A VEIN ATTEMPT AT TESTING HYPOTHESES IN EARLY ANGIOSPERM ECO-MORPHOLOGICAL EVOLUTION: CANALIZATION OF LEAF ARCHITECTURE

In 2004, Feild, Arens and colleagues (Paleobiology 30:82-107) proposed that early angiosperms gained a roothold in Cretaceous ecosystems in shady, wet and disturbed habitats. In 2005, Feild and Arens (New Phytologist 166:383-408) proposed that a key innovation allowing angiosperms to break out of the “dark and disturbed” ancestral habitat was a regular pattern of leaf veins capable of efficiently meeting the heightened water demands of full sun. This implies canalization of vein architecture as angiosperms diversified into sunny habitats. To test this hypothesis I selected 191 representatives of early diverging angiosperm lineages: Amborella, the ASIT clade (Austrobaileyaceae, Schisandraceae, Illiciaceae, Trimeniaceae), magnoliids (Magnoliales, Laurales, Piperales, Canellales), Ceratophyllaceae, and early-diverging eudicots (Ranunculales and Sabiales). Thirty-three fossil leaves from the Early Cretaceous Potomac Group were also included. For each leaf, I measured five parameters: distance between secondary veins, distance between tertiary veins, angle of secondary vein divergence, angle of tertiary vein divergence, and areole area. To assess regularity, variance of each parameter was calculated for each leaf. These data were standardized and ordinated. Basal angiosperms (Amborella, ASIT clade) are widely dispersed in the resulting plot, consistent with the predicted high variation in vein architecture early in lineage history. In contrast, magnoliids cluster tightly within the ancestral distribution, suggesting significant canalization of leaf architecture. Early-diverging eudicots and Ceratophyllum occupy a still smaller region within the magnoliid distribution. Potomac Group fossils from stratigraphic Zones I and II plot with the magnoliids; Zone III leaves plot with early-diverging eudicots. This suggests that the major phase of angiosperm leaf architecture canalization was already complete by Potomac Group time.