of the UK area during the Early Jurassic. Most of the recovered remains are washed-in and belong to conifers, mainly of the Cheirolepidiaceae. This indicated that the climate at that time must have been rather warm (Vakhrameev, 1991). 3. PLANT LIFE IN THE MID JURASSIC Compared with the Early Jurassic, a great deal more is known about the plant life of the UK area during the

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Mid Jurassic. It is generally believed that during the Early and Mid Jurassic 'greenhouse' conditions prevailed world-wide, with a relatively high atmospheric CO 2 level and high global temperatures. Only towards the end of the Mid Jurassic Epoch might the temperature have been lower (Vakhrameev, 1991; Cleal et al., 2001). In the UK area, the Mid Jurassic climate was probably also humid and subtropical, with little seasonal changes (Morgans, 1999; Cleal et al., 2001), and plant life flourished. Yorkshire The Yorkshire Jurassic (mainly Mid, i.e. late Pliensbachian-Bathonian) flora is one of the classic Jurassic floras and has been studied since the early nineteenth century. More than 300 taxa have been described, and the best overview can be found in Harris' detailed descriptions (Harris, 1961a, 1964, 1969,1979; Harris et al., 1974), van Konijnenburg-van Cittert & Morgans (1999) and Cleal et al. (2001). It would be out of the scope of this review to describe each of the taxa, but a general impression of this well-known flora will be given. Only a few bryophyte taxa are known from the Yorkshire Jurassic and all of these are thalloid liverworts (thalloid means without differentiation into roots, stems and leaves); no moss-like fossils have been found. Four species of Hepaticites are known, but only H. arcuatus (Lindley et Hutton) Harris occurs at more than one locality. Moreover, this species has been recorded from many localities in Europe and the Middle East (Schweitzer et al., 1997). Considering the excellent state of preservation of a large number of Yorkshire plant fossils, it is remarkable that more bryophyte fossils are not known. Lycopods (Lycophyta) are represented by only one macrofossil taxon, Selaginellites falcatus, formerly known as Lycopodites Jalcatus Lindley et Hutton (Schweitzer et al., 1997), and some associated reproductive organs (Harris, 1961a). Moreover, a number of megaspore species have been ascribed to the genus Triletes; some of these show affinities with Selaginella, others more with Isoetes. A number of sphenophyte (horsetail) species have been found at various Yorkshire localities. The most common one and, indeed, the most common fossil in the Yorkshire flora, Equisetum columnare Brongniart, has been found both as a compression fossil and as an erect stem in growth position (Fig. 2a). Strangely enough, only a few reproductive organ fragments have been found of this very common taxon, and still nothing is known about the possible attachment of these cones to the stems. Two other Equisetum species, the large E. beanii (Bunbury) Harris (stems 10-12 em wide) and the much smaller E. laterale Phillips (stems usually 1-2 em wide), are locally abundant but less well understood. No fertile remains have been found so far (Harris, 196Ia). Other horsetail taxa worth mentioning

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Fig. 2. (a) Equisetum columnare Brongniart in growth position; Saltwick Formation ; near Hayburn Wyke, York shire; S1347; x 0.5. (b) Osmundopsis sturii (Raciborski) Harris; detail of fertile frond; Gri sth or pe Plan t Bed, Cloughton Form ation; Cayton Bay, York shire; V63876a; ap prox . x 3.5.

are N eoca lamites nathorstii Erdtrnan , N. hoeren sis (Schimper) Halle, S ehizon eur a stenophy lla Harris and Annulariopsis simps onii (Phillips) Har ris. All of these Calami tes-like species are ra re and occur only in the older beds, i.e. of late Pliensbachian and mainly Aalenian age (Ha rris, 1961a), Ferns are very common in the York shire Jurassic; the majority probably grew as under storey flora below the larger cycad and bennettitalean trees, or along river bank s. Representatives of all the 'older' fern families can be found in the Yorkshire flora, from the Mar atti aceae and Osmundaceae to the Schizaeaceae, Dicksoniaceae, Matoniaceae and Dipt eridaceae, but most families are represented only by a few taxa each. It is also noteworthy that fertile fern fossils ar e just as comm on as sterile ones. The Palaeozoic important orde r Mar att iales is repre sented in Yorkshire by only M arattia anglica (Thomas) Harris (occurring abundantl y at four Aalen ian localiti es but nowhere else) and Angiopt eris blaek ii van Cittert (known fro m two Aa lenian localities only). Both these species have been ascribed to extant genera as no difference can be recogn ized (Ha rris, 1961a; van Konijnenburg-van Cittert, 1975: Hill, 1987). Osmundaceous remains have been attributed to three genera. Fertile Osmundaceae have been placed either in the genus Todites, when the whole lower surface of the pinnule s (the finest segments of a fern frond) was covered with spora ngia (repro ductive structures in which spores ar e produced), or in Osmundopsis when the lamin ar part of the fertile frond is reduced

