Lake

[SY 978 908]

Introduction

This is one of only two sites still yielding fossil plants from lower Eocene beds traditionally called the 'Dorset Pipe Clays'. These deposits are of about the same age as the London Clay, but are fluvial rather than marine deposits and thus largely lack the mangrove-palm-dominated vegetation seen in the London Clay. Lake yields the most diverse of the Dorset Pipe Clay floras, containing 69 species of mainly angiosperms and ferns, 30 of which (and 3 genera) are unique to the site.

In the western part of the Hampshire Basin, the London Clay Formation sensu stricto is mostly missing, and is replaced by fluviatile sediments. These sediments have included a number of historically important palaeobotanical sites, including Alum Bay on the Isle of Wight (see La Harpe and Salter (in Bristow, 1862), Gardner (in Reid and Strahan, 1889) and Crane, 1977, 1978) and Corfe and Studland in Dorset (Brodie, 1853; Gardner, 1877, 1886a; Gardner and von Ettingshausen, 1879; Chandler, 1962). These leaf beds now yield very little material, but in recent years it has been discovered that there are also fruit and seed deposits, which are much more productive. The site at Lake, on the shore of Poole Harbour, was discovered in 1938 by E. St J. Burton and most of what we know about the flora here is based on his collection (now stored at the Natural History Museum, London). Chandler (1955, 1962) has described the fossils from here, and some were briefly mentioned by Collinson (1980a), who also illustrated a Ficus fruit (Collinson, 1989). Fruits of the dogwood family from here were also used in a chemical investigation of fossil resins (van Aarssen et al., 1994).

Description

Stratigraphy

The low cliffs here (Figure 8.19) show fluviatile sands and muds of the Lower Eocene (Ypresian) (Plint, 1988). For recent correlations see Hooker and Collinson in Collinson (1996b). They were originally assigned to the Lower Bagshot Beds, but were renamed as they are older than the true Bagshot Formation in the Thames Valley (Curry et al., 1978). Most of the section consists of poorly consolidated sands with bands of red and white mottled clay, traditionally known as the Dorset Pipe Clay 'Series'. These are sometimes now referred to as the 'Code Member' of the Poole Formation, but we have here continued to refer to them as the 'Dorset Pipe Clays'. In the sands are numerous lenses and thin bands of carbonaceous material, mainly consisting of woody tissue but also containing fruits and seeds.

Palaeobotany

Lake has yielded nearly 70 species of plant fossil, the full assemblage being shown in (Table 8.2).

The enigmatic 'Scirpus' lakensis (see (Figure 8.20)), together with an extinct member of the caper family (Palaecleome) and pyrenes of Ehretia dominate the flora. There are also numerous seeds of lianas, especially of the icacina, moonseed and grape families. Most species are represented as carbonaceous fossils that have probably suffered significant shrinkage. They reveal no internal anatomical detail, although details of the sclerotic tissue are often preserved. Lake is the type locality for 44 species and 3 genera.

Interpretation

Lake has produced by far the most diverse and well-preserved fossil flora from the Dorset Pipe Clays. The only comparable site is Arne, but that has yielded only just over half the number of species. The Arne fossils also often suffer from the coarse overgrowth of pyrite crystals, which can mask the morphology of the fruits and seeds.

The dominant plant fossils at Lake are what Chandler (1962) identified as isolated fruits of Scirpus. However, essentially identical specimens in the Eocene strata at Messel occur within receptacular fruiting heads, showing that they are seeds and thus totally unlike Scirpus but more similar to modern Cyclanthaceae (Panama hat family) (Collinson, 1982b, 1988, 1996b). Collinson (1996b) showed that these fossils came from plants that grew along rivers and lakeshores. Further evidence of aquatic conditions is the presence of the water lily, Palaeonymphaea (Collinson, 1980a). The relatively high proportion of icacina, moonseed and grape family members probably reflects a dense swathe of lianas, growing on trees surrounding the rivers and lakes, whose fruits dropped directly into the water.

