Introduction

By the late Jurassic, present-day plant groups had begun to appear and terrestrial floras were composed predominantly of woody gymnosperm groups and herbaceous pteridophytes (Wing & Sues, 1992). Generally, the climate was warm and equable, in the sense of an Earth with a more even latitudinal distribution of temperatures, and ice-free poles (Valdes et al., 1996) with high latitude floras. Seasonal aridity occurred at middle to low latitudes, particularly in southern Eurasia (Hallam, 1984, 1985, 1993) and parts of northern Europe (Parrish, 1993). The geochemical models considered previously predict an atmospheric CO2 concentration of 1800 ppm (Berner, 1994) and an O2 content of 22% (Berner & Canfield, 1989), a combination that would be expected to produce very low rates of photorespiration and high photosynthetic productivity. There is, however, some uncertainty surrounding the changes in the concentration of atmospheric CO2 at the boundary between the Triassic and Jurassic (T–J boundary) (Hallam & Wignall, 1997) and this represents a time suitable for investigation by the study of stomatal characters of fossil leaves (McElwain et al., 1999). These authors studied the fossilised remains of terrestrial vegetation spanning the T–J boundary from plant beds in Sweden and Greenland, with the aim of reconstructing past changes in atmospheric CO2.