Arne

[SY 970 892]

Introduction

This exposure on the shore of Wareham Channel is one of only two sites still yielding fossil plants from the Lower Eocene beds traditionally called the 'Dorset Pipe Clays'. It complements the more diverse Lake flora by containing a number of species not found there (see (Table 8.2)).

The site was not discovered to be of palaeobotanical interest until some time after the better-known site at Lake. The only account of the fossils from the Arne exposures is by Chandler (1962).

Description

Stratigraphy

The low cliff and foreshore at Arne exposes sands and clays of the Dorset Pipe Clays (Poole Formation), which is early Eocene (Ypresian) in age. Within the sands are thin seams of carbonaceous material, probably deposited near the limits of a flood channel, and which contain numerous fossil fruits and seeds. There was also exposed (now apparently covered by silt) a band of densely packed fossil ferns.

Palaeobotany

Chandler (1962) has reported 39 species from Arne, which are listed in (Table 8.2). Most are angiosperm fruits and seeds, but there are also schizaeacean and pteridacean ferns, and foliage of a redwood conifer. As at Lake, the dominant fossil is usually the fruit of the enigmatic plant 'Scirpus' (Figure 8.20), although the pteridacean fern Acrostichum dominates at least one level. The fossils are mainly carbonaceous fragments. Coarse pyrite overgrowth sometimes occurs but unfortunately does not preserve the anatomy of the plants, as in the London Clay flora.

Interpretation

Arne is only the second site now known to yield fossil fruits and seeds from the Dorset Pipe Clays (the importance of the plant fossils from these deposits is discussed in the account of the Lake site). It is thus of national importance for understanding the paratropical forests that grew over much of southern Britain during early Palaeogene times. The Arne flora is neither as diverse nor as well preserved as the Lake flora. Arne is nevertheless of considerable interest as it yields taxa not found at Lake, including members of the pteridacean, arum, flacourtia, nightshade, moonseed and sapodilla families. Arne is the type locality for 11 species.

The presence here of the pteridacean fern Acrostichum, also recorded in a nearby borehole (Collinson, 1978b), is of considerable interest as it is far outside its current geographical distribution. It is also notable that these Eocene examples were growing in freshwater swamps, whereas today it is normally found in mangrove settings (Collinson, 1996a, in press a).

Conclusions

The exposures at Arne have yielded a nationally important assemblage of early Eocene fruits and seeds from the Dorset Pipe Clays, about 50 Ma old. It is not as diverse or well preserved as the flora from Lake, but includes a number of plant groups not found there, such as the pteridacean ferns, arums, flacourtia, sapodilla and nightshade families.

References