and co nsists only of veins covered with sporangia (Fig, 2b), as in living Osmunda. Two rare Osmundopsis species are known from Yorkshire (van Konij nenburgvan Cittert, 1996) and four much more comm on Todit es species. of which T. williamso nii (Brongniart) Seward and T. denticula tus (Bron gniart) Kr asser ar e found abunda ntly at a number of localities not only in York shire but all over the world (Harris, 196I a). Sterile remains are included in the morph ogenu s Cladophlebis when no fertile parts are kn own. Two Cladop hlebis species occur in York shire. Both have relatively large fronds and have been recorded from a numb er of other Eurasian localit ies as well (Harris, 1961a; Schweitzer et al., 1997). The family Schizaeaceae is represent ed by the common Ju rassic genera, Klukia and Stachypteris , each with one species. The Dicksoniaceae are ofte n found in the York shire Jurassic; they include six species of the genus Coniopteris; C. hymenophylloides (Bron gniart) Seward, C. m urray ana (Brongniart) Brongniart , C. simplex (Lindley et Hutton) Harris and C. bella Harris are quite comm on . C. hymenophylloides , or iginally described from Yorkshire , has been record ed from Jurassic sediments all over the world. Kylikipt eris arguta (Lindley et Hutton ) Harris. Eboracia lobifolia (Phillips) Thom as and two species of the extant genus Diek sonia have also been found. No ne of these last named is common in the Yorkshire Ju rassic, but E. lobifolia has been found world-wide. Th e fam ily Mat oniaceae is represented by the genera Phlebopt eris (with thre e species), Ma tonidiu m (with one species) and Matonia (also with one species), but none of these is really common in Yorkshire. Three genera of the fam ily Dipteridaceae occur, each with one species, i.e. Dicty ophyllum rugosum Lindley et Hutton (commo n), Clathropteris obo vata Oishi (only in lower Aaleni an beds) and Hausmannia dichot oma Dunker (ra re. only occurring at two Aaleni an localities). The famous seed fern order Caytoniales was named after the locality Ca yton Bay (near Scarborough) in Yorkshire. Th is group of plants has long been the subject of debat e. The leaves tSagenopteris) , with two pairs of leaflets and a net venation (Fig. 3a), were first included in the waterfern s because of their resembl ance to the extant genus Marsilea . When the associated fructifications (Cay ton ia, Fig. 3b) were found, Th omas (1925) believed them to be angio sperm in affinity but Harris (1940) dem on strated that the fruits were not closed but possessed a 'mouth' that was open until the ovules were pollinated by sma ll bisaccatc pollen from the staminate pollen organs Caytonanthus (a sta men consists of a stalk and one or more ant hers). Nowa da ys, the Cay to niales are considered to be a highly developed but isolated gymnosperm gro up. Anoth er gro up of seed ferns in York shire, the Corystospermales. is represented by two species of Pa chypteris, i.e. the type species P. lanceolata Bron gniar t (found at man y localities but never common ) and P. papillosa (Thomas et Bose) Harris. The latt er has been found together with its associa ted male

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Fig. 3. (a) Sagenopt eris colpodcs Ha rr is; complete la rge leaf and fragment s of small leaves; Gristhorpe Plant Bed, Cloughton Forma tion ; Cay ton Bay, Yor kshi re; S9639; x I. (b) Cayt onia sewardii T homas; seed-bea ring c upule; Gri stho rpe Plant Bed, Cloughton For ma tion; Cay ton Bay, Yorkshire; 52313; x 3.

