Richard G. Olmstead, Helen J. Michaels, Kathy M. Scott, Jeffrey D. Palmer, Monophyly of the Asteridae and Identification of Their Major Lineages Inferred From DNA Sequences of rbcL, Annals of the Missouri Botanical Garden, Vol. 79, No. 2 (1992), pp. 249-265
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Olmstead et al. (1992) studied this question.