Fossil evidence for the onset of xeromorphy and scleromorphy in Australian Proteaceae

Abstract

The morphological response of leaves to low nutrient levels and low phosphorus (scleromorphy) in particular, has become confused in the literature with the response to low water levels (xeromorphy). However, the two can be reconciled to some degree, particularly when it becomes clear that the earliest Proteaceae in the fossil record are scleromorphic, but occurred in very wet climates, where excessive water on the leaf surface was probably a major problem. Unequivocal xeromorphic characters are interpreted as those that increase the boundary layer and thus reduce water loss per unit of leaf surface area without improving water repellancy from the surface. The clearest characters in this regard are the presence of stomata in pits, stomata individually enclosed by raised structures or revolute leaf margins. None of these characters appears prior to the Late Eocene in south-western Australia and the Oligocene in south-eastern Australia, suggesting that xeromorphy arose relatively late, at least in the areas wherefossil deposits occur. A dense covering of trichomes, often interpreted as a xeromorphic response, is here considered to have had the primary function of keeping water off the leaf surface, although it may have been exapted to a xeromorphic function when dry conditions arose. Thus scleromorphy and xeromorphy appear to have arisen at distinctly different times in Australian Proteaceae, with the latter not being a convincingly pre–Late Eocene phenomenon.