org an Pteroma. P. papillosa is a very abundant plant in the lower Aalenian beds, often occurring in whole layers, as well as in small num bers at some Bajocian localities. It is often associa ted with marine microfossils (Ha rris, (964) and is co nsidered to have been a mangrove bush. A few other seed ferns should be mentioned: th ree species of Ctenozamites occur at a number of localities but never a bundantly. No fert ile organs are known for this genus but it closely resembles P tilozarnites and , of the latter genus, the associated male organ Harrisotheca is known. The organization of that organ places both leaf genera into the seed ferns ra ther than into any other group (Harr is, 1964). Fin ally, although no fertile organs are known, three species of the genus R haphidopteris (formerly assigned to St enopt eris; see Harris, 1964 and Barale, 1972) are considered to be seed ferns, based on their leaf morphology and their cuticle (Barale, 1972). Cycads, and especially the or der Nilssoniales, are also an importan t comp onent in the Yorkshire Jurassic flora. T oday the order Cycad ales is a rela tively rar e gro up of mainly tropical plants, but a num ber of cycad leaves and fertile organs have been found in York shire sediments. In particular, repr esentatives of Ni lssonia (nine species) are quite comm on and diverse (Harris, 1964 ; van Konijnenburg-van Cittert & Mor gans , 1999). The ir leaves were usually pinna te (a feather-like arra ngemen t of leaf segmen ts, see also Fig. 6) but in some species, they were entire. It is believed that at least N. compta (Phillips) Bronn had the leaves

arranged on short shoots (Fig . 4). Associated male t Androst robus mallis Harris) and female (Beania gra cilis Car rut hers) orga ns occur in the York shire sediment s as well, and the species has been reconstructe d as a branched tree (cont rary to living cycads) with a crow n of leaves and hanging fertile strobili (Ha rris, 1961 b). A numb er of other N ilssonia species has associated male and/or female or gans as well. Th e rar e Paracy cas cteis Harris has long nar row pinna e (firstorder divisions of a pin nat e leal), but no fertile or gans have been found so far. This is no t the case for some species of Ctenis and Pseudoctenis. Ps eudoctenis lanei Th omas (common in the olde r beds) is associated stro ngly with the male cone Androstr obus prisma Th omas et Harris, and Ctenis sulcicaulis (Phillips) Ward with A . szei Harri s (Harris, 1964; Cleal et al., 200 I). Th e oth er three Pseudocteuis species are relatively rare and no fertile orga ns have been found, just as for the othe r Ctenis species. Ctenis is distinguished fro m Pseudoctenis by its occasionally anastomosing (fused) venation. Quite a numb er of leaves that look remar kabl y like those of the Cycad ales belong, in fact, to the Bennettitales. Th e right assignment can often be determined only by cuticular analysis. A cycad cuticle can be recognized by its straight epidermal (epiderm is is the outermos t cell layer) cell walls and monocheilic stomata, while a bennettitalean cuticle has sinuo us cell walls and syndetocheilic sto mata (for explanatio n see Fig. 5). Both Cycadales and Bennettit ales ar e

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Fig.4. Nilssonia compta (Phillips) Bronn;several leaves on a short shoot;Gristhorpe Plant Bed,Cloughton Formation; Cayton Bay, Yorkshire; S8511; approx. x 0.6.

dominant elements in the Yorkshire flora, with numerous species. In the Yorkshire Mid Jurassic flora, Williamsoniaceae is the only bennettitalean family. This family consisted of shrubs and/or small trees with slender, branched stems with tufts of leaves at the end. The leaves can be assigned to various morphogenera, such as Zamites, Ptilophyllum and Otozamites for some pinnate leaves (distinguished by their leaf base) and Nilssoniopteris for undivided leaves (Fig. 6). The first three genera bore female fructifications named Williamsonia, consisting of a dome-shaped receptacle whose lower part bears numerous seeds and interseminal scales (Fig. 7e). The upper part does not bear seeds and is termed the corona. The whole was surrounded by a number of bracts (Cycadolepis) that fell off after maturation. The male inflorescences (=the flowers of a plant collectively) (Weltrichia) look like 'flowers' and were cup-shaped with an outer part divided into lobes (Fig. 7d). The lobes bear rows of

Fig. 5. Stomata of (a) bennettitalean foliage with two subsidiary cells and sinuous cellwalls and (b)cycadalean foliage with four or more subsidiary cells and straight cell walls (after Cleal et al., 2001, fig. 3.6; courtesy of JNCC).

