Chapter 3 The Jurassic palaeobotany of Yorkshire

B.A. Thomas and D.J. Batten

Introduction

The Cleveland Basin in northern England (see (Figure 3.2) and (Figure 3.12) is one of the classic areas in the world for Mesozoic palaeobotany. This is partly because it is located in Europe where much of the early research in palaeobotany was centred. However, it also reflects the diversity and fine preservation of the floras, which are among the best in the European Palaeoarea (sensu Vakhrameev et al., 1978) that extended over most of the northern mid-latitudes in Early and Middle Jurassic times, including much of Europe and North America. Over 300 species have been identified from Yorkshire, including spore-producing clubmosses, horsetails and ferns, and several groups of seed plants such as cycads, bennettites, caytonias, ginkgos and conifers. Many of these species have their type locality in Yorkshire.

Incorporation of plant material into the accumulating delta sediments led to the fossiliferous plant beds that have been found at over 500 sites throughout Yorkshire. Inevitably some deposits are richer in species than others and it is these that have yielded the most information through recognition and association of separated parts from the plants of one species. The preservation is often very good, permitting the preparation of cuticles and the recovery of spores and pollen from fructifications. A complete list of the described species from the GCR sites is given in (Table 3.1) (pp. 32–6).

History of research

Fossil plants from the Yorkshire coast were first described and rather crudely figured in 1822 by Young (a Nonconformist minister) and Bird (an artist who for a time was also curator at the Whitby Museum). Although this work had little scientific merit it did, however, encourage others to search for fossils in the area. Further collecting and the interest it generated led to the establishment of both the Whitby and Scarborough museums.

William Bean and his nephew John Williamson were the two most active collectors of Yorkshire Jurassic plants in the early 19th century. Bean was an incredibly enthusiastic collector. He exchanged and sold specimens to such an extent that they can be found in many other British and foreign museums. Williamson, through his enthusiasm, became the curator of the Scarborough Museum. He knew William Smith, who has been described as 'the Father of English Geology', Smith's nephew John Phillips and Sir Roderick Murchison. Smith was originally a surveyor and the author of the famous Map of the Strata of England and Wales published in 1815. In 1813 he visited north-east Yorkshire to examine some of the coal pits that were scattered through the central moorlands in order to report on their economic importance. Coal had been mined spasmodically in the area from 1648, but, by the mid-18th and early 19th centuries was concentrated in the Coxwold–Gilling Trough. The largest mine was the Birdforth Colliery c. [SE 481 772], which was sunk to a depth of 46 metres in 1760 and employed more than 30 men until it closed in 1798. Most mines were, however, much smaller adits that were worked for fuel to burn lime, which was very much in demand as a soil fertilizer. In the summer of 1821, Smith published his geological map of Yorkshire in four sheets. In 1824 he moved, with his nephew, to live in Scarborough. Then, in 1829, Phillips published his Illustrations of the Geology of Yorkshire, which for the first time placed the geology of east Yorkshire on a sound scientific basis. He included descriptions and drawings of fossil plants, but the latter were far from accurate.

At the same time, the renowned French palaeobotanist Adolphe Brongniart was working on his great publication Histoire des Végétaux Fossile, which was published in Paris (Brongniart, 1828b-1838). He described and figured 22 species of Jurassic plants from the Yorkshire coast in the second of these works.

The important Fossil Flora of Great Britain by Professor John Lindley and William Hutton was published in parts between 1831 and 1837. It contained drawings and descriptions of several Yorkshire Jurassic plants supplied by Bean, Phillips and Dr Murray Dunn (secretary of the Literary and Philosophical Society of Scarborough) but most importantly by John Williamson's son, William Crawford Williamson (1816–1895), who was later to make his name in Carboniferous palaeobotany after becoming Professor of Botany and Geology at the University College in Manchester. He is now known as the founder of modern palaeobotany in Britain.

Williamson drew the Yorkshire Jurassic specimens for the Fossil Flora of Great Britain, which came as a surprise to Lindley when Williamson enrolled as a student of his in London (Williamson, 1896). One species figured by Lindley and Hutton illustrates especially well how they relied on the Scarborough collectors. Bean was mentioned several times in their book as supplying the fossils and they named one, Cyclopteris beanii, after him. Interestingly they pointed out that it was in fact John Williamson who had collected it, William Williamson who had described and drawn it, and Mr Dunn who had given an account of its usual appearance, while William Bean had merely sent it to Hutton. Brongniart (1849) later transferred this species to the genus Otozamites.

(Table 3.1) Records of plant fossils from the Yorkshire Jurassic GCR sites. These records have been gleaned from published accounts, largely by Harris (1961a, 1964, 1969, 1979a,b; Harris et al., 1974), Hill et al. (1985), Hill and van Konijnenburg-van Cittert (1973), Spicer and Hill (1979), van Konijnenburg-van Cittert (1971, 1975a,b, 1978, 1981, 1987, 1989), and van Konijnenburg-van Cittert and Morgans (1999), from archived field notes in the Natural History Museum (London), and from examining collections in that museum and the National Museum and Gallery Cardiff. Records known to fall outside the boundaries of the sites have been omitted, but those over which there is some doubt have been included.

A significant discovery made by Lindley and Hutton was that cuticles could be prepared from these fossils. This should have enabled the study of the flora to be developed in a significant way, using anatomical data of a type that was then rarely used in palaeobotany. However, for reasons that are difficult to explain, their discovery seems to have been effectively ignored and it was not until nearly a century later that H. Hamshaw Thomas started properly to develop cuticle studies on these fossils (see below).

