The reconstruction of ancient plants and the environments they shaped remains as much a "forbidding pursuit" as when Joseph Hooker labeled it as such in the mid-19th century. Fortunately, paleobotanists are not easily discouraged and their remarkable narrative of the plant history continued to expand in 2003.
Earliest Land Plants
Knowledge of early land plants is based primarily on vascular plants from the
Silurian and Early Devonian. New evidence indicates that earlier fossils may
represent bryophytes, a view supported by the find of sporangial fragments with
spores showing liverwort-like ultrastructure from the Ordovician (Wellman and
colleagues, Nature, v. 425, p. 282-285). Also, chemical analysis of the
Lower Devonian Rhynie Chert plants by Charles Kevin Boyce and others (International
Journal of Plant Sciences, v. 164, p. 691-702) indicates that the conducting
cell walls in many early vascular plants were not lignified.
Gymnosperms
Sharon D. Klavins and others (International Journal of Plant Sciences,
v. 164, p. 1007-1020) described an anatomically preserved cycad pollen cone
from the Triassic. This study appeared concurrently with phylogenetic analyses
of extant cycads (Hill et al. International Journal of Plant Sciences,
v. 164, p. 933-948; Rai et al. Molecular Phylogenetics and Evolution,
v. 29, p. 350-359), raising the hope that integrated phylogenetic studies of
cycads may soon be possible.
Insights into pteridosperms were prominent. Michael T. Dunn and colleagues (Review
of Palaeobotany and Palynology, v. 124, p. 307-324) described a medullosan
from the Mississippian, which indicates that the earliest forms were vines.
Evidence for the extensive exploitation of the vine habit by Paleozoic pteridosperms
was reviewed by Michael Krings and others (The Botanical Review, v. 69,
p. 204-221).
It's becoming clear that the Cordaitales underwent a more extensive diversification
in the Paleozoic of Asia than indicated by Euro-American floras. Shi-Jun Wang
and others (International Journal of Plant Sciences, v. 164, p. 89-112)
supported this view with a reconstruction of the Chinese cordaite, Shanxioxylon.
Opponents of fossil use in phylogenetics note that many features of extinct
plants will remain unknown due to preservational limitations. Paleobotanists,
however, continue to document remarkable preservation, such as the demonstration
of pollen tubes and flagellated sperm cells in a glossopterid ovule by Harufumi
Nishida and colleagues (Nature, v. 422, p. 396-397). Pollen tubes and
embryos were also described by Ruth Stockey and others (International Journal
of Plant Sciences, v. 164, p. 251-262) from the cycadeoid Williamsonia.
Angiosperm origins
The origin of angiosperms continues to be controversial. Else Marie Friis and
others (Trends in Plant Science, v. 8, p. 369-373) present a reconsideration
of the early Chinese fossil angiosperm, Archaefructus. The putative flowers
of this plant were originally interpreted as primitive in having conduplicate
carpels and stamens occurring along an elongate floral axis. Friis and colleagues
stress the radiometric dates supporting a Lower Cretaceous age for Archaefructus,
which place it temporally close to other early angiosperms. They also question
the conduplicate carpel morphology, suggesting that the supposed carpels and
stamens are reduced flowers along an inflorescence axis. Archaefructus
could then be a relatively derived aquatic angiosperm. Regardless of the outcome
of this controversy, the great phylogenetic distance between modern angiosperms
and gymnosperms means that answers to the question of angiosperm origins will
depend on the interpretation of critical fossils like Archaefructus.
In recent decades, our knowledge of the Cretaceous angiosperm radiation has
benefited from the discovery of floral mesofossils. Nearly all of this research
has been Northern Hemisphere-based, however. In the Review of Paleobotany
and Palynology (v. 127, p. 187-217), Helena Eklund described the first Cretaceous
flowers from Antarctica.
Documentation of Tertiary angiosperm radiation was enhanced by studies from
classic localities, such as the reconstruction of a new genus of Salicaceae
from the Green River Formation by Lisa Boucher and colleagues (American Journal
of Botany, v. 90, p. 1389-1399) and Smith and Stockey's analysis of aroid
seeds from the Princeton Chert (International Journal of Plant Sciences,
v. 164, p. 239-250).
Paleoecology
Paleobotany continues to provide important data for reconstructing regional
and global climates. For example, Ulh and others (Review of Paleobotany,
v. 126, p. 49-64) and Elizabeth Kowalski and David Dilcher (Proceedings of
the National Academy of Sciences, v. 100, p. 167-170) refined techniques
for estimating paleoclimate from leaf assemblages; Nestor Ruben Cúneo
and others (Palaeogeography, Palaeoclimatology, and Palaeoecology, v.
197, p. 239-261) described a Triassic forest from Antarctica which once grew
in an area where paleoclimate models that are based on physical data had indicated
was too cold for extensive plant growth.
Paleobotany also provided evidence related to controversies over the age of
the tropical rainforest, often characterized as geologically young (Neogene
or Pleistocene). Peter Wilf and colleagues (Science, v. 300, p. 122-125)
reported on rich fossil plant assemblages from Patagonia. The Eocene age of
these deposits shows that high levels of plant diversity in warm regions of
South America are very ancient.
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