pollen sacs. In some cases, glands have been found inside the cup, the function of which may have been to attract insects for pollination. The undivided Nilssoniopteris leaves are associated with the bisexual flowers Williamsoniella. In this case, the gynoecium (the female parts of a flower comprising the receptacle, seeds and interseminal scales) was surrounded by microsporophylls (male organs bearing the microsporangia or pollen sacs) each carrying four pollen sacs (Fig. 7a, b). Two other, related, pinnate leaf morphogenera occurring in the Yorkshire flora are Anomozamites and Pterophyllum (see Fig. 6). They show straight to slightly expanded leaf bases; Anomozamites has more or less equidimensional pinnae while those in Pterophyllum are at least twice as long as broad. Their female fructifications are named Bennetticarpus and so far have not been found surrounded by bracts. Although now only represented by one species (Ginkgo biloba Linnaeus), the order Ginkgoales had an almost world-wide distribution in the Jurassic and a number of species has been found in the Yorkshire flora, although they were not so dominant as the cycads and bennettitales. Leaves are attributed to such genera as Ginkgo, Ginkgoites, Baiera, Sphenobaiera and Eretmophyllum, depending on how often and how deeply the leaves are divided (Fig. 8). The species Ginkgo huttonii (Sternberg) Heer is attributed to the living genus Ginkgo based on its male and female fructifications that are remarkably like those of the extant one. Stems that could be attributed to the ginkgos with confidence have never been found, but it is generally believed that Jurassic ginkgos were trees like the extant one. The order Czekanowskiales is a group of gymnosperms that are still poorly understood and cannot be attributed to any of the major groups. They were originally placed in the Ginkgoales because their leaves are borne on short shoots. Those short shoots are surrounded by small scale leaves. The leaves are in

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All pinnae usually with stomata confined to abaxial surface (hypostomatic)

e-f . Otozamites acroscopic basal angle forming auricle

g-hPtiJophyl/um basiscopic basal angle decurrent

k. Anomozamites basal angles usually expanded. pinna about as wide as long

i-j. Pterophyl/um basal angles usually expanded. pinna longer than wide

I. Dictyozamites reticulate venation stomatal groove

m. Nilssoniopteris leaf entire

o

o. Pseudocycas stomata confined to abaxial central groove

n. Sphenozamites pinna rhomboid. attached laterally

Fig. 6. Some bennettitalean lea r shapes (afte r Cleal et al., 2001, fig. 3.9; co urtesy or JN CC).

bundles and are long , narrow and undivided (So lenites) or repeatedly bifurcatin g (Czekan owski a) (Fig . 9a). The female fructificatio ns (Leptostrobus) consist of an elongated axis with scale leaves below and loosely arranged seed-containing capsules a bove. Th e almos t sessile capsu les are broadly wedge-shaped and co nsist of similar lower and upper halves with usually five seeds (Fig. 9b). Pr esumed ma le fructifications belong to the genus Ixo strobus. Conifers vary in size from shru bs to large trees, and are cha racterized by woody tissue form ing stems and shoots. M ost of the living conifer families were already represented in the Yorkshire J urassic, but there were also several conife rs th at are attributed to extinct groups especially the family Cheirolepidiaceae. That fami ly was a very successful one duri ng the Mesozoic (and is the most common one in York shire) but died out a t t he end of the Cretaceous (va n der Ham et al.,

2003). Cheirolepidiaceous folia ge falls in the mo rph ogenera Brachyphyllum and Pagiophy llum (distingui shed only by the length of the free pa rt of the needle; see Fig. 10; Kendall, 1947, 1948). A nu mber of species occur in the Yorkshire flora , including their fructificatio ns. Fem ale cones are named Hir m eriella; the y exhibit bracts with axillia ry five-lobed cone-scales bea ring two ovules (un fertilized seeds) tha t are partly covered by a flap-like structure (Clemen t-Westerhof & van Konijne nburg-van Cit ter t, 1991). The sma ll male cone s (Classostrobu s s yield a very characteristic type of pollen , Classopo llis, which is kn own world-wide as an indica tor of warm climates dur ing the Mesozoic (Vakhrameev, 1991). Most living conifer families are rep resented in the Yorkshire J urassic flora by foliage and cones. such as the Araucariaceae tBruchyphyllurn mamillare Lindley et Hutt on with attached male co nes and the fema le A raucarit es phillip sii Carruthers) , the

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d Fig. 7. (a) Williamsoniella coronata Thomas; bisexual flower; Gristhorpe Plant Bed, Cloughton Formation; Cayton Bay, Yorkshire; S1349; x 2. (b) Schematic drawing of Williamsoniella flower with A the receptaculum bearing ovules and interseminal scales, B the corona, C the rnicrosporophylls, and 0 the surrounding bracts. (e) Schematic drawing of Williamsonia flower with A, Band 0 as in (b) (after van Konijnenburg-van Cittert & Morgans, 1999, text-fig. 29; courtesy of Palaeontological Association). (d) Weltrichia whitbtensis (Nathorst) Harris; male flower; Whitby Plant Bed, Saltwick Formation; Yorkshire; V3688; x I. (e) Williamsonia hildae Harris; part of receptaculum with ovules and interseminal scales; Saltwick Formation; Hasty Bank, Yorkshire; S2964; x 1.5.

