The fossil cuticle as a skeletal record of environmental change

The plant cuticle with its stomatal pores represents an important interface between the plant and its surrounding environment. The potential of cuticular features such as cuticle thickness, stomatal density, stomatal index and stomatal ratio to signal the environment in which they grew and developed have been reviewed. In particular, new stomatal data from three Yorkshire Middle Jurassic species, Brachyphyllum crucis Kendall, Brachyphyllum mamillare Lindley and Hutton and Ginkgo huttonii (Sternberg) Heer, have been compared with those of two selected nearest living equivalent (NLE) species Athrotaxis cupressoides and Ginkgo biloba, in an attempt to deduce the atmospheric carbon dioxide concentration from that time. It appears that the development of a thick cuticle can represent an adaptation to more than one kind of environmental constraint and evidently is a feature of certain tamnomi groups. It was concluded therefore that cuticle thickness, taken on its own, was not a suitable palaeo-ecological indicator. In contrast however stomatal parameters of fossil plants seem to have great potential as palaeo-atmospheric indicators of carbon dioxide and in this sense as "skeletal evidence of palaeo-ecological change." The stomatal density and index results of the Jurassic species were significantly lower (P < 0.0001) than those of their selected NLE species, therefore indicating elevated atmospheric CO (sub 2) concentrations for the Middle Jurassic. In addition the stomatal ratios of the Jurassic species were in agreement with those of previous Devonian and Carboniferous stomatal ratio results. These results are consistent with the evidence from carbon cycle modelling and carbon isotopic data which infer elevated atmospheric CO (sub 2) concentrutions during the Middle Jurassic of 4 to 5 times and 6 to 10 times the present atmospheric level respectively.