why does leaf shape record temperature?
Significance: The shape of fossil leaves has long been used as a `palaeo-thermometer´ of ancient climates, especially the fraction of toothed and untoothed leaves in an assemblage¹. The basis for this proxy is the observation that the proportion of woody angiosperm species with toothed leaf margins in contemporary floras increases linearly with latitude. Yet no clear mechanistic explanation for this pattern has yet been established. Hypotheses: It is widely hypothesized, on the basis of limited measurements, that tree species with toothed-margin leaves have higher rates of transpiration early in the growing season than those with entire margins². This may favour the refilling of airspaces (embolisms) that accumulate in stem xylem during winter freeze-thaw episodes³. If correct, toothed-leaves might be an adaptation conferring ecological advantages for coping with freezing conditions more frequently encountered at higher latitudes. Further, species-dependent differences in xylem vulnera
