Thorness Bay

[SZ 436 926]–[SZ 463 948]

Introduction

This is the classic site (Figure 9.26) for yielding plant remains from the Bembridge Marls, from the Eocene–Oligocene transitional interval (see 'Stratigraphical Background' in the introductory part of this chapter). Over 100 species of mainly angiosperms have been found here, and many are unique to this site. The preservation in some levels is particularly fine, allowing delicate structures such as seed wings and plumes to be studied. It provides important evidence of the vegetational and climatic changes occurring in Europe during the Eocene–Oligocene transition, and is thus of international importance for both palaeobotanical and palaeoclimatic studies. Uniquely in Britain, the Insect Limestone preserves plants and insects in association, allowing evidence from both to be compared in a palaeoenvironmental study. The Bembridge Limestone from here also yields an important flora.

The Insect Limestone in the Bembridge Marls (Figure 9.27) is exposed along a stretch of the Isle of Wight coast in Thorness and Gurnard Bay. It has been known as a rich source of palaeobotanical remains for over a century, with early records by Gardner (1883–1886a, 1886c, 1888). However, much of our knowledge of this flora arose from the collecting activities of an amateur geologist, James A'Court Smith, who spent some 30 years amassing a large collection of fossils from these beds. This collection eventually found its way to the British Museum (Natural History) and was the subject of the first extensive study of the flora, by Reid and Chandler (1926), who also give the historical background to A'Court Smith's work. The flora was reviewed again by Chandler (1963a), who renamed some of the species from here. Fowler (1975) described whole, fertile plants of Azolla from here, one of the few examples in the world and the only one from England (Collinson, 1991, 1996a, in press a). Collinson (1999) emphasized the importance of plumed seeds from here as the best-published example of Palaeogene seeds with a pappus (hair tuft) facilitating dispersal by wind.

Chandler (1963a) also reported a small flora from the Bembridge Limestone that underlies the Bembridge Marls. Collinson (1980b) included specimens from here in her revision of the Tertiary Nymphaeaceae. Fossils from both the Bembridge Marls and the underlying Bembridge Limestone were figured and named by Collinson (1978a) and Collinson et al. (1993a). Collinson (1983a) recorded the monocot fruit Alismaticarpum from Gurnard Bay, while van Bergen et al. (1994a,b) used specimens from here in studies on the chemistry of seed coats of fossil water plants (see also reviews in van Bergen et al., 1995, 2000). Collinson and Hooker (2000) recorded rodent-gnawed seeds from here (Figure 9.28). Jones et al. (1996) used charophytes from here in an isotopic study showing that such fossils can potentially eluci date the geochemistry of ancient water bodies.

Description

Stratigraphy

Daley (in Daley and Balson, 1999) discuss the stratigraphy of this site and its significance. The exposed sequence is less than 30 m thick, and consists mainly of 6.7 m of Bembridge Limestone Formation overlain by 21.5 m of Bembridge Marls (including, near the base, the Insect Limestone) the basal member of the Bouldnor Formation (Figure 9.29). The rocks represent brackish to freshwater deposits formed under relatively quiet conditions.

Palaeobotany

Despite the long history of research on the flora from the Insect Limestone, not all of it has been described and named. Chandler (1963a) records that at that time, 113 taxa had been discovered, but only 54 had been identified to species. Angiosperms dominate the flora and the 44 that have been identified to species are given in (Table 9.3). In addition, Chandler (1963a) lists nine ferns (Acrostichium lanzeanum (Vsiani) Chandler, ?Anemia sp. (= ?Ruffordia subcretacea (Saporta) Barthel — see Collinson, 1996a, in press a), ?Lygodium sp., and Azolla prisca Reid and Chandler emend. Fowler, 1975, plus five others), one horsetail (Equiseum lombardianum Saporta) and nine conifers 'Araucarites' gurnardi Florin, ?Pinus dixonii (Bowerbank) Gardner, P vectensis Gardner, two other Pinus spp., Pityospermum ambiguum Reid and Chandler, Sequoia couttsiae Heer (see Footnote 2 to (Table 8.2), this volume), Cupressus sp. and ?Libocedrus sp.). The fruits and seeds are mainly preserved as moulds or are carbonaceous, while the leaves may be compressions with preserved cuticles. The preservation is often remarkable, showing very fine structures such as seed and fruit wings and plumes, and whole Azolla plants.

Other horizons within the Bembridge Marls have also yielded plant remains. These are mainly fruits and seeds of aquatic plants, and include Rhamnospermum bilobatum, Stratiotes neglectus and Sabrenia chandlerae (Collinson, 1983a).

