assume that the available fossil information only provides a poor estimate for the timing of
divergence, but in others one might consider the molecular estimate to be wrong,
overestimating the ‘true’ taxon age. Extending the temporal range of a taxon further back
in time does imply the existence of a gap in the fossil record, but given that fossils only
provide minimum age estimates (Doyle and Donoghue 1993), we should expect a gap of
some size to exist. The question then is, when is a gap large enough to indicate
incongruence? In the zoological community, at least two different methods have been
suggested for evaluating whether the fossil record is complete enough to discard the
existence of such gaps (Marshall 1998; Foote et al. 1999), but to our knowledge, neither
one has been used by the palaeobotanical community. It would, for example, be
interesting to apply these methods to the origin of eudicots. Intuitively, it is often argued
that the fossil record of triaperturate pollen (indicating the eudicot origin) is good enough
to discard the possibility of anything but a minor gap, but it would perhaps be worthwhile
to demonstrate this using either one of these methods.
Origins of angiosperms and eudicotsThe crown-group of extant angiosperms is resolved to have originated in the Early-Middle
Jurassic (158–179 Ma), and eudicots are indicated as Late Jurassic-mid-Cretaceous (131–
147 Ma). Despite the conservative age estimate for our calibration point, these estimates
are older then nearly all fossil-based estimates.
Claims of a pre-Cretaceous angiosperm diversification have been made before based
both on fossil evidence (Cornet and Habib 1992; Cornet 1993), and molecular clock
estimates (Ramshaw et al. 1972; Martin et al. 1989, 1993; Wolfe et al. 1989; Brandl et al.
1992; Goremykin et al. 1997). Such claims, however, have generally been rejected, and
Crane et al. (1995) considered the appearances of angiosperms in the Valanginian,
eudicots around the Barremian-Aptian boundary, and rosids and hamamelids in the Early
Cenomanian (Figure 8.1) as an orderly sequence, and one that argues against a pre-
Cretaceous origin. From a molecular point of view, it is not the sequence of appearances
that poses a problem, but the ages themselves. The fossil evidence indicates that the time
intervals separating basal branches are short and that major angiosperm lineages diverged
within a comparatively short time span (Hickey and Doyle 1977; Lidgard and Crane
1988; Crane and Lidgard 1989; Taylor and Hickey 1990; Crane et al. 1995).
Nevertheless, we see a substantial amount of nucleotide change on those branches
(Figure 8.1) and, in our molecular-based estimates, angiosperm and eudicot origins are
pushed further back in time than the fossil-based estimates (Figure 8.3).
If claims of a pre-Cretaceous angiosperm diversification need to confront the orderly
sequence of appearance seen in the fossil record, claims of a Cretaceous diversification
need to confront the long branch lengths observed on our molecular phylogenetic trees
(Soltis et al. 1999, 2000). There may, of course, be alternative explanations for those long
branch lengths. They may be incorrectly inferred, and true branch lengths might be
considerably shorter. Such an explanation, however, would have serious consequences
with respect to our phylogenetic analyses implying that support for basal branches is based
on spurious and incorrectly inferred evidence. Other, non-molecular, lines of evidence
corroborating these phylogenetic analyses indicate that this is unlikely (Nandi et al. 1998;
156 ANGIOSPERM DIVERGENCE