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This
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Ancient carbon cycles and carbon-dioxide
levels
Fossil plant data are proving to be invaluable for estimating past carbon-dioxide
levels and providing insights on the functioning of ancient carbon cycles. Paleontological
data for the diversity of marine animals and land plants was integrated elegantly
with a concurrent measure of stable carbon-isotope fractionation for the last
400 million years (D.H. Rothman, Proceedings of the National Academy of Sciences,
USA, v. 98, p. 4305). Rothman's study suggests that long-term fluctuations
of carbon dioxide levels can be linked to complementary changes in the biological
and fluid reservoirs of carbon, and illustrates the value of the paleontological
records of biodiversity for estimating fluctuations of ancient of carbon dioxide
levels.
Paleobotanists continue to use analyses of stomatal density (SD) or stomatal
index (SI) from fossil leaves as indicators of paleoatmospheric carbon dioxide
concentration. The reliability of these methods was assessed by D.L. Royer (Review
of Palaeobotany and Palynology, v.114, p.1) and SD-based estimations, which
are strongly influenced by environmental stresses, were found likely to be less
accurate than SI-based carbon-dioxide reconstructions. Furthermore, Royer's
work casts some doubt on the belief that stomata are unable to respond to carbon-dioxide
concentrations above present-day levels. Clearly paleobotany will play a significant
role in interpreting not only plant responses to changes of carbon dioxide levels
in the past but in predicting the consequences of future fluctuations in carbon
dioxide levels.
Plant-insect associations
Once again, the year 2001 found many paleobotanists focusing on plant-insect
interactions. The role of paleobotany in helping to elucidate interactions within
paleocommunities was well illustrated by several contributions made in a topical
session entitled "New Frontiers in the Fossil Record of Insects and Terrestrial
Arthropods" during the Annual Meeting of the Geological Society of America
in Boston. Sara Lubkin (Cornell University) reported on a well-preserved Turonian-age
insect assemblage associated with a diverse array of angiosperm flowers. This
material has the potential to provide a valuable glimpse of plant-insect interactions
during the Cretaceous. During the same session, Conrad Labandeira (National
Museum of Natural History) presented presence-absence data for 51 plant-insect
associations on 13,441 fossil plant specimens from the K/T boundary in southwestern
North Dakota. Labandeira's data show a primary and simultaneous demise of both
plants and their herbivores at the K/T boundary. This is in marked contrast
to the Paleocene-Eocene boundary that is characterized by no net loss of associations.
Wilf and colleagues (Proceedings of the National Academy of Sciences, USA,
v. 98, p. 6221) examined insect damage on fossil leaves from the central Rocky
Mountains and documented herbivore responses to regional climate and vegetation
change during the late Paleocene through the middle Eocene. The significance
of this study is that on a regional level it appears that climate and plant
defense had linked effects on herbivory. In general, a significant component
of insect species avoided evergreen taxa and favored more palatable, short-lived
foliage. Very few botanists will argue that insects were not a driving force
influencing plant evolution. Ongoing paleobotanical work will continue to assess
insect and plant interactions in the context of both paleoecological and plant
evolutionary studies.
Early land plants and the
Devonian floras
Recent studies have extended the record of evidence for early terrestrial plants
back to the Ordovician and have provided new information about Devonian plant
systematics and diversity. A fine synthesis of our current knowledge of early
land plants in the context of chemical, geological, and physical data is presented
through a series of 13 papers published in Plants Invade the Land (Columbia
University Press, 2001), edited by P. Gensel (USA) and D. Edwards (UK). An entire
issue of Review of Palaeobotany and Palynology (v. 116, edited by P.
Gerrienne and C. Berry) was dedicated to recent studies of Devonian plants.
Papers included in the issue greatly expand the geographic range of Devonian
floras and include reports from Argentina, Brazil, Morocco, and China. Noteworthy
contributions include Roth-Nebelsick's innovative approach to modeling heat
transfer in rhyniophytic plants (p. 109) and Hueber's classification of the
enigmatic Prototaxites in the Kingdom Fungi (p. 123).
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