Leckenby (1864) described and figured some new specimens, most of which are now in the Sedgwick Museum in Cambridge. Other scattered publications make reference to fossil plants from the Yorkshire coast, but the next major piece of work was by the Swedish palaeobotanist Alfred Nathorst. This was an unillustrated account, in Swedish, of his visit to English museums and the coast in 1880. Fox-Strangways and Barrow (1892) published a long list of plant fossils in the second edition of the Geological Survey Memoir on the region. Interestingly an acknowledgement is given in this work to the help given by the palaeobotanist Clement Reid who is better known for work on Tertiary plant fossils.

Albert C. Seward (1863–1941) was probably the most important British palaeobotanist living around the turn of the 20th century (for bibliographical details see Harris, 1941c). He had, in fact, studied with William Williamson in Manchester in 1886, although he subsequently concentrated on Mesozoic plants for most of his career. In 1900, Seward catalogued the fossil plants in the British Museum (Natural History) (now the Natural History Museum, London) that had been collected from the Yorkshire coast, although in so doing he included details of specimens that he had examined in other British and continental museums (Seward, 1900a). Seward was a prolific writer and included details of Yorkshire Jurassic plants in his seminal four-volume work, Fossil Plants (1898–1919). Unfortunately his catalogue showed no fundamental difference in technique from that of Brongniart. The fossils to both of them were 'impressions with carbon' and, although Seward knew how to prepare cuticles and must have been aware that Lindley and Hutton had already prepared them from Yorkshire fossils, he did not believe that they could be of any value (Harris, 1961a).

This idea that fossils were merely 'pictures on the rock' changed with the work of the Swedish palaeobotanist Nathorst and his colleague Thore Halle. They prepared cuticles from specimens that they had collected themselves and showed that cuticle characters were of supreme importance, especially when studying reproductive organs. As Harris (1961a) has pointed out, the large slabs of foliage that previously had always been sought after were suddenly no longer as important. Collecting had passed into the hands of the researcher, and Nathorst and Halle, and then Rudolf Florin, all came from Stockholm to Yorkshire for the purposes of collecting material for research.

Hamshaw Thomas (1885–1962) was, like Seward, at Cambridge for most of his career, but, unlike Seward, Thomas believed passionately in fieldwork and collecting for himself (for bibliographical details, see Harris, 1963). He had worked for a while with the Scottish palaeobotanist Robert Kidston and then with Nathorst in Sweden where he appreciated fully the value of cuticle studies. In fact, Thomas came back with the extreme view that the fossil was not merely a black mark on the rock but a whole organ. If he could not see every cell of such a compressed organ then it was a failure of technique or observation. Thomas went to Yorkshire to collect for himself, although the localities were forgotten and he was told that they were worked out. Collecting cannot have been easy at first and he only rediscovered the famous Gristhorpe Bed when he had given up looking for plant fossils and was collecting a seaweed.

The seaweed was firmly attached to a piece of rock that broke off with it revealing a plant fossil. This was an incredible piece of luck considering the wealth of material that has subsequently come from this bed. Another important discovery of his was a layer of thick leathery brown leaves (Pachypteris lanceolata) found in a landslide at Roseberry Topping. He even stuck them onto Christmas cards for his friends. Thomas collected a great number of specimens between 1910 and 1914, often with the help of an amateur, the Reverend George Lane of the Cleveland Naturalists. The Great War interrupted Thomas' research but he returned repeatedly to Yorkshire for several years afterwards.

Thomas published a number of papers on Yorkshire Jurassic plants. In 1913, with Bancroft, he studied cycadophyte leaves and divided them into two groups on epidermal characters they saw in cuticle preparations. This division into the cycads and what has become known as the cycadeoids is still good today. Perhaps his best-known work, however, was on the Caytoniaceae when he showed that the seed-bearing Caytonia, the pollen organ Antholithus and the leaves Sagenopteris all belong to the same plant (Thomas, 1925). This was based on frequent association in some strata, their complete absence from others, and the close similarity of epidermal structure in all their petiole-like bases. His work on interpreting Caytonia itself involved collecting hundreds of specimens, dissecting some, sectioning others and preparing cuticles from yet more. This was his greatest work. Unfortunately, Thomas was too much of a perfectionist, like Kidston who influenced him. He believed that he should collect for himself, study, and then go back for more until he had sufficient material to confirm all the points on which he would publish. As a result, only a fraction of his work on the plant fossils he discovered appeared in print. He stopped collecting around 1925, although from about that date staff at the British Museum (Natural History), notably Maurice Wonnacott, started collecting, eventually amassing a great amount of material.

Next on the scene was Tom Harris (1903–1983), who went to Cambridge and ended up working with Seward. After studying the Greenland Rhaeto–Liassic flora and visiting Stockholm to learn cuticle preparation from Halle, he eventually made the Yorkshire Jurassic succession his own (for bibliographical details, see Chaloner, 1985). Harris moved to Reading in 1935 where he became Professor of Botany and stayed for the rest of his career. He started working on the Yorkshire Jurassic material during the early years of World War II, having realized that Hamshaw Thomas had not published on the flora since 1925. After the war he visited Yorkshire many times, collecting from all the localities that he could find, often with research students or on holiday with his family, and travelling around on his bicycle. His aim was to look for plant fossils in every exposure, and he spent a great deal of time walking the length and breadth of northern Yorkshire. In so doing he found over 500 localities, although very few of them yielded assemblages worthy of study. Fortunately, Harris' approach, although rigorous, was not perfectionist. In 1947 he expressed the view that knowledge of the Yorkshire Jurassic flora should be brought up to date (Harris, 1947), so he went ahead and did just that. He published his work in a long series of papers mainly in the Annals and Magazine of Natural History (Harris, 1941a, 1942a,b, 1943a,b, 1944a,b, 1945a–c, 1946a–c, 1948, 1949a,b, 1950, 1951, 1952a,b, 1953). Later, he compiled all the species accounts, together with new work, in the five-volume Yorkshire Jurassic Flora (1961a, 1964, 1969, 1979a; Harris et al., 1974). Among his papers Harris cleared up a problem that Thomas had conveniently sidestepped; he recognized two species of Caytonia. Harris not only showed that there were two species of Sagenopteris and then that there were two species of the male pollen-producing organs, but also that there were good associations between the two respective sets. He also cleared away the possibility that Caytonia was angiosperm-like when he found gymnospermous bisaccate pollen inside it. Thomas had found pollen attached to the lip, which he thought to be the stigma of a fruit. However, Harris' work demonstrated quite clearly that the pollen that had landed on Thomas' stigma' were the failures, not the successes. The successful gymnospermous pollen grains were inside what Harris was now calling a cupule.