Taxodiaceae (with two species of Elatides including attached male and female cones), the Podocarpaceae (with at least one species, Cyparissidium blackii Harris with male and loose female cones), Pinaceae (with Pityocladus scarburgensis Harris and its female cones) and the Taxaceae (with Marskea jurassica (Florin) Harris and its female cone). Some sterile conifer foliage is classified tentatively at family level based on its similarities to known taxa. Others (such as Lindleycladus) are assigned to an extinct family (the Podozamitaceae) whose taxonomic position is not yet clear, or are not completely understood (Bilsdalea dura Harris) even though the female fructifications are known. Southern England

The Yorkshire flora is not the only Mid Jurassic flora of the UK; in southern England, the Stonesfield flora

(Oxfordshire) is also well known, especially after the recent revision by Cleal & Rees (2003). Some 25 species, dominated by conifers, Bennettitales and the gymnosperm Pelourdea (unknown affinity), constitute this flora in the Taynton Limestone Formation (Bathonian). Contrary to Yorkshire, where the vegetation probably grew in fluvio-deltaic habitats, this flora represents mainly coastal vegetation (including mangrove plants); the Pelourdea leaves might have drifted in from more upland areas (Cleal & Rees, 2003). Floras similar to those from Stonesfield but with less diversity can be found at Eyeford (Huntsman's Quarry) and Sevenhampton, Gloucestershire (Cleal et al., 2001). Brachyphyllum remains similar to those from Stonesfield occur as compressions in mudstone in the Oxford Clay (Callovian) of Christian Malford, Wiltshire (Kendall, 1947). The Stonesfield assemblage is also similar to some contemporaneous floras preserved in marine deposits in France

JURASSIC FOSSIL PLANTS OF THE UK

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a Fig. 8. (a) Gingko huttonii (Sternberg) Heer; specimen with a number of leaves; Scalby Formation; Scalby Ness, Yorkshire; S12618; x 0.25. (b) Baierafurcata (Lindley et Hutton) Braun; single leaf; Scalby Formation; Scalby Wyke, Yorkshire; S1259; xl.

Fig. 9. (a) Solenites vimineus (Phillips) Harris and one capsule of Leptostrobus cancer Harris; Cloughton Formation; Cloughton Wyke, Yorkshire; S6380; x 2. (b) Schematic drawing of part of Leptostrobus cancer Harris strobilus (after van Konijnenburgvan Cittert & Morgans, 1999, text-fig 38; courtesy of Palaeontological Association).

(Barale, 1981) and northern Italy (e.g. Wesley, 1956, 1958). Clubmosses and horsetails are absent from the Stonesfield flora, while ferns are represented only by a few taxa (one species each of Dictyophyllum, Phlebopteris and Coniopterisy. Seedferns were more common, including Sagenopteris colpodes Harris, cf. Ctenozamites leckenbyi (Leckenby) Nathorst, Pachypteris macrophylla (formerly also known as

Sphenopteris? macrophylla and Hymenophyllites macrophylla) and Komlopteris speciosa (formerly known as Thinnfeldia speciosa); none of these species is very common. Cycad ales are rare as well; only one specimen of Ctenis cf. sulcicaulis and one of Ctenis sp. have been recorded. Bennettitales are more common, with three leaf species: Ptilophyllum pectiniformis with over 25 good specimens, P. cf. hirsutum with one specimen and Sphenozamites bellii with six specimens.

68

.I . H . A . VAN KONI.INENB UR G-VAN C I TT E R T

Fig. 10. Pagiophyllum maculosum Kendall; several shoots; Cloughton Formation; Cloughton Wyke. Yorkshire; S311 9; x l.