The Bembridge Limestone in the lower part of the succession has yielded a small but expanding flora. Chandler (1963a) reported a small collection made by J.F. Jackson, whilst others were figured by Collinson et al. (1993a). The species list now includes Azolla prisca, Sparganium multiloculare, Stratiotes neglectus, S. cf. headonensis, Caricoidea cf. maxima, Sabrenia chandlerae, Brasenia spinosa, Potamogeton pygmaeus, P tenuicarpus, Alrovanda intermedia, Sambucus parvula, ?Pilea sp., yuncus sp., Carpolithes sp., Rhamnospermum bilobatum, Dictyophyllum pinnatifidum, undetermined genera of the Alismataceae and Sabiaceae, a possible taxodiaceous seed, and several other undetermined seeds. Collinson et al. (1993a) also reported palynomorphs from the upper part of the Bembridge Limestone.

Collinson and Hooker (2000) described a few specimens of Stratiotes seed from here (out of hundreds that were studied) which carried trace fossils of rodent gnaw marks (Figure 9.28). Thorness Bay is only the second site at which these have been found, and shows the persistence of this feeding behaviour in glirid rodents from the time of deposition of the Totland Bay Member (Hordle) to the Bembridge Limestone.

Charophyte remains occur in both the Bembridge Limestone and the Bembridge Marls on the Isle of Wight. Groves (1926) described several specimens in the A'Court Smith collection that came from Thorness Bay but did not specify the exact horizon (see also Feist-Castel, 1977). The species described are Harrisichara tuberculata (Lyell) Grambast, Stephanochara vectensis (Groves) Grambast and 'Chara' vespiformis Groves. From the upper Bembridge Limestone, Collinson et al. (1993a) and Jones et al. (1996) also documented Nitellopsis latispira Feist-Castel, H. tuberculata (Lyell) Grambast, H. vasiformis (Reid and Groves) Grambast and Grovesichara distorta (Reid and Groves) Grambast.

Interpretation

This site has yielded by far the most diverse fossil floras from Bembridge Limestone and Bouldnor Formations in Britain, including the remains of aquatic plants, herbaceous plants, climbers and rare trees of the neighbouring forests, and charophytes. Among the vascular plants, only just over a half have been fully described and named. Nevertheless, for 39 of these species, Thorness Bay is the type locality. In addition to the 32 new angiosperm species described by Reid and Chandler (1926) (see (Table 9.3)), it is the type locality for Stephanothara vectensis, 'Chara' vespiformis, Azolla prisca, Araucarites' gurnardii, Pinus vectensis and Pityospermum ambiguum. It is the only British Tertiary site to yield examples of Equisetum, Pityospermum, Najas, Epipirenznites, Costus, Palaeocarya, Hooleya, Clematis, Myosurus, Ranunculus, Papaver, Zizyphus, Phyllanthera, Tylophora, Catalpa, Incarvillea, Radermachera, Dipelta and Flabellicula. It is also the only British site for fossils of the trumpet-creeper, milkweed, water nymph, poppy and buttercup families. With the exception of the walnut-tree family, and Dipelta (see below) and Myosurus (see Mai and Walther, 1978, 1991) these potentially exciting fossils have yet to be re-examined using modern approaches. The continued collecting opportunities at Thorness Bay will be vital to accomplish the full potential of this site.

(Table 9.3) Angiosperm floras from the Bouldnor Formation. Species descriptions or references to them may be found in Chandler (1963a) and Collinson (1980b, 1983a) unless otherwise referenced. The family classification used here is summarized in Chapter 1 of the present volume. (Note: records of Tagus and Quercus by Reid and Chandler (1926) are here considered indeterminate.)

The flora shows marked changes from the earlier floras of the British Tertiary record, in both the aquatic and forest components. The bulrush Typha latissima and the bur-reed Sparganium multiloculare become the most abundant of the aquatic species, replacing the cyperaceans as the common reed-like plants. The combination of Typha and the leather fern (Acrostichum) is a characteristic of these Eocene–Oligocene transition floras in southern Britain (Collinson, 1983a; Collinson and Hooker, 1987). Potamogeton tenuicarpus is progressively replacing P. pygmaeus, although some examples of the latter can still be found in the Bembridge Marls. Stratiotes headonensis is replaced by S. neglectus. These are all part of the progressive change in the aquatic flora of southern Britain that occurred during the Palaeogene Period (Collinson et al., 1981, 1993a; Collinson and Hooker, 1987; Collinson, 1990b, 1992).

