Divaricate growth habit in Williamsoniaceae (Bennettitales): unravelling the ecology of a key Mesozoic plant group

Abstract

Representatives of Williamsoniacae (Bennettitales) are usually restored as small-leafed shrubs or low-growing trees with densely interlaced stems bifurcating or trifurcating at broad angles—a growth form referred to as divaricating. A divaricate plant architecture has evolved independently in at least 18 modern plant families, of which the majority occur in New Zealand, where they constitute more than 10 % of the flora. Botanists favour two or three hypotheses on the benefits of a divaricating habit for modern plants. One hypothesis favours the evolution of this habit to protect the foliage and reproductive structures from browsing by large mammals or large flightless birds, such as the recently extinct moa or other ratites. Another argues that this habit evolved in response to a dry, windy or frosty climate, whereas a third regards divarication as having evolved to optimise foliar light harvesting. Our evaluation of these hypotheses with respect to the ecological pressures known to have been experienced by Williamsoniaceae in the mid-Mesozoic reveals that although defence against browsing tetrapods cannot be excluded as a selective pressure that promoted divarication in Williamsoniaceae, many of the anatomical and morphological features of this family appear to represent responses to local environmental conditions. In this context, representatives of Williamsoniaceae have many characters that are convergent with members of Banksiinae (Proteaceae), suggesting adaptation to open vegetation communities on nutrient-deficient soils.