One possible ray of a male bennettitalean flower was described by Cleal & Rees (2003) as ? Weltrichia sp. Two specimens of entire cycadophyte leaves of unknown affi nity have been designated as Taeniopteris vittata. Finally . Cleal & Rees (2003) re-introduced the genus Conites, not as a genus for cones of unknown affinity but as a senior synonym of Bucklandia, used for casts and compressions of cycadoph yte stems . Its type species, Conit es bu cklandii, is known from Stone sfield. Ginkgoales are represented in the Ston esfield flora by one specimen of Ginkgo aff. longifo lius; the order Czekanowskiales is absent. In contra st. conifers are a very common element in the flora. Brach yph yllum expansum with leafy shoo ts is represented by c. 25 specimens and associated male cones . Moreover, larger associated cone s were interpreted formerl y as the female cone s of this taxon (Seward, 1904; Kendall , 1949b), without an y evidence of ovules (unfertilized seeds) being present. Cleal & Rees (2003) considered these cones to be male (Classostrobus sp.) and of cheirolepidiaceous affinity, but there is no evidence of pollen sacs. so thi s attribution is doubtful as well. The family Araucariaceae is represented by abundant female cone scales, A raucarites brodiei, and one probable male cone. Finally, three specimens of Elat ocladus cf. laxus (of unknown family) are recorded . The most common taxon in the Stone sfield flora is Pelourdea megaphylla (Fig. II a), a gymnosperm genus of possible coniferalean affi nity and known mainl y from Triassic sediments (see Cleal & Rees , 2003). At first glance, the leaves resemble those of the Palaeozoic genus Cordaites but no cordaitalean reproductive structures are known from the Mesozoic. A possible Pelourdea seedlin g resembles those of the Tri assic herbaceous conifer A etophyllum stipulare (G ra uvogel-Stamm, 1978). Apart from a number of seeds placed in Carpolithes, one more fossil taxon deserves attention: a leaf attributed to the morphogenu s Phylli tes sp. This entire-margined leaf (Fig. II b) look s like a dicot yledonous leaf with three primary veins radiating from a point ju st above the leaf base. The central primary extends to the apex, the lateral ones curve slightly in their upper regions and appea r to

Fig. II. (a) Pelourdea megaphylla (Phillips)Seward; small leaf with part of stem still attached at base of leaf; Taynton Limestone Formation; Stonesfield, Oxfordshire: V4652; x l. (b) Phyllit es sp.; complete leaf with petiole and angiospermlike venation; Taynton Limestone Formation; Stonesfield, Oxfordshire; V4 1385; x I (both courtesy of Dr Chris Cleal).

end at the leaf marg in. A couple of secondary veins can be seen on the out side of one of the primaries. There is no evidence that this fossil represents an angiosperm but the possibility cannot be ruled out. Finally , a recently discovered permineralized fossil cone found in a nodule on the floor of a gravel pit in Ashton Keynes near Cirencester in Lower Callovian marine sediments, should be mentioned . Classostrobus crossii Rothwell et aI. is the first cheirolepidiaceous pollen cone with preserved internal anatomy (Ro thwell et al. , 2007). So far , no other plant fossils have been found at this localit y. Scotland

As well as the very fine floras from York shire (fluviodeltai c) and Sto nesfield (coastal), a numb er of smaller floras have been described from Scotland . Fir st, a florule from the Bathonian Brora Coal (Stope s, 1907; Ha rris & Rest. 1966) has many similarities with the Mid Jurassic York shire flora. Some 12 fossil plan t species were record ed from a drifted plant bed outcropping on the beach (Stopes , 1907). The assemblage is dom inated by hor seta ils (three species of Equisetit esi and ferns (two species of Osrnundaceae, two

J UR A S SI C FOSS I L P L A N T S OF TH E UK

Coniop teris species and Dictyophyllum sp.), suggesting tha t it represents vegeta tion growi ng close to the site of deposition . Also present a re the bennettit alean Zam ites sp. and Otozam ites sp., as well as a cheirol epidiaceous conifer (recorded as Ch eirolep is sp.), and Gingk o digitata (Brongniart) Heer leaves which are the best preserved fossils in the assembl age. Harris et al. (1974) suggested, however, th at th ese leaves resembled Sp henobaiera gy ron from Hasty Bank (Yorkshire) ra ther than Ginkgo digitat a. Har ris & Rest ( 1966) also described a florule fro m the Bror a Co al, comprisi ng fusai ned wood (mainly con ifer) and cuticle fragments and lycop od (club moss) megaspores (five species of Trilet es). Th e commonest cuticle fragments belon g to Equise ti tes species; cuticles of th e Yorkshire conifer species Farn dalea jragilis Bose were also found, and a new Pterophyllum species, P. cyc adites Harri s et Rest, which was also recorded from Yorkshire. A recentl y discovered florule from the Isle of Skye sho uld be mentioned. It comprises 15 morphospecies (origina ting from 8-11 whole plant species), including calcareo usly perm ineralized cyca dophyte leaves and conifer strobi li (Batema n et al., 2000). Ther e are at least two distinct associa tio ns: a coas ta l/deltaic community, domin ated by th e araucarian con ifer complex Brachyphy llum cf. mamillare, cf. A raucarites and Taxodioxylon, admixed with some cheiro lepidiaceo us rema ins of B. cf. crucis; and an inlan d communi ty represented primarily by fusai ned remains of the matoniaceous fern genus Phlebopteris.