Although Harris published a great deal and appears to have found out so much, he always maintained that his work was certainly nowhere near finished and that further collecting would turn up new discoveries. He believed that the collector was more likely to become exhausted than the locality.

A number of papers were published by Harris' research student Mabel Kendall (1947, 1948, 1952), and by some of his undergraduates in projects that partially fulfilled the requirements of their BSc degrees. Others workers have continued Harris' approach to the Yorkshire Jurassic succession. Christopher Hill, then at the Natural History Museum, London, was fortunate in having material from a 'new' site north of Whitby (Runswick) brought to his notice by Ron Williams (an amateur collector). Han van Konijnenburg-van Cittert (Utrecht University) has also been very active in palaeobotanical research on the Yorkshire Jurassic flora, and more recently Helen Morgans (Leicester University) has studied the conifer woods from the succession (Morgans, 1999). Together, these two have written a field guide to the Yorkshire Jurassic flora of four coastal sites (van Konijnenburg-van Cittert and Morgans, 1999).

It is perhaps interesting at this point to contemplate one of Harris' own reflections on his work. In 1974, he suggested that he should have attempted to continue with his earlier Greenland studies rather than turning to the Yorkshire Jurassic succession, on which the most obvious work had already been done. He even went so far as to suggest that he should have concentrated his efforts on the 10 good localities rather than spending about 30 two-week trips walking many thousands of miles searching the hills for indifferent localities. This reflection on his choice of study area was probably meant to be thought-provoking rather than a damning assessment of his major work. His comments about the comparative value of the sites is, how ever, very pertinent to GCR work, for only those localities yielding a selection of floras have been judged to be worthy of GCR status and described in the present volume.

Palaeogeographical setting

During Early and Middle Jurassic times, most of the world's land remained connected together in the Pangea Supercontinent (Figure 3.1). It was a time of 'greenhouse' conditions, with relatively high atmospheric CO₂ levels and high global temperatures, although towards the end of the Middle Jurassic Epoch there is evidence of a lowering of temperatures (Frakes et al., 1992). There was also a relatively low temperature gradient between low and high latitudes, with little ice being present at the poles.

There was some latitudinal variation in land vegetation, but nothing like that seen today; there were, for instance, no equivalents of today's tropical rain forests or tundra vegetation (Ziegler et al., 1994). Consequently, it is not normal to recognize different palaeokingdoms in the floras of this time. Three palaeoareas were, however, delineated by Meyen (1987): the Siberian, in north-eastern Europe, Siberia, Kazakhstan and northern China; the European, which includes North America, northern South America, most of Europe, the Caucasus, Middle and south-eastern Asia, and southern China; and the Notal encompassing the southern Gondwana continents. In addition, a large part of the tropical land area was a desert. In this palaeofloristic scheme, Yorkshire lies within the Europe Palaeoarea.

Alexander (1992) suggested that the British climate during the Middle Jurassic Epoch was warm, wet and subtropical because of the wide range of sand-body shapes that were generated as a result of variations in fluvial channel form, migration, and aggregation patterns. The common occurrence of fine sediment fractions in the channel deposits is also characteristic of humid, and subtropical, marine-influenced accumulations, because flocculation of fine sediments occurs at lower salinity in warmer climates. This interpretation of the climate is supported by Morgans' (1999) observation that Jurassic conifer woods from Yorkshire show distinct growth rings with narrow late wood and low to moderate annual sensitivity, all characters of little seasonal change and no shortage of water. Using multivariate analyses of spore and pollen assemblages, Hubbard and Boulter (1997) concluded that the Middle Jurassic climate was subject to regular, but not extreme, oscillations with perhaps increasing humidity. Temperatures dropped slightly near the Aalenian-Bajocian transition and then increased gradually until the Callovian. The Callovian climate was probably warmer and more humid than that of the Bathonian Age.

The high global temperatures and absence of significant polar ice meant that sea levels were relatively high. Consequently, much of Britain was below sea level during the Early Jurassic Epoch. However, a significant regression of the sea occurred in Britain during late Aalenian times, owing to the influx of significant quantities of clastic deltaic sediment into the area (Bradshaw et al., 1992). It was on some of this sediment, derived from the Mid North Sea High, that the Yorkshire Jurassic swamp vegetation developed (Figure 3.2).

Stratigraphical background

The Jurassic succession of the Cleveland Basin is lithologically varied. The Lower Jurassic Series is entirely marine, whereas the Middle Jurassic Series reflects a continuance of fully marine conditions in the south but predominantly freshwater conditions in the north. The stratigraphy of the northern part of the basin is further complicated by lateral changes in facies resulting from extensive input of deltaic sediment from the Mid North Sea High. The general stratigraphical scheme used in this chapter is summarized in (Figure 3.3).

