Stepwise evolution of Paleozoic tracheophytes from South China: Contrasting leaf disparity and taxic diversity

Abstract

During the late Paleozoic, vascular land plants (tracheophytes) diversified into a remarkable variety of morphological types, ranging from tiny, aphyllous, herbaceous forms to giant leafy trees. Leaf shape is a key determinant of both function and structural diversity of plants, but relatively little is known about the tempo and mode of leaf morphological diversification and its correlation with tracheophyte diversity and abiotic changes during this remarkable macroevolutionary event, the greening of the continents. We use the extensive record of Paleozoic tracheophytes from South China to explore models of morphological evolution in early land plants. Our findings suggest that tracheophyte leaf disparity and diversity were decoupled, and that they were under different selective regimes. Two key phases in the evolution of South Chinese tracheophyte leaves can be recognized. In the first phase, from Devonian to Mississippian, taxic diversity increased substantially, as did leaf disparity, at the same time as they acquired novel features in their vascular systems, reproductive organs, and overall architecture. The second phase, through the Carboniferous-Permian transition, saw recovery of wetland communities in South China, associated with a further expansion of morphologies of simple leaves and an offset shift in morphospace occupation by compound leaves. Comparison with Euramerica suggests that the floras from South China were unique in several ways. The Late Devonian radiation of sphenophyllaleans contributed significantly to the expansion of leaf morphospace, such that the evolution of large laminate leaves in this group occurred much earlier than those in Euramerica. The Pennsylvanian decrease in taxic richness had little effect on the disparity of compound leaves. Finally, the distribution in morphospace of the Permian pecopterids, gigantopterids, and equisetaleans occurred at the periphery of Carboniferous leaf morphospace. Keywords• Leaf morphology;• Disparity;• Diversity;• Tracheophytes;• Floral provinciality;• Paleozoic IntroductionThe origin and diversification of tracheophytes (vascular plants) in the Paleozoic was a key event, as life moved from the water to colonize land (Kenrick and Crane, 1997, Vecoli et al., 2010 and Kenrick et al., 2012. The earliest known tracheophyte megafossils, from the Late Silurian-Early Devonian, are characterized by a wide distribution, low taxic diversity, and simple morphological organization (Edwards et al., 1992 and Gensel, 2008). It has been hypothesized that the increase in Paleozoic tracheophyte diversity throughout the Paleozoic was triggered by several key innovations, including increased vasculature complexity, monopodial stem branching, secondary xylem growth, formation of sporangium clusters, leaves, and heterospory (Niklas et al., 1983, Knoll et al., 1984 and Niklas, 1988.In turn, such innovations are thought to have promoted greater morphological variety and a rapid exploration and colonization of new niches (Bateman et al., 1998 and Hao and Xue, 2...

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