The Dorset Pipe Clay flora at Lake is important because it complements the more diverse London Clay flora, such as found at Sheppey, Bognor Regis and Herne Bay. Whereas the Nypa–Ceriops mangrove vegetation that fringed the coast dominates the London Clay flora, the Dorset Pipe Clay flora represents more inland vegetation. Only one example of a Nypa fruit has been recorded from the Dorset Pipe Clays, from a temporary exposure near Stoborough, Dorset (Collinson, 1993, 1996b). There are nevertheless many similarities between the floras, especially at the family level. Putting the dominant diagnostic elements of this interval aside (Nypa in the London Clay and 'Scirpus' for the Dorset Pipe Clays — Collinson, 2000a), both assemblages include species of the dillenia, dogbane, dogwood, flacourtia, grape, icacina, moonseed, sapodilla, spurge, sumac, sweetleaf and tea families. There are groups present at Lake and not in the London Clay, including the caper, elder, ebony and styrax families. Nevertheless, it seems that behind the effects of the 'local' mangrove versus lake or river margin vegetation, both floras are representing basically similar, paratropical forests.

(Table 8.2) Composition of floras from the Dorset Pipe Clays, Hampshire Basin. Species descriptions, or references to them, can be found in Chandler (1962), unless otherwise referenced. Discussions on some of these species can also be found in Manchester (1994), Mai and Walther (1978, 1985), Mai (2000) and Collinson (1996b, in press a). The family classification used here is summarized in Chapter 1 of the present volume

As suggested by Collinson (1983b), a superficial appraisal of the London Clay and Dorset Pipe Clay fruit and seed assemblages (compare Tables 8.1 and 8.2) suggests marked differences at the rank of species. This may, however, be as much to do with the differences in preservation as with differences in the composition of the original vegetation (Collinson, 1983b). For instance, Dunstania glandulosa, Iodes acutiformis, Icacinicarya inornata and Palaeobursera lakensis from the Dorset Pipe Clay may well be the same as D. multilocularis, I. comiculata, I. platycarpa and P bognorensis from the London Clay, respectively (Chandler, 1962). The classic London Clay palaeobotanical sites (e.g. Sheppey) produce pyrite petrifactions that often yield the type of anatomical detail not seen in the Lake fossils. Furthermore, they are less vulnerable to shrinkage during fossilization, which can produce apparently dramatic differences in size and morphology Work on the sites that yield carbonaceous London Clay-type fruits and seeds (e.g. Walton-on-the-Naze) might help resolve this problem (Collinson, 1983b).

Another difference between the floras is the apparently greater abundance of smaller fossils at Lake, such as Ficus lucidus, Ebretia lakensis, Palaeocleome lakense, Capparidispermum eocenicum and Alatospermum lakense. However, this is probably mainly because of tidal winnowing of the seeds at Sheppey removing the smaller fraction rather than any significant difference in the original vegetation. Collinson (1983b, p. 16) noted a comparable difference between surface-picked larger fruits and smaller fruits in sieved concentrates at Sheppey.

Associated with the angiosperm fruits are very small fragments of fern foliage. They show extremely fine preservation of the reproductive structures, from which Chandler (1955) was able to establish that they all belong to the now mainly tropical family, the Schizaeaceae. They demonstrate that Lygodium, Anemia and the Anemialike Ruffordia had a much wider geographical range during the Eocene Epoch than they have today (Collinson, 1996a, in press a), a situation similar to that which we observe in the angiosperms.

Conclusions

The coastal exposures at Lake have yielded the most diverse assemblages of fossil fruits and seeds from the Lower Eocene Dorset Pipe Clays, about 50 Ma old. It preserves evidence of vegetation probably surrounding lakes or rivers, together with paratropical forest trees and lianas. It complements the floras of the similar-aged London Clay, as it lacks evidence of the Nypa mangrove-palm vegetation fringing the coasts. In contrast, it is dominated by 'Scirpus' lakensis from a river or lake marginal plant. Nypa and 'S.' lakensis are together diagnostic for early and middle Eocene floras in Britain. It also includes remains of the fern family Schizaeaceae, far outside the geographical range of their living relatives.

References