Red Cliff (Gristhorpe Bay/Cayton Bay)

[TA 083 842]

Introduction

This site (Figure 3.37) is probably the most famous Middle Jurassic plant locality in the world. Its prolific flora has been the centre of attention for geologists ever since the early days of Williamson, Phillips, and Young and Bird. Over 100 taxa have been described from these beds in the Cloughton Formation (Bajocian), of which nearly 30 are type species. The best-known member of this flora is Caytonia, whose parent plant has been reconstructed using specimens from this site.

The fossiliferous deposits are sometimes referred to as the 'Cayton Bay plant beds', but they are more correctly referred to as the 'Gristhorpe plant beds'. They crop out in an extensive foreshore exposure (Figure 3.38) and (Figure 3.39) and, although usually only accessible at low tide, have been collected from many times over the years. As Harris (1969) stated, 'its flora though large must be one of the best known [in the world]'. Young and Bird (1822, 1828) were the first to figure and describe material from here, shortly followed by Phillips (1829, 1835, 1875) and Lindley and Hutton (1837). Significantly, Lindley and Hutton showed that cuticles were still preserved in these fossils, although it was nearly a century before cuticle studies were really used in the study of the Yorkshire Jurassic floras.

(Table 3.5) The locations of the plant beds identified by T.M. Harris (in manuscript) at Red Cliff (Gristhorpe Bay).

Interest in the site waned significantly towards the end of the 19th century and by the early 20th century its exact location seems to have become forgotten. As explained earlier, Hamshaw Thomas rediscovered the plant bed in the 1920s. His work on a small ovuliferous structure, which he named Caytonia after the locality, made the locality one of the best known in Yorkshire. Harris (1941b, 1942a–c, 1943a,b, 1944a,b, 1945c, 1946a, 1948, 1949b, 1951, 1952a,b) later described or redescribed many of the species, and they formed a major part of his monograph (Harris, 1961a, 1964, 1969, 1979a; Harris et al., 1974). Kendall (1947, 1948) described a number of conifers from the bed. More recent studies have been by van Konijnenburg-van Cittert (1968, 1981, 1989) and Morgans (1999), who included the site in a field guide to the Yorkshire Jurassic floras (van Konijnenburg-van Cittert and Morgans, 1999).

Description

Stratigraphy

Immediately north of Yons Nab, the Cloughton Formation is exposed at low tides as landward dipping beds (Figure 3.40). The succession is shown in (Figure 3.41), including the position of the main plant bed. The oldest beds exposed (only at the lowest tides) belong to the Sycarham Member, and these are overlain by the marine Cayton Bay Formation (Millipore Bed). These in turn are overlain by the Gristhorpe Member, which represents a return to marshy, delta-top conditions and paralic sedimentation. Stacked tabular sheet sandstones with poorly developed channelling characterizes the unit, with burrows of the trace fossil, Diplocraterion, indicating marine conditions in the lower sandier horizons. Hancock and Fisher (1981) reported a palynological change up through the member in which marine dinoflagellate cysts, gave way to acritarchs and leiospheres, and finally tasmanitids. They suggested that this indicates gradual shallowing of a sublittoral environment, which coincided with an increase in the amount of plant material being deposited and preserved.

Harris, Kendall and Hill all collected extensively from the exposure, and several different plant beds were identified (Table 3.5), although the classic Gristhorpe Plant Bed, which is approximately 0.5 m thick and exposed continuously for over 100 metres, is the most prolific. It occurs near the base of the Gristhorpe Member and represents a channel abandonment facies (Livera and Leeder, 1981). It consists of three layers: a basal claystone rich in plant debris; a soft, grey-white claystone yielding the best specimens; and an overlying hard, micaceous silty claystone in which U-shaped burrows of Diplocraterion indicate marine influence. The composition of the flora and the preservation of the plants changes laterally every few metres.

The best-preserved leaves and fructifications occur in pale grey to white clays while the more fragmentary plants tend to be common in medium to dark grey siltstones.

Palaeobotany

The complete list of species is given in (Table 3.1) (see also (Figure 3.42), (Figure 3.43), (Figure 3.44), (Figure 3.45)). It includes representatives of all the major plant groups known to be present in the Yorkshire Jurassic succession (bryophytes, horsetails, clubmosses, ferns, caytonias, 'pteridosperms', cycads, bennettites, ginkgophytes, czekanowskiaeans and conifers).

The Gristhorpe Bed is the type and only known locality for Androstrobus manis, Otozamites thomasii, Bennetticarpus diodon and Elatocladus zamioides. It is also the type locality for another 33 species: Hepaticites arcutus, H. hymenoptera, H. wonnacottii, Aspidites thomasii, Todites thomasii, Amphorispermum pullum, Caytonanthus arberi, C. onocodes, Caytonia nathorstii, C. sewardii (Figure 3.42), Sagenopteris colpodes (Figure 3.43), S. phillipsii, Ctenis reedii, C. sulicaulis, Ctenozamites leckenbyi, Androstrobus szei, A. wonnacottii, Deltolepis crepidota, Nilssonia tenuicaulis, Pseudoctenis locusta, Stenopteris nitida, Anamozamites nilssonii, Cycadolepis nitens, C. stenopus, Otozamites beanii, Williamsoniella coronata, Eremetophyllum pubescens, Czekanowskia microphylla, C. thomasii, Solonites vimineus, Geinitzia divaricum, G. rigida, Masculostrobus harrisii and Pagiophyllum insigne.

