Figure 18.25   The evolution of angiosperm leaf shape and paleoclimate. (a) Samples of typical leaf shapes from North American fl oras spanning the mid-Cretaceous, showing variations in length, margins and shapes. The average leaf size declines, suggesting an increase in temperature. (b) The leaf size index (percentage of entire-margined species and average leaf size) for low-latitude North American fl oras through the Late Cretaceous shows fl uctuations. These are interpreted as the result of changes in temperature. (Based on information in Upchurch & Wolfe 1987.)

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