Evolution__3rd_Edition

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Figure 18.8
The rise of the angiosperms.
Angiosperms (flowering plants)
have gradually expanded in
diversity since the Cretaceous.
Gymnosperms (conifers,
cycads, and ginkgos) have
declined, as have pteridophytes.
For some species it is not
known which group they
belong to, and they are called
“incertae sedis.” Redrawn, by
permission of the publisher,
from Niklas (1986).
. . . then seeds ...
. . . and flowers
Number of species
600
500
400
300
200
100
Cycadophyta
Ginkgophyta
Taxales
Coniferophytes
Pteridophyta
CHAPTER 18 / The History of Life 539
Non-vascular and
incertae sedis
Angiosperms
Neo. B. Apt. Alb. Cen. Tur. Co. Sa. Cam. Pal. Eoc. Olig. Mio. Pli. P.
Lower Cretaceous Upper Cretaceous Paleogene Neogene Q
Cretaceous Tertiary
140 120 100 80 60 40 20 0
Time (Myr BP)
through the evolution of roots. Roots enhance weathering, and weathering removes
large amounts of carbon dioxide from the atmosphere. This reduction in carbon dioxide
set up a force of selection in favor of leaves a and the evolution of leaves, and more
powerful photosynthesis, further reduced the carbon dioxide levels.
The seed plants make up the main modern land plant groups. Fossil seeds exist in the
Carboniferous, when the coal deposits were formed. However, seed plants were a
minor group at that time. Coal is mainly formed from fossil pteridophytes. The two
groups of seed plants a gymnosperms (conifers) and angiosperms (flowering plants) a
proliferated later (Figure 18.8).
Angiosperms first clearly appear in the fossil record in the early Cretaceous, about
125 million years ago (Sun et al. 2002). Darwin once remarked that the origin of the
angiosperms was an “abominable mystery.” Modern molecular phylogenetics has
helped work out the relations within the angiosperms, and between angiosperms
and gymnosperms (see, for example, Figure 15.24, p. 461). But molecular studies of
angiosperm phylogeny have created a new abominable mystery in their turn. If we use
the molecular clock to estimate the time of angiosperm origins we find a figure of
perhaps 200–250 million years ago, well before the earliest fossils. As always with a