DOYLE, JAMES A. Section of Evolution and Ecology, University of California, Davis, CA 95616. - Congruence of molecular phylogenies and the Early Cretaceous angiosperm record.
Although molecular phylogenetic analyses refute morphological
cladistic inferences that angiosperms are related to Gnetales and
leave angiosperm outgroup relationships as unclear as ever, they
provide increasingly strong evidence on rooting of the angiosperms,
with Amborella, Nymphaeales, and a clade including Illiciales,
Trimeniaceae, and Austrobaileya as the first three branches of
the angiosperm tree, possibly followed by Chloranthaceae. These
results suggest that previously reported similarities between
pre-Albian angiosperms and these taxa are more significant than
originally recognized, whether as confirmation of molecular results or
as evidence that the Early Cretaceous radiation may be close to the
origin of crown-group angiosperms. Similarities between Aptian leaves
and the presumed basal lines include chloranthoid teeth and variable
stomata, as noted by Upchurch. According to molecular phylogenies,
the first angiosperms had columellar exine structure, and a reticulate
tectum arose soon after, whereas the supposedly primitive granular and
"atectate" exines of Magnoliales are derived. This is
further evidence against the homology of granular exine structure in
Bennettitales, Gnetales, and angiosperms. Thus the
reticulate-columellar monosulcates that dominate Hauterivian,
Barremian, and Aptian angiosperm pollen floras need not have been
preceded by long phase with granular monosulcates. Hauterivian
verrucate monosulcates described by Hughes as "CACTISULC",
which resemble pollen of Amborella, could represent the
pre-reticulate stage. Molecular trees imply that ascidiate rather
than plicate carpels are ancestral, and exotestal seeds are common in
the basal lines; this is consistent with the abundance of such carpels
and seeds in the oldest Cretaceous mesofloras described by Friis and
others. Groups appearing in the Albian belong to more deeply nested
"magnoliid" clades and the first few lines of eudicots in
molecular phylogenies (Ranunculales, Nelumbonaceae, Platanaceae,
Buxales).
Key words: angiosperms, Cretaceous, molecular systematics, paleobotany