The first geological survey of the area by Fox-Strangways and Barrow (1892) provided what was to become, for many years, the standard comprehensive stratigraphical account of the region. His divisions are still in use although their names have changed. The top part of the Lower Jurassic Series belongs to the Blea Wyke Sandstone Formation (Lias Group). This is conformably overlain by up to 12 m of the Dogger Formation, the lowest unit of the Middle Jurassic Series in Yorkshire, which is Aalenian in age. The Dogger is a marine facies and contains no plant remains. Where it is exposed on the foreshore to the south of Whitby, ammonites of the opalinum Zone have been found. Specimens from other sites have been referred to the murcbisonae Zone, which suggests that the various elements of the Dogger span a considerable period of time (Cope et al., 1980a).

The Hayburn (Saltwick) Formation (the 'Lower Deltaic Series' of earlier authors), which overlies the Dogger, consists of fluvio-deltaic deposits including channel sandstones and levee accumulations. They are predominantly sandstones with fossiliferous shales and poor quality coals. The flora is well known from Runswick Bay, the Whitby to Saltwick coast, Broughton Bank, Roseberry Topping and Hillhouse Nab. Subsequent flooding of the area by marine water from the open sea to the east led to the deposition of the Eller Beck Formation with its large fauna of bivalves.

The sediments of the succeeding Cloughton Formation (Middle Deltaic Series' of earlier authors) have been described as coal-measures facies cut by channel sandstones (Cope et al., 1980a). They have been referred to two members. The lower Sycarham Member consists of thick-bedded sandstones with fragmentary remains of plants. It is capped by marine deposits that have been referred to the Lebberston Member, although Cope et al. (1980a) renamed it the 'Cayton Bay Formation', and subdivided it into the Millepore Bed and Yons Nab Bed. Cope et al. (1980a) suggested that the Cloughton Formation should be restricted to the non-marine sandstones comprising the Sycarham and Gristhorpe Members. The beds of the Gristhorpe Member include sandstones, shales and the coals that were once worked in the area. Some of the lacustrine shales are very fossiliferous and include most of the famous coastal exposures south of Saltwick, such as the Gristhorpe beds at Cayton Bay, Hayburn Wyke and Cloughton Wyke. Extensive sheet sands/sandstones of crevasse-splay origin overlie the shales and are themselves channelled with sandstone infill.

The fourth marine transgression led to the limestone, sandstone and shale succession of the Scarborough Formation, which is the highest and best-developed marine sequence in the Bajocian strata. Different associations of marine faunas suggest a changing coastal environment from shallow marine to open-marine shelf and open marine conditions. This period of marine deposition was in turn succeeded by the last, mainly Bathonian, deltaic phase, represented by the Scalby Formation (Upper Deltaic Series' of earlier authors). The problem is that, apart from the White Nab Member, which has yielded Teloceras blagdeni Subzone faunas, the rest of the Scarborough Formation is devoid of fossils that would permit correlation with the standard marine section. There is still some debate about the depositional environments represented; this is centred on the influence of saline tides. The Long Nab Member of the Scalby Formation contains current-bedded sandstones with many small sand-filled channels that are sometimes packed with the remains of tree trunks. The overlying silty micaceous shales contain locally abundant Ginkgo leaves, ferns and conifer cone scales that are easily seen at Scalby Ness.

Fox-Strangways (1892) suggested that the non-marine beds were estuarine in origin, but Kendall and Wroot (1924) and Black (1929) preferred a deltaic interpretation. Since then the debate has centred on deltaic versus coastal alluvial plain environments (Rawson and Wright, 1992).

Plant fossils are often valuable stratigraphical markers and have been used to great effect in the Carboniferous System. This is not the case in the Yorkshire Jurassic succession. Although floral changes do occur, there is nothing to compare with the fine zonation seen in the ammonites (Harris, 1952c).

Yorkshire Jurassic vegetation

Bryophytes

Very few bryophytes are known from the Yorkshire Jurassic succession and all are thalloid. Harris (1961a) referred those with characters exclusive to the hepatic thalloid liverworts to four species of Hepaticites. Fossils that could be either algae or thalloid liverworts were placed in Thallites. No moss-like remains have ever been found. Harris concluded that the bryophytes are unsatisfying fossils because they fail to provide evidence that could lead to a better understanding of the inter-relationships of the living families.

Lycophytes (clubmosses)

Lycopodites falcatus is the only species of lycopsid shoot known from the Yorkshire Jurassic succession. The fossils have straight main branches (2-mm thick) with small leaves in pairs or whorls. The lateral branches (1-mm thick) fork unequally in one plane and have spreading lateral leaves and much smaller adpressed ventral and dorsal leaves. Several associated cones are known, but none has yielded spores.

A number of lycopsid megaspores have been described from bulk maceration of rock. Some probably came from lycopsids that grew at some distance from the swamps and pools where the spores were trapped, but others are likely to represent plants that inhabited these perennially wet places. Harris (1961a) referred them to 15 species of Triletes, but this megaspore genus is now regarded as being too broadly based to have any taxonomic value. All of the species have since been transferred to other genera that are based on more narrowly defined morphological characters, as follows: Aneuletes Patera, Bacutriletes corynactis, B. onodios, Echitriletes hispidus, Erlansonisporites sparassis, Horstisporites areolatus, H. casses, H. harrisii, H. kendalliae, Minerisporites richardsonii, M. volucris, Paxillitriletes phyllicus, Trilites candoris and T. murrayi.

