Patterns of floral evolution in the early diversification of non-magnoliid dicotyledons (eudicots)

Recent cladistic analyses of angiosperms based on both morphological and molecular sequence data recognize a major clade of dicotyledons defined by triaperturate or triaperturate-derived pollen (non-magnoliids/eudicots). Evidence from morphology, as well as the atpB and rbcL genes (cpDNA), indicates that extant Ranunculidae (e.g., Papaverales, Lardizabalaceae, Berberidaceae, Menispermaceae, Ranunculaceae) as well as “lower” Hamamelididae [e.g., Eupteleaceae (allied to Ranunculidae), Hamamelidaceae, Myrothamnaceae, Platanaceae, Trochodendraceae] and several other families (e.g., Gunneraceae, Nelumbonaceae, Proteaceae, Sabiaceae) are basal in this group. The earliest records of diagnostic eudicot pollen are of mid-late Barremian age (c. 126myr BP) and by around the latest Albian (c. 97 myr BP) several basal eudicot groups (e.g., Trochodendrales, Platanaceae, Buxaceae, and perhaps Circaeasteraceae, Myrothamnaceae, and Nelumbonaceae) are recognizable in the fossil record. Possible Hamamelidaceae and perhaps Proteaceae are present by the Turonian (c. 90 myr BP). Among basal eudicots, flowers are generally bisexual although unisexual flowers are also common. In some groups (e.g., Myrothamnaceae, Buxaceae, certain Berberidaceae), delimitation of the flower is not always clear and there is a more or less gradual transition between tepals and inflorescence bracts. Plasticity in floral form at this level of angiosperm evolution is predominantly encompassed by dimerous and trimerous cyclic floral organization and transitions from one to the other are common. Spiral floral phyllotaxis of numerous stamens and carpels is more or less restricted to the Ranunculaceae. The basic condition of the perianth in eudicots appears to lack differentiation into sepals and petals, and petals appear to have arisen independently numerous times from stamens. Based on the generality of its systematic distribution, cyclic floral architecture is probably basic for eudicots as a whole, and at this level of angiosperm evolution flowers with numerous, helically-arranged stamens and/or carpels (e.g., many Ranunculaceae) almost certainly reflect processes of secondary multiplication that have occurred independently many times.