The ages of major lineages of
green plants, including the age of land plants, vascular plants,
seed plants, angiosperms, and various groups within the angiosperms
(e.g., eudicots), have been debated extensively. In the
past 10-15 years attempts have been made to provide such estimates
based on the hypothesis of a molecular clock. Considering
the angiosperms alone, relevant references include Martin et al.
(1989, 1993), Wolfe et al. (1989), Goremykin al. (1997), and Sannderson
and Doyle (2001). Recent reviews of such efforts for seed
plants and land plants are provided in Sanderson et al. (2000),
Sanderson and Doyle (2001), and Soltis et al. (submitted).
Molecular clock estimates typically conflict
with both the fossil record and each other, often substantially
so and typically provide estimates that are "too old"
(i.e., older than any available fossil evidence; reviewed by Sanderson
and Doyle 2001). Some molecular clock estimates appear to
be way too old. For example, in a recent paper (Heckman
et al., 2001) a molecular clock estimate of 700 million years
ago (mya) was provided for the first land plants, while the oldest
known fossils are only about 400-450 mya. This type of spurious
result calls into question the utility of a molecular clock for
Recent efforts have attempted to address the
sources of error in estimating the age of lineages using molecular
data (Sanderson and Doyle, 2001; Soltis et al. submitted).
Sources of error include inaccurate calibration, an incorrect
topology, heterogeneous rates of evolution, and inadequate sampling
of taxa (reviewed in Sanderson and Doyle, 2001; Soltis et al.,
submitted). Alternative methods, designed to accommodate
rate inconstancy, have been proposed, such as non-parametric rate
smoothing [NPRS (Sanderson, 1998), likelihood methods (e.g. Yoder
and Yang, 2000), and Bayesian methods (e.g. Huelsenbeck et al.,
In considering the age of a lineage, it is
also important to keep in mind that any extant group has two ages:
the age at which its stem lineage (see B in figure below) branched
from the line leading to its extant sister group and the age of
the crown group (see A in figure below), which is the age of the
most recent common ancestor of the living members (fig. 1).
This issue can have a major impact on age considerations; however,
age estimates are often provided without consideration of this
issue (see Sanderson and Doyle, 2001).
1. Crown group (A) and stem lineage (B).
The most careful consideration of the use of
molecular data in estimating the age of the angiosperms and sources
of error is that of Sanderson and Doyle (2001). They obtained
a wide range of estimates from 68-281 mya, but most are 140-190
mya (Early Jurassic - Early Cretaceous), which overlaps estimates
based on the fossil record (Sun et al., 1999, submitted).
Wikstrom et al. (2001) used NPRS to provide estimates of
the ages of most angiosperm clades. Some of these estimates
are provided in Table 1. Whereas some estimates are close
to those based on the fossil record, many of the molecular estimates
are older than fossil estimates (see below).
Table 1. Age of eudicot clades
Minimum age for
major eudicot clades, as documented by their earliest
occurrence in the fossil record. Earliest records for
clades are taken from the earliest appearance of the oldest
family included in each clade, from Collinson et al. (1993),
and other sources (see main text). Time scale is based
on Harland et al. (1989). Eudicot clades without a known
fossil record are not included. Names in parentheses after
the age indicate the specific taxon on which minimum age
is based. Table modified from Magallón-Puebla et
||Late Miocene (pollen)
Santonian (Normapolles flowers)
||Early Eocene (Caprifoliaceae)
Goodeniaceae [pollen], Menyanthaceae)
||Middle Eocene (Eucommia)
||Early Eocene (Boraginaceae)
||Early Eocene (Convolvulaceae)
||Early Eocene (Apocynaceae,
||Late Eocene (Scrophulariaceae,
||Early Eocene (Olacaceae)
Lycophytes (377.4 mya;
also less conservatively 400 mya). The earliest record of
lycopsids (which includes the extant genera Huperzia, Selaginella,
and Isoetes) and also the more inclusive lycophytes is provided
by the genus Baragwanathia from the Upper Silurian (Ludlow, 424-410.7
mya), of Australia (Kenrick and Crane, 1997). From the lowermost
Devonian onwards (Lochkovian, 408.5-396.3 mya), both lycophytes
and lycopsids are extensively represented. Based on the
topology presented by Kenrick and Crane (1997), a conservative
estimate for the crown group lycopsids is given by the first appearance
of plants referable to the ligulate lycopsids, such as Leclerquia
(Grierson and Bonamo, 1979), which is known from the Middle Devonian
(Givetian, 380.3-377.4 mya). [This material is from Soltis et
Gymnosperms (290 mya). The most
ancient seed plants are known from the uppermost Devonian (Fameunnian,
367-362.5 mya) based on well-preserved permineralized seeds (Rothwell
and Scheckler, 1988). However, phylogenetic analyses based
on morphological data suggest that the common ancestor of extant
seed plants may be considerably younger (e.g., Crane, 1985; Doyle,
1996). A more conservative estimate of the age of crown
group seed plants (or gymnosperms in topologies in which this
group is monophyletic) is provided by the first appearance of
an extant seed plant lineage. Scattered reports of conifer
leaves and shoots are known from the Westphalian B (approximately
equivalent to the Kashirskian, 309.2-307.1 mya), but unequivocal
conifers with well-preserved female cones (e.g., Emporia lockardii;
Mapes and Rothwell, 1984, 1991) are first recorded from around
the Carboniferous-Permian boundary (290 mya). Subsequently
conifers are extensively represented through the Permian and Mesozoic.
Fossil cycad megasporophylls also first appear in the fossil record
during the early Permian (Asselian, 290-281.5 mya). [This material
is from Soltis et al., submitted.]
Angiosperms (125-131.8 mya). A conservative
estimate is provided by a variety of fossils from the Barremian-Aptian
boundary (125 mya) in the Early Cretaceous. A less conservative
estimate of 131.8 mya is provided by pollen grains from the Hauterivian
(Doyle, 1992; Hughes, 1994; Friis et al., 1999; Brenner, 1996).
[This material is from Soltis et al., submitted.] Sun et al. (1998)
proposed an origin of 144 mya for the angiosperms based on the
discovery of Archaefructus, which would place flowering plants
in the late Jurassic. However, this record has been redated
as Early Cretaceous (Swisher et al., 1999). Sun et al. (submitted)
later revised the age for Archaefructus to range from 144 mya
to a youngest estimate of 125 mya. 125 mya remains the most
conservative estimate of the age of the angiosperms.
Eudicots Magallón et al. (1999) compiled estimates
of the minimum ages of major eudicot clades based on the earliest
well-documented appearance of these groups in the fossil record.
Many of these estimates are provided on the best current estimate
of angiosperm phylogeny (Soltis et al., 2000) in Figure 2 and
also in Table 1. Dates for several lineages of basal angiosperms
have also been added to Figure 2.
2. Minimum ages of major eudicot clades and several lineages of
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