Sphenophytes (horsetails)

The horsetail, Equisetum, is sometimes found upright or rooted in the swampy sediment where it grew, but more often horsetail remains have been transported to the site of fossilization. The vast majority of the plants colonized either drier river banks or habitats further inland above the level of the water table. Fragments of the plants were either shed naturally or lost following death, while those growing in the lowlands were sometimes carried away after erosion and collapse of the river banks. Incorporation of horsetails into the accumulating sediments led to the fossiliferous plant beds that are found in over 500 localities throughout Yorkshire. Harris (1961a) recognized nine species that he deliberately referred to the living genus because he did not detect any recognizable morphological differences between them. Inevitably some deposits are richer in species than others, and it is these that have yielded the most information through recognition and association of separated parts from the plants of one species. Equisetum columnare is the most common, being abundant throughout much of the succession. Its cuticles are found in over half the successful bulk macerations of rock and many of the coal seams seem to be composed entirely of this one species.

Some larger stems have been included in the genera Neocalamites and Calamites, and whorls of small lanceolate leaves have been named as Annulariopsis simpsonii.

Pteridophytes (ferns)

Ferns are abundant and so regularly encountered in the Jurassic strata that they must have been the dominant herbs on land. However, Harris (1956a) commented that the number of recognized species has diminished as work has proceeded. In 1875, Phillips recorded 45 species in Yorkshire, but Seward (1900a) reduced this to 15. However, about 30 are now recognized, the additional species representing new discoveries that have been carefully worked on. Several other Jurassic floras have more ferns and the number of Yorkshire ferns also appears low compared with the gymnosperms. The latter comparison is misleading, however, because the records of gymnosperms are augmented by those of their cuticles obtained from bulk macerations. Ferns do not have cuticles so their presence can never be recorded in this way. As a result, there are over 500 localities for gymnosperms but only about 50 for ferns (Harris, 1961a).

The Marattiaceae, which was so common in the latter part of the Palaeozoic Era, is only represented by three species: Angiopteris blackii, A. neglecta and Marattia anglica. They are placed in genera of living ferns because no recognizable morphological differences between the modern and fossil ferns have been recognized at generic level.

Two families of ferns more or less maintained their position during the early part of the Mesozoic Era. These were the Matoniaceae (Phlebopteris, Selenocarpus and Matonidium) and Osmundaceae (Todites for fertile, and Cladophlebis for similar, sterile fragments; Osmundopsis for fertile pinnae with reduced pinnules). A reconstruction of a typical Jurassic osmundacean fern is shown in (Figure 3.4).

One section of the Dipteridaceae (the Camptopteridae with Dictyophyllum and Clathropteris) rapidly declined almost to extinction, but the other section (Dipterideae with Hausmannia) was always present in small numbers. The Schizaeaceae had also declined, with only one species, Klukia exilis, represented. In great contrast, the Dicksoniaceae was described by Harris as 'surging in like an invader' by appearing during the Middle Jurassic Epoch in many parts of the world. The Dicksoniaceae is the largest family of Yorkshire Jurassic ferns and, of these, Coniopteris hymenophylloides is by far the most common. Both sections of the family (subfamilies Dicksonioideae and Thyrsopteridoideae) are known from the Jurassic System onwards although their proportions become reversed. There are about 40 living species in the Dicksonioideae and only one in the Thyrsopterioideae but in the Yorkshire flora the numbers are two (in Dicksonia) and nine (in Coniopteris, Kylikipteris and Eboracia) respectively.

Aspidistes thomasii was included in the Aspideae by Harris (1961a) because a protective flap (indusium) covers its sporangial masses (sori) as in many modern ferns.

Caytoniaceae

This isolated family consists of no more than one type of plant that is represented in the fossil record by isolated organs (Figure 3.5). The leaves, called Sagenopteris, bear two pairs of leaflets on petioles. The fruits, called Caytonia, were borne in nearly opposite pairs on short axes. They were fleshy and edible, and their remains have been found in coprolites. Thomas (1925, 1934) thought they were allied to the angiosperms but Harris (1940, 1960) later showed them to be gymnospermous fruits containing several seeds. The pollen organs, called Caytonanthus, had pollen sacs terminally attached to forked branchlets on a main rachis.

Dispersed pollen similar to that in Caytonanthus has been found inside Caytonia fruits. This intimate association of pollen (known as Vitreisporites) and fruit is supported by further regular occurrences of fruits and leaves, which led Harris (1964) to recognize three groupings representing the remains of three different plant species. There is, however, no reliable reconstruction of the whole plants although they were most probably shrubs.

Caytonia probably evolved from the glossopterids, which dominated the Southern Hemisphere during the later part of the Palaeozoic Era. The Caytoniales died out at the end of the Mesozoic Era, leaving no near relatives.

Cycadales

Many isolated leaves and fragments of leaves found throughout the deposits of the Mesozoic Era were at one time thought to be the remains of cycads. Today this is a relatively rare group of mainly tropical plants; it includes Cycas, Zamia, Macrozamia, Encephalartos and a few other, less common genera. They have a succulent, usually unbranched, trunk which gives the plants the appearance of a small palm tree. Some species are rather different in appearance, having a small tuberiferous underground stem.

We now know that there are in fact three groups of Jurassic plants with leaves of this type: cycads, bennettites and a heterogeneous group informally known as pteridosperms. They can be separated using the following key, although it must be stressed that this is merely a guide because there are doubtless exceptions.

i. Rachis never forked; vein branching is simply pinnate and the veins are nearly parallel; lamina of pinnae never strongly decurrent on to the rachis, see ii;

Rachis may be simple or forked: vein branching is two or more times pinnate with diverging veins; there is a tendency for the lamina of a pinna to be strongly decurrent on to the rachis = pteridosperms;

ii. Stomata haplocheilic (Figure 3.6)A = cycads;.

Stomata syndetocheilic (Figure 3.6)B = bennettites.

