‘Basal’ nodesIf we treat the fossil-based estimates in a conservative way, only comparing our estimates
with the more inclusive groups, such as the Aristolochiaceae-Lactoridaceae clade
(Table 8.1), the molecular-based ages are uniformly older than thecorresponding fossil-
based estimates. However, the fact that the fossil-based estimates concern crown-groups
and not stem-groups (Magallón et al. 1999; Magallón and Sanderson 2001) implies that
this comparison may not be appropriate, and there is considerably more congruence if we
instead compare our estimates with the less inclusive groups, such as Lactoridaceae
(Table 8.1, Figures 8.3–8.6).
This congruence, between molecular and fossil-based estimates of ‘basal’ angiosperm
divergence times, should perhaps be a concern for those who reject the idea of pushing
the angiosperm and eudicot origins back in time (Crane et al. 1995). The means by which
ages of groups such as the Nymphaeaceae, crown-group Chloranthaceae (Hedyosmum), and
Illiciaceae (Figure 8.3) as well as the Platanaceae, Tetracentraceae, and Buxaceae
(Figure 8.4) are resolved in the molecular analyses ultimately depend on how the NPRS
analyses have resolved the evolutionary rates among those ‘basal’ nodes. If the analysis is
doing a reasonable job for these groups, it would seem illogical to conclude that it does so
poorly with respect to the angiosperm and eudicot origins. The fossil-based arguments
(Crane et al. 1995) imply that a rapid morphological expansion of early angiosperm
lineages was accompanied by equally rapid molecular evolution. However, looking at the
tree (Figure 8.1), such rapid molecular evolution along the spine of the tree has passed
without any trace among living early branching taxa. NPRS analyses use an optimization
scheme to resolve changes in evolutionary rates, and without any visible trace of such
changes, the angiosperm and eudicot origins are being pushed back in time.
Terminal nodesThe molecular estimates tend to underestimate ages for more terminal nodes in the tree.
This is true if we compare our estimates with the fossil-based estimates from Table 8.1
and Figures 8.3–8.6 (e.g. Zingiberaceae, Poaceae, Cyperaceae, Rutaceae, Araliaceae),
and also if we extend our comparison to other more terminal nodes with reliable fossil-
based estimates. Examples include the Moraceae, Salicaceae, and Aceraceae (Collinson et
al. 1993), and the list could no doubt be expanded through more comprehensive
comparisons.
One possible explanation for this general pattern could be that we are underestimating
the true age for our calibration point. Uncertainties surrounding both the precise
relationships of Protofagaceae and Antiquacupula within the Fagales clade, and the
documentation of Normapolles pollen from corresponding or even older ages than our
calibration point (Sims et al. 1999) may be seen to support such a view. However,
recalibrating our tree, fixing the Normapolles clade (Figure 8.5) in the late Santonian,
would increase all our ages by about 37 per cent pushing the eudicot origin into the Early
Jurassic and the angiosperm origin into the Triassic. Considering the fossil record, these
ages seem unlikely and maybe the incongruence seen among more terminal nodes simply
indicates that the NPRS analyses fail to resolve the evolutionary rates in a reasonable way.
A second, and partial, explanation for this pattern relates to the resolution of homoplasy
160 ANGIOSPERM DIVERGENCE