PATTERNS OF FLORAL EVOLUTION IN THE EARLY DIVERSIFICATION OF NON-MAGNOLIID DICOTYLEDONS (EUDICOTS)

Recent cladistic analyses of angiosperms based on both morphological a nd molecular sequence data recognize a major clade of dicotyledons def ined by triaperturate or triaperturate-derived pollen (non-magnoliids/ eudicots). Evidence from morphology, as well as the atpB and rbcL gene s (cpDNA), indicates that extant Ranunculidae (e.g., Papaverales, Lard izabalaceae, Berberidaceae, Menispermaceae, Ranunculaceae) as well. as ''lower'' Hamamelididae [e.g., Eupteleaceae (allied to Ranunculidae), Hamamelidaceae, Myrothcamnaceae, Platanaceae, Trochodendraceae] and s everal other families (e.g., Gunneraceae, Nelumbonaceae, Proteaceae, S abiaceae) are basal in this group. The earliest records of diagnostic eudicot pollen are of mid-late Barremian age (c. 126 myr BP) and by ar ound the latest Albian (c. 97 myr BP) several basal eudicot groups (e. g., Trochodendrales, Platanaceae, Buxaceae, and perhaps Circaeasterace ae, 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 general ly 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 flo ral form at this level of angiosperm evolution is predominantly encomp assed by dimerous and trimerous cyclic floral organization and transit ions from one to the other are common. Spiral floral phyllotaxis of nu merous stamens and carpels is more or less restricted to the Ranuncula ceae. The basic condition of the perianth in eudicots appears to lack differentiation into sepals and petals, and petals appear to have aris en 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 evolut ion flowers with numerous, helically-arranged stamens and/or carpels ( e.g., many Ranunculaceae) almost certainly reflect processes of second ary multiplication that have occurred independently many times.