An. Drinnan et al., PATTERNS OF FLORAL EVOLUTION IN THE EARLY DIVERSIFICATION OF NON-MAGNOLIID DICOTYLEDONS (EUDICOTS), Plant systematics and evolution, 1994, pp. 93-122
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.