4. LATE J URASSIC Knowledge abo ut Lat e J urassic flora s comes mainly fro m Scotland (Kimmeridg ian to Portl andian ) and from the Purbeck Group (Po rtlandian) in southern England . Scotland Late Jurassic floras are generally rare in Europe (Vakhrameev, 1991), so the presence of a well-developed flora in Scotland is an exception. For most of the Jurassic, Scotland was an upland area with little sedimenta tion and thus no palaeobot anical record. However, sedimenta tion took place around the eastern and western coasta l areas, preserving at least partial evidence of vegetation growing in th e vicinity (see also the Mid Jur assic floras from Bror a and Skye, discussed above). On the east coas t, the Kimmeridgian plant- bea ring deposits of Sutherla nd belong mainly to two lithostratigra phical units: the Lothbeg Siltstone (ma rine beds with dri fted-in plant rema ins) and th e Helmsdale Boulder Beds, of which th ose at Culgowe r have the largest plan t fossil diversity (van Konijnenburg-van Citte rt & van der Burgh, 1996; Cleal et al., 200 1). M ost of the plant s were prob abl y growi ng along or near to riverb an ks in a lowland a rea . Th e flora of the Lothb eg Siltstone comp rises material th at was washed into the

69

Fig. 12. Hausmanni a buchii Andrae; leaf fragment pre served as fusain; Lothbeg Siltstone; Loth beg Point. Sutherl and; S13463; x 2.

sea and broken up by wave action ; hence the presence of relatively small fragme nts of sto ut plant s and the absence of large a nd thin leaves. Ferns form a major compo nent of the flo ra. Fragment s of Gleicheni tes cycadina (Schenk) Sewa rd, Phlebopteris dun k eri (Schenk ) Schenk a nd Hausmannia buchii (An d rae) Seward are especially common, and it is notewo rth y th at they often occu r as the coal component fu sain , which explains th eir preservation in the marine sha les (Fig . 12). It is believed tha t these ferns were eithe r par t of a salt-brackish marsh vegeta tion or that they grew on heathl and (van Konijnenburg-van Cittert & van der Burgh, 1996). In the Helmsdale Bould er Beds at CuIgower , remains of A ngiop teris boweri (Seward) van Konijn enburg-van Cittert et van der Burgh , Hausmannia dichot oma Dunker and Todites williamsonii, representing fern s from a moist-lu sh vegetatio n, are comm on , together with some rarer species. Two seed ferns are common elements in the Sutherland floras, especially in the Boulder Beds: Sagenopteris ph illipsii (Brongnia rt) Presl and Pachypteris lanceolata Brongniart. Fragments of the very thick-l eaved Cycadop teris jurensis (Kur r) Hirmer have been recorded onl y fro m th e Lothbeg Siltsto ne. Th is plant may have been a man gro ve or salt marsh species. Cycadophytes (including two Nilssonia species, two to th ree Pterophyllum species , a Zamites and a Wi lliam sonia) also occur. Th e type species of the genus Pseudoctenis, P. eathiensis (Richa rds) Sewa rd was named after Eathie (in Ros s & Cro mar ty (Black Isle» , where a permineralized Kimmeridgia n flora was found (see below and , for example, Seward , 1911) but also occur s in the Loth beg Siltstone and at Culgower. Large leaf fragmen ts are found at the latter localit y (Fig. l3a). The orde r G inkgoales is rare in Sutherla nd bu t two czeka nowski alean taxa are commo n, especia lly in the marine beds, where even complete leaves of

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Fig. 13. (a) Pseudoctenis eathiensis Seward; fragment of a large leaf; Helmsdale Boulder Beds; Cuigower, Sutherland; S14933; x 0.75. (b) Elatides curvifolia (Dunker) Nathorst; several branched shoots; Helmsdale Boulder Beds; Cuigower, Sutherland; S13487; x 0.66.