Pappus hairs are very rare in the Palaeogene record and those from the Insect Limestone are among the best examples (Collinson, 1999). Seeds of Apocynospermum from here have a pappus up to 13 mm long. Similar, unpublished examples occur in the German Eocene succession (Collinson, in press b). Manchester (1999) mentions other examples, and draws attention to the fact that the name 'Echitonium' has priority for these seeds. The fine-grained limestone has also enabled the preservation of whole plants of the water fern Azolla — the only examples from England, and the only examples of this age in the world (Collinson, 1991, 1996a, in press a).

The forest component of the fossil flora also appears markedly different and is very rare. Chandler (1964) interpreted this as being in part taphonomic, as the Insect Limestone preferentially preserved wind-transported fruits and seeds with wings or plumes. Many of the tropical-subtropical families found in the British Eocene fossil record are absent here, despite the depositional environment not being so different. Even the palms, although still present, are of low diversity and rare. Instead we see the remains of wing nuts (Juglandaceae), and of elder and birch families, all of which are also very rare. The decline in the tropical-subtropical elements in the Bembridge Marls flora, give the clear impression of cooling conditions.

The species of the walnut-tree family (Juglandaceae) from the Insect Limestone belong to Palaeocarya of the Engelhardiae (modern tropical trees of Asia and Central America) and Hooleya of the Platycaryae (modern trees in broad-leaved forests of eastern China and Japan). Both had wind-dispersed small winged nutlets, and were discussed in detail in a wide-ranging review of the fossil history of the family by Manchester (1987).

The record of Dipelta (Caprifoliaceae, a family of modern deciduous trees of central and south China) was critically re-appraised and accepted by Manchester and Donoghue (1995). However, the same authors rejected all records of Abelia from here, excluding all of them from the Caprifoliaceae.

Fossils of the Bignoniaceae (trumpet creeper family) are very rare and the seeds from the Insect Limestone are the earliest examples (Collinson et al., 1993b). Meyer and Manchester (1997) described a similar seed of Catalpa (Indian Bean Tree) from the Oligocene deposits of Oregon. However, the Insect Limestone fossils are now in very poor condition and all three genera are each based on only a single specimen. New collections are needed to confirm the recognition of this family in the Eocene–Oligocene transitional strata.

A single partial impression of a nut assigned tentatively to Quercus by Reid and Chandler (1926) was not considered by Kvaček and Walther (1989) in their revision of European Fagaceae fossils, or by Manchester (1994) who described the oldest fossil acorn. The Insect Limestone specimen is best considered indeterminate. The leaf tentatively referred to Fagus was also not discussed by Kvaček and Walther (1989) and, as no cuticle details are known, is also probably indeterminate.

This was one of the main sites to yield material used in the study of the composition and diagenesis of the seed coats of fossil water plants by van Bergen et al. (1994a,b) (material was also obtained from Headon Hill and Bouldnor Cliff). This has helped considerably in our understanding of how these fossils are preserved and the degree to which they have become altered during fossilization. The analytical techniques used in this study are amongst a number of newly developing procedures (e.g. carbon isotope analysis for understanding palaeoatmospheric compositions) that require newly collected specimens. Without sites such as Thorness Bay, where fresh fossils can be collected in situ, work of this kind would be impossible.

Collinson et al. (1993a) analysed the differences between the floras found in the Bembridge Limestone and the Bembridge Marls, which they interpret as due to differing ecologies. The Bembridge Limestone was formed in calcareous-rich ponds or lakes in relatively dry surroundings, while the Bembridge Marls represent marshlands with more immediately adjacent woodlands. This is argued as possibly being a reflection of a fluctuation in temperatures during deposition.

Conclusions

Thorness Bay is the best site for studying the plant fossils from the Eocene–Oligocene transition beds, which are about 34–35 Ma years old. The flora consists of over 100 species and for many of them this is the only known locality. The flora consists mainly of aquatic plants, especially of bulrushes and leather ferns, but there are also rarer remains of plants from the surrounding forests, including wing nuts and elders. The flora is thus important for understanding the broader environmental history of Britain during the Palaeogene Period and its relationship to the global cooling of the climate that was then taking place. The association of plants and insects is valuable for environmental interpretation. Trace fossils on seeds have proved rodent gnawing and seed predation by glirid rodents. The high quality of preservation in the Insect Limestone at Thorness Bay provides one of the very few examples of Palaeogene seeds with a hairy pappus for wind dispersal, as well as the only known examples of whole Azolla (water fern) plants in the English Tertiary record.

References