Of the genera of cycad-like leaves known from the Yorkshire Jurassic succession, Nilssonia, Ctenis and Paracycas are really cycads. A good many genera of other isolated organs can, however, be classified on more or less good evidence as cycads. A number of associations of separate organs permit some of them to be linked together. Male and female organs are known for Nilssonia and male organs for Pseudoctenis. However, perhaps the best example of a reconstructed cycad is the plant usually referred to as Beania (Figure 3.7). Unlike living cycads, it had branched stems, with each branch terminating in a cluster of elongated, very irregularly divided leaves called Nilssonia tenuinervis. Both male and female cones in living cycads tend to be stiff structures that stand proud of the crown of leaves, but in Beania they hung from the crowns rather like catkins (Harris, 1961b). The male organs are called Androstrobus wonnacottii and the female organs Beania mamayi from which the whole plant gets its name. Harris (1961b) suggested, on the basis of the local abundance of Nilssonia leaves, that Beania must have been a dominant tree of the swamps or river banks.

Pseudoctenis lanei is associated with the massive male cone Androstrobus prisma in three localities in Yorkshire, strongly suggesting to Harris (1961b) that they belonged to the same parent plant. Later he referred to the possibility of Androstrobus szei having been borne by a plant with Ctenis sulcicaulis leaves (Harris, 1964). This was supported by Hill (1990) who, as a result of further collecting from the Gristhorpe Bed, found both of these species in intimate association. However, the regular anastomosing venation of Ctenis separates it from the other cycad leaves. Androstrobus szei is also unusual in having fibrous microsporophylls and Ginkgo-like sculpture on its pollen, which led Hill (1990) to suggest that a new family is needed for Ctenis sulcicaulis and Androstrobus szei.

Pteridosperms

The informal group known as the Pteridosperms includes the foliage genera Ctenozamites, Stenopteris and Pachypteris (e.g. (Figure 3.8)). A little is known about the reproductive methods of the pteridosperms. For example, the two species of Pachypteris have leathery fern-like leaves and their pollen-producing organ is thought to be Pteroma, which has simple pinnate structures about 3 cm long, its short pinna terminating in fertile heads. Because of this, Pachypteris is now placed in the Corystospermaceae, a family better known from the lower Mesozoic deposits of southern high latitudes, such as South Africa and Australia.

Bennettitales

Many cycad-like leaves found in the Yorkshire Jurassic succession, including Anomozamites, Dictyozamites, Nilssoniopteris, Pterophyllum, Ptilophyllum, Otozamites and Zamites, in fact belong to the bennettites (Figure 3.9). Within these genera, division into species often relies on epidermal studies. However, although the combination of visual and microscopic characters permits satisfactory species determinations to be made, the differences are small and they form a nearly continuous sequence. The few remains of stems believed to be bennettitalean are included in the genus Conites.

Bennettitalean reproductive organs are often called flowers because of their superficial resemblance to angiosperm inflorescences. They cause great problems because hardly any two of them are known equally well and in comparable ways (Harris, 1969). Williamsonia Carruthers is a genus of female 'flowers'; bisexual 'flowers' are referred to Williamsoniella Thomas and male 'flowers' to Weltrichia Braun (Figure 3.10). All simple scale leaves with bennettitalean stomata are included in Cycadolepis (Harris, 1969). All female organs that are not fully known, and cannot be included with certainty in Williamsonia or Williamsoniella, are referred to Bennetticarpus.

The plants themselves varied considerably in form and we believe that there is sufficient evidence to recognize two families within the group. The Williamsoniaceae consisted of shrubs with slender, profusely branched stems bearing terminal tufts of leaves. The fructifications were borne in the forks of the branching stems and could be either unisexual (male or female) or bisexual. The Cycadeoidaceae, which lived during later part of the Jurassic Period and the Cretaceous Period, consisted of plants having robust, unbranched trunks covered in the scaly bases of lost leaves, and a terminal crown of leaves. Some had tall trunks, others were short and stockier and must have resembled large pineapple fruit. The reproductive organs are usually referred to as 'cones', although they are sometimes called 'flowers'. They grew in the axils of the leaves and, initially at least, were sunk deep within the leaf bases and scales. The generally accepted idea is that they were self-pollinated, although Crepet (1974) has suggested that insects may have aided pollination.

Several associations of leaves and reproductive organs have been found in fossil assemblages, suggesting that they came from the same parent plant. These are as follows:

• Anomozamites nilssonii–Bennetticatpus diodon–Cycadolepis stenopus;
• Nilssoniopteris major–Williamsoniella papillosa;
• Nilssoniopteris vittata–Williamsoniella coronata;
• Otozamites beanii–Williamsonia himas–Weltrichia setosa–Cycadolepis eriophous;
• Otozamites falsus–Cycadolepis pelecus;
• Otozamites gramineus–Weltrichia spectabilis–Cycadolepis spheniscus;
• Otozamites graphicus–Williamsonia himas–Cycadolepis eriophous – Bennetticarpus fragram and/or B. litchii;
• Pterophyllum thomasii–Cycadolepis rugosa;
• Ptilophyllum pecten–Weltrichia pecten–Williamsonia leckenbyi–Cycadolepis nitens–Bucklandia pustulosa;
• Ptilophyllum pectinoides–Weltrichia whitbiensis–Williamsonia hildae–Cycadolepis hypene–Bucklandia pustulosa.
• Zamites gigas–Williamsonia gigas–Weltrichia sol–Bucklandia gigas.

Ginkgoales

Ginkgos lived throughout the Mesozoic world, but just one species is extant. This is Ginkgo biloba, the maidenhair tree that only grows wild in a few remote valleys in the Zhejiang Province of eastern China.