Czekanowskia rigida Heer and Phoenicopsis gunnii Seward have been found. Czekanowskialean plants are considered to be indicators of a moderately warm climate (Vakhrameev, 1991). Two conifer species are very common elements in the flora: Elatides curvifolia (Dunker) Nathorst found together with attached male and female cones mainly in the Boulder Beds of Culgower (Fig. l3b), and Tritaenia scotica (van der Burgh et van Konijnenburg-van Cittert), which is the commonest element in the whole flora and especially in the Lothbeg Siltstone (Fig. 14a). Leaves of this conifer, of unknown affinities but possibly taxodiaceous as suggested by the pollen of the associated male cone Masculostrobus zeilleri Seward, occur in large quantities at all the localities. Brachyphyllum eathiensis Seward is another relatively common conifer at Culgower, and its female cone scales Araucarites milleri Carruthers have been found as well. The two conifers Elatocladus jeffreyi (Seward) van Konijnenburg-van Cittert et van der Burgh, and Podozamites sp. occur rarely at CuIgower. The latter looks like a 'normal' Podozamites leaf but with a petiole (leaf stalk) (Fig. 14b), a feature not encountered in other Podozamites species. There is a slight possibility that these leaves are of angiosperm rather than gymnosperm nature (van Konijnenburg-van Cittert & van der Burgh, 1989).

Finally, a few words on the permineralized Kimmeridgian flora from Eathie, mentioned above; this flora consists mainly of cycad (e.g. Pseudoctenis eathiensis, Nilssonia orientalis Heer), bennettitalean (Zamites spp.) and conifer (e.g. Brachyphyllum eathiensis with Araucarites milleri, Elatides curvifolia and Sphenolepidium cf. kurrianum Dunker) remains. Most of these plants occur at CuIgower as well. Of particular interest are the bennettitalean flowers Williamsonia scotica Seward (Seward, 1912). Southern England The lower Purbeck Group of Dorset is of Portlandian age, representing the youngest part of the Jurassic Period. The quarries on the Isle of Portland are famous for a silicified forest of stumps and branches of the cheirolepidiaceous conifer Protocupressinoxylon purbeckensis Eckhold (Francis, 1983). Most of the stumps are preserved in situ and a number are encircled by algal stromatolites. The algae probably grew around the trees when they were drowned by rising water. Francis (1984) calculated that the trunks were spaced on average between 3 m and 5 m apart, suggesting an open forest, and with well-defined growth rings indicating a seasonal climate in a semi-arid environment. The Isle of Portland is also famous for its

JURASSIC FOSSIL PLANTS OF THE UK

71

Cycadeoidea gigantea Seward. Diamond-shaped ridges, representing leaf petioles, cover its surface.

5. CONCLUSIONS

b

a Fig. 14. (a) Tritaenia scotica (van der Burgh et van

Konijnenburg-van Cittert) Wilde; complete leaf; Lothbeg Siltstone; Lothbeg Point, Sutherland; S13508; x I. (b) Podozamites sp.; leaf with petiole;Helmsdale Boulder Beds; Culgower, Sutherland; SI4926; x I.

cycadeoid stems (from the family Cycadeoidaceae, in the order Bennettitales). Seward (1897) described a well-preserved one (1.18 m high and 1.7m in girth) as

During the Early and Mid Jurassic, the UK area was situated in the subtropical belt of the Northern Hemisphere (Vakhrameev, 1991), which had a mainly humid climate. The woody vegetation was dominated by conifers (especially the families Cheirolepidiaceae and Araucariaceae) and the order Ginkgoales. Representatives of the orders Cycadales and Bennettitales were very diverse, with both weakly ramified columnshaped small trees having a crown of palm-like leaves, and thin-stemmed branching trees. The understorey was formed by, for example, ferns and seed ferns, the latter often as small shrubs. The climate during the Early and Mid Jurassic (see Sellwood & Valdes, 2008) was not completely uniform and this can be detected from the plant fossils. For instance, the onset of the Toarcian coincides with the beginning of a warming interval; an increase in cheirolepidiaceous conifers, both macrofossils and pollen, that are indeed indicators ofa warm (sub)tropical climate occurs at this level (Vakhrameev, 1991). At the outset of the Late Jurassic, a prolonged warming set in. This is reflected in the flora from southern England with its forest of cheirolepidiaceous conifers. The Scottish flora, however, shows a few fossil plants that are considered to be indicators of a more temperate climate, such as the czekanowskialean Phoenicopsis gunnii (Vakhrameev, 1991). The number of conifer remains in this flora is relatively high and a number of fossil plants occur, mainly as fusain, indicating the occurrence of wildfire and probably more arid habitats.

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Manuscript received I September 2006; revised typescript accepted 20 August 2007