Using fossil leaves as evidence for open vegetation
The ability to discriminate between open vegetation and closed forest in the fossil record is constrained by a paucity of suitable proxies. Taxonomy-based proxies (especially the floristic composition of fossil pollen assemblages) provide the main tool for inferring vegetation type but this approach can be confounded by evolutionary changes in ecology, especially for ancient fossil assemblages. This paper considers a range of indicators of open vegetation that can be observed on fossil leaves. We show that the presence of amphistomatic leaves (i.e. leaves with stomata on both upper and lower surfaces) is unambiguously associated with open vegetation in the ecologically diverse family Proteaceae. This linkage shows very high levels of evolutionary convergence and the relationship is not explained by climate. Similar relationships are also present in a wide range of other plant groups. The combination of these empirical correlations and physiological theory suggests that there is a strong functional link between amphistomatic leaves and open vegetation. Theory says that amphistomy increases the efficiency of water transport and gas exchange by allowing two layers of high performance photosynthetic tissue to be proximal to sources of both carbon dioxide (stomata) and water (veins). However these benefits come at the cost of requiring thick leaves, which are inefficient in shaded environments. Other leaf characteristics observable on fossils (especially very thick cuticles, the presence of thick lignified hypodermal layers and the presence of deeply encrypted stomata) may also indicate the presence of open vegetation, although the functional links between these traits and vegetation type are more indirect. We propose that amphistomatic fossil leaves of dicotyledonous angiosperms provide a strong proxy for open vegetation but any inference will be enhanced by evidence from alternative proxies. We also provide an example of the application of this proxy.