The origin of the ginkgos is unknown but they may be derived from a group of Palaeozoic pteridosperms known as the Callistophytales (Thomas and Spicer, 1987). Although we are a long way from understanding the evolutionary history of the group, the fossil record shows that by the Jurassic Period some of the leaves were so like those of the living Ginkgo biloba that they are included in the modern genus. In one Yorkshire locality (Scalby Ness), ovules and pollen organs have been found with such leaves, an association that further supports close comparison with living Ginkgo.

Other genera of Jurassic leaves found in Yorkshire are Eremetophyllum and Pseudotorellia, which are tongue-shaped, and Baiera and Sphenobaiera, which are deeply segmented. The range of different leaf morphologies among the Yorkshire Jurassic ginkgophytes is shown in (Figure 3.11). Seeds have been attributed to Baiera furcata.

Czekanowskiales

The species included in this order have leaves that were borne in limited numbers on short shoots surrounded by scale leaves. The foliage leaves are either simple or linear (Solonites) or consist of a fan-shaped system of slender, forking segments (Czekanowskia and Sphenarion).

Until reproductive organs were found, Czekanowskia was confidently placed in the Ginkgoales (Harris, 1974). Leptostrobus has been described from Yorkshire as a capsule comparable with a Czekanowskia dwarf shoot, but with a shorter stem and just two leaves that bear ovules on their upper surfaces. This is very different from the seed-bearing organ of Ginkgo. Ixostrobus is a male cone bearing microsporophylls in its upper part, each of which has a short stalk bearing four pollen sacs and a small scale-leaf. It may be the pollen organ of Czekanowskia or Desmiophyllum, a genus of ribbon-shaped leaves found in isolation from any stem.

Pinales (conifers)

The first conifers appeared in the later part of the Palaeozoic Era, but were different from modern species particularly in having their ovules borne on short leafy shoots within the cone. Most of these primitive conifers became extinct at the end of the Palaeozoic Era. During the very early part of the Mesozoic Era, conifers of a more modern aspect appeared; their ovules were attached to small seed-scales arranged in cones.

In the final volume of his Yorkshire Jurassic Flora, Harris (1979a) described about 30 taxa of vegetative and reproductive organs representing about 23 natural species of conifers. Of these the best known have leafy shoots linked directly or indirectly with female cones and sometimes also with male cones. Other shoots, although sterile, are well preserved and relatively intact. The least-known species are represented merely by fragments of leaves.

Fossil conifer wood has also been described from the Lower and Middle Jurassic series of Yorkshire by Seward (1904, 1919), Holden (1913) and recently by Morgans (1999). The cell structure is commonly well preserved and shows distinct growth rings. Morgans found that these often include narrow latewood and false rings, which she interpreted to reflect fairly consistent growth throughout the growing season and from year to year. Water supply is thought to have been the main limiting factor affecting growth. It was probably more restricted during Bathonian times than earlier in the Jurassic Period because wood collected from the Scalby Formation shows a transition towards narrower rings of more variable width, proportionally wider latewood and more false rings.

Harris referred the conifer remains in the Yorkshire Jurassic flora to eight families, basing his definite attributions on evidence of reproductive organs and his tentative ones on cuticle characters. He also recorded specimens of Bilsdalea dura, whose family affinities could not be determined. Morgans (1999) identified a ninth family based on wood remains.

Araucariaceae

Today this family is restricted to the Southern Hemisphere. It includes the monkey puzzle and the Norfolk Island Pine. In the Mesozoic Era, however, it had a worldwide distribution. In the Yorkshire Jurassic flora, it is represented by Brachyphyllum mamillare with its female cone Araucaria pbillipsii. Morgans (1999) also reported the araucariacean wood Araucarioxylon lindlei.

Cephalotaxaceae

This family has a very limited fossil record, and today all six living species of Cephalotaxus are restricted to eastern Asia. The cones are very unusual in that only one ovule develops, forming a large olive-like seed with a stony layer surrounded by an outer fleshy one. There is just one species in the Yorkshire Jurassic succession, Elatocladus zamioides.

Cheirolepidiaceae

Harris (1979a) referred to this family as the Hirmeriellaceae but this has now been superseded by Cheirolepidiaceae. It was perhaps the most successful conifer family in the Mesozoic Era although it became extinct at the end of the Cretaceous Period. In Yorkshire, it is represented by Pagiophyllum (Hirmeriella) kurii, P. (H.) masculosum and the shoots Brachyphyllum crucis with their so-far unnamed male cones. Also possibly belonging here are Brachyphyllum ardenicum, Genitzia rigida, Pagiophyllum insigne and P. ordinatum.

Pinaceae

Today representatives of the Pinaceae are the most common conifers in the Northern Hemisphere. They include pines, larches and cedars. For many years, the fossil record of the family was thought to extend back to the Late Triassic Epoch but most of these early records are now thought to be spurious (Millar, 1998). Harris (1979a) suggested that the Yorkshire Jurassic fossil Pityocladus scarburgensis and its probable female cone scales, Schizolepis liasokeuperanus, might belong to the Pinaceae, but he also noted several differences from living members of the family. Morgans also reported species of the wood genera Cedroxylon (recorded as Cedroxylon spp.) that bear some similarity to the Pinaceae.

Podocarpaceae

The Podocarpaceae is regarded by many botanists as the most primitive living conifer family and it may be significant that the oldest known examples are Early Triassic in age, hence older than any known example of other conifer families with living representatives. The female cone Scarburgia blackii (and its probable male cone Pityanthus scalbiensis) and S. hillii belong here.

Podozamitaceae

The Podozamitaceae is a family of Mesozoic conifers with cycad-like leaves, and includes Lindleycladus lanceolatus in Yorkshire.