Van Konijnenburg-van Cittert (1981, 1989) described the in-situ spores of Coniopteris hymenophylloides, C. murrayana, Dicksonia mariopteris, Eboracia lobifolia, Kylikipteris arguta, Todites denticulatus, T. princeps, T. thomasii and T. williamsonii from specimens collected here. Some specimens of the fern Phlebopteris polypodioides have recognizable fungal spots on their pinnae (Harris, 1961a). Morgans (1999) has recently described pyritised conifer wood from wood-rich layers as Cupressinoxylon spp. and Xenoxylon phyllocladoides Gothan 1906.

Interpretation

The Gristhorpe Bed contains the most diverse assemblage of plant fossils in the whole of the Yorkshire Jurassic succession. It has yielded an enormous amount of material and many accounts have been written of specimens found here. One of the best-known studies is that by Thomas (1925) on a small ovuliferous structure, which he named Caytonia after the locality, as noted above. This made Gristhorpe one of the best-known palaeobotanical localities in Yorkshire, particularly because Thomas argued that Caytonia might be an ancestral angiosperm. His reconstruction of the plant was based on the constant association of these ovuliferous structures with the male fructification Caytonanthus and the palmate leaf Sagenopteris. He thought Caytonia was a carpel enclosing ovules and possessing a stigmatic lip on which he had observed pollen. His work generated much interest in the plant, but Harris (1951, 1958) subsequently showed Caytonia to be an open pteridosperm cupule rather than a closed carpel. Pollen gained entry to the ovules through a canal below the lip; hence the grains observed by Thomas were those that had failed to enter the cupule rather than become successfully attached to a stigma. This revelation did not diminish interest in the site, and the Caytonia plant remains one of the best known of the now-extinct Mesozoic gymnosperms.

Two other important associations recorded from the Gristhorpe Bed have had important consequences for reconstructing Mesozoic plants. The most widely quoted is that of the cycad leaf Nilssonia compta, the ovuliferous cone Beania gracilis and the pollen-producing cone Androstrobus manis (Harris, 1941b, 1942b, 1964). This was used by Harris (1961b) to reconstruct the Beania tree (see (Figure 3.7)), a distinctive Jurassic cycad that is quite different from living cycads. Many palaeobotanists place this plant in its own family, the Nilssoniaceae, and it has played an important role in attempts to unravel the early evolutionary history of the cycads.

Four gynoecia, surrounded by persistent, narrow, inwardly curved bracts called Bennetticarpus diodon have been collected from the Gristhorpe Bed and nowhere else. It is, therefore, a very rare plant organ, particularly in view of the fact that it comes from what Harris (1969) has described as 'one of the most collected plant beds in the world'. Harris also recovered the scale leaf Cycadolepis stenopus from shale bearing the locally abundant bennettitalean leaf Anomozamites nilssonii (both species having this as their type locality) and suggested the two belonged together. Although the scales are twice as long as any previously described specimens, they are half the size of B. diodon. Nevertheless, similarity of epidermal structure is highly suggestive of the two being biologically linked. Unfortunately the base of B. diodon is unknown, although if it were complete it would have to be referred to either Williamsoniella or Wielandia. Indeed it was just for such uncertainty that the genus Bennetticarpus was created.

The number of species known from this site must, in part, be a result of excellent local conditions for preservation. The change in lithology from the basal claystone to the softer grey-white claystone is accompanied by a change in the quality of the plants preserved. Indeed one of the notable features of collecting in the Gristhorpe Bed is the softness of the shale and the beautiful appearance of the black plant remains. It would be interesting to know if there is any corresponding change in species content between the two claystone bands.

The assemblage is interpreted as having been deposited in a lagoon. The flora surrounding it must have been diverse for such a varied fossil assemblage to have been preserved. It includes the most diverse pteridophyte flora of any of the Yorkshire sites, although unusually, no horsetails have been recorded. Instead the rare lycophyte Lycopodites falcatus is present as are three species of the very rare thalloid liverwort genus Hepaticites. Like the liverworts, Lycopodites was probably a creeping plant so there must have been open ground for them to spread over. Where this might have been is a difficult question to answer. It is unlikely that it was around the lagoon because such an area would probably have been quickly colonized by ferns and gymnosperms. Perhaps Lycopodites and the liverworts grew on the levee banks and were washed in when parts of the bank collapsed or were eroded away. It is highly likely that the assemblage overall consists of the remains of a flora that was being carried to the site of deposition by a river system, although the good state of preservation of the plant remains suggests that they did not travel a great distance. Limited transport would have reduced damage to specimens and also permitted different organs of the same plant to become entombed together. Transportation over longer distances would have caused leaves and seeds to become separated because of their different buoyancy and waterlogging characteristics. The association of the leaves, female and males organs of Caytonia, and of the conifer Elatides williamsonii with female cones and seeds strongly suggests that the parent plants must have been growing not far from the lagoon.

Conclusion

The Gristhorpe Bed is the most famous locality in the British Jurassic succession and is widely known internationally. It has yielded a large and well-researched flora including the 'type' material for 37 species. The site has proved enormously valuable for linking dispersed organs biologically, which in turn has helped in developing whole-plant reconstructions. The most famous of these are the Caytonia and Beania plants. Even though so much appears to be known about the assemblage there is still considerable potential for further research into whole-plant reconstructions and palaeoecological interpretations.

References