Taxaceae

The earliest member of this family was found in the Lower Jurassic Series of Sweden; today it includes the yew. Some botanists separate this group from the conifers because, although the foliage is conifer-like, the ovules are quite different, being borne singly at the ends of shoots and partly surrounded by a fleshy fruit-like structure called an all. In Yorkshire, the family includes Marskea jurassica, and possibly Elatocladus punctatus, E. ramosus (which might be linked to the ovuliferous dwarf shoot Poteridion hallei), Torreya gracilis and T. valida.

Taxodiaceae

Today the Taxodiaceae has a scattered distribution, with each of the seven genera restricted to a particular continent. It includes the Californian redwoods and the dawn redwood, Metasequoia, which was described as a fossil before being found living in a remote part of China. In Yorkshire, Elatides williamsonii, E. thomasii and possibly Pagiophyllum fragilis and Elatocladus laxus are thought to belong here. Morgans (1999) also reported taxodiacean wood of the genera Taxodioxylon and Xenoxylon.

Cupressaceae

Morgans (1999) reported the fossil wood genus Cupressinoxylon, which probably belongs to this family.

Palaeobotanical sites in the Yorkshire Jurassic succession

Any study of Yorkshire Jurassic plants soon reveals that there are many professionally collected specimens with no labels or, even worse, with incorrect labels. There have also been comparatively few accounts that list species by localities. Thomas (1913) and Black (1929) were the first. Spicer and Hill (1979) attempted to quantify data from the Broughton Bank locality using 'Principal Components Analysis'. The new field guide to Yorkshire Jurassic plant fossils gives lists for only four sites. Even in his monograph, Harris does not always document all the known sites for many species. Therefore, in order to provide species lists here it proved necessary to look at all the original papers, the field notes of Harris and Hill (archived in the Natural History Museum, London), and the collections in the Natural History Museum and the National Museums and Galleries of Wales. Even these lists (Table 3.1) should not be regarded as complete, because further collecting will inevitably yield new records or even new species.

Although Harris never produced floral lists for the any of the large number of sites from which he collected, he summarized his views on the distribution of plants within the succession (Harris, 1952a). The flora is distributed more or less evenly over the whole area although the composition of individual plant beds may differ quite considerably. Floral change is commonly attributable to fluctuation rather than to 'sufficiently final' events that may form the basis of a zonation. Harris recognized five ranges of species, and van Konijnenburg-van Cittert (1971) added a sixth.

1. Species ranging through the whole succession with no striking change in abundance, e.g. Brachyphyllum mamillare and Coniopteris hymenophylloides.
2. Species common in the lower three formations but absent in the Scalby Formation, e.g. Equisetum columnare.
3. Species confined to the 'Lower Deltaic Series' (= Saltwick Formation), e.g. Spenobaiera pecten.
4. Species common in both the 'Lower and Upper Deltaic Series' (= Saltwick and Scalby Formations) but rare in between, e.g. Baiera furcatas, B. gracilis, Pachypteris lanceolata, Ptilophyllum pectinoides and Zamites gigas. Some, such as Stenopteris nana and Pseudoctenis oleosa, are rare in the Lower and Upper Deltaics and so far unknown in between.
5. Species more or less abundant in the 'middle divisions' (Sycarham–Gristhorpe Members), but rare or absent below and above, e.g. Ptilophyllum pecten, Weltrichia pecten and Williamsonia leckenbyi.
6. Species characteristic of the Scalby Formation that are rare or absent from the lower beds, e.g. Czekanowskia blackii and Ginkgo buttonii.

Harris' view was that the entire flora should be treated as a single entity. He identified five types of fossiliferous beds although he stressed that they intergrade (Harris, 1952a):

1. Autochthonous beds where plants are preserved in the position of growth. The vast majority are of stands of Equisetum, sometimes with vertical sandstone casts of the stems formed when the swampy places in which they grew were suddenly overwhelmed by sand through the diversion of a river (e.g. Beast Cliff). More frequently they consist of beds of truncated roots, the upper parts having been eroded by river action (e.g. Cloughton Wyke, Gristhorpe). Many of the very thin coal seams probably represent the accumulated remains of Equisetum stems in a marsh because they yield cuticles attributable to the genus by the million. The thicker coal seams differ in being formed of logs of coniferous trees, a varying amount of Equisetum and leaves. Such deposits were allochthonous although the plant material may not have been transported far.
2. Lagoonal and sluggish river channel beds in which the prevailing sediment is fine mud (e.g. Gristhorpe and Broughton Bank).
3. River channel beds that have a higher proportion of water-worn plants (e.g. Whitby Long Bight plant bed).
4. Drifted accumulations in which the prevailing sediment is sand and all but the smallest plants are severely water-worn (e.g. Black's drifted plant bed at Scalby Wyke).
5. Redeposited plant beds where the characteristic fossils are tough cuticles that were often already partially macerated at the time of deposition. Reworking would have only involved plants from slightly older sediments because all of the erosion channels in the Middle Jurassic succession are shallow.

Based partly on these factors, 12 sites have been chosen from the 500 or so known, for several reasons. All yield good assemblages of a reasonably large number of species, and all are sufficiently different from each other to warrant inclusion. They also incorporate sites from the entire known stratigraphical range of plant fossils in the region. Some, such as Scalby Ness, are classic sites from which fossil plants have been collected since the beginning of the 19th century. Others, such as Hillhouse Nab, have only been known for a comparatively short time. All yield compression fossils that often give excellent cuticles, spores and pollen. Three dimensionally preserved specimens are rarely encountered, but have been found recently in the Runswick Bay locality. The general location of these sites in the Cleveland Basin is shown in (Figure 3.12).

References